Significant Achievements in Space Bioscience 1958-1964
by United States. National Aeronautics and Space Administration · English · Gutenberg #40268 · 377,704 chars · public domain (US)
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Title: Significant Achievements in Space Bioscience 1958-1964
Author: United States. National Aeronautics and Space Administration
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*** START OF THE PROJECT GUTENBERG EBOOK SIGNIFICANT ACHIEVEMENTS IN SPACE BIOSCIENCE 1958-1964 ***
NASA SP-92
Significant Achievements in
Space Bioscience
1958-1964
_Scientific and Technical Information Division_ 1966
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
_Washington, D.C._
For sale by the Superintendent of Documents,
U.S. Government Printing Office
Washington, D.C., 20402—Price 55 cents
_Foreword_
This volume is one of a series which summarize the progress made during
the period 1958 through 1964 in discipline areas covered by the Space
Science and Applications Program of the United States. In this way, the
contribution made by the National Aeronautics and Space Administration
is highlighted against the background of overall progress in each
discipline. Succeeding issues will document the results from later
years.
The initial issue of this series appears in 10 volumes (NASA Special
Publications 91 to 100) which describe the achievements in the following
areas: Astronomy, Bioscience, Communications and Navigation, Geodesy,
Ionospheres and Radio Physics, Meteorology, Particles and Fields,
Planetary Atmospheres, Planetology, and Solar Physics.
Although we do not here attempt to name those who have contributed to
our program during these first 6 years, both in the experimental and
theoretical research and in the analysis, compilation, and reporting of
results, nevertheless we wish to acknowledge all the contributions to a
very fruitful program in which this country may take justifiable pride.
_Homer E. Newell_
_Associate Administrator for_
_Space Science and Applications, NASA_
_Preface_
This summary of certain aspects of the space biology program of the
National Aeronautics and Space Administration brings together some
results of NASA research and NASA-sponsored research under grants and
contracts from 1958 through 1964. Closely related research even though
not sponsored by NASA is also included.
The space biology program has had a late start in comparison with the
space physics program, and only a token program existed before 1962.
Much of the present research involves preparation of space-flight
experiments and obtainment of adequate baseline information. Perhaps
half the research results reported are derived from the NASA program.
Additional information is included from many other sources, especially
the U.S. Air Force with its long history of work in aviation and
aerospace medicine.
Relatively few biological space-flight experiments have been undertaken.
These have been to test life-support systems and to demonstrate, before
manned space flight, an animal’s capability to survive. Few critical
biological experiments have been placed in orbit by NASA, but a
biosatellite program will soon make a detailed study of the fundamental
biological effects of weightlessness, biorhythms, and radiation.
The search for extraterrestrial life has been limited to ground-based
research and planning for planetary and lunar landings. Life-detection
experiments have been developed and tested, and an important and
exciting program is being planned to detect and study extraterrestrial
life, if it exists.
Interest in space biology has been slow in developing, and there has
been some caution and controversy in the scientific community. However,
increased interest is starting to push forward the frontier of this new
and important scientific field, and future outlook appears to be
optimistic.
This summary was written and compiled by the members of the Bioscience
Programs Division of the Office of Space Science and Applications. The
report was edited and chapters 1, 3, 6, and 7 were written by Dale W.
Jenkins, Chief, Environmental Biology; chapter 2, by Gregg Mamikunian,
Staff Scientist, Exobiology; chapters 4 and 8, by Richard E. Belleville,
Chief, Behavioral Biology; and chapter 5, by George J. Jacobs, Chief,
Physical Biology.
_Contents_
page
1 BACKGROUND ................................................... 1
2 EXOBIOLOGY ................................................... 5
3 ENVIRONMENTAL BIOLOGY ........................................ 23
4 BEHAVIORAL BIOLOGY ........................................... 43
5 MOLECULAR BIOLOGY AND BIOINSTRUMENTATION ..................... 57
6 FLIGHT PROGRAMS .............................................. 65
7 MANNED SPACE FLIGHT .......................................... 77
8 SIGNIFICANCE OF THE ACHIEVEMENTS ............................. 111
REFERENCES ..................................................... 119
chapter 1
_Background_
The biological program of the National Aeronautics and Space
Administration had a late start. A small life sciences group, organized
in 1958, was concerned with life support and use of primates for system
and vehicle testing for the Mercury program. Three small suborbital
flights of biological materials were flown in space.
The Bioscience Program Office of the Office of Space Science and
Applications was organized in 1962. The goals of the Bioscience Program
are: (1) to determine if extraterrestrial life exists anywhere in the
solar system and to study its origin, nature, and level of development,
if it is present; (2) to determine the effects of space and planetary
environments on Earth organisms, including man; (3) to conduct
biological research to develop life support and protective measures for
extended manned space flight; and (4) to develop fundamental theories in
biology relative to origin, development, and relationship to
environment. Research and development has been carried out to design
life-detection experiments and instruments for future flights to Mars
and to develop experiments to study the effects of the space environment
on living organisms. A biosatellite program, started in 1963, has the
first of six flights scheduled for 1966.
Space exploration has demanded a rigorous development, especially in the
biosciences area. Investigation of the solar system for exotic life
forms, the environmental extremes to which Earth organisms (including
man) are being exposed, the possibilities for modification of planetary
environments by biological techniques yet to be developed, and the
problems of communication in biosystems are areas which have required
refinement of the theoretical framework of biology before progress could
be made rapidly enough to keep pace with technological advances in
transportation.
Of all the sciences, biology alone has not yet benefited from
comparisons with the universe beyond Earth. It is reasonable to suppose
that breakthroughs might be made in biology on the basis of comparisons
with life from other worlds. Organisms elsewhere may have found
alternatives to processes we think of as basic characteristics of life.
In contrast, physical science has advanced sufficiently to provide a
great body of laws which may be expressed in mathematical terms, and by
which phenomena may be predicted with complete accuracy. A well-known
characteristic of biological phenomena is variability. The Darwinian
concept of evolution is perhaps the only pervading generalization in
biology. This concept has been supported by evidence of a hereditary
mechanism in the discovery of genes and gene mutations.
Space bioscience represents the convergence of main disciplines with a
single orientation, whose direction is determined by the problems of
manned space travel which have, in turn, created a host of
bioengineering problems concerned with supporting man in space.
Foremost among these questions is the possibility of the existence of
extraterrestrial life. The field which is concerned with the search for
extraterrestrial life has come to be called "exobiology." In addition to
the challenge of great technological problems which must be solved,
exobiology is so closely related to the central scientific questions in
biological science that it is considered by some to be the most
significant pursuit in all of science.
One of the major opportunities already presented by the advances in
propulsion systems is the ability to escape from the influence of the
Earth, which has made possible the study of organism-environment
relationships, particularly the role that environmental stimuli play in
the establishment and maintenance of normal organization in living
systems.
Transcending even these formidable objectives of space bioscience is an
objective shared by all life sciences, the discovery of nature’s scheme
for coding the messages contained in biological molecules.
Extraterrestrial biology seeks to find not only evidence of life now
present, but the vestigial chemicals of its previous existence. The ways
and means have already been made available to study molecules on whose
long, recorded messages is written the autobiography of evolution—the
history of living organisms extending back to the beginnings of life. On
this same basis, it is now within the realm of science to foresee the
means of predicting the development of life from primordial, nonliving
chemical systems. Closely allied to the search for extraterrestrial life
is research which seeks to identify the materials and the conditions
which are the prerequisites of life.
Space bioscience research is now extending human knowledge of
fundamental biological phenomena, both in space and on Earth, just as
the physical sciences explore other aspects of the universe. The
accomplishment of bioscience objectives is totally dependent upon
advances in the technology of space flight. A highly developed
launch-vehicle capability is essential to accomplish the long-duration
missions required in the search for extraterrestrial life.
Life on other planets in the solar system (with emphasis on Mars) will
be investigated by full exploitation of space technology which will
allow both remote (orbiter) and direct (lander) observations of the
planetary atmosphere, surface, and subsurface. Certain characteristics
of terrestrial life, such as growth and reproduction, provide a basis
for relatively simple experiments which may be used on early missions to
detect the existence of life on Mars. Later missions will provide
extensive automatic laboratory capabilities for analyzing many samples
taken from various depths and locations. Because of the hypothetical
nature of current experiment designs, it is likely that visual
observations of the planet will be required. Many technical problems are
involved in storing and transmitting the large amounts of data over
planetary distances. Such visual observations might very well be crucial
in interpreting results from other experiments. Critical to all
exploration of the Moon and planets are the requirements to: (1) prevent
contamination of the environment with Earth organisms and preserve the
existing conditions of the planet for biological exploration; (2)
provide strict quarantine for anything returned to Earth from the Moon
and planets.
The biological exploration of Mars is a scientific undertaking of the
greatest significance. Its realization will be a major milestone in the
history of human achievement. The characterization of life, if present,
and study of the evolutionary processes involved and their relationship
to the evolution of terrestrial life would have a great scientific and
philosophical impact. What is at stake is nothing less than knowledge of
our place in nature.
Extended Earth orbital flights with subhuman specimens will be used to
determine the effects on Earth organisms of prolonged weightlessness,
radiation, and removal from the influence of the Earth’s rotation. Such
flights of biosatellites and other suitable spacecraft are expected to:
(1) establish biological specifications for extending the duration of
manned space flight; (2) provide a flexible means of testing unforeseen
contingencies, thus providing an effective biological backup for manned
missions; (3) yield experimental data more rapidly by virtue of the
greater number and expendability of subjects; (4) anticipate possible
delayed effects appearing in later life or in subsequent generations,
through use of animal subjects with more rapid development and aging;
(5) develop and test new physiological instrumentation techniques,
surgical preparations, prophylactic techniques, and therapeutic
procedures which are not possible on human subjects; and (6) provide a
broad background of experience and data which will permit more accurate
interpretations of observed effects of space flight on living organisms,
including man.
chapter 2
_Exobiology_
The possibility of discovering an independent life form on a planet
other than Earth presents an unequaled challenge in the history of
scientific search. Therefore, the detection of life within the solar
system is a major objective of space research in the foreseeable future.
The scientific data presently available concerning the possible
existence of a Martian life form and the chemical constitution of the
surface of Mars are disappointingly few. In fact, it is impossible to
make a statement about any of the many surface features, other than the
polar caps, with any degree of certainty. The observational results have
been accounted for by many conflicting hypotheses which can only be
resolved by the accumulation of new evidence.
The arguments supporting the existence of Martian life ([ref.1]) are
based on the following observations:
(1) The various colors, including green, exhibited by the dark areas
(2) The seasonal changes in the visual albedo and polarization of the
dark areas
(3) The ability of the dark areas to regenerate after an extensive
"duststorm"
(4) The presence of absorption bands at 3.3µ-3.7µ, attributed to
organic molecules
Conflicting interpretations of the above observations have been
advanced. The argument based on the colors is inconclusive, and several
workers have suggested that the color is a contrast effect with the
bright-reddish continents. The meager quantitative data have been
discussed by Öpik ([ref.2]) who has reduced Kozyrev’s photometric
observations of the very dark area of Syrtis Major to intrinsic
reflectivities by allowing for the estimated atmospheric attenuation and
reflectivity. Kuiper ([ref.3]) similarly demonstrated the absence of the
near-infrared reflection maximum, which is characteristic of most green
plants, indicating that chlorophyll was not responsible for the color.
The second and third arguments remain the most cogent. However, serious
limitations are imposed on the second if the severity of the Martian
climate is considered. Föcas ([ref.4]) has photometrically measured the
seasonal changes in the fine structure of the dark areas of Mars and
concludes that—
(1) The dark areas of Mars show periodic variation of intensity
following the cycle of the darkening element
(2) The average intensity of the dark area, not including the action
of the darkening waves, increases from the poles toward the
equator
(3) The action of each of the darkening waves decreases from the poles
toward the equator. This decrease is balanced in the equatorial
zone by the combined action of the two darkening waves alternately
originating at the two poles. The mechanism of the
darkness-generating element seems to be constant for all latitudes
during the Martian year.
The variation in intensity has been explained recently by nonlife
mechanisms for Depressio Hellespontica (an area showing one of the
greatest seasonal changes) ([ref.2]). Similar nonlife mechanisms may be
applicable to the other dark regions, and, thus, the "darkening" can be
used only as circumstantial evidence in support of a Martian life form.
If inorganic interpretations of the seasonal albedo variation are
accepted, then an inorganic interpretation must also be advanced for the
polarization variation. Two possibilities can be suggested:
(1) A change in surface texture, caused by varying absorption of
atmospheric constituents, causing both the albedo and polarization
to change in the manner observed
(2) A change in surface texture, in which the surface material becomes
rougher, which also explains the observed polarization data
([ref.5])
The third argument against the regenerative feature of the dark areas
being a life process has been advanced by Kuiper ([ref.6]). It is based
on atmospheric circulation causing dust, presumably lava, to be blown on
the dark areas of Mars during the late summer, autumn, and winter, and
then removed during the spring. Mamikunian and Moore have recently
advanced the similar explanation that carbonaceous chondrites or
asteroidal matter may induce the observed phenomenon if they are
abundant on the planet’s surface. The pulverized chondritic material
will exhibit a high degree of opacity due to localization and, hence, a
change in polarization characteristics and a decrease in polarization
following mixing of the chondritic material with indigenous surface
minerals.
The fourth observational argument, the Sinton bands ([ref.7]), has been
shown to be at least doubtful. Rea, Belsky, and Calvin ([ref.8])
recorded infrared reflection spectra for a large number of inorganic and
organic samples, including minerals and biological specimens, for the
purpose of interpreting the 3µ-to-1µ spectrum of Mars. These authors
state that a previous suggestion that the Martian "bands" be attributed
solely to carbohydrates is not a required conclusion. At the same time
they fail to present a satisfactory alternate explanation, and the
problem remains unsolved. More recently, Rea et al. ([ref.9]) noted the
similarity between the 3.58µ and 3.69µ minima in the Martian infrared
spectra and those of D₂O-HDO-H₂O mixtures and, particularly, of HDO.
With all this marked disagreement in interpreting the observational data
concerning Mars, it becomes clearly evident that an experimental
approach to the detection of life on Mars should provide the maximum
positive information possible. Some life-detection experiments developed
with NASA support have been summarized by Quimby ([ref.10]).
The schema of the biological exploration of a planet is to conduct a
series of complementary experiments proceeding from general to specific.
The general experiments will examine gross characteristics of the
planet’s environment and surface for determining the probability of an
active biota (life). Data from the general experiments will be
significant in—
(1) Defining the nature of specific experiments in which life
detection is the major objective; and
(2) Providing a high degree of confidence in undertaking specific
experiments, since indications from the gross characterization of
the planet in question will influence the choice and design of the
specific experiments.
The biological exploration of planets is then to be defined as the
search for those parameters relevant to the origin, development,
sustenance, and degradation of life in a planetary environment. This
definition will give rise to a critical question for each progressively
specific and complex experiment to determine—
(1) The existence of life on the planet
(2) The degree of similarity or dissimilarity (structure and function)
with respect to terrestrial life
(3) The origin of this planetary life
The immediate objective of the biological explorations of the planet is
to define the state of the planetary surface, which may exhibit the
following properties:
(1) A prebiota (defined as the absence of life)
(2) An active biota (defined as the presence of life)
(3) An extinct biota (defined as evidence of former life)
The identification and the detailed characterization of each of the
above stages of planetary development constitute the subject matter of
the biological exploration of the planets and, specifically, Mars.
THE EXPERIMENTAL INVESTIGATION OF CHEMICAL EVOLUTION
Attempts have been made to simulate and approximate models of primitive
Earth conditions for abiogenic synthesis, and successful synthesis of
essential biochemical constituents necessary for maintaining life has
been partly accomplished.
Urey ([ref.11]) has clearly pointed out the possible role of a reducing
atmosphere in the synthesis of prebiological organic molecules. Miller
([ref.12]) synthesized a variety of amino acids in a reducing atmosphere
by means of an electrical discharge. A variety of organic compounds have
been synthesized by the action of various energy sources upon reducing
atmospheres, and several investigators have extended the
Urey-Miller-type reactions to synthesize nucleic acid components
([ref.13]), adenosine triphosphate ([ref.14]), and a host of
biologically essential organic compounds.
It is likely that in the synthesis of organic moieties, simple and
specific molecules were first produced when the planets had a reducing
atmosphere. Further complexity or degradation of the organic compounds
produced varied, depending on the geochemical changes of the planet’s
surface, the atmospheric constituents, the degree of interaction between
surface and atmosphere, and the rate of the organic synthesis. Oparin
([ref.15]) presented the most detailed mechanisms for the spontaneous
generation of the first living organism arising in a sea of organic
compounds synthesized in a reducing atmosphere on Earth.
It is generally accepted that, under favorable conditions, life can
arise by spontaneous generation. A primary requirement for this
initiation is that there be abundant organic compounds concentrated in
one or more specific zones. These simple organic molecules would undergo
modification to develop a greater structural complexity and specificity,
finally giving rise to a "living" organism. Therefore, because of the
ease with which organic compounds can be synthesized under reducing
conditions, planetary surfaces may contain an abundant source of similar
organic matter. However, difficulties arise in postulating steps for
further organization or modification of the above synthesized organic
matter into a living state. Most of the original organic matter produced
in the primary reducing atmospheres of the various planets may have been
quite similar. However, major variations between planets, in chemical
evolution beyond the prebiotic stage, must have been the rule rather
than the exception.
The primary interest in this area of research has been the realization
of the possible existence of organic molecules on planetary surfaces
and, particularly, Mars. Pertinent synthesis may be either biological or
abiological. Research conducted in the simulation of cosmochemical
synthesis has used most of the available solar spectrum. Simulation
experiments devised to study the effects of these energies on the
assumed early atmosphere of the Earth have yielded products that play a
dominant role in molecular and biochemical organization of the cell.
Calvin ([ref.16]) irradiated water and carbon dioxide in a cyclotron,
obtaining formaldehyde and formic acid. Miller ([ref.17]) found that
when methane, ammonia, water, and hydrogen were subjected to a
high-frequency electrical discharge, several amino acids were produced
along with a variety of other organic compounds.
Corroborating experiments established that the synthesis of amino acids
occurred readily. The apparent mechanism for the production of amino
acids is as follows: aldehydes and hydrogen cyanide are synthesized in
the gas phase by the electrical discharge. These substances react
together and also together with ammonia in the water phase of the system
to give hydroxy and amino nitriles, which are then hydrolyzed to hydroxy
and amino acids. Among the major constituents were aspartic acid,
glutamic acid, glycine, α-alanine, and β-alanine.
The "Miller-Urey" reaction mixture has been extended and several
modifications introduced. Oró ([ref.18]) introduced hydrogen cyanide
into the system as the primary gas component. Adenine was obtained when
Oró heated a concentrated solution of hydrogen cyanide in aqueous
ammonia for several days at temperatures up to 100° C. Adenine is an
essential component of nucleic acids and of several important coenzymes.
