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NASA Science News for February 7, 2003 9:00:00 AM

The Pull of HyperGravity
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A NASA researcher is studying the strange effects of artificial
gravity on humans. 

February 7, 2003:  Want to know what 3-g feels like?

Go to a carnival.

There's a circular ride there that spins dizzyingly fast. Standing
inside it, your back is pressed against the wall. It spins faster and
faster until, suddenly, the floor falls away. But you don't fall with
it. You remain in place, pinned to the wall by centrifugal forces.
The force that holds you up "can be as much as 3-g -- or three times
the normal force of gravity," says Malcolm Cohen, chief of the Human
Information Processing Research Branch at NASA Ames. 

During the past few summers, Cohen has been spinning research subjects 
in something far more impressive than a carnival ride. He's been 
studying engineers, mountain climbers, teachers and other paid
volunteers as they live for up to 22 hours in a giant, 58-foot
diameter centrifuge. His goal? To learn how humans adjust to changes
in gravity--particularly strong gravity. 

NASA is interested because it's not just microgravity that astronauts
experience in space. They're exposed to hypergravity, too: up to 3.2-g 
at launch, and about 1.4-g on reentry. "Under these conditions," Cohen 
points out, "fluid weighs more." The heart has to change the way it 
operates, pumping faster, and working harder to push the blood all the 
way to the brain. This could cause astronauts to become dizzy or even, 
in extreme cases, to pass out.

By spinning people in his centrifuge, Cohen hopes to learn whether the 
heart's response can be conditioned. Perhaps if astronauts were
exposed to controlled doses of hypergravity before launch or reentry,
then they might be able to tolerate high g forces better than they
otherwise would have. 

An easier ride to space is not the only potential benefit. Here on
Earth, hypergravity could be used to train athletes, providing an
environment in which exercises could be conducted with more benefit in 
shorter time. People who suffer from muscle atrophy might be exposed 
to it, to stress their muscles more effectively. 

Centrifugal force could be key to long-term space travel, too. That's
because microgravity causes the body to deteriorate in a multitude of
ways: cardiovascular deconditioning, loss of muscle mass, loss of bone 
density, and a host of other problems. Artificial gravity could
prevent all that--and centrifuges are one plausible way to generate
artificial gravity. 

The participants in Cohen's study have to be less than 5'8" tall--
that's because the outer dimensions of the centrifuge cabin are only
7'7" deep by 5'11" wide. "With its padded walls, the subjects barely
have enough room to lie down on the cabin's built-in cot," he
explains. The cramped cabin is outfitted with a toilet, a TV, and a
laptop loaded with computer games, tests and questionnaires. While
they're spinning, participants answer questions about stress, fatigue
and motion sickness; they perform complex reasoning tasks; and their
vital signs, head movements, and general activity are monitored by
sensors and cameras. 

"Artificial gravity is a potentially useful tool," notes Cohen, "but
it's not a universal panacea." Centrifugal force is not exactly the
same as gravity, he explains. If you have a small centrifuge--say, one 
that might fit in a spaceship--you have to spin it pretty fast to
create g levels high enough to be effective. But there's a problem:
across the radius of a small centrifuge, g levels change rapidly. 
"Suppose you're lying on a short-radius centrifuge, with your head
near the center, and your feet at the outside, and suppose you have
1-g at your feet. Your head would feel only about 0.2-g, or even
less." That's not quite what you would experience in Earth's
gravitational field!

Rapid spinning creates another concern: if you move your head too
quickly while you're inside a fast-moving centrifuge, you might feel
uncomfortably like you're tumbling head over heels. This can happen
when balance-sensing fluids in the semicircular canals of your inner
ear become "confused." Some experiments using centrifuges often
include devices that fix the subjects' heads in place, just to prevent 
that illusion. Traveling through space, however, with your head fixed 
in place is not practical. 

Cohen ticks off ways to make centrifugal gravity feasible:

Perhaps engineers could develop a centrifuge with a radius of several
kilometers, large enough to generate high artificial gravity without
rotating fast enough to trigger the tumbling illusion. Rather than
using small onboard centrifuges, space travelers might slowly rotate
their entire spaceships instead. 

Alternately, perhaps subjects could be taught to adapt to a rotating
environment. The brain is unaccountably good at interpreting strange
sensations after they're been around for a while. Witness the way
astronauts can be disoriented when they first arrive in space, but
soon learn to function in a weightless environment. If humans are spun 
for long enough, says Cohen, the strange effects of rotation might 
become familiar. 

For now, though, Cohen is still trying to determine how different
kinds of activities done in hypergravity affect cardiovascular
conditioning. Cohen found that his centrifuge riders spent a lot of
time lying down, in part because it was more comfortable, and in part
because spinning made them drowsy--an effect called "the sopite
syndrome." Cohen noted that he was surprised at how strong it was.
Going forward, he'd like to examine what happens when they perform a
range of predetermined activities, such as standing, in which the
g-force places more stress on the heart.

Much more research remains to be done. "There are so many options for
how best to implement hypergravity most effectively," says Cohen. "Low 
intensity for long durations, high intensity for short durations,
short radius centrifuges, rotating an entire spaceship." We know a
lot, he says, but there's much more to learn. It is, after all, a
weighty subject. 

Credits & Contacts
Author: Karen Miller 
Responsible NASA official: Ron Koczor 
Production Editor: Dr. Tony Phillips 
Curator: Bryan Walls 
Media Relations: Steve Roy

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