As people move outward into space, the adaptation of the human body to this new environment is one of the basic - and poorly understood - challenges they face. To quote Philip C. Johnson Jr., chief of the Medical Research Branch at the Johnson Space Center in Houston: ''... astronauts remain pioneers, providing medical investigators with the information humans will need someday to live safely in space or on another planet with a different gravity.''
That is why the new record for spaceflight endurance set recently by Soviet cosmonauts deserves more than the passing mention it has generally received in the Western press.
Both United States and Soviet spaceflight services are gaining experience in coping with the disorientation, dizziness, lassitude, and other aspects of the so-called ''space adaptation sickness,'' which affects many astronauts when first going onto orbit. This seems to be a short-term problem, however, which fades after a few days. Currently, only the Soviets are gaining knowledge of the long-term bodily adaptations that are crucial to the future development of the space frontier.
When Cosmonauts Leonid Kizim, Vladimir Solovyev, and Oleg Atkov landed at 1: 57 a.m. Moscow time Oct. 2, they had logged a record-setting 237 days on orbit. They returned in good physical shape, having gotten on ''well over the lengthy stay in zero gravity,'' according to the Soviet news agency Tass. Film clips did show them to be somewhat unsteady on their feet, a normal aspect of readapting to Earth's gravity. Western experts may not learn many details of the cosmonauts' physiological history for an indefinite time. But the mere fact that Soviet space medicine specialists are again permitting such extensive missions suggests that they believe they have learned enough to be reasonably satisfied about the cosmonauts' safety. They had suspended marathon missions in 1981 because of uncertainties about the long-term bone loss of crew members.
The puzzle physiologists are trying to solve is not one of pathology so much as of normality. The human body appears to be acting normally in adapting to a new environment. If the astronauts were to live weightless in space forever after, the bodily changes might well turn out to be advantages. They become disadvantages when astronauts return to Earth or visit the moon and other gravitating bodies.
Astronauts stand 3 percent taller in space, because spinal disks and the lower spine are no longer compressed by gravity. With less need to support the kind of forces associated with weight, parts of the bone mass dissolve. Some muscles, especially leg muscles, atrophy to a degree. Floating around weightless , the body doesn't need that extra mass and gets rid of it. Astronauts regain much of the lost mass back on Earth. Not all of the bone material is replaced, however. Johnson says that ''for each 90 days spent in space, ... an astronaut can expect a permanent loss of 0.5 percent of his total body skeleton, and about 5 percent in certain critical areas.''
To summarize the situation this way is not to imply that the challenge of adapting to life in space is well grasped at all. Whatever dangers and benefits there may be have yet to be fully defined, as Johnson explains in reviewing the subject in the current issue of American Scientist. ''The long-term effects of multiple short space flights and the short- and long-term consequences of time spent away from the Earth are still little understood,'' he says.
Meanwhile, both Soviet and US experts experiment with countering some bodily responses to weightlessness by having crew members do regular exercise, using such special equipment as treadmills with hold-down straps to simulate the forces of weight. This does seem to help maintain muscle tone and bone mass, although the full effectiveness of the exercise has yet to be demonstrated. But the time spent amounts to many hours during a long flight. It seriously cuts into work efficiency on the mission. As James E. Oberg, a leading analyst of the Soviet space program, puts it, on some of their longest flights, cosmonauts have exercised to a degree that ''practically added up to climbing on foot into space.''
As both the US and the Soviet Union prepare to maintain permanently manned space stations and as they plan for possible missions to the moon, and perhaps Mars, early in the next century, the adaptation of human beings to the space environment has become a critical subject. Both programs are proceeding on the assumption that the bodily resilience and technical ingenuity that have enabled humanity to inhabit most of the Earth's land environments, and even to survive beneath the sea, will also enable people to develop the space frontier. But right now, only the Soviets are gaining the experience with long-duration spaceflight needed to test that assumption practically.
A Thursday column.