Guanine and urea were the two other products identified in the hydrogen
cyanide reaction. Oró further obtained guanine and uracil as products of
nonenzymatic reactions by using certain purine intermediates as starting
materials.
Ponnamperuma ([ref.19]) also obtained adenine upon irradiation of
methane, ammonia, hydrogen, and water, using a high-energy electron beam
as the source of energy of irradiation. These results indicate that
adenine is very readily synthesized under abiotic conditions. Adenine,
among the biologically important purines and pyrimidines, has the
greatest resonance energy, thus making its synthesis more likely and
imparting greater radiation stability to the molecule.
The formation of adenine and guanine, the purines in RNA and DNA, by a
relatively simple abiological process lends further support to the
hypothesis that essential biochemical constituents of life may have
originated on Earth by a gradual chemical evolution and selection. In
this respect, the examination of planetary surfaces—specifically
Mars—presents practical implications for current research on the problem
of chemical evolution.
When Ponnamperuma et al. ([ref.14]) exposed adenine and ribose to
ultraviolet light in the presence of phosphate, adenosine was produced.
When the adenine and ribose were similarly exposed in the presence of
the ethyl ester of polyphosphoric acid, adenosine diphosphate (ADP) and
adenosine triphosphate (ATP) were produced. The abiological formation of
ATP was a major stride along the path of chemical evolution, since ATP
is the principal free energy source of living organisms.
Oparin ([ref.15]) postulated that α-amino acids could have been formed
nonbiologically from hydrocarbons, ammonia, and hydrogen cyanide at a
time when the Earth’s atmosphere contained these substances in high
concentrations. Oparin’s hypothesis has received strong experimental
support, as evidenced by the work of Miller ([ref.12]). Bernal
([ref.20]) has emphasized the role played by ultraviolet light in the
formation of organic compounds at a certain stage of the Earth’s
evolution.
Generally it has been believed that the first proteins or foreprotein
were nonbiologically formed by the polycondensation of preformed free
amino acids ([ref.21]). Akabori ([ref.22]) proposed a hypothesis for the
origin of the foreprotein and speculated that it must have been produced
through reactions consisting of the following three steps.
The first step is the formation of aminoacetonitrile from formaldehyde,
ammonia, and hydrogen cyanide.
CH₂O + NH₃ + HCN ————> H₂N—CH₂—CN + H₂O
The second is the polymerization of aminoacetonitrile on a solid
surface, probably absorbed on clay, followed by the hydrolysis of the
polymer to polyglycine and ammonia.
x H₂N—CH₂—CN ————> (—NH—CH₂—C—)ₓ
||
||
NH
|
| + x H₂O
|
V
(—NH—CH₂—CO—)ₓ + x NH₃
The third step is the introduction of side chains into polyglycine by
the reaction with aldehydes or with unsaturated hydrocarbons. Akabori
has demonstrated experimentally the formation of cystinyl and cysteinyl
residue in his above-postulated mechanism.
Fox’s theory of thermal copolymerization ([ref.23]) suggests that
proteins or like molecular units could have been formed in the Earth’s
crust, under geothermal conditions. The accumulated amino acids were
heat polymerized and transported into the primary oceans for further
modifications. Fox has obtained polymers consisting of all 18 amino
acids usually present in proteins. The polymerization is generally done
at 160° C to 200° C, although in the presence of polyphosphoric acid it
can be accomplished at temperatures below 100° C. Molecular weights
increased from 3600 in a proteinoid made at 160° C to 8600 in one made
at 190° C.
Fox showed that when hot saturated solutions of thermal copolymers
containing the 18 common amino acids were allowed to cool, large numbers
of uniform, relatively firm, and elastic spherules separate. These range
from 0.2µ to 60µ in diameter and are quite uniform within each
preparation. Various chemical observations suggest the presence of
peptide bonds in the structural organization of these proteinoids.
Continuing observations of these microspheres have established further
characteristics that point to the possibility of their interpretation as
a kind of primitive protein macromolecule with self-organizing
properties, such that a primitive form of cell, with boundary and other
properties, might form.
In laboratory experiments the behavior of gram-negative and
gram-positive microspheres in dilute alkali parallels that of
gram-negative and gram-positive bacteria ([ref.23]). Furthermore,
time-lapse studies indicate that the proteinoid microspheres undergo a
septate kind of fission, mimicking cell division as shown in figure 1.
Cytochemical studies show that the microsphere’s boundary is
membranelike in having a primitive selectivity. Electron micrographs of
sections of stained microspheres also indicate the presence of a
boundary.
Oparin ([ref.15]) states that the type of organization peculiar to life
could only result from the evolution of a multimolecular organic system
separated from its environment by a distinct boundary but constantly
interacting with this environment. In his concept of coacervates as
precell models, Oparin ([ref.24]) indicates that present-day protoplasm
possesses a number of features similar to coacervate structure. These
coacervates could represent the starting point for evolution leading to
the origin of life. Moreover, in the course of their evolution the
initial systems may gradually become more complex. Oparin also showed
([ref.15]) that mixing solutions of different proteins and other
substances of high molecular weight produced these coacervate droplets.
These droplets are characterized by the formation of a surface layer
with altered structure and mechanical properties, thus providing a
somewhat selective barrier in which to house a molecular system capable
of replication. However, these coacervates are unstable structurally.
[Illustration: Figure 1.—_Protenoid microspheres undergoing septate
fission. Small microspheres and filamentous associations thereof are
also shown ([ref.25])._]
The NASA program has further provided considerable impetus for
continuing research with respect to the chemical evolution of life,
since its life-detection experiments may encounter prebiological
molecules in their search for extraterrestrial life on other planetary
surfaces.
In the area of exobiological research, the significant accomplishments
to date have been—
(1) The reconstruction of some of the pathways which may have led to
the origin of life, by means of laboratory simulation of processes
yielding prebiological organic molecules
(2) The developments in experimental and theoretical biology;
specifically, the role of nucleic acid-protein interactions in
storage and transmission of information both within living cells
and from generation to generation of cells
(3) The suspected role of DNA in information storage and the
development of new concepts of the coding mechanism in DNA that
may lead to a universal biological theory embracing evolutionary,
as well as homeostatic, adaptation to environment and learned
behavioral systems
With the essential biochemical constituents of life and the mechanism of
replication beginning to be understood, the challenge for the synthesis
of living matter by abiogenic experimental techniques has become to many
scientists the ultimate goal of the scientific era.
NASA has established an exobiology laboratory at Ames Research Center in
addition to the sizable support of research at various academic centers
of excellence for the continuation of abiogenic synthesis.
Although research on organochemical evolution is in its infancy, the
data from relatively few experiments have already created an immense
enthusiasm for knowledge of the biochemical pathways of evolution. This
kind of research will ultimately elucidate the terrestrial evolution of
life and, perhaps, the nature of life on other planetary bodies and the
distribution of life in our galaxy.
This program, with its vast demands on the scientific community at
large, is coordinated with related endeavors of a number of Federal
agencies. It is allied with certain biochemical studies at the National
Institutes of Health for the eventual elucidation of the dynamic
pathways in cosmochemical synthesis of life’s essential biochemical
constituents.
METEORITES AND ORGANIC GEOCHEMISTRY
Meteorites
A significant area of exobiological research is the investigation of a
special class of stony meteorites known as "carbonaceous chondrites." It
is increasingly apparent that almost all life-detection concepts rely on
the eventual analysis of the solid materials that may be available on
Mars and other planetary surfaces. Cosmic dust and meteorites are two
classes of material bodies that reach the Earth from outer space. The
carbonaceous chondrites are the only extraterrestrial materials known to
contain organic carbon.
The study of meteorites has generated an astonishing diversity of
hypotheses. There is agreement at only one point: that meteorites are
preserved chunks of very ancient, perhaps primordial, planetary matter
and that when we are able to understand the curious structures and
chemical and isotopic variations in the meteorites, we will also know a
great deal about early planetary (and perhaps preplanetary) history.
Meteorites provide a more representative sample of average planetary
matter than the highly differentiated crust of the Earth. Although it is
known that the meteorite parent bodies ceased to be geochemically active
shortly after their formation, some 4½ billion years ago, there is no
consensus on the nature of the meteorite parent bodies, not even on such
basic properties as size, location, and multiplicity. This is not
surprising because the meteorite samples commonly available for study
represent only about 10⁻²³ to 10⁻²⁶ of the parent body.
Carbonaceous Meteorites
Analysis and characterization of the chemical constituents (organic) of
carbonaceous chondrites, including the possible mechanism of their
formation, may be expected to improve methods of analyzing samples from
the Moon and planets and of interpreting remote automated biological
analyses on the planets’ surfaces.
Carbon has been detected in all meteorites analyzed; however, both the
amount and forms present vary considerably. Among the forms of meteorite
carbon are diamond, graphite, cohenite (Fe,Ni,Co)₃C, moissanite SiC,
calcite CaCO₃, dolomite (Ca,Mg)CO₃, bruennerite (Mg,Fe)CO₃. A summary of
the results of carbon analyses on large numbers of meteorites is given
in table I ([ref.26]).
Table I.—_Meteorite Carbon_
-----------------------------------------------------------------
Meteorite group Number Mean carbon content,
analyzed percent by weight
-----------------------------------------------------------------
Pallasites 10 0.08
-----------------------------------------------------------------
Ureilites 2 .69
-----------------------------------------------------------------
Bronzite chondrites 12 .05
-----------------------------------------------------------------
Hypersthene chondrites 8 .04
-----------------------------------------------------------------
Enstatite chondrites 8 .29
-----------------------------------------------------------------
Carbonaceous chondrites 16 2.04
-----------------------------------------------------------------
Most meteorites possess only traces of carbon, and studies of this
carbon indicate that it is composed largely of graphite, cohenite, and
moissanite, with some diamond. However, studies of the carbon in the
carbonaceous chondrites have failed to detect any of these forms. Some
carbonates are present in a minority of the carbonaceous group, but
account for only a small percentage of the total carbon (perhaps about
10 percent of the total C in type I only).
The carbonaceous chondrites contain organic carbon. The word "organic"
is not used in a biological sense, merely as a chemical term to describe
compounds of carbon other than carbonates, bicarbonates, and carbides.
No evidence has been found of any form of carbon other than organic,
except for traces of carbonates.
Various studies have demonstrated possible methods of estimating the
total amount of organic matter present in meteorites. Wiik ([ref.27])
has suggested that organics can be estimated by measuring the loss of
weight on ignition. Unfortunately, this method has several disadvantages
and gives very low values. Corrections must be made for weight gains due
to oxidation of reduced constituents, such as FeO, Fe, Ni, and Co, and
for weight losses due to H₂O, S, etc. The water loss is exceedingly
difficult to estimate, as part comes from the combustion of organic
hydrogen and part comes from the loss of mineral-bound water. The carbon
also proves difficult to combust completely, and high temperatures (over
1000° C) are required for efficient conversion to CO₂.
In one study the major fraction of organic matter removed proved to have
a carbon content of about 47 percent ([ref.28]). Thus, if all the
meteorite carbon is present as organic matter of approximately this
composition, total organics must be approximately double the carbon
content; that is, 2 percent by weight carbon indicates 4 percent by
weight organic matter. This estimate may be too low, for Mueller
([ref.29]) has extracted a major organic fraction containing only 24
percent carbon; however, this work has not been confirmed for other
meteorites.
Briggs and Mamikunian ([ref.26]) have pointed out that only 25 percent
of the organic matter has been extracted, and only about 5 percent of
this has been chemically characterized. Most of this 5 percent is a
complex mixture of hydroxylated aromatic acids together with
hydrocarbons of the aliphatic, napalicyclic, and aromatic series. Small
amounts of amino acids, sugars, and fatty acids are also present.
Thus far, these chemical analyses point to an abiogenic origin for the
organic matter, and no conclusive evidence exists of biological activity
on the meteorite parent body. Microbiological investigations of samples
of the carbonaceous chondrites have yielded only inconclusive evidence
on the problem of "organized elements."
Several of these microstructures from different carbonaceous chondrites
are illustrated in a paper by Mamikunian and Briggs ([ref.30]). It has
been difficult to identify the organized structures, and most do not
have morphologies identical to known terrestrial micro-organisms.
However, they may prove to be a variety of mineral grains, droplets of
organic matter and sulfur, as well as a small amount of contaminating
terrestrial debris.
A comparison between the photographs of the organized elements observed
in the Orgueil and Ivuna meteorites and the synthetic proteinoid
microspheres observed by Fox ([ref.25]) point to similarities between
the two. One inference from this finding is that the organized elements
in carbonaceous chondrites were never alive but, rather, should be
considered as natural experiments in molecular evolution. Also, these
similarities strengthen the belief that the laboratory experiments are
similar to the natural experiments in space.
In cooperation with the Smithsonian Astrophysical Observatory, NASA has
a network to track meteors in the Midwest (South Dakota, Nebraska,
Kansas, Oklahoma, Iowa, Missouri, and Illinois). Photographs of meteor
trails are used for scientific study, and attempts are made to track and
recover meteorites for examination for traces of organic material of
extraterrestrial origin.
Fundamental research in terrestrial organic geochemistry has shown that
ancient sediments and drill core samples subjected to organic analysis
contain certain stable biochemical components of past life. This
preserved record is significant not only in studies of early-life
chemical pathways but also in studies of the interaction of organic
matter with the geological factors. Since life on any planetary body
will interact with the soil, or surface material, it is of interest to
understand the relationship.
CONCEPTS FOR DETECTION OF EXTRATERRESTRIAL LIFE
It is not possible to present completely convincing evidence for the
existence of extraterrestrial life. The problem often reduces to
probabilities and to estimates of observational reliability. In almost
all cases the evidence is optimistically considered strongly suggestive
of—or, at the worst, not inconsistent with—the existence of
extraterrestrial life. Alternatively, there is a pessimistic view that
the evidence advanced for extraterrestrial life is unconvincing,
irrelevant, or has another, nonbiological explanation.
In studies of the laboratory synthesis of life-related compounds and its
significance concerning the origin of life, several results seem to
suggest that organochemical synthesis is a general process, occurring
perhaps on all planets which retain a reducing atmosphere. The
temperature ranges must be such that precursors and reaction products
are not thermally dissociated. The reaction rates for the synthesis of
more complex organic molecules diminish to a negligible value when the
temperature range is below 100° C.
Besides the planetary parameter of temperature, an even more fundamental
necessity for a living state exists—a liquid solvent system. For
terrestrial life forms, water serves this purpose. Water has this and
other properties of biological significance because of hydrogen bonding
between adjacent molecules in the liquid state.
Ultraviolet radiation could serve as an extraterrestrial energy source
for organic synthesis. Research shows that, while an atmosphere is
important, living systems can survive a wide range of ambient pressures
and are little affected by a wide range of magnetic field strengths.
Oxygen is not a prerequisite for all living systems. While it is
sometimes concluded that free oxygen is needed for all but the simplest
organisms, less efficient metabolic processes coupled with higher food
collection efficiency—or a more sluggish metabolism—would seem to do
just as well. Earth is the only planet in the solar system on which
molecular oxygen is known to be present in large amounts. Since plant
photosynthesis is the primary source of atmospheric oxygen, it seems
safe to infer that no other planet has large-scale plant photosynthesis
accompanied by the production of oxygen.
The possibility of the existence of extraterrestrial life raises the
important question of man’s being able to detect it. Research on
extraterrestrial life detection is predicated on the ability to develop
ways to detect it even when the living systems are based on principles
entirely different from those on Earth.
The substitution of various molecules for those of known biological
significance to living organisms as we know them has been investigated;
the substitution of NH₂ for OH in ammonia-rich environments leads to a
diverse, and biologically very promising, chemistry. The hypothesis that
silicon may replace carbon does not support the construction of
extraterrestrial genetics based on silicon compounds. (Silicon compounds
participate in redistribution reactions which tend to maximize the
randomness of silicon bonding, and the stable retention of genetic
information over long time periods is thus very improbable.)
Evidence relevant to life on Mars has been summarized by Sagan (ch. 1 of
[ref.10]):
_The Origin of Life_
In the past decade, considerable advances have been made in our
knowledge of the probable processes leading to the origin of
life on Earth. A succession of laboratory experiments has shown
that essentially all the organic building blocks of contemporary
terrestrial organisms can be synthesized by supplying energy to
a mixture of the hydrogen-rich gases of the primitive
terrestrial atmosphere. It now seems likely that the laboratory
synthesis of a self-replicating molecular system is only a short
time away from realization. The syntheses of similar systems in
the primitive terrestrial oceans must have occurred—collections
of molecules which were so constructed that, by the laws of
physics and chemistry, they forced the production of identical
copies of themselves out of the building blocks in the
surrounding medium. Such a system satisfies many of the criteria
for Darwinian natural selection, and the long evolutionary path
from molecule to advanced organism can then be understood. Since
nothing except very general primitive atmospheric conditions and
energy sources are required for such syntheses, it is possible
that similar events occurred in the early history of Mars and
that life may have come into being on that planet several
billions of years ago. Its subsequent evolution, in response to
the changing Martian environment, would have produced organisms
quite different from those which now inhabit Earth.
_Simulation Experiments_
Experiments have been performed in which terrestrial
micro-organisms have been introduced into simulated Martian
environments, with atmospheres composed of nitrogen and carbon
dioxide, no oxygen, very little water, a daily temperature
variation from +20° to -60° C, and high ultraviolet fluxes. It
was found that in every sample of terrestrial soil used there
were a few varieties of micro-organisms which easily survived on
"Mars." When the local abundance of water was increased,
terrestrial micro-organisms were able to grow. Indigenous
Martian organisms may be even more efficient in coping with the
apparent rigors of their environment. These findings underscore
the necessity for sterilizing Mars entry vehicles so as not to
perform accidental biological contamination of that planet and
obscure the subsequent search for extraterrestrial life.
_Direct Searches for Life on Mars_
The early evidence for life on Mars—namely, reports of vivid
green coloration and the so-called "canals"—are now known to be
largely illusory. There are three major areas of contemporary
investigation: visual, polarimetric, and spectrographic.
As the Martian polar ice cap recedes each spring, a wave of
darkening propagates through the Martian dark areas, sharpening
their outlines and increasing their contrast with the
surrounding deserts. These changes occur during periods of
relatively high humidity and relatively high daytime
temperatures. A related dark collar, not due to simple dampening
of the soil, follows the edge of the polar cap in its
regression. Occasional nonseasonal changes in the form of the
Martian dark regions have been observed and sometimes cover vast
areas of surface.
Observations of the polarization of sunlight reflected from the
Martian dark areas indicate that the small particles covering
the dark areas change their size distribution in the spring,
while the particles covering the bright areas _do_ not show any
analogous changes.
Finally, infrared spectroscopic observations of the Martian dark
areas show three spectral features which, to date, seem to be
interpretable only in terms of organic matter, the particular
molecules giving rise to the absorptions being hydrocarbons and
aldehydes. [However, see p. 7 and Rea et al. ([ref.9]).]
