At special facilities in the United States, France and the Soviet Union, groups of healthy volunteers have spent from one week to one year lying in bed and living at a 6-degree, head-down slant. They eat by propping themselves up on one elbow. They shower in bed using hand-held nozzles, and they use bedpans. To exercise, they move their legs in the air or do isometrics.

The experiments are the closest scientists have come to simulating on the ground the effects of prolonged weightlessness in space. The results of this kind of research may hold the key to when, or even if, human beings will be able to explore the planets.

Although the United States, the Soviets and the European space agency have indicated increasing interest in a manned mission to Mars, scientists caution that they do not yet know how travelers will be able to stay healthy and function productively for the long periods of weightlessness that such a journey, expected to take six to eight months each way, will require.

Soviet officials have indicated they intend to carry out a mission to Mars early in the next century and have been engaged for some time in studying the physiological problems of prolonged space travel. In this country, the independent National Commission on Space has recommended a manned Mars flight by the year 2015 and the government, although it has not committed itself to the mission, has vowed to step up its languishing research on the subject.

Astronaut Sally K. Ride emphasized the need for such a step at a recent international conference here on space life sciences. Probably the most important research to be done in the 1990s, she said, "would be life sciences research."

The engineering challenges along humanity's path into the solar system are formidable, but scientists warn that the physiological stumbling blocks may be more serious. After millions of years of adaptation to gravity, the human body in the weightless state quickly develops a number of problems, according to space life science researchers:

The body starts to get rid of fluids; the heart pumps less blood and so must beat faster to keep up; the muscles, no longer resisting gravity, begin to atrophy; the bones start to lose calcium almost immediately, and subtle changes occur in the body's immune system. In addition, half of all space fliers develop profound motion sickness for up to four days at the beginning of a flight, a condition that may return toward the end or even after the flight is over.

Scientists also express concern about psychological or behavioral problems for a crew of six to eight people crammed into quarters the size of a couple of house trailers for perhaps two years. The Soviets have discovered that their crews develop cabin fever and their work performance falls off after five to six months, the experts say.

More questions surround the problems caused by the increased level of radiation to which space fliers are exposed. Unless ways are found to protect the astronauts, "one trip to Mars could be an astronaut's whole space career" in terms of body tolerance to such radiation, said Charles Redmond, a spokesman for the National Aeronautics and Space Administration's office of space science.

But changes in the body during weightlessness are, for the life scientists, the largest concern.

"We know the major physiological changes that take place over two to four weeks, but we do not understand the mechanisms underlying all these changes," said John Billingham, chief of the life science division at NASA's Ames Research Center in California. "And the key thing is that we do not know the extent to which these changes will continue to progress over longer and longer periods of weightlessness."

Reproducing the effects of weightlessness on the ground is difficult, he said. "The only effective way we have of doing so for long periods is to put people in the horizontal position."

Researchers around the world have adopted the 6-degree head-down slant as the best way to simulate many of the effects of weightlessness on the body, he said. They have found this degree of slant most closely duplicates the volumes of pooled blood and other effects that occur in space. Volunteers maintain the position using methods developed for chronically ill hospital patients, with the aid of a nursing staff.

Ames has conducted studies of up to a month, he said, and a bed-rest program at NASA's Johnson Space Center in Houston has done studies lasting three or four months. U.S. studies in the past have reflected the fact that shuttle flights are limited to seven to 10 days.

The Soviets have conducted far more research into long-duration manned space flight, and recently completed a one-year bed-rest study -- the record to date -- using about a dozen healthy volunteers in a Moscow facility, according to American and Soviet officials.

With the advent of the proposed U.S. space station, where crews will stay aloft longer, and also with increasing interest in manned planetary exploration, Billingham said, "our approach is now changing" in favor of longer-term studies similar to the Soviets'.

The more costly alternative research method is, of course, to put people into space. Since 1961, Soviet cosmonauts have progressed from two hours in space to an eight-month sojourn. The two-man crew aboard the Soviet space station Mir is now in the midst of a 10-month tour of duty, a period expected to become routine for Mir crews. Two Soviet cosmonauts are also the only humans who have repeated long-duration space flights. One, Yuri Romanenko, is now aboard Mir for his third long space flight.

