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Chapter 3Acceleration and Free Fall
head’s blood vessels are not able to constrict as effectively. Weightless
astronauts’ blood tends to be expelled by the constricted blood vessels of the
lower body, and pools around their hearts, in their thoraxes, and in their
heads. The only immediate result is an uncomfortable feeling of bloatedness
in the upper body, but in the long term, a harmful chain of events is set in
motion. The body’s attempts to maintain the correct blood volume are most
sensitive to the level of fluid in the head. Since astronauts have extra fluid in
their heads, the body thinks that the over-all blood volume has become too
great. It responds by decreasing blood volume below normal levels. This
increases the concentration of red blood cells, so the body then decides that
the blood has become too thick, and reduces the number of blood cells. In
missions lasting up to a year or so, this is not as harmful as the musculo-
skeletal effects, but it is not known whether longer period in space would
bring the red blood cell count down to harmful levels.
Reproduction in space
For those enthralled by the romance of actual human colonization of
space, human reproduction in weightlessness becomes an issue. An already-
pregnant Russian cosmonaut did spend some time in orbit in the 1960’s,
and later gave birth to a normal child on the ground. Recently, one of
NASA’s public relations concerns about the space shuttle program has been
to discourage speculation about space sex, for fear of a potential taxpayers’
backlash against the space program as an expensive form of exotic pleasure.
Scientific work has been concentrated on studying plant and animal
reproduction in space. Green plants, fungi, insects, fish, and amphibians
have all gone through at least one generation in zero-gravity experiments
without any serious problems. In many cases, animal embryos conceived in
orbit begin by developing abnormally, but later in development they seem
to correct themselves. However, chicken embryos fertilized on earth less
than 24 hours before going into orbit have failed to survive. Since chickens
are the organisms most similar to humans among the species investigated so
far, it is not at all certain that humans could reproduce successfully in a
zero-gravity space colony.
Simulated gravity
If humans are ever to live and work in space for more than a year or so,
the only solution is probably to build spinning space stations to provide the
illusion of weight, as discussed in section 9.2. Normal gravity could be
simulated, but tourists would probably enjoy g=2 m/s
2
or 5 m/s
2
. Space
enthusiasts have proposed entire orbiting cities built on the rotating cylin-
der plan. Although science fiction has focused on human colonization of
relatively earthlike bodies such as our moon, Mars, and Jupiter’s icy moon
Europa, there would probably be no practical way to build large spinning
structures on their surfaces. If the biological effects of their 2-3 m/s
2
gravitational accelerations are as harmful as the effect of g=0, then we may
be left with the surprising result that interplanetary space is more hospitable
to our species than the moons and planets.