Taken together, these observations suggest, but do not
conclusively prove, that the Martian dark areas are covered with
small organisms composed of familiar types of organic matter,
which change their size and darkness in response to the moisture
and heat of the Martian spring. We have no evidence either for
or against the existence of more advanced life forms. There is
much more information which _can_ be garnered from the ground,
balloons, Earth satellites, Mars flybys, and Mars orbiters, but
the critical tests for life on Mars can only be made from
landing vehicles equipped with experimental packages....
Results of Kaplan et al. ([ref.31]) indicate that Mars has no detectable
oxygen, but does contain small amounts of water vapor, more abundant
carbon dioxide, possibly a large surface flux of solar ultraviolet
radiation, and estimated daily temperature variations of 100° C at many
latitudes. Studies have shown that terrestrial micro-organisms can
survive these extremely harsh environments. Furthermore, a variety of
physiological and ecological adaptations might enable the biota to
survive the low nighttime temperatures and intracellular ice
crystallization.
Less evidence is available to support the possibility of
extraterrestrial life on other planets. The Moon has no atmosphere, and
extremes of temperature characterize its surface. However, the Moon
could have a layer of subsurface permafrost beneath which liquid water
might be trapped. The temperatures of these strata might be biologically
moderate.
Studies by Davis and Libby ([ref.32]) on the atmosphere of Jupiter
support the possibility of the production of organic matter in its
atmosphere in a manner analogous to the processes which may have led to
the synthesis of organic molecules in the Earth’s early history. It is
difficult to assess the possibility that life has evolved on Jupiter
during the 4- or 5-billion-year period in which the planet has retained
a reducing atmosphere.
The question of extraterrestrial life and of the origin of life is
interwoven. Discovery of the first and analysis of its nature may very
well elucidate the second.
The oldest form of fossil known today is that of a microscopic plant
similar in form to common algae found in ponds and lakes. Scientists
know that similar organisms flourished in the ancient seas over 2
billion years ago. However, since algae are a relatively complex form of
life, life in some simpler form could have originated much earlier.
Organic material similar to that found in modern organisms can be
detected in these ancient deposits as well as in much older Precambrian
rocks.
Although the planets now have differing atmospheres, in their early
stages the atmospheres of all the planets may have been essentially the
same. The most widely held theory of the origin of the solar system
states that the planets were formed from vast clouds of material
containing the elements in their cosmic distribution.
It is believed that the synthesis of organic compounds preceding the
origin of life on Earth occurred before its atmosphere was transformed
from hydrogen and hydrides to oxygen and nitrogen. This theory is
supported by laboratory experiments of Calvin ([ref.16]), Miller
([ref.33]), and Oró ([ref.34]).
The Earth’s present atmosphere consists of nitrogen and oxygen in
addition to relatively small amounts of other gases; most of the oxygen
is of biological origin. Some of the atmospheric gases, in spite of
their low amounts, are crucial for life. The ultraviolet-absorbing ozone
in the upper atmosphere and carbon dioxide are examples of such gases.
Significant in the search for extraterrestrial life are the data (e.g.,
planet’s temperature) transmitted by Mariner II, which was launched from
Cape Canaveral on August 27, 1962, and flew past Venus on December 14,
1962. Mariner II’s measurements showed temperatures on the surface of
Venus of the order of 800° F, too hot for life as known on Earth.
The question "Is life limited to this planet?" can be considered on a
statistical basis. Although the size of the sample (one planet) is
small, the statistical argument for life elsewhere is believed by many
to be very strong. While Mars is generally considered the only other
likely habitat of life in our solar system, Shapley ([ref.35]) has
calculated that more than 100 million stars have planets sufficiently
similar in composition and environment to Earth to support life. Of
course, yet unknown factors may significantly reduce or even eliminate
this probability.
SPACECRAFT STERILIZATION
The search for extraterrestrial life with unmanned space probes requires
the total sterilization of the landing capsule and its contents.
Scientists agree that terrestrial organisms released on other planets
would interfere with exobiological explorations (refs.
[ref.36]-[ref.43]). Any flight that infects a planet with terrestrial
life will compromise a scientific opportunity of almost unequaled
proportions. Studies on microbiological survival in simulated deep-space
conditions (low temperature, high ultraviolet flux, and low dose levels
of ionizing radiation) indicate that these conditions will not sterilize
contaminated spacecraft (refs. [ref.44]-[ref.48]). Furthermore, many
terrestrial sporeformers and some vegetative bacteria, especially those
with anaerobic growth capabilities, readily survive in simulated Martian
environments (refs. [ref.49]-[ref.54]). It has been estimated that a
single micro-organism with a replication time of 30 days could, in 8
years of such replication, equal in number the bacterial population of
the Earth. This potential could result not only in competition with any
Martian life, but in drastic changes in the geochemical and atmospheric
characteristics of the planet. To avoid such a disaster, certainly the
first, and probably many succeeding landers on Mars, must be
sterile—devoid of terrestrial life ([ref.55]). Since the space
environment will not in itself kill all life aboard, the lander must
leave the Earth in a sterile condition.
The sterility of an object implies the complete absence of life. The
presence of life or the lack of sterility may be proven; but the absence
of life or sterility cannot be proven, for the one viable organism that
negates sterility may remain undetected. Many industrial products which
must be guaranteed as sterile cannot be tested for sterility in a
nondestructive manner. A similar situation exists in determining the
sterility of a spacecraft. Certification of sterility—based on
experience with the sterilizing process used, knowledge of the kinetics
of the death of micro-organisms, and computation of the probability of a
survivor from assays for sterility—is the only accurate approach to
defining the sterility of such treated items.
Macroscopic life can be readily detected and kept from or removed from
the spacecraft, but the detection and removal of microscopic and
submicroscopic life is an extremely difficult task. The destruction of
micro-organisms can be achieved by various chemical and physical
procedures. Sterilizing agents have been evaluated not only for their
ability to kill microbial life on surfaces and sealed inside components,
but also for the agents’ effects on spacecraft reliability as well
(refs. [ref.56]-[ref.59]). Of the available agents, only heat and
radiation will penetrate solid materials. Radiation is expensive,
hazardous, difficult to control, and apparently damages more materials
than does heat. Heat, therefore, has been selected as the primary method
of spacecraft sterilization and will be used, except in specific
instances where radiation may prove to be less detrimental to the
reliability of critical parts ([ref.60]).
The sterilization of spacecraft is a difficult problem if flight
reliability is not to be impaired. The development of heat-resistant
parts will enable the design and manufacture of a heat-sterilizable
spacecraft. Without careful microbiological monitoring of manufacture
and assembly procedures, many bacteria could be trapped in parts and
subassemblies. To permit sterilization at the lowest temperature-time
regimen that will insure kill of all organisms, the microbiological load
inside all parts and subassemblies must be held to a minimum.
The role of industrial clean rooms in reducing the biological load on
spacecraft is currently being defined. NASA-supported studies indicate
that biological contamination in industrial clean rooms for extended
time periods is about 1 logarithm less (tenfold reduction), compared
with conditions in a well-operated microbiological laboratory
([ref.61]). With the use of clean-room techniques and periodic
decontamination by low heat cycles or ethylene oxide treatment, it
should be possible to bring a spacecraft to the point of sterilization
with about 10⁶ organisms on board ([ref.60]).
The sterilization goal established for Mars landers is a probability of
less than 1 in 10 000 (10⁻⁴) that a single viable organism will be
present on the spacecraft. Laboratory studies of the kinetics of
dry-heat kill of resistant organisms show that at 135° C the number of
bacterial spores can be reduced 1 logarithm (90 percent) for every 2
hours of exposure (refs. [ref.58] and [ref.62]). The reduction in
microbial count needed is the logarithm of the maximum number on the
spacecraft (10⁶) plus the logarithm of the reciprocal of the probability
of a survivor (10⁴), or a total of 10 logarithms of reduction in
microbial count. Thus, with an additional 2 logarithms added as a safety
factor, a total of 12 logarithms of reduction in count has been accepted
as a safe value which can be achieved by a dry-heat treatment of 135° C
for 24 hours. This is the heat cycle that is currently under study and
being developed for use in spacecraft sterilization ([ref.60]). However,
other heat treatments at temperatures as low as 105° C for periods of
300 hours or longer are under study ([ref.63]).
Based on results to date, it is reasonable to believe that a full
complement of heat-sterilizable hardware will be available when needed
for planetary exploration. Every effort is being made to improve the
state of the art to a point where spacecraft can not only withstand
sterilization temperatures, but will be even more reliable than the
present state-of-the-art hardware that is not heated.
chapter 3
_Environmental Biology_
BIOLOGICAL EFFECTS OF WEIGHTLESSNESS AND ZERO GRAVITY
High priority has been given to studies of weightlessness. Gravity is
one of the most fundamental forces that acts on living organisms, and
all life on Earth except the smallest appears to be oriented with
respect to gravity, although certain organisms are more responsive to it
than others. The gravity force on Earth is 1 g, but this force may be
experimentally varied from zero g, or weightlessness, to many thousands
of g’s.
Zero gravity or decreased gravity occurs during freefall, in parabolic
trajectory, or during orbit around the Earth. Gravitational force
decreases by the square of the distance away from the Earth’s center. It
is reduced about 5 percent at about 200 nautical miles’ altitude.
Gravitational force greater than 1 g can be obtained by acceleration,
deceleration, or impact. It also can be increased by using a centrifuge
which adds a radial acceleration vector to the 1 g of Earth.
On the ground, the biological effects of gravity have been studied at 1
g, and experimentally, forces of many g have been produced. In addition,
modifications of the effects of the 1-g force have been induced by
suspension of the organism in water or by horizontal immobilization of
an erect animal such as man. The biological effects of such modification
have been of significant value in understanding some of the possible
consequences of human exposure to the zero-g environment of space.
Weightlessness in an Earth-orbiting satellite occurs when the continuous
acceleration of Earth’s gravity is exactly counterbalanced by the
continuous radial acceleration of the satellite. In such a weightless
state, organisms are liberated from their natural and continuous
exertion against 1 g, but this liberation may carry with it certain
serious physical penalties.
Some of the physical processes which probably have the greatest
biological effects are (1) convective flow of fluid, e.g., protoplasmic
streaming, transport of nutrient materials, oxygen, waste products, and
CO₂ from the immediate environment of the cell, and (2) sedimentation
occurring within cells; substances of higher density sediment in a
gravitational field, and those of lighter density rise. A separation of
particles of different densities probably occurs. The removal of gravity
would change a distribution of particles like mitochondria by 10 percent
([ref.64]).
Gravity has effects on the physical processes involved in mitosis and
meiosis. Study under weightlessness might contribute to our
understanding of the general cellular information-relay process.
A gravitational effect is known in the embryonic development of the frog
_Rana sylvatica_. After fertilization, the eggs rotate in the
gravitational field so that the black animal hemisphere is uppermost.
Development becomes abnormal if this position is disturbed. If the egg
is inverted following the first cleavage and held in this position, two
abnormal animals result, united like Siamese twins. This phenomenon
appears to be related to the gravitational separation of low- and
high-density components of the egg. The size of the egg is about 1 to 2
mm and is suspended in water of about the same density. This system is
very sensitive to gravity; and, under weightlessness, the separation of
different density components might be irregular, leading to aberrant
development. When certain aquatic insect eggs are inverted, subsequent
development results in shortened abnormal larvae.
The directional growth of plant shoots and plant roots is probably due
to this sedimentation phenomenon, particularly the effect on movement of
auxins ([ref.65]).
Free convection flow is a major transport process, and under its
influence the mixing of substances is much more effective than when
diffusion operates alone. Free convection flow is a macroscopic
phenomenon which increases not only with g, but varies also
approximately with the five-fourths power of the bulk concentration
involved. Whether or not convection is important at the microscopic
level remains an experimentally unsolved question. The Grashoff number
limits free convection to the macroscopic domain. It would appear in
weightlessness that the contribution of free convective flow would be
small and that only diffusion should occur. This phenomenon would cause
equilibration to occur much more slowly than that occurring with free
convection and diffusion. The absence of convective transfer raises a
problem as to how nutrients may be obtained and waste products removed
in living cells during weightlessness. In a liquid substrate, nutrients
and oxygen would be depleted, and waste products would accumulate around
the cell.
Absence of gravity may have far-reaching consequences in the homeostatic
aspects of cell physiology. The outstanding characteristics of living
cells which are most likely to be influenced by the absence of gravity
are the ability of the cell to maintain its cytoplasmic membrane in a
functional state, the capacity of the cell to perform its normal
functions during the mitotic cycle, and the capacity of the cytoplasm to
maintain the constant reversibility of its sol-gel system ([ref.66]).
Two-phase systems, e.g., air-in-water and air-in-oil, possess entirely
different characteristics at zero g than at 1 g. These physical
differences in phase interaction could well be suspected of interfering
with the orientation and flow pattern of cell constituents, thus
hindering the cellular processes involved in the movement, metabolism,
and storage of nutrients and waste.
On the basis of theoretical calculations, weightlessness can be expected
to have some effect even on one individual cell if its size exceeds 10
microns in diameter ([ref.64]). Cell colonies might be affected. In
larger cells there may be a redistribution of enzyme-forming systems
which give rise to polarization. The low surface tension of the cell
membrane lends itself to hydrostatic stress distortion, implying an
alteration in permeability and thus an almost certain alteration of cell
properties under low gravity conditions.
Another aspect of gravity that affects the growth and development of
living organisms is the directionality of the gravitational field. In
fact, some plants are so sensitive that they are able to direct their
growth with as little stimulus as a 1×10⁻⁶ gravitational field.
Investigations of plant growth in altered gravitational fields are
underway at Argonne National Laboratory and Dartmouth College.
The Argonne Laboratory has designed and developed a 4-pi, or
omnidirectional, clinostat. By rotating a plant so that the force of
gravity is distributed evenly over all possible directions, the
directional effects of gravity are eliminated, simulating some aspects
of the zero-g state. It was shown that certain plants grew more slowly
and had fewer and smaller leaves, while others had about 25 percent
greater replication of fronds and had greater elongation of certain
plant parts. It will be extremely interesting to compare these effects
under zero-g conditions in orbiting spacecraft.
The effect of gravity in transporting growth hormones in plants has been
demonstrated at Dartmouth College using radiocarbon-labeled growth
hormones. Plant geotropisms and growth movements have been studied and
biosatellite experiments developed.
Anatomy is considered a derivative adaptation to gravity ([ref.67]). A
large background of plant research exists on the effect of orientation
on plant responses. Information from clinostat experiments is considered
susceptible of extrapolation to low gravity conditions because the
threshold period for gravitational triggering is relatively long.
Once over critical minimum dimensions, the major effects of low gravity
would be assumed to occur in those heterocellular organisms that develop
in more or less fixed orientation with respect to terrestrial gravity
and which respond to changes in orientation with relatively long
induction periods; these are the higher plant orders. On the other
extreme are the complex primates which respond rapidly, but whose
multiplicity of organs and correlative mechanisms are susceptible to
malfunction and disorganization. It may be suggested that the
heterocellular lower plants and invertebrates will be less affected.
Perturbations of the environment to which the experimental organism is
exposed must be limited or controlled to reduce uncertainties in
interpretation of the results. At the same time, the introduction of
known perturbations may assist in isolating the effects due solely to
gravity. Study of _de novo_ differentiation and other phenomena
immediately after syngamy may be of particular importance. Study of
anatomical changes after exposure of the organism to low gravity is
important.
BIOLOGICAL EFFECTS OF SPACE RADIATION¹
¹ This section includes part of the Summary of the Panel on Radiation
Biology of the Environmental Biology Committee Space Science Board,
NAS/NRC (1963), and results of research by the Bioscience Programs,
NASA.
Radiation sources in space are of three types: galactic cosmic
radiation, Van Allen belts, and solar flares with an intense proton
flux. Cosmic radiation has higher energy levels than radiation produced
by manmade accelerators.
The Panel on Radiation Biology, while recognizing the need for
radiobiological studies of an applied nature with reference to manned
flight programs, stated that it would be shortsighted for the United
States to confine its efforts to the solution of immediate problems
since, in the long run, successful exploration of space will be aided by
the contributions of basic research. Both the immediate biological
research program and the continuing program for basic studies should be
built upon the large body of existing knowledge of radiation effects.
The attitude that all radiobiological experiments need be repeated in
the space environment should be resolutely rejected. Since fundamental
radiobiology cannot be performed easily in space, it has been
recommended that, wherever possible, these investigations be carried out
in ground laboratories in preference to flying laboratories.
Space environment does vary from the terrestrial environment, but the
variations are not so great as to lead to the expectation of strikingly
different biological effects of radiation in space. However, it is
conceivable that radiations whose effects are well known under
terrestrial conditions may have some unsuspected biological effects when
combined with unusual features of the space environment: e.g., zero g.
Previous space radiobiological studies have depended solely on very low
and inaccurately measured doses of ambient space radiation. It has been
difficult to distinguish between the observed response levels and the
random noise; thus, experiments have been inconclusive.
Biological Effects of Heavy Ions and Mesons
The biological effects of heavy ions (especially Z>2) and mesons are of
specific interest to space radiobiology.
Controlled Radiobiological Experiments in Space
There is the remote possibility that the radiobiological response may be
modified by factors as yet unknown and perhaps not susceptible to
terrestrial study. Experiments have been designed to settle this matter
including the exposure of biological materials during space flight which
meet the following criteria of reliability: (1) the use of well-known
biological systems, e.g., mutation induction or chromosome breakage; (2)
the use of a sufficient number of individuals in the experiment to
guarantee statistical precision on the results; (3) the exposure of the
system to known quantities and qualities of radiation; (4) the use of
adequate controls.
High-altitude balloon ascents of the 1930’s initiated study of the
biological effects of cosmic rays. They were limited to the exploration
of secondary cosmic radiation effects. After World War II, the research
extended to the use of V-2 rockets fired from the White Sands Proving
Ground. Interest returned to balloons and a significant program was
underway by 1950, first using mice and then hamsters, fruit flies, cats,
and dogs. These flights gave no evidence of radiation damage. However,
it was realized that the flights were too far south to obtain a
significant exposure, and more northerly flights began in 1953. Mice and
guinea pigs were flown on these later flights. Chase ([ref.68]) showed
the most unequivocal results to that time, a statistically significant
increase in light hairs on black animals and the streaks of white hair
up to 10 times wider than expected. Brain lesions were detected in the
guinea pigs flown on Man High in 1957. Many other types of biological
material were sent aloft in an effort to further corroborate existing
information and to investigate genetic and developmental effects of
cosmic radiation.
From the earlier V-2 rocket flights to the Jupiter missile launchings of
the monkeys Able and Baker, cosmic-ray research was continued, but the
short flight durations of these vehicles did not provide substantial
information. The USAF Discoverer satellite program has given impetus to
cosmic-ray research and provided for longer "staytimes."
