The Complete Book of Spaceflight: From Apollo 1 to Zero Gravity

A branch of aerospace medicine concerned specifically
with the physiological and psychological effects
of spaceflight. Some of the potential hazards of space
travel, such as acceleration and deceleration forces,
the dependence on an artificial pressurized breathable
atmosphere, and noise and vibration, are similar to
those encountered in atmospheric flight and can be
addressed in similar ways. However, space medicine
must embrace a number of other issues that are
unique to living and exploring beyond Earth’s atmosphere.
The first information concerning the potential
effects of space travel on humans was compiled in
Germany in the 1940s under the direction of Hubertus
Strughold. However, these seminal data are
tainted by their purported link with Nazi atrocities.
Both the United States and the Soviet Union conducted
rocket tests with animals (see animals in
space) beginning in 1948. In 1957, the Soviet Union
put a dog, Laika, into Earth orbit, and, shortly after,
the United States began sending primates on suborbital
flights. These early experiments suggested that
few biological threats were posed by short stays in
space. This was confirmed when human spaceflight
began on April 12, 1961, with the orbital flight of Yuri
Gagarin.
Space motion sickness became a fairly regular,
though not serious, side effect of lengthier missions.
Of greater concern were the consequences of weightlessness
that first became apparent in the 1970s and
1980s, when Soviet cosmonauts began spending
months at a time in gravity-free environments aboard
Salyut and then Mir. These included loss of bone
matter (see bone demineralization in space) and loss
of muscle strength. The atrophy of certain muscles,
particularly those of the heart, was seen to be especially
dangerous because of its effect on the functioning
of the entire cardiovascular system. During
extended spaceflight, the heart becomes smaller and
pumps less blood with each beat. One way to try to
counter this is by regular exercise on treadmills or
bicycles. But some cardiovascular change appears
inevitable. The blood itself is also affected, with a
measurable decrease in the number of oxygencarrying
cells. To what extent these physiological
changes are reversible is still not clear. The bones and
space medicine
muscles of most space travelers have been observed to
fully recover within weeks of their return. However, in
1997, serious effects on heart function were reported
in some Russian cosmonauts who had served for
unusually long periods in orbit.
The absence of gravity is particularly damaging to
biological development. An early indication of this
came from a seven-day Shuttle mission in 1985,
involving 24 rats and 2 monkeys. Post-flight examination
revealed not only the expected loss of bone and
muscle strength but a decrease in the release of growth
hormone as well. More recent findings point to a pervasive
effect of gravity—or the lack of it—on cell
metabolism, brain development, and DNA synthesis.
A study of 18 pregnant mice launched into space carrying
some 200 mouse fetuses at varying stages of
development suggests that nerve cells, and possibly
every cell in our bodies, may need gravity cues to
grow and function properly. Profound changes were
noted when the space fetuses were compared with
carefully matched counterparts on Earth. Cell death, a
normal aspect of development, slowed down in space,
as did cell proliferation. Tiny structures that have to
move about within the cells and that normally travel
at high speed slowed to a crawl. Without gravity to
guide the migration of nerve cells that form the outer
layer of the cortex, the space-grown brains wound up
smaller, and although they appeared to be normal
otherwise, they turned out to have fewer nerve cells
than normal mouse brains. Just what functional
importance this would have in an adult animal awaits
further study. But it appears as if the brain struggled to
adapt as it developed, only to mask what could be
deleterious changes. Women are already forbidden to
head into orbit if they are pregnant. These new findings
suggest that children born, or even conceived, in
space might suffer permanent nervous-system damage
unless exposed to Earth-like gravity at key points during
their early development. At the very least, children
who grew up under zero-gravity or low gravity
(for example, on Mars) might have trouble walking on
Earth, because their nervous systems would be permanently
wired for a nonterrestrial environment.
Another concern on longer-duration spaceflights is
radiation exposure. Short orbital flights result in exposures
about equal to one medical X-ray. The crews on
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