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Weightlessness
or microgravity is a new experience for the human body.
Therefore in space, all the human systems that have
evolved to cope with the constant tug of Earth's gravity
no longer function in the same way. But as long as a
spacecraft contains a carefully controlled atmosphere
to enable normal breathing, adequate temperature, and
sufficient shielding to guard against the dangerous
radiation levels in space, humans can survive under
microgravity conditions.
STS-99 Crew
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In weightlessness, there is
no up or down as we know it. You don't even
know the orientation of your body at first because
it has no weight for you to feel and sense where
it is. In space, your body becomes confused
by the sudden change in what it has learned
to expect.
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When entering
weightlessness, nearly all astronauts are troubled to
some extent by a condition called space
motion sickness, which is similar to car sickness
or seasickness. Because the human brain on Earth has
learned how to process signals about the position of
different parts of the body in relation to the world
around it, this sudden input of confusing signals causes
many astronauts to feel sick. Within the inner ear,
there is a balance organ called the vestibular
organ. The brain identifies or determines the
body's position, relative to the pull of gravity, by
collecting and analyzing information received from the
muscles; joints; the senses of touch and sight; and
the vestibular organ. In space, where there is
virtually no gravity, the signals from the vestibular
organ combined with what is seen and felt by the other
body sensors are all giving conflicting information
to your brain. Space sickness usually goes away after
a few hours or days of acclimating to the weightless
environment.
The most serious consequence
of weightlessness, however, is the deconditioning
(weakening) of physiological systems such as
the cardiovascular system. On Earth, the heart
must operate against gravity to sustain blood
flow and proper functioning of the cardiovascular
system. Under zero-gravity conditions, the heart
lessens its pace to achieve an equilibrium appropriate
to decreased demands. Reduced output of the
heart, decreased heart rate, decreased heart
size, and diminished blood volume regulation
result.
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Astronaut Bonnie
Dunbar exercising on the bicycle
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Diminished gravity is also a significant problem in the way it affects
the musculoskeletal system. Reduced weight-bearing in space leads
to bone "disuse" symptoms including loss of calcium, nitrogen, and
phosphorus and decreased bone size and volume. As discovered on the
Mir space station, 1.2 % of bone mass in the lower hip and spine is
lost per month in microgravity! Decreased muscle tone and strength,
weakened reflexes, and decreased tolerance for physical work are further
negative consequences of a zero-g
environment. The last thing we want for future Mars explorers
is for them to arrive on the planet unable to walk or work because
of a weakened heart or muscular system.
Bed-rest
tests on Earth, which simulate some of the effects
of zero g on the body, are used in conjunction with
studies on the astronauts living in space to help determine
how we can help keep crews fit during their flight to
Mars. Exercise and nutritional supplements are currently
used to help counteract some of the negative effects
of zero g.
Artificial
gravity?
One possibility that has been suggested as a countermeasure to the
effects of weightlessness is the use of artificially
produced gravity aboard spacecraft. Artificial gravity could be
accomplished either by rotating the entire vehicle or by including
an onboard centrifuge.
Rotating a vehicle to the degree that
it can be developed to approximate a normal gravitational
environment would produce the more comfortable living
arrangement for long-duration space crews. Unfortunately,
a large vehicle is necessary to produce rotation simulating
Earth gravity. There are also rotational sickness problems
with the rotation of the ship. If you go closer to or
away from the center of rotation people will feel it
and probably not like it.
How
can crews adapt from zero g to Mars g?
On its surface, Mars has one-third
the gravitational force of Earth. Long-term
exposure to Mars
gravity can be as harmful as long-term exposure
to microgravity when astronauts are planning
to return to a 1-g environment on Earth.
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The Martian gravity is low enough that bone integrity will probably
still be affected, and it is too high to be ignored. There will be
symptoms that occur due to the transition between zero g and
Mars g. How severe the transition will be is unknown. Will it last
a few days or longer? Mars explorers who are on a short surface stay
mission will need to accomplish their surface activities quickly since
any adaptation problems could impact the fulfillment of their mission.
The design of any mission will have to include a strategy to deal
with this possibility. To read the Mars Society of CalTech Human
Exploration of Mars Endeavor mission design plan click
here.
It is thought
that Martian gravity may perhaps be sufficient to counter
some of the deconditioning that endangers astronaut
health during the in-flight zero-g environment. If this
is the case, expeditions that include a longer surface
stay may be better because they would deflect some of
the physical conditions caused during in-flight zero-g
exposure. Crews might be able to rebuild their tolerance
to gravity in this gentler gravity environment, thereby
restoring their bodies to near-preflight conditions
before embarking on the mission back to Earth. However,
the effect of prolonged exposure to Mars g over time
is still an unknown factor. Astronauts who live on the
surface of Mars for a long period of time may have unanticipated
reactions to prolonged exposure to 1/3 g. Click
here to look at the effects of gravity
on the Earth versus Mars.
Questions
to think about:
- How do you feel after staying in bed for
several days with a cold? How do your muscles feel?
- How do you think the 1/3 g-force on Mars
will feel to astronauts after a six-month flight in
zero g?
- How do you think you would adapt to living
in Mars g for a long period of time? What changes
in your body might occur?
- How hard would it be to
readapt to Earth gravity after an extended period
of time on Mars?
Next...
Keeping Fit (pg.
7 of 17) |