It has been difficult to separate radiation effects from other
space-flight factors: therefore, some of the alterations observed are
still subject to debate. Vibration, acceleration, and weightlessness
appear to be the three most important additional parameters.
Measurements of radiation dosage have been made by chemical and
photographic dosimetry, ion chambers, and biological dosimetry. All
evidence to date indicates that radiation exposure levels are not
hazardous to man at present orbital altitudes up to 200 nautical miles.
Most biological materials flown so far have been for the express purpose
of investigating space-radiation levels and effects. The biological
materials have ranged from tissue cultures to entire organisms and from
phage and bacterial cells to man. The studies have required much of the
space and weight resources allotted biology by the U.S.S.R. and the
United States. They have been accompanied by ground-based controls.
The Vostok series provided the following data:
(1) A small, but statistically significant, increase was observed in
the percentage of chromosome aberrations in the rootlet cells of
air-dried wheat and pea seeds after germination. In this case
only, the increase did not depend on flight duration.
(2) Lysogenic bacteria exhibited an increase of genetic alterations
and increased phage production. Length of flight was associated
with increased bacteriophage production by the lysogenic bacteria.
There was an increase of recessive lethals coupled with
nonconvergence of chromosomes (sex linked) in the fruit fly. A
stimulation of cell division in wheat and pea seeds was observed.
Cultures of human cells exposed to space-flight factors did not
differ significantly from terrestrial controls with respect to
such indicators as proliferation rate, percentage of mortality and
morphological, antigenic, and cultural properties. Repeated
flights of the identical HeLa cells revealed that there was a
longer latent period for restoration of growth capacity than in
cells carried into space once or not flown at all.
(3) The most definite radiation effects observed were only revealed in
genetic tests. No harmful influence on those characteristics
affecting the viability of the organism has been discovered.
The Air Force Discoverer series launched from the west coast had a few
successful flights incorporating organisms. With severe environmental
stress and long recovery times, data on radiation exposure were
equivocal up to Discoverer XVII and XVIII when cultures of human tissue
were flown, recovered, and assessed for radiation exposure effects.
Comparison with ground-based controls revealed no measurable
differences.
Radiation dosimetry from the Mercury series established that minimal
exposures were encountered at those orbital altitudes. A typical example
is the MA-8 flight of W. M. Schirra, Jr., during which the body surface
dosage was less than 30 millirads.
NASA has supported fundamental radiation studies at the Oak Ridge
National Laboratory and the Lawrence Radiation Laboratory. Emphasis has
been placed on the biological effects of high-energy proton radiation
and particulate radiation from accelerators.
At the NASA Ames Research Center extensive fundamental studies are being
carried out on the effects of radiation, especially in the nervous
system. It has been demonstrated that deposits accumulate in the brain
following exposure to large doses of ionizing particle radiation as well
as after X-irradiation. These deposits, referred to as a "chemical
lesion," result from an accumulation of glycogen. The formation of these
deposits during exposure to large doses of X-irradiation was not
increased in environments of 99.5 percent oxygen and increased
atmospheric pressure.
SIMULATION OF PLANETARY (MARTIAN) ENVIRONMENTS
Attempts have been made to simulate to some degree the various
parameters of the Martian environment, such as atmospheric composition,
pressure, radiation flux, temperatures, and the day-night as well as
seasonal cycles. Certain factors for Mars cannot yet be simulated, such
as soil composition, gravitational field, magnetic field, and electrical
field.
Caution is required in interpreting all simulation experiments. How
Earth organisms respond to simulated Martian environments probably has
nothing to do with life on Mars, but these experiments may show whether
or not anything in the environment of Mars makes life as we know it
impossible. We must expect that on Mars, life will have evolved and have
adapted over long periods of time under conditions which are quite
different from conditions on Earth. The simulation experiments also
provide some information about the possibility of contaminating the
planet Mars, or any planet, with organisms from Earth. In addition, they
give us some clues about the possibilities of adaptation and evolution
of life under these conditions.
From an evolutionary point of view, if life has developed on Mars, we
expect it to have evolved at least to a microbial stage. On Earth,
micro-organisms are the most ubiquitous and numerous forms of life. This
fact should be considered in studying extraterrestrial bodies.
Micro-organisms have been selected as the best test organisms, and
bacteria and fungi have been used because they are durable and easy to
grow. Also, because of their rapid growth, many generations can be
studied in a relatively short period of time. The organisms include
chemoautotrophic bacteria, which are able to synthesize their cell
constituents from carbon dioxide by energy derived from inorganic
reactions; anaerobic bacteria, which grow only in the absence of
molecular oxygen; photoautotrophic plants such as algae, lichens, and
more complex seed plants; and small terrestrial animals.
Organisms have been collected from tundra, desert, hot springs, alpine,
and saline habitats to obtain species with specialized capabilities to
conserve water, balance osmotic discrepancies, store gases, accommodate
to temperature extremes, and otherwise meet stresses. An attempt is made
in these simulation experiments to extend these processes across the
possible overlapping microenvironments which Earth and Mars may share.
Scientists have developed various special environmental simulators,
including "Mars jars" and "Marsariums." These have made possible
controlled temperatures, atmospheres, pressures, water activities, and
soil conditions for duplicating assumed Martian surface. A complex
simulator, developed by Young et al. ([ref.52]), reproduces the
formation of a permafrost layer with some water tied up in the form of
ice beneath the soil surface. This simulator serves as a model to study
the wave of darkening, thus supporting the hypothesis that the
pole-to-equator wave of darkening is correlated with the availability of
subsurface water. The simulator is a heavily insulated 2-cu-ft capacity
chamber with an internal pressure of 0.1 atm. The chamber contains a
soil mixture of limonite and sand and an atmosphere of carbon dioxide
and nitrogen. With the use of a liquid nitrogen heat exchanger at one
end and an external battery of infrared lamps at the other end, the
temperature simulates that of Mars from pole to equator. Thermocouples
throughout the soil monitor the temperatures in the chamber.
Zhukova and Kondratyev ([ref.69]) designed a structure measuring
100×150×180 cm. Micro-organisms were placed at the surface of a copper
bar made in a special groove separated by glass cloth. Copper was
selected as one of the best heat-conduction materials permitting a rapid
change of temperature. The lower end of the bar was immersed into a
mixture of dry ice and ethyl alcohol, which made it possible to create a
temperature of -60° C. Heating was performed by an incandescent spiral.
As the knowledge concerning the Martian environment becomes more
refined, scientists can more accurately simulate this environment under
controlled conditions in the laboratory. Determination of the effects of
the Martian environment on Earth organisms will permit better
theorization on the forms of life we might find on Mars and will permit
us to estimate the potential survival of Earth contaminants on Mars.
However, until the environmental conditions of Mars are defined more
accurately, the experiments must be changed continually to fit newly
determined conditions. Therefore, existing simulation data are made less
valid for comparison. The data resulting from the simulation experiments
for Mars have been compiled in table II, and the experiments are
summarized below.
The earliest simulation studies were carried out by the Air Force, and
the studies during the past 6 years have been supported by NASA.
Recently, these studies have received less support or have been
terminated in favor of critical studies on the effects of biologically
important environmental extreme factors on Earth organisms. These
critical studies permit establishing the extreme environmental factor
parameters in which Earth life can grow or survive. These data will have
valuable application to the consideration of life on any planet, to the
design of life-detection instruments, to the sterilization of space
vehicles, and to the problem of contamination of planets.
Some exploratory experimental studies are in progress to study the
capabilities of organisms to grow under the assumed conditions on
Jupiter. These include studies at high pressure with liquid ammonia,
methane, and other reducing compounds.
Early experiments simulating Martian conditions using soil bacteria were
carried out by Davis and Fulton ([ref.70]) at the Air Force School of
Aviation Medicine, San Antonio, Tex. Mixed populations of soil bacteria
were put in "Mars jars" with the following conditions: 65-mm Hg
pressure, 1 percent water or less, nitrogen atmosphere, sandstone-lava
soil, and a temperature day-night cycle of +25° to -25° C. The moisture
was controlled by desiccating the soil and adding a given amount of
water. Experiments, conducted up to 10 months, demonstrated that
obligate aerobes died quickly. The anaerobes and sporeformers survived.
Although a small increase in the total number of organisms indicated
growth, the increases in the number of bacteria may have been due to
breaking up clumps of dirt.
Roberts and Irvine ([ref.71]) reported that, in a simulated Martian
environment, colony counts of a sporeforming bacterium, _Bacillus
cereus_, increased when 8 percent moisture was added. Moisture was
considered more important than temperature or atmospheric gases inasmuch
as a simulated Martian microenvironment containing 8 percent moisture
permitted germination and growth of endospores of _Clostridium
sporogenes_. Increases in colony counts of _Bacillus cereus_ appeared to
be influenced by temperature cycling ([ref.72]).
Table II.—_Survival and Growth of Organisms in Simulated Planetary
(Martian) Environments_
------------------------------------------------------------------
Species Survival, Moisture Temperature,
months °C
------------------------------------------------------------------
Conditions on Mars: 14µ±7µ -70 to +30
------------------------------------------------------------------
Anaerobic 6 Low, -60 to +20
sporeformers (CaSO₄)
_Clostridia_,
_Bacillus
planosarcina_
------------------------------------------------------------------
Anaerobic 6 Low, -60 to +20
nonsporeformers (CaSO₄)
_Pseudomonas_,
_Rhodopseudomonas_
------------------------------------------------------------------
Anaerobes Growth Very wet -75 to +25
_Aerobacter
aerogenes_,
_Pseudomonas sp._
------------------------------------------------------------------
_Clostridium_, 10 1 -25 to +25
_Corynebacteria_ percent
"Thin short rod" or less
------------------------------------------------------------------
_Bacillus cereus_ 2 0.5 -25 to +25
percent
soil
------------------------------------------------------------------
_Clostridium sporogenes_ 1 8.4 -25 to +25
(growth) percent
------------------------------------------------------------------
_Clostridium botulinum_ 10 Lyophilized -25 to +25
------------------------------------------------------------------
_Klebsiella pneumoniae_ 6 Lyophilized -25 to +25
------------------------------------------------------------------
_Bacillus subtilis_ var. 4 2 percent -25 to +25
_globigii_
------------------------------------------------------------------
_Sarcina aurantiaca_ 4 0.5 percent -25 to +25
------------------------------------------------------------------
_Clostridium tetani_ 2 or less 1 percent -60 to +25
------------------------------------------------------------------
_Aspergillus niger_ Over 6 hr Very dry -60 to +25
------------------------------------------------------------------
_Aspergillus oryzae_ Over 6 hr Very dry -60 to +25
------------------------------------------------------------------
_Mucor plumbeus_ Over 6 hr Very dry -60 to +25
------------------------------------------------------------------
_Rhodotorula rubra_ Over 6 hr Very dry -60 to +25
------------------------------------------------------------------
Pea, bean, tomato, rye, 0.3 Moist +25
sorghum, rice.
------------------------------------------------------------------
Winter rye 0.6 Moist -10 to +23
------------------------------------------------------------------
Table II.—_Survival and Growth of Organisms in Simulated Planetary
(Martian) Environments_
----------------------------------------------------------------------
Species Atmospheric N₂, CO₂, Substrate
pressure, percent percent
mm Hg
----------------------------------------------------------------------
Conditions on Mars: 85, 3 to 30
25±15, 11
----------------------------------------------------------------------
Anaerobic 76 95 5 Air-dried
sporeformers soil
_Clostridia_,
_Bacillus
planosarcina_
----------------------------------------------------------------------
Anaerobic 76 95 5 Air-dried
nonsporeformers soil
_Pseudomonas_,
_Rhodopseudomonas_
----------------------------------------------------------------------
Anaerobes 760 100 (?) Difco
_Aerobacter infusion
aerogenes_, broth
_Pseudomonas sp._
----------------------------------------------------------------------
_Clostridium_, 65 100 (?) Soil
_Corynebacteria_
"Thin short rod"
----------------------------------------------------------------------
_Bacillus cereus_ 65 94 2.21 Sandstone
soil
----------------------------------------------------------------------
_Clostridium sporogenes_ 65 94 2 Enriched
soil
----------------------------------------------------------------------
_Clostridium botulinum_ 65 95 0 to Lava soil
0.5
----------------------------------------------------------------------
_Klebsiella pneumoniae_ 65 95 0 to Lava soil
0.5
----------------------------------------------------------------------
_Bacillus subtilis_ var. 85 95 0.3 Media
_globigii_
----------------------------------------------------------------------
_Sarcina aurantiaca_ 85 95 0.3 Desert
soil
----------------------------------------------------------------------
_Clostridium tetani_ 85 95 0.3 Soil
----------------------------------------------------------------------
_Aspergillus niger_ 76 95.5 0.25 Glass
cloth on
copper
bar
----------------------------------------------------------------------
_Aspergillus oryzae_ 76 95.5 0.25 Do.
----------------------------------------------------------------------
_Mucor plumbeus_ 76 95.5 0.25 Do.
----------------------------------------------------------------------
_Rhodotorula rubra_ 76 95.5 0.25 Do.
----------------------------------------------------------------------
Pea, bean, tomato, rye, 75 100 0 Filter
sorghum, rice. paper
----------------------------------------------------------------------
Winter rye 76 98 0.24 Soil
----------------------------------------------------------------------
Studies of the effects of simulated Martian environments on sporeforming
anaerobic bacteria were carried out by Hawrylewicz et al. ([ref.49]).
They showed that the encapsulated facultative anaerobe, _Klebsiella
pneumoniae_, survived under simulated Martian atmosphere for 6 to 8
months, but were less virulent than the freshly isolated organisms.
Spores of the anaerobe _Clostridium botulinum_ survived 10 months in the
simulator. Hagen et al. ([ref.53]) found that the addition of moisture
to dry-simulated Martian soil did not improve the survival of _Bacillus
subtilis_ or _Pseudomonas aeruginosa_. _Bacillus cereus_ spores
survived, with added organic medium plus moisture, but no germination of
the spores resulted.
Hawrylewicz et al. ([ref.49]) put rocks from Antarctica bearing various
lichens in simulated Martian conditions in a large desiccator. They
found that the algal portion of a lichen, _Trebouxia erici_, showed only
slight resistance to the Martian environment. They also pointed out the
effect moisture had on the physical condition of lichens. The
undersurface of a lichen has great water-absorbing capability, and the
slightest amount of moisture on a rock surface is absorbed by the lichen
which can turn green in 15 minutes.
Scher et al. ([ref.51]) exposed desert soils to simulated environmental
conditions and diurnal cycles of Mars. The atmosphere consisted of 95
percent nitrogen and 5 percent carbon dioxide (no oxygen) and was dried,
using calcium sulfate as a desiccant. The total atmospheric pressure was
0.1 atm. The temperature ranged from -60° to +20° C in 24-hour cycles.
One hour was spent at the maximum and at the minimum temperatures. The
chambers were irradiated with ultraviolet, 2537 Å, with a dose of 10⁹
ergs/cm², which is comparable to a daily dose found on Mars, and easily
exceeds the mean lethal dose for unprotected bacteria. Soil aliquots
were removed weekly and incubated at 30° C. The scoring was done both
aerobically and anaerobically. Sporeforming obligate and facultative
anaerobes, including _Clostridium_, _Bacillus_, and _Planosarcina_, and
nonsporeforming facultative anaerobes, including _Pseudomonas_ and
_Rhodopseudomonas_, were found. The experimental chambers were frozen
and thawed cyclically up to 6 months. Organisms that were able to
survive the first freeze-thaw cycle were able to survive the entire
experiment. The ultraviolet irradiation did not kill subsurface
organisms, and a thin layer of soil served as an ultraviolet shield. All
of the samples showed survivors.
Young et al. ([ref.52]) assumed that water is present on Mars, at least
in microenvironments, and that nutrients would be available. The primary
objective of their experiments was to determine the likelihood of
contaminating Mars with Earth organisms should a space probe from Earth
encounter an optimum microenvironment in terms of water and nutrients.
The experiments used bacteria in liquid nutrient media. The environment
consisted of a carbon dioxide-nitrogen atmosphere, and the temperature
cycling was -70° to +25° C, with a maximum time above freezing of 4½
hours. _Aerobacter aerogenes_ and _Pseudomonas sp._ grew in nutrient
medium under Martian freezing and thawing cycles. Atmospheric pressure
was not a significant factor in the growth of bacteria under these
conditions.
Silverman et al. ([ref.47]) studied bacteria and a fungus under
extreme—but not "Martian"—conditions. Spores of five test organisms (_B.
subtilis_ var. _niger_, _B. megaterium_, _B. stearothermophilus_,
_Clostridium sporogenes,_ and _Aspergillus niger_) and soils were
exposed while under ultrahigh vacuum to temperatures of from -190° to
+170° C for 4 to 5 days. Up to 25° C there was no loss in viability; at
higher temperatures, differences in resistivity were observed. At 88° C,
only _B. subtilis_ and _A. niger_ survived in appreciable numbers; at
107° C, only _A. niger_ spores survived; none were recoverable after
exposure to 120° C. _B. subtilis_ survived at atmospheric pressure and
90° C for 5 days, but none of the other spores were viable alter 2 days.
Four groups of soil organisms (mesophilic, aerobic, and anaerobic
bacteria, molds, and actinomycetes) were similarly tested in the vacuum
chamber. From one sample only actinomycetes survived 120° C, while one
other soil sample yielded viable bacteria after exposure to 170° C.
Several organisms resisted 120° C in ultrahigh vacuum for 4 to 5 days.
When irradiated with gamma rays from a cobalt 60 source, differences
were observed between vacuum-dried spores irradiated while under vacuum
and those exposed to air immediately before irradiation. A reduction of
from one-third to one-ninth of the viability of spores irradiated in
vacuum occurred with vacuum-treated spores irradiated in air.
Siegel et al. ([ref.73]), in approximate simulations of Martian
environments, studied tolerances of certain seed plants, such as
cucumbers, corn, and winter rye, to low temperatures and lowered oxygen
tensions. Lowered oxygen tensions enhanced the resistance of seedlings,
particularly cucumber and rye to freezing, and lowered the minimum
temperature required for germination. Germination of seeds in the
absence of liquid water has also been studied. In this case, seeds of
xerophytes have been suspended in air at 75-mm Hg pressure above water.
The air was thus saturated. Germination was slow but did occur.
Siegel et al. (refs. [ref.73] and [ref.74]) found that the growth rate
of several higher plants was enhanced by certain gases usually thought
to be toxic, such as N₂O. This finding is significant inasmuch as the
presence of nitrogen oxides in the Martian atmosphere has been cited as
evidence for the nonexistence of plants on that planet by Kiess et al.
([ref.75]). Exploratory survival tests showed that various mature
plants, as well as the larvae, pupae, and adult specimens of a
coleopteran insect, were undamaged when exposed to at least 40 hours of
an atmosphere containing 96.5 percent N₂O, 0.7 percent O₂, and 2.8
percent N₂.
