SCIENCE FILE: An exploration fo issues and trends affecting science, medicine and the environment. : A Tough Re-Entry : With an Eye Toward Longer Space Lab Missions and a Possible Trip to Mars, NASA Cannot Take Astronauts’ Problems With Weightlessness Lightly

Upon returning to Earth, astronauts seem the picture of health: They climb out of the space shuttle orbiter, smiling and waving, check the craft for damage and even kick the tires.

But behind the scenes, most space travelers suffer in-flight and post-flight syndromes that NASA scientists are only now beginning to comprehend.

After weeks in space, U.S. astronauts and Russian cosmonauts are unsteady and dizzy, a cosmic version of a sailor’s sea legs unable to adjust to land, with some symptoms lasting longer than a week--almost two months in the case of lengthy missions. Their muscles are weak and visibly shrunken, regardless of the amount of exercise they did in space. And during their flights, most suffer bouts of nausea, headaches, lethargy, impaired concentration and appetite loss.

National Aeronautics and Space Administration scientists say exposure to the micro-gravity conditions of space can throw the human body’s central nervous system and balance out of whack. Although the symptoms do not seem severe or permanent, NASA worries that the afflictions will be magnified during long periods of weightlessness--such as yearlong space lab missions expected as soon as 1999, or journeys to Mars.

“For short-duration flights, it can be a minor impact, but if we’re considering flights to Mars, for example, then we have to worry about whether people can withstand a two-year duration and do anything useful after coming back,” said Jacob Bloomberg, a research physiologist at Johnson Space Center’s neuroscience laboratories in Houston.

Last summer, NASA did its first extensive tests of the health effects of long-term flights, using two Russians and one American who spent nearly four months in orbit on a historic joint mission aboard the Mir space station.

Scientists have begun to understand the underlying causes of many of the problems and are making headway in finding ways to alleviate astronauts’ discomfort. Diagnosing and preventing space travel problems also could help doctors treat people on Earth suffering similar symptoms, such as muscular dystrophy or the imbalance problems of senior citizens.

“The advantage of doing space research is you have a reversible syndrome,” Bloomberg said. “You can test someone preflight, produce motor-control changes and plot their recovery.”

Side effects have hampered astronauts and mystified NASA since the shuttle debuted in 1981. But crew medical records are confidential, and only recently have some civilian crew members openly discussed their motion sickness and post-flight problems.

“In the early days, [all astronauts were] military pilots, who traditionally are sort of macho and don’t want people to know they are sick,” said NASA senior scientist Deborah Harm, a space motion sickness expert. “But so many people had symptoms that it became more accepted.”

According to NASA reports, seven of every 10 astronauts suffer space motion sickness, with malaise, headache, nausea and sometimes vomiting, usually beginning within an hour of orbit and lasting 24 to 72 hours. Only in extreme cases are they unable to work, although the more they move, the sicker they feel.

NASA scientists believe that the space illness, similar to car or sea sickness, stems from the astronauts’ conflicting sensory signals--mainly from their eyes and ears--when they move their heads or bodies in weightless environments.

“Any time you change the sensory information or the system gets information beyond its capability of interpreting, it basically rearranges the relationship among all of them,” Harm said. “That first few days is a time when they are adapting very rapidly and the central nervous system is beginning to learn what you might call a new set of rules.”

Astronauts cannot know in advance whether they will be stricken, since NASA simulators fail to produce the same symptoms, perhaps because the simulated weightlessness lasts only for short spurts.

But NASA has recently developed training equipment that replicates the illness--including a 12-foot sphere that uses virtual reality to create an environment that causes the same head movements and sensory conditions as space.

By making astronauts sick before they fly, Harm hopes to help their nervous systems adjust and lessen the severity of their illnesses in space. Her team plans to expose all shuttle crews to the training beginning this year or next.

Upon the astronauts’ return to gravity, the same sensory confusion triggers equilibrium problems, making it difficult for crew members to stand straight or move normally, especially after long flights.

In the micro-gravity of space, crew members learn that signals from their inner ear are askew, so they rely instead on vision to maintain their posture. Upon landing, they must switch back to trusting their inner ear, and during the transition, they are unstable.

For the crew of the 115-day Mir 18 mission, balance problems persisted for six to seven weeks--compared to eight days after a typical one- or two-week shuttle flight, according to Bloomberg’s data.

In one series of tests, the three crewmen stood on moving platforms. They fell much more frequently after the flight than before. Acute problems lasted the first few days and tapered off gradually, and the severity varied widely from person to person. For example, the two Russians, able to walk but wobbly, were carried off the shuttle on stretchers, while the U.S. crew member, Norman E. Thagard, walked off.

Understanding the variability among the crews is the key to unlocking why the problems happen and how to avoid them. Bloomberg, for instance, finds that returning astronauts who avoid swiveling their heads tend to have a smoother transition to gravity.

In other Mir 18 tests, scientists learned that human muscles are not damaged, as they originally feared.

Using sophisticated checks for biochemical changes in blood enzymes, the researchers detected no muscle tears or other fiber damage, said Dan Feeback, who heads the Johnson Space Center’s muscle research laboratory. Previous tests on rodents in space had shown signs of damage, which worried NASA.

“One of the key differences is when humans go into micro-gravity, they know what to expect,” Feeback said. “When rats get up there, they are totally confused. They have no up or down. You have a stress response, you have struggling. All those things could lead to muscle damage.”

Although muscles are uninjured, the size of the crew members’ muscles does substantially change. On Earth, muscles work even during sitting and standing to resist gravity. But in space, they atrophy from lack of use regardless of the amount of exercise.

Using magnetic resonance imaging, Feeback’s team detected a 20% loss of certain leg muscles in the three Mir 18 astronauts.

That is substantial shrinkage, but less than feared. In eight-day missions, 5% to 6% of muscle mass disappears, so NASA worried that crews could lose more than half their muscle during flights 10 times longer. Instead, the loss seems to plateau--”good news for missions of very extended duration,” Feeback said.

While in space, one Mir 18 crew member worked out with a bungee cord, one wore an anti-gravity suit, and all three did aerobic exercises. “They varied in exercise from very little to quite a bit, but there wasn’t a statistically significant difference in the amount of muscle lost,” Feeback said.

More resistive exercise probably would help, Feeback said. Since hauling Nautilus equipment into space isn’t exactly practical, NASA is developing special pulley devices.

Although post-flight symptoms appear temporary, scientists cannot rule out permanent effects.

“I don’t think anyone unequivocally can say . . . there are no structural changes,” Bloomberg said. “That’s what we are hoping to find out with this research.”