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.