Lichens are of interest because of their ability to survive and thrive
under extreme environmental conditions on Earth. Biological activity of
slow-growing lichens was detected by metabolic gas exchange, CO₂
detection being especially convenient. Siegel points out that this
method is sensitive and nondestructive, to be preferred to staining
techniques, which at present are limited because they are only
semiquantitative, subjective, and destructive of the lichen.
A Russian study of simulated planetary environments has been performed
with good simulation but for periods of only 2 to 6 hours. Comments on
simulation experiments made by Zhukova and Kondratyev ([ref.69]) are
presented as follows:
On the basis of modern conceptions on Martian conditions it is
difficult to imagine that higher forms of animals or plants
exist on the planet. A Martian change of seasons similar to that
of our planet empowers us to think that there is a circulation
of an organic substance on Mars, which cannot exist without
participation of microbic forms of life. Microorganisms are the
most probable inhabitants of Mars although the possibility is
not excluded that their physiological features will be very
specific. That is why the solution of the problem concerning the
character of life on Mars is of exceptional interest. But still
the answer to this question can be verified only by simulating
Martian conditions, taking into account the information obtained
from astrophysicists.
Experiments aimed at creating artificial Martian climatic
conditions have been started quite recently; their number is not
large since they cannot be combined with the results of numerous
experiments investigating the effect of extreme factors on
microorganisms. The result of the effect of such physicochemical
parameters of the medium as pressure, sharp temperature changes,
the absence of oxygen and insolation, depends on their
combination and simultaneity. These examples convincingly show
that while simulating Martian conditions one should strive to
the most comprehensive complex of simultaneously acting factors.
The creation of individual climatic parameters acting
successively leads to absolutely different, often opposite
results. It should be mentioned also that refusal to imitate
insolation and the performance of experiments with specimens of
soil which itself has protective effect on cells of
microorganisms, but not with pure culture of bacteria, are usual
shortcomings in the bulk of studies on this problem.
It appears that organisms from Earth might survive in large numbers when
introduced to Martian environment. Whether these organisms will be
capable of growth and explosive contamination of the planet in a
biological sense or not is highly questionable. The likelihood of an
organism from Earth finding ideal conditions for growth on Mars seems
extremely low. However, the likelihood of an organism from Earth serving
as a contaminant for any life-detection device flown to Mars for the
purpose of searching out carbon-based life is considerably higher. The
chance that life has originated and evolved on Mars is a completely
separate question and much more difficult to answer.
It would be interesting to attempt to determine possible evolutionary
trends which might occur on a planet by means of selection of organisms
in a simulated planetary environment. Rapid genetic selection combined
with radiation and chemicals to speed up mutation rate under these
conditions should reveal possible evolutionary trends under the
planetary environmental conditions. This could be attempted after the
planetary environments are more accurately defined.
EXTREME AND LIMITING ENVIRONMENTAL PARAMETERS OF LIFE
The question of the existence of extraterrestrial life is one of the
most important and interesting biological questions facing mankind and
has been the subject of much controversial discussion and conjecture.
Many of the quantitative, and even qualitative, environmental
constituents of the planets also are still subjects of controversy and
speculation. Best guesses about a relatively unknown planetary
environment, combined with lack of information about the capabilities of
Earth life to grow in extreme environments, do not provide the basis for
making informed scientific estimates.
Life on Earth is usually considered to be relatively limited in its
ability to grow, reproduce, or survive in extreme environmental
conditions. While many common plants and animals (including man) are
quite sensitive to, or incapable of, surviving severe chemical and
physical changes or extremes of environment, a large number of
micro-organisms are highly adapted and flourish in environments usually
considered lethal. Certain chemoautotrophic bacteria require high
concentrations of ammonia, methane, or other chemicals to grow.
Anaerobic bacteria grow only in the absence of oxygen.
Besides adapting to the extremes of environments on Earth, life is also
capable of growing and reproducing under extreme environmental
conditions not normally encountered: e.g., from a few rad of radiation
in normal habitats to 10⁶ or more rad from artificial sources, from 0.5
gauss of Earth magnetism to 167 000 gauss in manmade magnetic fields,
and from 1-g force of gravity to 110 000 g. The extreme ranges of
physical and chemical environmental factors for growth, reproduction,
and survival for Earth micro-organisms are phenomenally large.
Life is ubiquitous on Earth and is found in almost every possible
environment, including the most severe habitats, from the bottom of the
ocean to the highest mountain tops and from cold Arctic habitats to hot
springs, as well as in volcanic craters, deep wells, salt flats, and
mountain snowfields. Earth life has become adapted to, and has invaded,
nearly every habitat, no matter how severe. The physiological and
morphological adaptations of life are exceedingly diverse and complex.
Surprisingly, the extreme parameters or ranges of the physical and
chemical environmental factors permitting growth, reproduction, and
other physiological processes of Earth organisms have not been
critically compiled. A partial compilation of certain selected
environmental factors has been made by Vallentyne ([ref.76]). A
compilation of available published data on certain environmental
extremes, particularly from recent NASA-supported research (compiled by
Dale W. Jenkins, in press), is presented in tables III to VI. These data
can serve as a starting point for a more intensive literature review by
specialists, critical evaluation, standardization of end points, and
especially to point out areas where critical experimentation is urgently
needed.
This critical compilation involves a review of a very broad and complex
range of subjects involved in many different disciplines with widely
scattered literature. Since the effects of many of the specific
environmental factors are harmful, it is difficult to select a point on
a scale from no effect to death and use some criteria to say that normal
or even minimal growth and reproduction are occurring. The effects of
environmental factors are dependent on (1) the specific factor, times,
(2) the concentration or energy, times, (3) the time of exposure or
application of the factor. Many reports, especially older ones, do not
give all of the necessary data to permit proper evaluation. A
complicating factor is that the effect of each factor depends on the
other factors before, during, and after its application. The condition
of the organism itself is a great variable. Proper evaluation requires
the critical review by a variety of biological specialists, physicists,
and chemists.
To determine the potential of Earth organisms to survive or grow under
other planetary environmental conditions, a number of experiments have
been carried out attempting to simulate planetary environments,
especially of Mars, as reviewed previously. While the results are of
real interest, they do not provide much basic information. Further, as
the Martian environment is more accurately defined, the experimental
conditions are changed. In addition, some experimenters have altered
certain factors, such as water content, to allow for potential
microhabitats or for areas which might contain more water at certain
times.
Table III.—_Extreme Physical Environmental Factors_
-----------------------------------------------------------------
Physical Minimum Organism
factors
-----------------------------------------------------------------
Temperature -30° C Algae (photosynthesis),
pink yeast (growth)
-----------------------------------------------------------------
Magnetism 0-50 gamma (=×10⁻⁵ Human
gauss)
-----------------------------------------------------------------
Gravity 0 g Human, plants, animals
-----------------------------------------------------------------
Pressure 10⁻⁹ mm Hg (5 days) _Mycobacterium_
_smegmatis_
-----------------------------------------------------------------
Microwave 0 W/cm²
-----------------------------------------------------------------
Visible 0 ft-c Animals, fungi,
bacteria
-----------------------------------------------------------------
Ultraviolet 0 erg/cm²
-----------------------------------------------------------------
X-ray 0 rad
-----------------------------------------------------------------
Gamma ray 0 rad
-----------------------------------------------------------------
Acoustic 0 dyne/cm²
-----------------------------------------------------------------
Table III.—_Extreme Physical Environmental Factors_
----------------------------------------------------------------------
Physical Maximum Organism Activity
factors
----------------------------------------------------------------------
Temperature 104° C (1000 _Desulfovibrio Grows and reduces
atm) desulfuricans_ sulfate
----------------------------------------------------------------------
Magnetism 167 000 _Neurospora_ 1 hr—no effect,
gauss _Arbacia_ _Arbacia_
_Drosophila_ development delayed
----------------------------------------------------------------------
Gravity 400 000 g _Ascaris_ eggs 1 hr—eggs hatch, 40
110 000 g _Escherichia coli_ days’ growth
----------------------------------------------------------------------
Pressure 1400 atm Marine organisms Growth
----------------------------------------------------------------------
Microwave 2450 Mc/sec _Drosophila_ 68 hr, growth not
0.3 to 1 affected
W/cm²
----------------------------------------------------------------------
Visible 50 000 ft-c _Chlorella_, Seconds,
17 000 ft-c higher plants recurrently
continuous
----------------------------------------------------------------------
Ultraviolet 10⁸ erg/cm², Bean embryos Suppressed growth
2537 Å
----------------------------------------------------------------------
X-ray 2×10⁶ rad Bacteria Growth
----------------------------------------------------------------------
Gamma ray 2.45×10⁶ rad _Microcoleus_ Continued growth
_Phormidium_
_Synechococcus_
----------------------------------------------------------------------
Acoustic 140 db or Man Threshold of pain
6500
dyne/cm² at
0.02 to 4.8
kcs/sec
----------------------------------------------------------------------
Table IV.—_Extreme Low and High Temperature Effects Permitting
Life Processes_
-----------------------------------------------------------------
Minimum Organism Activity or condition
temperature,
°C
-----------------------------------------------------------------
-11 Bacteria Growth (on fish)
-----------------------------------------------------------------
-12 Bacteria Growth
-----------------------------------------------------------------
-12 Molds Growth
-----------------------------------------------------------------
-15 _Pyramidomonas_ Swimming
-----------------------------------------------------------------
-15 _Dunaliella salina_ Swimming
-----------------------------------------------------------------
-18 Mold Growth
-----------------------------------------------------------------
-18 Yeast Growth
-----------------------------------------------------------------
-18 _Aspergillus Growth (in glycerol)
glaucus_
-----------------------------------------------------------------
-18 to -20 Mold Growth (in fruit juice)
-----------------------------------------------------------------
-18 to -20 _Pseudomonads_ Growth (in fruit juice)
-----------------------------------------------------------------
-20 Bacteria Growth
-----------------------------------------------------------------
-20 Bacteria Growth
-----------------------------------------------------------------
-20 Bacteria Luminescence development
accelerated
-----------------------------------------------------------------
-20 to -24 Insect eggs
(diapause)
-----------------------------------------------------------------
-30 Algae Photosynthesis
-----------------------------------------------------------------
-30 Pink yeast Growth (on oysters)
-----------------------------------------------------------------
-30 Lichens Photosynthesis
-----------------------------------------------------------------
-20 to -40 Lichens and conifers Photosynthesis
-----------------------------------------------------------------
-44 Mold spores Sporulation and germination
-----------------------------------------------------------------
Table IV.—_Extreme Low and High Temperature Effects Permitting
Life Processes_
------------------------------------------------------------------
Maximum Organism Activity or condition
temperature,
°C
------------------------------------------------------------------
73 Thermophilic organisms Growth (P³² metabolism)
------------------------------------------------------------------
73 _Phormidium_ (alga) Acclimatized
------------------------------------------------------------------
70 to 73 _Bacillus calidus_ Growth and spore
germination
------------------------------------------------------------------
70 to 74 _Bacillus cylindricus_ Growth and spore
germination
------------------------------------------------------------------
70 to 75 _Bacillus tostatus_ Growth and spore
germination
------------------------------------------------------------------
80 _Bacillus Cultured in laboratory
stearothermophilus_
------------------------------------------------------------------
83 Sulfate-reducing Found in a well
bacteria
------------------------------------------------------------------
89 Sulfate-reducing Found in oil waters
bacteria
------------------------------------------------------------------
65 to 85 Sulfate-reducing Cultured in laboratory
bacteria
------------------------------------------------------------------
89 Micro-organisms Found in hot springs
------------------------------------------------------------------
95 _Bacillus coagulans_ In 80 min. sporulation
activation
------------------------------------------------------------------
110 _Bacillus coagulans_ In 6 min, sporulation
activation
------------------------------------------------------------------
104 _Desulfovibrio Grow and reduce sulfate
desulfuricans_ at 1000 atm
------------------------------------------------------------------
Table V.—_Extreme Temperature Limits of Survival_
--------------------------------------------------
Minimum Organism
temperature
°C
--------------------------------------------------
-190 Yeast bacteria, 10 species
--------------------------------------------------
-197 _Trebouxia erici_ from lichens
--------------------------------------------------
-197 Protozoa, _Anguillula_
--------------------------------------------------
-252 Yeasts, molds, bacteria, 10 species
--------------------------------------------------
-253 Black currant, birch
--------------------------------------------------
-273 Bacteria, many species
--------------------------------------------------
-273 Bacteria, many species
--------------------------------------------------
-272 Desiccated rotifers
--------------------------------------------------
-269 Human spermatozoa
--------------------------------------------------
Table V.—_Extreme Temperature Limits of Survival_
------------------------------------------------------------------
Maximum Organism Time of exposure
temperature
°C
------------------------------------------------------------------
140 Bacterial spores 5-hr immersion
------------------------------------------------------------------
170-200 Desiccated rotifers 5 min
------------------------------------------------------------------
151 Desiccated rotifers 35 min
------------------------------------------------------------------
150 _Clostridium tetani_ 180 min
------------------------------------------------------------------
170 Aerobic bacteria, molds. 5 days at
actinomycetes 6×10⁻⁹mm Hg
------------------------------------------------------------------
127 (dry) Bacteria (in activated charcoal) 60 min
------------------------------------------------------------------
110 (wet) _Bacillus subtilis_ var. _niger_ 400 min
------------------------------------------------------------------
120 _Bacillus subtilis_ var. _niger_ 400 min
------------------------------------------------------------------
141 _Bacillus subtilis_ var. _niger_ 70 min
------------------------------------------------------------------
160 _Bacillus subtilis_ var. _niger_ 15 min
------------------------------------------------------------------
180 _Bacillus subtilis_ var. _niger_ 2 min
------------------------------------------------------------------
188 _Bacillus subtilis_ var. _niger_ 1 min
------------------------------------------------------------------
120 (wet) _Bacillus stearothermophilus_ 25 min
------------------------------------------------------------------
120 (dry) _Bacillus stearothermophilus_ 100 min
------------------------------------------------------------------
141 _Bacillus stearothermophilus_ 12 min
------------------------------------------------------------------
160 _Bacillus stearothermophilus_ 2 min
------------------------------------------------------------------
166 _Bacillus stearothermophilus_ 1 min
------------------------------------------------------------------
Table VI.—_Extremes of Chemical Environmental Factors
Permitting Growth or Activity_
--------------------------------------------------------
Chemical Minimum Organism
factor
--------------------------------------------------------
O₂ 0% HeLa cells, _Cephalobus_,
anaerobic bacteria
--------------------------------------------------------
O₃ (ozone) 0%
--------------------------------------------------------
H₂ 0%
--------------------------------------------------------
H₂O Aw 0.48 _Pleurococcus vulgaris_
------------------------------------------
Aw 0.5 _Xenopsylla cheopis_
(prepupae)
--------------------------------------------------------
H₂O₂ 0%
--------------------------------------------------------
He 0%
--------------------------------------------------------
CO 0%
--------------------------------------------------------
CO₂ 0%
--------------------------------------------------------
CH₄ 0%
--------------------------------------------------------
CH₂O 0%
--------------------------------------------------------
CH₃OH 0%
--------------------------------------------------------
N₂ 0%
--------------------------------------------------------
NO 0%
--------------------------------------------------------
NO₂ 0%
--------------------------------------------------------
N₂O 0%
--------------------------------------------------------
Ar 0%
--------------------------------------------------------
NaCl,
Na₂SO₄,
NaHCO₃
--------------------------------------------------------
H₂S 0%
--------------------------------------------------------
H₂SO₄ 0%
--------------------------------------------------------
Cu⁺⁺
--------------------------------------------------------
Zn⁺⁺
--------------------------------------------------------
pH 0 _Acontium velatum_
_Thiobacillus thioodixans_
--------------------------------------------------------
Eh -450 mV Sulfate-reducing bacteria
at pH 9.5
--------------------------------------------------------
Table VI.—_Extremes of Chemical Environmental Factors
Permitting Growth or Activity_
----------------------------------------------------------------------
Chemical Maximum Pressure, Time, Organism Activity
factor atm days
----------------------------------------------------------------------
O₂ 100% 1 Plants, Growth
animals
----------------------------------------------------------------------
O₃ 100 ppm 5 _Armillaria Growth
(ozone) --------------------------- mellea_ -----------------
500 ppm 5 Light emission
----------------------------------------------------------------------
H₂ 100% Various Germination
plants
----------------------------------------------------------------------
H₂O Aw 1.0 1 Various Growth
aquatic
organisms
----------------------------------------------------------------------
H₂O₂ 0.34% Rye Germination
enhanced
----------------------------------------------------------------------
He 100% Wheat, rye, Germination
rice
----------------------------------------------------------------------
CO 100% Rye Germination
--------------------------------------------------------------
80% 1.1 4 _Hydrogenomonas_ Growth
----------------------------------------------------------------------
CO₂ 100% 1.1 4 Rye Growth and
germination
----------------------------------------------------------------------
CH₄ 100% 1.1 4 Rye Germination
----------------------------------------------------------------------
CH₂O 50% Rye Germination
----------------------------------------------------------------------
CH₃OH 50% Rye Germination
----------------------------------------------------------------------
N₂ 100% .1 10 Various plants Germination and
root growth
----------------------------------------------------------------------
NO 18% .018 10 Sorghum, rice Germination and
root growth
----------------------------------------------------------------------
NO₂ 18% .018 10 Rye, rice Germination and
root growth
----------------------------------------------------------------------
N₂O 100% 1.2 4 Rye Germination
--------------------------------------------------------------
96.5% 1.7 Rye Germination
------------------------------------
_Tenebrio Survival
molitor_
----------------------------------------------------------------------
Ar 100% 1.2 2 Rye Germination
----------------------------------------------------------------------
NaCl, 67% Photosynthetic Growth
Na₂SO₄, bacteria
NaHCO₃
----------------------------------------------------------------------
H₂S 0.96 _Desulfovibrio Growth
g/liter desulfuricans_
----------------------------------------------------------------------
H₂SO₄ 7% _Acontium Growth
velatum_
-------------------------------
Thiobacilli Growth,
reproduction
----------------------------------------------------------------------
Cu⁺⁺ 12 _Thiobacillus Growth
g/liter ferrooxidans_
----------------------------------------------------------------------
Zn⁺⁺ 17 _Thiobacillus Growth
g/liter ferrooxidans_
----------------------------------------------------------------------
pH 13 _Plectonema Growth
nostocorum_
-------------------------------
_Nitrobacter_ Growth
-------------------------------
_Nitrosomonas_ Growth
----------------------------------------------------------------------
Eh 850 mV Iron bacteria Growth
at pH 3
----------------------------------------------------------------------
chapter 4
_Behavioral Biology_
EFFECTS OF THE SPACE ENVIRONMENT ON BEHAVIOR
NASA was established in 1958, shortly after the Russian launching of the
second Earth satellite Sputnik II, the first vehicle to carry life into
orbit around the Earth. This accomplishment was preceded by the
pioneering work of Henry et al. ([ref.77]), in which animals were
exposed briefly to low-gravity states in Aerobee rockets. A
motion-picture camera photographed the behavior of two white mice in
rotating drums during this series of flights, which marked the first
time that simple psychological tests were made on animals in the
weightless condition. While this behavioral experiment was relatively
simple, it provided the basic concepts for recent studies which involved
rotation of animals during the weightless state. Subsequent flights such
as Project MIA (Mouse-in-Able) reflected a preoccupation with
physiologic measures (refs. [ref.78] and [ref.79]), although the flights
of Baker and Able included preflight and postflight performance studies
([ref.80]). Able’s behavior was recorded in detail on in-flight film,
but none of the behavior was programed or under experimental control.
