BY MARK SHWARTZ
If you've ever wanted to experience the sensation space shuttle astronauts feel during liftoff and landing -- or if spinning around in circles all day inside a large NASA centrifuge is your idea of a good time -- then Malcolm Cohen wants to hear from you.
A consulting professor in Stanford's Department of Human Biology and chief of NASA's Human Information Processing Branch, Cohen is looking for participants in a groundbreaking study to determine the extent to which people can tolerate prolonged exposure to increased gravitational force -- or hypergravity.
"The human body has evolved and adapted to G forces that are relatively constant, except for brief periods of acceleration in planes, cars, merry-go-rounds and so forth," Cohen said. "But there has never been a comprehensive study of the long-term effects of hypergravity on humans."
To remedy this lack of data, Cohen and his colleagues at the NASA Ames Research Center in Mountain View, Calif., are seeking men between the ages of 18 and 35 who are willing to experience G forces up to two times greater than those normally found on Earth. The study begins in July and will run about nine weeks for each participant. Evaluations of G-tolerance levels in women are expected to be the subject of future experiments.
Those selected for the study will have to endure long hours sitting, and occasionally standing, inside NASA Ames' 20-G centrifuge -- a 58-foot-long spinning machine featured in the film Space Cowboys. The centrifuge simulates increasing levels of hypergravity as it rotates faster and faster. For example, to experience the effects of 2 G -- twice Earth's gravitational force -- a passenger is spun at nearly 15 revolutions per minute.
Gravity shapes life
A physiological psychologist, Cohen began studying the effects of hypergravity on military jet pilots in the 1960s. He joined the staff of NASA Ames in 1982 -- the same year he began teaching a course titled "Astrobiology and Space Exploration" in the Human Biology Program at Stanford.
"Gravity shapes life," Cohen observed. "It defines the character of the nervous system, our reflexes and our bones. Interesting things happen to astronauts when they are weightless for long durations. They become de-conditioned -- that is, their tolerance for G forces is reduced."
Astronauts returning from space sometimes experience fainting spells when they try to stand up -- the result of a sudden drop in blood pressure caused by the failure of the cardiovascular system to provide an adequate supply of blood to the brain. Some space travelers are unable to stand for long periods of time when they return to Earth, while others undergo leg muscle atrophy -- a classic case of "use it or lose it," Cohen noted.
During a typical shuttle flight, astronauts spend less than a half hour in hypergravity conditions -- up to 3.2 G during takeoff and 1.4 G on re-entry. Most of their journey is spent weightless as they orbit the Earth.
According to Cohen, if astronauts were exposed to hypergravity while in space, they might have an easier time re-adjusting to gravity on Earth -- or on other planets -- without suffering bouts of muscle atrophy, fainting and other common side effects.
"Is it possible that if you're exposed to increased gravity over time, your ability to cope with hypergravity could be enhanced?" he asked. "That's one question our centrifuge experiment will try to address."
In the next few decades, space travelers are likely to spend long periods of time exploring Mars, where the gravitational force is .38 G -- or about 40 percent of Earth's gravity. Instead of remaining weightless during their long trip to the Red Planet, Cohen said that astronauts may have a better chance of adjusting to the Martian environment if they are exposed to artificial gravity en route -- in this case, .38 G or even higher.
Life in the centrifuge
Those willing to join the NASA study must be willing to ride up to 22 hours nonstop inside one of three experimental cabins on board the centrifuge. Because the passenger cab is just 7.6 feet long and 6 feet wide, NASA only will accept applicants who are 5-foot-8 or shorter.
The experiment will consist of seven habitation sessions in the centrifuge -- five of which will last 22 hours. The sessions will be spread over several weeks and could total 188 hours. Test subjects will be paid an hourly rate to be determined later.
The centrifuge cab will be equipped with food, water, a cot, a toilet and a television set. During the daylong sessions, participants will conduct normal routines, such as eating, excreting, sleeping, reading and watching TV. Closed-circuit video cameras will monitor all activities.