"The organism 'remembers' weightlessness and during repeat missions adaptation proceeds less painfully," said Tamara Breus of the Institute of Space Research of the Soviet Academy of Sciences in Moscow. Romanenko has reported having a much easier time adapting to weightlessness than his less-experienced crewmate, Breus said.

The longest any American astronaut has spent in space is just under three months (84 days), on the last Skylab mission. No women have flown for more than 10 days.

"The data the Soviets are accumulating is very, very valuable data. Nobody else is doing it," said Arnauld E. Nicogossian, director of life sciences for NASA at its Washington headquarters. "Our Skylab data was very valuable. We are still living off it."

There is a limited flow of information from the Soviet program, American researchers say. "We talk with them, scientist to scientist. We understand what problems they're studying. We get certain publications," Nicogossian said. "But to have a working relationship where you can comb through their data, ask the questions -- it doesn't happen."

The purpose of the research is to develop countermeasures -- such as special diets, excercise, fluid-loading, gravity suits and the like -- that make it possible for space fliers to stay healthy and keep functioning while they are in space and when they return to gravity.

To counteract the muscle deterioration, for example, the Soviets require their Mir cosmonauts to exercise on a treadmill for at least two hours a day, an undertaking that reportedly generates a shroud of sweat.

The loss of bone minerals presents a serious short-term problem in addition to its effects on bones. "That calcium ends up in your blood and increases the potential for kidney stones, and this could happen even within hours," said John Charles of the Johnson Space Center's Space Biomedical Research Institute. No astronaut has yet suffered a kidney stone problem during a mission. But it is painful and, he said, "and there is little you can do about it except surgery."

There are unconfirmed rumors such a case occurred on a long-duration Soviet flight, he added.

The loss of fluids is caused when, in the absence of gravity, the body's blood and other fluids drift into the upper regions of the body where they impinge on key sensors. Informed by these sensors, the brain assumes the unusually high amount of fluids accumulating in and around the chest exists throughout the body, so it starts getting rid of them through sweating, urination and other means, researchers say.

The Soviets and the Americans have tried applying "negative pressure" that sucks blood backs to the legs, Nicogossian said, and the Soviets have tried replenishing fluids. They have also tried compression suits that attempt to duplicate gravitational forces, Nicogossian said.

The most notorious space affliction is space motion sickness (SMS), experienced first-hand by Sen. Jake Garn (R-Utah) on a shuttle trip in 1985. Symptoms, which afflict at least 50 percent of a crew, almost always include lack of initiative and irritability but may range from headaches and lethargy to vomiting, experts say. The problem has occurred more frequently as space fliers have had bigger spacecraft in which to move around.

The symptoms can begin within minutes, or take an hour or two to develop, once the astronaut is weightless. They intensify for a few hours and then stabilize. They may last as briefly as 12 hours or as long as 72.

During space flight, the body's systems adjust somewhat to the absence of gravity, researchers theorize, and must undergo another adjustment upon return to it. The returnees may be susceptible to fainting, weakness and other changes. American astronauts have not experienced any serious health problems after their relatively short-term flights, but researchers express concern that irreversible changes may occur in the body during prolonged or repeated exposure to space flight.

Soviet cosmonauts, after long-duration stays in orbit, are hoisted out of capsules, slide down a chute and are carried around in sedan-chairs for days and sometimes weeks before they begin to function normally, according to American specialists.

The long-term effectiveness of the various countermeasures is not clear, in part because the Soviets use several at once rather than separating the effects of any one measure; for ethical reasons they have not provided a "control group," where no countermeasures are taken, for fear of endangering cosmonauts, experts say.

One possible alternative is to create artificial gravity for long space flights, probably by spinning the vehicle. But research in this area is in its infancy.

"It's an awful long way to Mars," Ride said. For now, the answers needed to get people there seem equally remote.