The first flights in which behavior or performance was explicitly
programed were those of Sam and Miss Sam in flights of the Little Joe
rocket with the Mercury capsule, launched from Wallops Island in 1959
and 1960 ([ref.81]). The first major space achievement in the behavioral
sciences was the successful in-flight measurement of the behavior of the
chimpanzee Ham in early 1961, in which the pretrained animal performed
throughout the flight. The second achievement along these lines was in
1962 when the chimpanzee Enos made several orbits around Earth and
performed continuously on a complex behavioral task. The tasks which the
animals performed during these flights have been described in detail by
Belleville et al. ([ref.82]), and the results of the in-flight
performance have been presented by Henry and Mosely ([ref.83]). These
early flights provided much of the technological framework on which
current biological experiments on organisms during flights of extended
duration are based. Due largely to the efforts of Grunzke (refs.
[ref.84] and [ref.85]), the apparatus needed to sustain animals during
space flight, such as zero-g watering and feeding devices, are now
commonplace ([ref.86]). Advanced systems of programing stimulus
presentations and recording responses, developed for Project Mercury,
may now be seen in many basic research laboratories throughout the
country.
Several other noteworthy advances have been made as an outgrowth of the
Mercury animal flights. Immediately before the orbital flight MA-5, in
which the chimpanzee Enos was employed, it was unexpectedly found that
this 5-year-old animal was hypertensive. Subsequent centrifuge studies
showed that its vascular responses exceeded those of a control group.
Consideration of the animal’s preflight experience led to speculation
concerning the origin of this hypertension. An explanation of the
high-blood-pressure responses detected in Enos has been pursued by
Meehan et al. ([ref.87]). Persistent hypertension has been produced in
other laboratory chimpanzees restrained in the same manner as those
participating in space flight and exposed to demanding performance
tasks, a demonstration which has important implications for prolonged
manned space flight and for cardiovascular medicine in general.
Studies more directly concerned with behavior and performance have been
extended from those of Project Mercury. These extensions have been in
the following directions: (1) the establishment and maintenance of
complex behavioral repertoires under conditions of full environmental
control, (2) the refinement of behavioral techniques for assessing
sensory and motor processes, and (3) the maintenance of sustained
performance under conditions of long-term isolation and confinement and
preliminary extension of such experimental analysis to man.
Numerous studies with primate subjects, including several at Ames
Research Center, have been devoted to developing methods for maintaining
optimum performance in environments with limited sources of stimulation.
Monkeys, baboons, and chimpanzees, for example, have been isolated for
periods of longer than 2 years with no decrement in performance on
complicated behavioral tasks ([ref.88]). The behavioral techniques used
in these studies are closely related to those employed on human subjects
under NASA sponsorship at the University of Maryland ([ref.89]). The
essence of these techniques is in the proper programing of environmental
stimuli ([ref.90]). It is not sufficient to provide the subject with his
physiological requirements for survival, but he must be given the
psychological motivation for using these provisions. This statement, of
course, is an oversimplification of the problem, but it serves to
illustrate the essence of these experimental programs.
Gravity has long been known as one of the major factors influencing
various life processes and the orientation of both plants and animals.
One of the most challenging problems of space research has been to
define this influence more precisely. Related to the effect of gravity
on living processes is the problem of the effects of weightlessness. Of
particular interest to psychologists are the possible modifications an
altered gravitational environment might produce in behavioral patterns
basic to the animal’s maintenance and survival, such as eating, sensory
and discriminative processes, development and maturation, and learning
capacity ([ref.91]).
One prominent method of studying gravitational effects is to simulate an
increase in gravity by centrifugation. Smith et al. ([ref.92]) and
Winget et al. ([ref.93]) have investigated the effects of long-term
acceleration on birds, primarily chickens, while Wunder (refs. [ref.94]
and [ref.95]) and his coworkers (refs. [ref.96]-[ref.99]) have used
fruit flies, mice, rats, hamsters, and turtles. The general findings are
that, when animals are subjected to a prolonged period of acceleration
of moderate intensity, they exhibit decreased growth, delayed
maturation, and an increase in the size of certain muscles and organs,
dependent on the species. With regard to the decreased growth effect,
the data of these investigators show some exceptions. When the
gravitational increase is kept below a certain limit, growth was greater
than that of controls in the fruit fly, turtle, mouse, and chicken. The
limit below which enhancement of growth was observed varied with the
species studied.
The data on food intake do not present a consistent picture. Wunder
([ref.94]) found that food intake in accelerated mice was markedly
reduced from that of nonaccelerated control animals. Smith, however,
found that in chickens, food intake increased up to 36 percent over
controls and has derived an exponential relation between food intake and
acceleration. After six generations of selective breeding, Smith has
produced a strain of chickens better adapted to prolonged exposure to
high g.
A very relevant finding of their research with birds was that exposure
to chronic acceleration in some way appears to interfere with
habituation to rotatory stimulation. Chickens who were being subjected
to chronic acceleration were given repeated rotatory stimulation tests
to estimate their labyrinthine sensitivity. This study revealed that
centrifuged animals showed a marked reduction in labyrinthine
sensitivity. This result appeared to persist after the acceleration was
terminated. In animals who developed gait or postural difficulties as a
result of acceleration, there was no evidence of a postnystagmus in
response to the rotatory stimulation test, which the investigators point
out may be evidence of a lesion in the labyrinth or its neural pathways.
Smith has implicated social factors as interfering with acceleration
effects. His subjects were typically accelerated four or six to a cage.
When groups were mixed midway through the experiment, they exhibited a
higher mortality rate and incidence of acceleration symptoms than did
groups whose constituency remained unchanged.
At the U.S. Naval School of Aerospace Medicine, numerous studies have
been conducted on the effects of slow rotation on the behavior and
physiology of humans and animals ([ref.100]). Rotation initially
produces decrements in performance, but adaptation to a rotating
environment ensues quite rapidly (refs. [ref.101]-[ref.103]). Perceptual
distortion, nystagmus, nausea, and other signs of discomfort are common
responses to slow rotation. These symptoms are generally reduced with
continued exposure (adaptation). Interestingly, however, adaptation is
delayed when the subjects are exposed to a fixed reference outside their
rotating environment.
At NASA-Ames, rodents have been used in experiments by Weissman and
Seldeen to delimit the stimulus effects of rotation. In these
experiments the subjects must discriminate between different speeds of
rotation in order to obtain food reinforcement. The results thus far
provide evidence that these animals are capable of discriminating
between the different speeds at which they are being rotated. The range
of speeds studied was 0-25 rpm, with tests of discrimination being made
at intervals of less than 5 rpm. Experiments such as these will lead to
the development of techniques for measuring rotational sensitivity in
many species, including man.
The optimum configuration of manned spacecraft will depend, in part,
upon biomedical considerations. A voluminous literature now exists on
the possible hazards to man of prolonged exposure to zero-g conditions.
Should prolonged weightlessness prove to be a serious detriment to
health, consideration must be given to design concepts which provide
artificial gravity.
No data exist on the minimum gravity requirements necessary to sustain
basic biological functions for extended periods. A limit of 0.2 g has
been given as the lower level at which man can walk unaided ([ref.104]).
It has also been recommended that angular velocity be maintained
at the lowest possible level in order to minimize the occurrence of
vestibular disturbances. These recommendations are based on human-factor
requirements, rather than upon biological considerations, which may
significantly modify these values. In recent studies, a technique has
been devised which promises to provide reliable criteria for biological
acceptability, since it is based on fundamental biological and
behavioral principles.
As animals progress up the evolutionary stale, their survival depends
less and less upon stereotyped physiological reactions which occur in
reflex fashion, in response to environmental stimulation. In higher
organisms, survival depends more upon the capacity of organisms to
modify their behavior. At the highest levels of functional efficiency,
the ultimate form of adaptation is seen—the manipulation of the
environment by the organism. Developments in behavioral science now
permit us to utilize the adaptive behavior of animals to investigate
many problems of biological interest. Recent studies on the
self-selection of gravity levels represent a further attempt to exploit
the adaptive capacities of animals, in order to provide information
relevant to problems of space exploration.
One such project allows animals to select their own gravity environment
in an apparatus designed to create g-forces through centrifugal action
by rotation at 60 rpm ([ref.105]). The surface of this centrifuge is
parabolic, so that the resultant of the centrifugal g and the Earth’s
gravity is always normal to the surface. When the animal moves away from
the center, increasing the radius of rotation, it is exposed to
increasing gravity. Motion toward the center reduces the gravity level.
By this means, an animal is free to select its own gravity environment.
When the animal moves toward or away from the center, he is moving from
one tangential velocity to another. He is therefore acted upon by a
third force—due to Coriolis acceleration. The effects of Coriolis forces
are a major problem difficult to eliminate in studies such as these, but
they must be taken into account in the design of spacecraft which
produce artificial gravity by rotation. Motion of the head in any
direction not parallel to the centrifugal force vector would result in
bizarre stimulation of the semicircular canals and consequent motion
sickness. This effect is likely to become even more pronounced if the
sensitivity of these organs is increased by prolonged exposure to
reduced gravity. Methods such as these are currently being developed for
conducting a refined psychophysical analysis of gravity, including
studies by Lange and Broderson on the perception of angular, linear, and
Coriolis acceleration.
The results of animal studies such as these will be of great value in
arriving at a decisive judgment concerning the need for artificial
gravity in a manned orbiting space station, or other vehicles designed
for long-term occupancy.
To aid in the interpretation of in-flight data, other studies are
underway to determine the functions of the vestibular system, as a
principal brain center related to orientation in space and to the
physiology of posture and movement, as well as with the influences of
acceleration, rotation, and weightlessness. Experiments are presently
being conducted on monkeys and cats in order to trace these complex
neurological connections and to determine their functional organization.
BIOLOGICAL INFORMATION SYSTEMS
The nature of memory has been the subject of considerable speculation in
the past. It has long been felt intuitively that retention of
information in the central nervous system involves either an alteration
of preexisting material or structure, or, alternatively, synthesis of
materials not present previously. The cellular site of operational
alteration was unknown but, again intuitively, was felt to be closely
associated with the synapses. The problems faced by early investigators
were great; but nevertheless much information relevant to the question
of biological information storage was obtained. With the relatively
recent advent of more refined tools and methodologies, there has been
rapid progress.
A significant amount of the work which has been conducted in the area of
biological information and communication systems is easily classified as
"basic research" (refs. [ref.106]-[ref.109]). This discussion will be
limited to those aspects closely related to the fields of molecular
biology and experimental psychology, which seem to have universal
application to all known animal life forms. Studies involving the basic
principles of acquisition, processing, storage, and retrieval of
information in living systems are emphasized.
Early Work
Early speculations on the operational nature of memory have been based
upon relatively little experimental evidence. Charles Darwin observed
that domestic rabbits had smaller brains than their wild counterparts,
and attributed this to lack of exercise of their intellect, senses, and
voluntary movements. Unfortunately, subsequent studies of the brains of
men with greatly differing intellectual capability did not substantiate
the hypothesis. Idiots sometimes had larger brains than geniuses. Later,
an idea proposed by Ramon y Cajal came into favor. Since brain cells did
not increase in number after birth, he proposed that memory involved the
establishment of new and more extended intercortical connections.
Unfortunately, methods were not available to test this hypothesis
adequately and it has remained until quite recently in the realm of
conjecture.
Another major hypothesis was that there were two or more stages in the
information storage process. The final form the information took in the
brain was called a brain engram, or memory trace. However, prior to the
formation of the engram, a transitory process denoted as
"reverberational memory" was postulated to exist for a relatively short
time (minutes to hours) (refs. [ref.106] and [ref.107]). This hypothesis
was used by Pauling to explain why an elderly chairman of a board could
brilliantly summarize a complex 8-hour meeting and yet, after its
conclusion and his return to his office, not even remember having
attended the meeting. Thus, this individual’s reverberational memory
functioned well, but advanced years had seriously impaired his brain’s
ability to form a permanent engram. Similar, although less dramatic,
observations in other situations are not uncommon. A wide variety of
experiments have been conducted to study this aspect of memory and to
relate it to the process whereby the information is transformed to a
more stable form (refs. [ref.110]-[ref.112]).
More recently, the concept of a specific biochemical activity during the
process of long-term storage of information has gained considerable
favor. Initially, neither the site nor the nature of the change was well
defined. Quite recent studies by Krech et al. (refs. [ref.113] and
[ref.114]), Bennett et al. ([ref.115]), Rosenzweig et al. (refs.
[ref.116] and [ref.117]) support the view that alteration of the levels
of acetylcholinesterase at cortical synapses play an important role in
information storage. These studies will be discussed in a later section.
However, these authors do not claim that the changes observed are
unambiguously related to the storage of memory. It may well be that the
alterations observed are in some way related to this process but are
still secondary to some other, more basic, process.
An alternative hypothesis is that the information resides in its
ultimate form in some more central structure of the neurone than the
synapse. (It has even been postulated that the basic information is
stored in nonneuronocortical material.) Perhaps Halstead was the first
to postulate the involvement of nucleoprotein in this process
([ref.107]). From the biochemist’s point of view, this is an extremely
attractive hypothesis. Both proteins and nucleic acids possess
sufficient possible permutations of structure to permit storage of a
lifetime’s accumulation of information in an organ the size of the
brain. From the previously known ability of the nucleic acids to code
genetic information, they are the prime suspects. However, from the
known regulatory ability of nucleic acids in specific protein synthesis,
it is possible that the final repository is protein.
Recent Biochemical Studies
Among the foremost investigators of the chemistry and biochemistry of
the central nervous system is Holger Hyden at the University of
Göteborg, Sweden. He and others (refs. [ref.118]-[ref.120]) have for
many years performed elegant microanalytical studies of single nerve
cells. The evidence which Hyden has obtained is consistent with the
hypothesis that the initial electrical reverberations in the brain
induce a change in the molecular structure of the ribonucleic acid (RNA)
of the neurones which, in turn, leads to a subsequent deposition of
specific proteins. It is well known from other investigations that a
major role of RNA in any type of cell is to specify and mediate
synthesis of the protein enzymes of the cells. Thus, in this hypothesis,
it is only necessary to postulate the modification of brain RNA by the
activities associated with reverberational memory. Particularly
pertinent to this hypothesis are observations that—
(1) Large nerve cells have a very high rate of metabolism of RNA and
proteins, and, of the somatic cells, are the largest producers of
RNA.
(2) Vestibular stimulation by passive means leads to an increase in
the RNA content of the Deiters nerve cells of rabbits ([ref.121]).
The protein content of these cells is also increased.
(3) Changes in the RNA composition of neurones and glia of the
brainstem occur during a learning situation. Animals were trained
over a period of 4 to 5 days to climb a steeply inclined wire to
obtain food. The big nerve cells and the glia of their lateral
vestibular apparatus were analyzed, since the Deiters neurones
present in this structure are directly connected to the middle
ear. The amount of RNA was found to be increased in the nerve
cells; and, more significantly, the adenine-to-uracil ratio of
both the nuclear RNA of nerve cells and glia cells became
significantly increased ([ref.119]). A variety of control
experiments were conducted. Although there was an increase in RNA
content of these cells in animals exposed to passive stimulation,
there was no change in the ratio of adenine to uracil. Nerve cells
from the reticular formation, another portion of the brain, had
only an increased content of RNA with no base-ratio change.
Animals subjected to a stress experiment involving the vestibular
nucleus showed only an increase in content of RNA. Littermates
living in cages on the same diet as learning animals showed no
change in content of RNA. Thus, it would appear that the change in
the base ratio of the RNA synthesized is not due to increased
neurone function per se, but is more directly related to the
learning process. The fact that this was nuclear RNA implies that
it was immediately related to chromosomal DNA.
(4) Neuronal RNA with changed cytosine-guanine ratios synthesized
during a short period of induced protein synthesis could be
blocked by actinomycin D. It was concluded, therefore, that the
RNA was immediately DNA dependent and directly related to the
genetic apparatus.
Rats which were normally right handed were forced to modify their
handedness in order to obtain food. The RNA of nerve cells in that part
of the cortex, whose destruction destroys the ability to transfer
handedness, was analyzed. A significant increase in RNA of nerve cells
of the fifth to sixth cortical layers on the right side of the brain was
observed. The corresponding nerve cells on the opposite side of the same
brain served as controls. There was an increase in RNA and a significant
increase in the purine bases relative to the pyrimidine bases in the
learning side of the cortex. When the animals were not forced to learn a
new procedure, only an increase of RNA was observed, with no change in
base ratio.
Frank Morrell, head of the Neurology Department at Stanford Medical
School, has also been active in this field during the past 6 years. He
has found that if a primary epileptic lesion is induced on one side of
the cortex, a secondary mirror lesion eventually develops in the
contralateral homologous cortex. This secondary lesion, which showed
self-sustaining epileptiform discharge, could be isolated, whereupon the
epileptiform discharge disappeared. This was interpreted as learned
behavior of the secondary lesion. From changes in the staining
properties of the secondary lesion, Morrell concluded that changes in
RNA had occurred in the cell. Changes in the composition of the RNA
could not be shown by these techniques.
At the University of California at Berkeley, Drs. Rosenzweig, Bennett,
and Krech have conducted extensive studies related to this topic. These
investigators have directed their efforts toward demonstrating
alterations in the cerebral cortex of animals exposed to continuing
learning situations or continuously deprived of sensory stimulation. In
a recent publication ([ref.116]), which also summarizes a considerable
amount of previous work, they report studies which demonstrate the
following:
(1) Rats given enriched experience develop, in comparison with their
restricted littermates, greater weight and thickness of cortical
tissue and an associated proportional increase in total
acetylcholinesterase activity of the cortex.
(2) The gain in weight of cortical tissue is relatively larger than
the increase in enzymatic activity. Acetylcholinesterase activity
increases in other portions of the brain even though tissue weight
decreases.
(3) The changes appear in a variety of lines of rats, although
differing in amount between strains.
(4) The changes are observed in both the young and adult animals.
The previous studies were comparisons between experience-enriched
animals and animals maintained in isolation. Animals which were housed
in colonies, but given no special treatment, showed intermediate effects
in those situations studied.