Each passenger will be fitted with a specially designed biosensor vest to measure his heart rate, blood pressure and oxygen levels. Urine samples will be collected and analyzed at a later date. NASA warns that in addition to motion sickness, participants may experience a number of "inconveniences, discomforts and risks": blackout, fainting, cardiac arrhythmias, disorientation and dizziness, among others.
Loss of vision and consciousness
The experiment will begin with a pretest to determine the subject's G-tolerance limit. Each man will be fitted with an EKG on his chest to record heartbeat and a Doppler sensor on his forehead to measure blood-flow velocity through his temporal artery. The centrifuge then will be rotated at a gradually accelerating rate to produce an increase of 1 G per 15 seconds.
"The increased force will tend to pull blood from your head and cause it to pool in your gut and your lower limbs," Cohen warns each candidate. "When the force is sufficiently strong, it will overcome the ability of your heart to pump blood to your head, and you will temporarily lose peripheral vision as your visual field begins to dim. The level of G at which this occurs will define your tolerance limit."
To determine that limit, a pair of lights will flash in the centrifuge cab as it rotates faster and faster. The lights will appear in the passenger's peripheral visual field and will be moved closer together as the G force increases. The centrifuge will be stopped when the lights are about 40 degrees apart -- the point at which peripheral vision begins to dim -- or when the Doppler sensor indicates that the passenger has undergone a significant reduction in blood flow to his eyes.
People tend to lose peripheral vision between 3 and 5 G, so the pretest should last one minute or less. However, as G forces increase, subjects face the risk of losing consciousness because of a lack of blood flow to the brain. For that reason, even if there are no symptoms, the pretest automatically will be stopped at 90 seconds, when the gravitational force reaches 7 G.
When the pretest is completed, participants will undergo five 22-hour sessions and two shorter multi-hour sessions once a week for seven consecutive weeks.
As a control, the first 22-hour session will be conducted at 1 G -- Earth's normal gravitational force -- while the centrifuge arm is at rest. During the next two sessions, the centrifuge will rotate nonstop for 22 hours to simulate higher levels of gravitational force at 1.25 and 1.5 G. If these tests are successfully completed, the subject will undergo a 12-hour session at 1.75 G the following week, then 1.75 G for 22 hours a week later, followed by a six-hour session at 2 G, and finally, a 22-hour session at 2 G.
Subjects will spend most of their time sitting or lying on a cot in a cabin with a specially built floor that tilts back on an angle to accommodate increasing G forces. Every four hours, participants will be required to perform three-minute sitting, standing and reclining exercises while their cardiovascular response is evaluated.
"At any time, and for any reason, participants will be able to terminate the session by pressing a button, or simply by asking the medical monitor to stop the session," Cohen noted.
A laptop computer with electronic games, questionnaires and behavioral tests loaded onto its hard drive also will be on board.
"Your use of this computer will be completely at your discretion," Cohen tells candidates.
At the end of each session, participants will take another test that is identical to the pretest, and then undergo a neurological and cardiovascular examination to determine if they are medically fit to return home for the week.
"This is not a benign test," Cohen observed. "I try to discourage candidates when I meet them, because I don't want people to participate unless they really want to do it."
In addition to providing much-needed data on the health effects of long-term space travel, Cohen said that the experiment could provide new insights into treating motion sickness -- and even help athletes improve their performance.
"Perhaps if they train in hypergravity, athletes will feel more comfortable lifting weights or running around the track at normal gravity," Cohen explained. "It's like a batter who swings three or four baseball bats when he's on deck. When he or she goes to the plate, a single bat will seem very lightweight and easier to swing."
Those interested in participating in the experiment can
contact Abigail Bautista at the NASA Ames Research Center: email
or telephone (650) 604-5464.
The 20 G Centrifuge at the NASA Ames Research Center in Mountain View, Calif is a 58-foot-long spinning machine. Men between the ages of 18 and 35 and under 5'8" tall who participate in the planned hypergravity study will have to ride up to 22 hours nonstop inside the centrifuge. Photos courtesy of NASA Ames Research Center
Stanford Report, March 20, 2002