The Berkeley group emphasized that the finding of changes in the brain
subsequent to experience does not prove that the changes have anything
to do with memory storage, but do establish the fact that the brain can
respond to environmental pressure. However, the results are compatible
with the hypothesis that long-term memory storage involves the formation
of new somatic connections among neurones. Calculations of the amount of
additional material required to permit this to exist are compatible with
the increases observed.
A number of investigators have studied the effects of antimetabolites
and drugs on the learning process. Since their specific metabolic
effects are known in other tissues, the rationale is that if these
materials do interfere with memory, then specific types of metabolic
activities may be implicated in the deposition of the engram.
One of the initial studies of this type was conducted by Dingman and
Sporn ([ref.122]), presently at the National Institute of Mental Health.
They showed that 8-azaguanine, a purine antagonist, injected
intra-cisternally was incorporated into the RNA of the brains of rats.
Associated with this incorporation was an impairment of the
maze-learning ability of the animals. These findings have been
confirmed.
Flexner and his associates injected puromycin, an inhibitor of protein
synthesis, into the brains of mice, which were then trained to perform
in a maze. Losses of short-term or long-term memory were obtained,
depending upon the site of the injection. The results indicate that the
hippocampal region is the site of recent memory.
The hippocampal region is of interest in connection with memory
processes for a number of other reasons. Adey et al. ([ref.123]) and his
group observed a transient fall in electrical impedance in this region
when cats learned to perform in a T-maze in response to a visual cue. It
was supposed that the electrodes were situated within glial cells of the
dendritic zone of the hippocampal pyramidal cell layer. Extinction of
the learned habit abolished the briefly evoked impedance changes, which
subsequently reappeared with retraining.
A number of other studies more or less indirectly implicate RNA in the
learning processes. For instance, in retinal cells of rabbits raised in
darkness, there was virtually no ribonucleoprotein as compared with
normal amounts in the cells of animals raised in light ([ref.124]).
Further, maintenance of normal electrical activity of isolated perfused
cat brains is highly dependent upon the presence of the ribonucleic acid
precursors, uridine and cytidine, in the perfusate ([ref.125]), and
severe derangements occur if any of a variety of pyrimidine antagonists
are added ([ref.126]). Brief electrical stimulation of cat cortical
tissue causes an increase in nucleic acid cytidine and adenine, thus
indicating a synthesis of altered polynucleotides. Finally, injections
of RNA in animals have shown interesting effects. When given at a dose
of 116 mg/kg daily for 1 month, rats showed an enhanced response and
greater resistance to extinction in a shock-motivated behavioral
response. It has been shown by another group that injections of RNA
enhance the ability of young animals to learn various tasks.
Planaria have been used in a variety of studies which seem to bear on
the problem of memory. Quite recent evidence by Bennett, Calvin, and
their associates has cast somewhat of a pall over the studies;
nevertheless, the work may have some validity. Interest in the use of
flatworms, particularly planaria, for study of memory began with a
demonstration by McConnell that these simple animals could undergo
conditioning ([ref.127]). Subsequently, it was found that some
conditioning was retained when the animal was transected and allowed to
regenerate. The retention of training was found in both new animals,
although the very simple brain, really only two ganglia, was in the head
section ([ref.128]).
Apparently, some diffusely distributed component of the animal was
responsible for retention of learning. Evidence has accumulated to
indicate that this material is RNA. Among this evidence is the
following:
(1) The two halves of a trained planaria were allowed to regenerate in
a solution containing RNA-destroying enzymes. Whereas the head
ends retained some training, no retention was observed in the
animals derived from the tail end ([ref.129]).
(2) When pieces of trained planaria were fed to untrained animals, the
untrained cannibal required a shorter time to become trained to a
criterion. It would appear that the digestive system of planaria
is so simple that the material responsible for the transfer of the
information was not broken down.
(3) When RNA, obtained from trained planaria, is injected into the
digestive tract of untrained animals, there is a transfer of
information.
NEUROPHYSIOLOGY²
² Excerpt from [ref.130].
Neurophysiological studies concern the functions of the nervous
system—in particular the central nervous system (CNS)—under normal,
simulated, and actual flight conditions. Of paramount importance is the
maintenance of equilibrium and orientation in three-dimensional space.
The ability of man and his close relatives among the vertebrates to
maintain these functions depends on an integrated sensory input from the
vestibular organ; the eyes; the interoceptors of the muscles, tendons,
joints, and viscera; and the exteroceptors of the skin.
Certain parameters of the environmental and space-flight conditions
drastically affect man’s ability to maintain equilibrium and spatial
orientation. Centrifugal forces modify or reverse the directional vector
of gravity. Linear acceleration may increase enormously, as may angular
stimulation. The sensory organs listed above are unreliable under such
conditions. The very organ which is designed specifically to furnish
information on spatial orientation may malfunction in man while he is in
flight. Thus, with respect to sensory orientation, these labyrinthine
organs are by no means precision instruments.
The use of classical histological methods and the observation of
equilibrium disturbances resulting from operative interference with the
internal ear have in the past been the two principal sources of
knowledge concerning the structure and function of the labyrinth, but
the answers given to various questions vary considerably in their value.
The development of electrophysiological techniques and the refinement in
recent years of the ultrastructural analysis by means of the electron
microscope may allow more precise experimental studies of the
correlation of function and structure.
Before considering vestibular impulses in their bulbar and descending
spinal pathways, a recent study concerning the generation of impulses in
the labyrinth must be mentioned. Von Bekesy’s finding ([ref.131]) of the
direct current potentials in the cochlea aroused speculation about the
existence of similar labyrinthine potentials. Such dc potentials were
also detected in the semicircular canal of the guinea pig by Trincker
([ref.132]), who measured the potential changes in the endolymph,
surface of the cupula, or side of the crista during cupular deflection.
It seems likely, however, that the effects do not represent the
physicochemical changes in the cupula but the electrical potentials in
the nerve and nerve endings of the crista. Attempts at differentiating
these effects have failed so far. Great expectations are brought by the
advances of microchemistry, microphysiology, and physical chemistry with
regard to the excitatory processes, the generation of the nerve impulse.
Quite apart from a need to understand vestibular nerve discharges and
patterns more adequately in such terms, the analysis of the vestibular
system has in the past revealed general biological principles which were
not readily discernible through the examination of other tissues
([ref.133]).
The neural connections of the vestibular organ consist of numerous
chains of neurons, reciprocally linked in many ways and having their
synapses in various anatomical nuclei. All the chains work in intimate
collaboration, and the final pattern of reflex responses is attributable
largely to the highly complex integrating activity of the center. The
labyrinthine function is automatic, carried out in a reflex fashion: in
other words, mostly below the level of consciousness. The brain centers
through which the labyrinth elicits the various appropriate muscular
reactions of the head, body, limbs, and eyes—the righting, the postural,
and the ocular reflexes—represent an intricate mechanism. Before we can
hope for a satisfactory understanding of their functional organization,
we will have to know their anatomy in more detail. Thus, we are
confronted with a fruitful field for the exploration of basic mechanisms
of neuronal activity. Major advances dining the last years have provided
us with new information about the neuroanatomy of the vestibular system
(refs. [ref.134]-[ref.137]).
Vestibular impulses entering the brainstem ascend and descend the
neuroaxis and cross the midline. It was previously believed that the
vestibular apparatus had only subcortical projections. Recently,
however, it has been established by means of electrophysiological
methods that the organ is represented by a projection area in the
cerebral cortex of some animals (refs. [ref.138]-[ref.141]). The use of
brief electrical stimulation of the vestibular nerve in order to elicit
a cortical response has been of great value for the mapping of these
areas.
Among a great variety of sensory receptors, the vestibular ones are
capable of evoking the most widespread somatovisceral effects throughout
the body. Moreover, vestibular effects seem to be imperious and less
dependent upon the state of readiness of the nervous system. As a
consequence of the extensive distribution of vestibular effects, there
are many opportunities for central integration. Proprioceptive and
vestibular systems are both known to be active in posture and
locomotion; streams of impulses arising from the receptors in each of
these systems must converge to influence the activity of the final
common path. The state of the motor centers of the spinal cord, as
affected by vestibular stimulation, has been tested by dorsal root and
other sensory input interventions. These experiments have provided us
with insight into the mechanisms concerned with the vestibular control
of spinal reflexes (refs. [ref.142]-[ref.146]).
It has long been known that the vestibular apparatus is essential for
the development of motion sickness. Commonplace subjective experience of
nausea relates to visceral changes mediated through autonomic efferent
pathways and may ultimately involve rhythmic somatic nerve discharges to
skeletal muscles responsible for retching and vomiting. However, very
little is known about the central nervous mechanisms responsible for
elaboration of the whole syndrome. Since the maintenance of vestibular
bombardment for some length of time seems essential for the development
of motion sickness, one would presume this to be an instance of slow
temporal summation. Experimental findings demonstrate a powerful effect
of temporal summation upon somatic motor outflow during vestibular
stimulation ([ref.147]), and not upon parasympathetic outflow.
The practical implication of these studies is closely related to
physiological effects of weightlessness. Based on experimental evidence
from short weightless periods obtained in aircraft, it was concluded
that "when the exposure becomes longer, there may develop minor
physiologic disturbances which, if cumulative or irritating, may cause
or enhance psychiatric symptoms" ([ref.148]). Although the zero-g
condition, per se, does not cause spatial disorientation if visual cues
are provided, the astronauts reported a temporary loss of orientation
during the orbital flight while they were engaged in activities which
diverted their attention. However, no disturbing sensory inputs were
observed during the weightless period. Violent head maneuvers within the
limited mobility of the helmet were performed in every direction without
illusions or vertigo. The subjective sensations of "tumbling forward"
after sustainer engine cutoff reported by the Mercury astronauts, and
Titov’s motion sickness attacks, which were particularly dismaying
during head movements, were well within the entire range of
psychosomatic experiences already obtained during aerodynamic
trajectories ([ref.149]). Interestingly enough it now appears that the
otolithic output in mammals including man is the differential of linear
acceleration, and therefore unaffected by zero g.
Of interest in this connection are the problems which may be encountered
during and following long-term exposure to weightlessness. Although
there is no evidence of adverse effects on operative behavior, the
possibility of biological disturbances on a cellular or subcellular
level, which may cause a deterioration of the somatic basis, has been
repeatedly stressed. Whether effects of this sort will occur or whether
the organism will be able to adapt is still an open question. Since
motion sensitivity based on vestibular stimulation differs widely among
individuals, the selection of astronauts may solve the problem of zero-g
vestibular disturbance. Reports from the MA-8 (Sigma 7) and Vostok III
and IV flights seem to support this assumption. Moreover, experiments
are being made in the slow rotation room at the Naval School of Aviation
Medicine to study the Coriolis effects which arise when "artificial
gravity" is produced by angular acceleration. Since man can adapt to
wave motion on shipboard within a few days, a similar process may be
expected to occur in the case of long-term weightlessness ([ref.150]).
chapter 5
_Molecular Biology and Bioinstrumentation_
To support biological investigations in space and to accumulate baseline
data needed for manned space flight, NASA has conducted a program in
laboratory research and theory. A multidisciplinary approach has
included such fields as ecology, physiology, organic and biological
chemistry, engineering, electronics, and optics. Emphasis in this
program has been placed on qualitative and theoretical rather than
purely descriptive research, and the investigation of fundamental
biological phenomena at all levels, from the molecular to the total life
form.
MOLECULAR BIOLOGY
Research in molecular biology has included chemical, physical,
biological, and theoretical investigations of prebiological conditions
on Earth and, possibly, on other planets; studies of cellular
inclusions; genetic material (DNA and RNA) and coding; as well as energy
transfer in biological systems.
The understanding of prebiological conditions on Earth, and possible
conditions on other planets, depends upon the nature of the complex
chemical species which might be encountered. Scientists have shown that
biologically important compounds, such as amino acids, can be generated
by applying an electrical discharge, ultraviolet radiation, or heat to a
gaseous mixture. Biologically interesting compounds can be removed from
such a system by condensation or absorption; however, in the limited
time and space available in such experiments, many compounds are not
produced in sufficient quantity to be measured.
The National Biomedical Research Foundation (NBRF) and the National
Bureau of Standards (NBS) are conducting an investigation on equilibria
in multielement systems. The distribution of molecular species at
equilibrium is independent of the way equilibrium was reached and is
dependent only on pressure, temperature, and elemental composition. Many
of the conditions which might have arisen naturally can be approximated
by thermodynamic equilibrium. Compounds which can be formed at
equilibrium need no special mechanism to explain their presence.
However, special mechanisms have to be sought for those compounds which
could not be so produced and which would have been required for the
structure and nutrition of the first living organisms.
In the absence of precise knowledge of the composition of the primitive
planetary atmospheres, equilibrium concentrations with a wide range of
temperatures, pressures, and elemental compositions are being
investigated by NBRF and NBS. These investigators have postulated that
the maximum atmospheric pressure may have approached 100 atm if the
primitive Earth was sufficiently hot and if an appreciable portion of
the water on Earth’s surface today was present on primitive Earth. (If
the present oceans were to evaporate, the surface pressure would be
approximately 300 atm.) Low pressures of 10⁻⁶ atm and temperatures
between 500° and 1000° K are being used.
A large range of N, O, C, and H compositions are being investigated for
interesting and plausible combinations of factors. In these calculations
an IBM 7090 computer is being used to obtain data on a very large number
of combinations of chemicals. Other chemical species will be added as
the research continues. Some results of this study give an insight into
the variety of biologically significant chemicals which might have
existed during Earth’s primitive prebiological condition or may now
exist on the surfaces and in the atmospheres of other planets (refs.
[ref.151]-[ref.153]). The general method described by White et al.
([ref.152]), minimizing the free energy of the system, was used. The
solution was approached by an iterative process, starting with an
initial guess of concentrations of the compounds. At each step, _M_+1
linear equations are solved where _M_ is the number of elements in the
system.
In addition to listing of the concentrations of all compounds included
in each problem, the results of three-element problems have been
expressed on a triangular composition diagram for convenience. A coarse
grid of 60 points is used to survey all elemental compositions, with
finer grids being used in regions of particular interest. The calculated
concentrations of the compounds at each composition are stored, and
finally a series of triangular diagrams is printed out, each showing the
concentrations of as many as four compounds at the grid points.
Figure 2 shows the results obtained in the C, H, and O systems. Organic
compounds in concentrations greater than 10⁻²⁰ mole fraction are found
everywhere except where free O₂, is present. Solid carbon theoretically
becomes stable along the lower dashed line at 500° K. However, reactions
producing it are very slow. The supersaturated region beyond the line of
potential carbon formation was also investigated. A threshold was found
where polynuclear aromatic compounds are sufficiently concentrated to
form a liquid phase. These conditions may have been involved in the
primordial formation of asphaltic petroleum.
[Illustration: Figure 2.—_Equilibrium diagram for the system C-H-O._]
Jukes and associates ([ref.154]) at the University of California at
Berkeley have been investigating the code for amino acids in protein
synthesis, the key for translating the sequence of bases in DNA into the
sequence of amino acids in proteins. The amino acid code was solely a
matter of theory until Nirenberg and Matthaei ([ref.155]) at the
National Institutes of Health carried out a crucial experiment. This
experiment bridged the last remaining gap separating theoretical
genetics and test-tube biochemistry. It now became experimentally
possible to search for codes for all 20 amino acids concerned in the
synthesis of proteins.
The amino acid bases of DNA are: A, adenine; C, cytosine; G, guanine; T,
thymine; and U, uracil, which replaces thymine in RNA. There are only 16
ways of arranging A, C, G, and T in pairs. For this and other reasons it
is thought that a triplet of three consecutive bases is needed to code
for each amino acid. The sequences of bases in a strand of DNA are known
to be unrestricted with respect to the order in which they occur;
apparently any one of the four bases can be next to any of the other
four, although, of course, each base must be paired with the
corresponding complementary base in the adjacent strand. Since the same
freedom is true of the amino acid sequences in the polypeptide chains of
proteins, any one of the 20 amino acids can occur next to any other.
Moreover, the sequences in DNA are subject to mutational changes in
which one base replaces another, or bases are added to or deleted from
the DNA. Such rearrangements plus the possibility of lengthening of DNA
molecules are numerous enough to account for all the genetics of living
forms since the first appearance of life on Earth.
Most of our knowledge is based on experiments with synthetic RNA carried
out with extracts of _E. coli_. The majority of the work has been at
Nirenberg’s laboratory at the National Institutes of Health and at
Ochoa’s laboratory at New York University ([ref.155]). Various
combinations of A, C, G, and U were used in preparing the synthetic RNA
molecules that are used in experiments to explore the code. These
molecules are made by incubating a mixture of ribonucleoside
diphosphates with a specific enzyme, polynucleotide phosphorylase. An
important property of this enzyme is that it condenses the nucleoside
diphosphates into polynucleotide strands containing random sequences
depending on the proportion of each base. For example, if the enzyme
were furnished with a mixture of 5 parts of A and 1 part of C, it would
make strands containing, on the average, 25 sequences of AAA, 5 of AAC,
5 of ACA, 5 of CAA, and 1 each of ACC, CAC, and CCA. The proportion of
triplets within the strands of a polynucleotide is reflected in the
proportion of amino acids in polypeptides that are obtained in the
cell-free system. Most of the present knowledge of the amino acid code
is based on this concept. All the proposed codes have been discovered by
this experimental approach where synthetic RNA molecules are used as
"artificial" messenger RNA.
Representative of another class of activities in molecular biology is
the examination of passive ion flux across axon membranes. This work is
being done by Goldman at the National Naval Medical Center. The question
of stimulus transmission by nerve tissue is far from simple, and the ion
concentrations associated with nerve membranes is a significant part of
the answer. Because the space environment may very well produce
alterations in these ion potentials, an investigation of their natures
and significance becomes extremely important. A working theory is now
being developed as a result of this study.
Vital cell processes, chemical transformations, and mechanisms that
provide energy for cell maintenance and activity have been studied by
Kiesow (refs. [ref.157] and [ref.158]) at the Naval Medical Research
Institute. The common objective of all phases of this project is the
elucidation of reaction steps in which energy and matter are transformed
in living systems. Compared with _photo_synthetic organisms,
_chemo_synthetic bacteria offer distinct advantages for the study of
energy assimilation. These studies have led to the following
experimental findings.
With the energy from oxidation of nitrite, NO₂— to nitrate, NO₃— as an
_inorganic_ source, and with added _organic_ chemical energy from the
hydrolysis of adenosinetriphosphate (ATP) to adenosinediphosphate (ADP)
and inorganic phosphate, chemosynthetic bacteria are capable of reducing
diphosphopyridinenucleotide (DPN⁺) to DPNH, in a coupled
oxidoreduction-dephosphorylation. Thus, in the crucial step of
chemosynthesis, _ATP is consumed, not produced_. However, in
simultaneously proceeding cell respiration, the energy donor, DPNH, is
oxidized and generates more ATP than is required for DPN⁺ reduction.
This "breeder cycle" for DPNH—with different ratios of cell respiration
and biosynthesis—results in a net production of either DPNH, or ATP, or
both. Production of DPNH in the cycle leads immediately to the
assimilation of C¹⁴ from HC¹⁴O₃—. These observations explain the
bacteria’s energy source without the classical hypotheses of either
direct phosphorylation or direct CO₂ reduction by inorganic chemical or
electromagnetic energy. The cycle transforms the free energy of nitrite
oxidation into the free energy of the organic compounds. Cell
respiration and elementary biosynthesis proceed through structure-bound
enzyme systems in the same fraction of subcellular particles. Three
components, two cytochromes and one flavoprotein, have been identified.
A thermodynamic analysis of the DPNH "breeder cycle" appears to be
attainable by measurements of redox potentials and calorimetric
determinations of heats of reaction.
Studies are also being conducted by Pollard and associates at
Pennsylvania State University in an attempt to formulate a theoretical
basis for the description of the processes of synthesis, growth,
division, and differentiation of the living cell. Such a theory would be
basic to an understanding of very primitive life forms or prebiological
material which might be found elsewhere in the universe. For these
purposes, studies are being undertaken in macromolecular reproduction
which differ from the studies involving cellular genetic material.
Theories concerning the problem of replication of cellular structures
and information storage in two-dimensional systems are being developed.
Theories are also being developed about the mechanisms which control and
regulate receptor and enzymatic activities within the cell.
One study involved the rate of mutation in cells and disposed of the
suggestion that the process of mutation consists of a "tunneling" of
proton from one base to another in DNA. Such a suggestion can no longer
be advanced as a major explanation of mutations.
Work is also being conducted on the centrifugation of cells of _E.
coli_. It has been shown that cells exposed to as little as 100 g have a
modification in their function. This has been looked at from the point
of view of thymine uptake, which would be concerned with the formation
of DNA, and also from the point of view of the induction of an enzyme,
which would correspond to the transcription of the DNA. Preliminary
experiments in the latter case indicate considerable centrifugation
effect. The thymine uptake is affected, but not nearly as much as
formerly thought. Further work is in progress in this area.
Important work has been completed on the cells of _E. coli_ grown on
maltose, which can be induced to produce betagalactosidase by the
addition of thiomethyl galactoside. If cells are irradiated shortly
after induction, the transcription of the DNA ceases and the enzyme
produced by the messenger RNA is observed to reach a maximum. This
enables the calculation of the half-life of unstable messenger RNA. The
half-life for this decay is readily measurable, and values are given
over a temperature range of 17° C (5.2 minimum) to 45° C (0.56 minimum).
These agree very well with half-lives measured by others by inducing for
short times and measuring the course of enzyme formation. The rate of
transcription is involved in the kinetics of cessation of enzyme
induction, and the rate of transcription can be measured. Arrhenius
plots for this rate and the rate of decay are given, and the activation
energies measured are about 16 000 cal/mole. The cessation of
transcription is linked to the degradation, possibly of only one strand,
of DNA.
Pollard has suggested that one important action of ionizing radiation is
concerned with the transcription of the genetic message into RNA.
Clayton and Adler ([ref.159]) showed that induced catalase synthesis in
_Rhodopseudomonas spheroides_ is inhibited by low doses of X-rays,
giving experimental support to the idea. Pollard and Vogler ([ref.160]),
using cells in which the process of induction involved permease, showed
that there is some sensitivity to gamma radiation. Novelli et al.
([ref.161]) found a reduced sensitivity as compared with colony
formation, but it is still a considerable sensitivity.
The process of induction of an enzyme indicates that the transcription
of the genetic message is repressed by something which can be acted on
by a small molecule, the inducer, to remove repression and permit the
formation of messenger RNA, which then acts to make the enzyme. The
messenger RNA undergoes decay through a process which is still not
clear. Very elegant measurements by Kepes ([ref.162]) show that for the
messenger RNA for betagalactosidase, the half-life is 1.02 min at 37° C
and 2.05 min at 25° C. The time of onset of enzyme formation after
induction was found to be about 3 minutes.
If the process of transcription is indeed sensitive to ionizing
radiation, then the irradiation of cells which have just been induced
should show formation of the enzyme to the extent of formation of new
messenger RNA within a few minutes, plus the formation of the enzyme
while the messenger RNA is decaying. This pattern was found by Clayton
and Adler. The experiments conducted by Pollard and associates amplify
and extend their work and also agree with the work of Kepes ([ref.162]).
BIOINSTRUMENTATION
Fernandez-Moran (refs. [ref.163]-[ref.165]), at the University of
Chicago, has devised a new multielectrode electrostatic lens which he
has incorporated into an electron microscope. This necessitated the
development of a novel high-voltage power source and voltage regulator
of extreme stability and accuracy. Some promising work has now been done
on superconducting lenses. In a series of experiments with a simple
electron microscope without pole pieces, using high-field
superconducting niobium-zirconium solenoid lenses in an open air core,
liquid helium Dewar, electron microscopic images of test specimens have
been recorded while operating at 32 200 gauss in a persistent current
mode, with regulated accelerating potentials of 4 to 8 kilovolts. These
preliminary experiments have demonstrated the exceptional stability of
the images (both short term and long term) over a period of 4 to 8 hours
and the relatively high quality of the images.
Progress has been made on the viscosimeter for high intrinsic
viscosities. This is now working, and the viscosity of DNA preparations
has been measured. It is hoped to use the viscosimeter to study the
variation in DNA viscosity as a function of the cell cycle.
An instrument is under development by Wald at the University of
Pittsburgh to automatically analyze cytogenetic material and, thus,
extend cytogenetic methodology both for research and as a biological
monitoring procedure, using automatic electronic scanning and computer
analysis of chromosomes. Chromosomal aberrations can thus be monitored
under unusual and abnormal conditions such as weightlessness and
radiation, since chromosomes are very sensitive to stress situations. In
this device a sample will be prepared and automatically inserted under a
microscope lens. The device will then scan, identify, and photograph on
35-mm film a predetermined number of mitotic cells and process the film.
The data will be recorded under the direct control of a digital
computer. The computer will perform a detailed quantitative analysis of
the pictorial data.
Significant effort has been expended in the development of
instrumentation for measuring and recording electrophysiological
information. One such instrument, developed by the Franklin Institute,
Philadelphia, Pa., is a temperature-sensing microprobe. This microprobe
is an implantable and remote broadcasting instrument. These developments
are associated, in part, with training programs so that competent
individuals may be trained not only in electronics but also in the
biological uses of the devices they construct.
A project of interest, conducted at the Stanford Research Institute, is
the investigation of the uses of an extremely sensitive method for
measuring magnetic susceptibility having the possibility of detecting
macroscopic quantum effects in macromolecules of biological interest.
Good progress has been made in the first 15 months of a project devoted
to the development and initial use of equipment specifically designed
for this purpose. A new superconducting circuit, together with
superconducting magnetic shields, has been constructed. This apparatus
can measure the magnetic susceptibility of small organic samples at
temperatures between 1° and 300° K in fields up to 40 000 gauss. It can
detect flux changes of 10⁷ gauss-cm², which is equivalent to detecting a
change in specific susceptibility of 1 in 10⁹ in a 100-mg sample under
an applied field of 10 000 gauss.
Several hundred preliminary measurements were made on samples of
coronene. The most reliable of these were in agreement with published
values of the magnetic susceptibility of coronene. Experience during
these measurements led to changes which have resulted in an apparatus
well suited to the measurements on macromolecules. An improved version
of the superconducting circuit now available shows promise of a further
improvement in sensitivity by a factor of more than a thousand
([ref.166]).
Living organisms possess many unique processes and systems which are
complex and poorly understood. The new theoretical approaches, combined
with laboratory studies, are expected to result in advances which will
expand both our scientific and technological horizons.
chapter 6
_Flight Programs_
BALLOONS
Biological and medical experiments carried out on balloon flights, both
manned and unmanned, antedate the establishment of NASA. Aside from the
early use of balloons in flights that could be called simply
flight-survival studies, balloons have made important contributions to
our present knowledge of the effects of cosmic radiation and to various
aspects of space travel.
The achievements of the Strato-Lab and Man High series by the U.S. Navy
and Air Force include a wealth of information on balloon travel and on
the survival of man at altitudes close to and above 100 000 feet.
Generally, balloon launches of animals, which reached a maximum in 1953
when 23 balloons were released, have established the feasibility of a
program of extended manned balloon flights to high altitudes.
Atmospheric life studies outside the area of cosmic radiation effects
have been comparatively few. Results from two manned flights, Strato-Lab
I and II, indicate that the flights did produce pronounced changes in
white blood cell count; however, the data suggest that these changes
were due to psychological rather than physical stress. Exposure to
altitudes above 90 000 feet for a total of 62 hours did not produce any
general behavioral change in two Java monkeys, according to other
balloon flights. Many of these flights were effective in testing
equipment, telemetering devices, and in pointing the way for other
flights.
Stratoscope I and II, originally undertaken by the Office of Naval
Research (ONR), are projects involving various astronomical observations
with the aid of a balloon-borne telescope and television and camera
systems. NASA cooperated with ONR on Stratoscope II (36-inch telescope
compared with Stratoscope I’s 12-inch telescope) which has already
resulted in significant discoveries about the nature of the planets and
stars. Water vapor has been identified in the atmosphere of cool red
stars and an analysis of the Martian spectra showed a greater abundance
of carbon dioxide than had previously been believed. Since the
balloon-borne telescope was carried beyond Earth’s obscuring atmosphere,
the Stratoscope projects have yielded valuable photographs of the Sun,
stars, and various planets.
ROCKETS AND SATELLITES
Historically, biological experiments aboard rockets and satellites have
been limited to a "piggyback" and "noninterference" basis on military
rockets. For the past few years, however, as the effort toward manned
space flight leading to lunar and Martian landings increased, more
attention was devoted to experiments designed to show the effects of the
space environment on living systems. As in the balloon flight programs,
the U.S. Army, Navy, and Air Force played an important role, reaching
what might be considered a high point with the successful launch and
recovery of a ballistic rocket experiment with monkeys Able and Baker.
Aerobee rockets as well as Thor IRBM’s carried biological payloads
consisting of mice and monkeys on six launches, contributing to our
knowledge of the effects of weightlessness and radiation on higher
animals.
Van der Wal and Young ([ref.78]) used Thor-Able combinations to serve as
boosters for lifting a 20-pound biocapsule to a peak altitude of 1400
miles and over a distance of about 5300 miles from Cape Canaveral to the
west coast of Africa. Weightlessness was attained for a period of almost
40 minutes. During reentry into the atmosphere, a peak deceleration of
about 60 g was reached. Each of the three capsules flown carried one
mouse (Mouse-in-Able); two of the mice were instrumented for heart-rate
telemetry. Although all three mice were lost, the two experiments with
Laska and Benji yielded physiological results.
The experimenters designed effective instrumentation for registering the
electrical activity of the mouse’s heart through a single commutated
telemetry channel. Records were obtained for both animals during various
portions of the flight. The results indicate that both animals were
alive when the nose cones hit the water.
Two South American squirrel monkeys (Gordo and Baker) and a rhesus
monkey (Able) were launched into space from Cape Canaveral in 1958 and
1959 by U.S. Army Jupiter missiles. The vehicles reached speeds of
approximately 10 000 mph and altitudes of 300 miles on flights which
lasted about 15 min.
Time courses of cardiac and respiratory rates ([ref.80]) of the two
squirrel monkeys showed that the noise of the engine at liftoff
immediately produced an increase in their heart rates. Respiration also
increased temporarily, but slowed later with increasing acceleration.
Heart rates fluctuated considerably during launch acceleration, which
reached about 15 g at cutoff.
The period of free flight and weightlessness was characterized by
pronounced fluctuations of heart activity in the postacceleration phase.
Thereafter, the heart rate of Baker remained relatively constant,
whereas the cardiac activity of Gordo fluctuated markedly and decreased
slowly almost to the end of his flight. Slight changes, which were
transient and not pathological in nature, were also noted in the
electrocardiogram. Gordo’s respiration was very shallow during maximum
launch acceleration, when Baker’s reached its highest value, only to be
approximated again during reentry when forces of about 35 g were
encountered.
Able’s cardiac and respiratory rates indicated that, after an initial
startle reaction, the heart rate dropped transiently and then increased
steeply, reaching a maximum of 259 during the 10-second interval at peak
acceleration. Respiration increased only slightly throughout the
launching phase. There was a period of tachycardia during
postacceleration weightlessness, after which the heart rate declined
steadily and was disturbed only by several startling missile events. At
the end of the subgravity phase, Able’s cardiac rate was slightly below
normal.
Although the periods of high g force and free flight were short, the
extremes were considerable, and the changes from one state to the next
were rapid. In spite of this, the cardiovascular, hemodynamic, and
electrocardiographic phenomena were remarkably well maintained.
Apparently the animals were not in serious plight at any time. That
psychological factors entered into the observed phenomena is clearly
evident from the increase in cardiac rate associated with the noise of
the engine prior to liftoff and also from the cinematographic record of
facial expressions. Nevertheless, the integrated responses indicated
that the animals’ physiological states remained sufficiently normal to
insure a safe flight.
LITTLE JOE FLIGHTS
The first step in an attempt at animal verification of the adequacy of
the Mercury flight program was the development of two tests by NASA in
collaboration with the U.S. Air Force School of Aviation Medicine in
which there would be a biomedical evaluation of the accelerations
experienced during the abort of a Mercury flight at and shortly after
liftoff. These flights were launched at the NASA Wallops Station with a
Little Joe solid-fuel launch vehicle.
Two Little Joe launches were made with activation of the escape rockets
during the boost phase to secure maximum acceleration; only a brief
period of weightlessness was attained. The first launch was on December
4, 1959, and the other on January 21, 1960. A 36 by 18-inch sealed,
125-pound, cylindrical capsule containing the subject, an 8-pound
_Macaca mulatta_, the necessary life-support system, and associated
instrumentation was flown in a "boilerplate" model of the Mercury
spacecraft. The rhesus monkeys were named "Sam" and "Miss Sam."
The flight profile included maximum accelerations of about 10 to 12 g
and periods of about 3 minutes at 0±0.02 g. The peak altitude obtained
in the last ballistic flight was about 280 000 feet. The experimental
capsule was pressurized at 1 atmosphere with 100 percent oxygen at the
start of the experiment and fell to just below a half atmosphere of
oxygen due to breathing during flight. The capsule temperature was kept
between 10° and 20° C in both flights.
The measurements taken from the rhesus monkeys were the
electrocardiogram, respiration, body temperature, eye movements, and bar
pressing, but only partial results were obtained in the first flight.
Oxygen tension, total pressure, capsule temperature, and relative
humidity were recorded. Both animals were recovered alive and did not
show pathologic alterations in their physiologic and psychological
reactions.
MERCURY ANIMAL TEST FLIGHTS
In the Mercury animal test program a Redstone missile carried the
chimpanzee Ham on a ballistic flight to a height of 155 miles to provide
animal verification of the success with which the Mercury system could
be applied to manned flight. The male chimpanzee was trained to perform
a two-phased reaction task during the 16 minutes of flight. The
chimpanzee Enos was put into orbit for 3 hours and 20 minutes. Results
of the two flights gave the following information:
(1) Pulse and respiration rates during both the ballistic (MR-2) and
the orbital (MA-5) flights remained within normal limits
throughout the weightless state. Effectiveness of heart action, as
evaluated from the electrocardiograms and pressure records, was
also unaffected by the flights.
(2) Blood pressures, both arterial and venous, were not significantly
changed from preflight values during 3 hours of the weightless
state.
(3) The performance of a series of tasks involving continuous and
discrete avoidance, fixed ratio responses for food reward, delayed
response for a fluid reward, and solution of a simple oddity
problem was unaffected by the weightless state.
(4) Animals trained in the laboratory to perform during simulated
acceleration, noise, and vibration of launch and reentry were able
to maintain performance throughout an actual flight.
From the results of the MR-2 and MA-5 flights, the following conclusions
were drawn:
(1) The numerous objectives of the Mercury animal test program were
met. The MR-2 and MA-5 tests preceded the first ballistic and
orbital manned flights, respectively, and provided valuable
training in countdown procedures and range monitoring and recovery
techniques. The bioinstrumentation was effectively tested and the
adequacy of the environmental control system was demonstrated.
(2) A 7-minute (MR-2) and a 3-hour (MA-5) exposure to the weightless
state were experienced by the subjects in an experimental design
which left visual and tactile references unimpaired. There was no
significant change in the physiological state or performance of
the animals as measured during a series of tasks of graded
motivation and difficulty.
(3) Questions were answered concerning the physical and mental demands
that the astronauts would encounter during space flight, and it
was shown that these demands would not be excessive.
(4) It was also demonstrated that the young chimpanzee can be trained
to be a highly reliable subject for space-flight studies.
The suborbital ballistic flight of Ham on January 31, 1961, was the
prelude to Alan R. Shepard’s suborbital space flight, while the orbital
flight of Enos on November 29, 1961, preceded the orbital flight of John
H. Glenn.
The fact that we now categorize these events as belonging to the rather
distant past, although they occurred only about 4 years ago, serves to
emphasize the pace of development in the exploration of space. While the
chimpanzee program may pale in the light of subsequent successes, its
scientific and technological contribution should not be overlooked.
The significance of this project can be fully appreciated, and its
contribution judged, only by considering the lack of knowledge existing
at the time of its conception. In addition to its essential training
function, this project verified the feasibility of manned space flight
through operational tests of the Mercury life-support system. It
demonstrated that complex behavioral processes and basic physiological
functions remained essentially unperturbed during brief exposures to
space flight. The Mercury chimpanzee program marked the first time that
physiological and behavioral assessment techniques were combined for
evaluating the functional efficiency of the total organism in space.
Perhaps the ultimate contribution of this program was in providing the
framework of knowledge upon which future scientific experiments on
biological organisms, exposed to flights of extended durations, must be
based. Biosatellite experiments designed to seek more subtle and elusive
effects of prolonged space flight on biological functioning will require
even more refined and difficult techniques, but will depend heavily on
the groundwork laid in these early steps of Project Mercury.
A summary of the more important animal suborbital and orbital flights
during the period 1957 to 1964 is presented in table VII.
In another NASA-supported flight, _NERV_ 1, various experiments were
carried in a suborbital flight of 20 minutes. _Neurospora_ molds showed
a surprisingly high level of mutation, but the control molds also had
high rates.
The Discoverer XVII and XVIII flights, to which the Air Force
contributed, resulted in many interesting findings relative to the
responses of living systems to space flight. On the Discoverer XVII
flight, samples of human gamma globulin and rabbit antiserum specific
for human gamma globulin showed an increase in reactivity, and samples
of synovial and conjunctival cells showed no changes in their
cytological characteristics.
Discoverer XVIII was launched during a massive solar flare which las
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