Space medicine is a branch of aerospace medicine concerned specifically with the physiological and psychological effects of spaceflight. Some of the potential hazards of space travel, such as acceleration and deceleration forces, the dependence on an artificial pressurized breathable atmosphere, and noise and vibration, are similar to those encountered in atmospheric flight and can be addressed in similar ways. However, space medicine must embrace a number of other issues that are unique to living and exploring beyond Earth's atmosphere.
The first information concerning the potential effects of space travel on humans was compiled in Germany in the 1940s under the direction of Hubertus Strughold. However, these seminal data are tainted by their purported link with Nazi atrocities. Both the United States and the Soviet Union conducted rocket tests with animals (see animals in space) beginning in 1948. In 1957 the Soviet Union put a dog, Laika, into Earth orbit, and, shortly after, the United States began sending primates on suborbital flights. These early experiments suggested that few biological threats were posed by short stays in space. This was confirmed when human spaceflight began on April 12, 1961, with the orbital flight of Yuri Gagarin.
Space motion sickness became a fairly regular, though not serious, side-effect of lengthier missions. Of greater concern were the consequences of weightlessness that first became apparent in the 1970s and '80s, when Soviet cosmonauts began spending months at time in gravity-free environments aboard Salyut and then Mir. These included loss of bone matter (see bone demineralization in space) and muscle strength. The atrophy of certain muscles, particularly those of the heart, was seen to be especially dangerous because of its effect on the functioning of the entire cardiovascular system. During extended spaceflight, the heart becomes smaller and pumps less blood with each beat. One way to try to counter this is by regular exercise on treadmills or bicycles. But some cardiovascular change appears inevitable. The blood itself is also affected, with a measurable decrease in the number of oxygen-carrying cells. To what extent these physiological changes are reversible is still not clear. The bones and muscles of most space travelers have been observed return to normal within weeks of their return. However, in 1997, serious effects on heart function were reported in some Russian cosmonauts who had served for unusually longer periods in orbit.
The absence of gravitational loading is particularly damaging to biological development. An early indication of this came from a seven-day Space Shuttle mission in 1985, involving 24 rats and two monkeys. Post-flight examination revealed not only the expected loss of bone and muscle strength but a decrease in the release of growth hormone as well. More recent findings point to a pervasive effect of gravity – or the lack of it – on cell metabolism, brain development, and DNA synthesis. A study of 18 pregnant mice launched into space carrying some 200 mouse fetuses at varying stages of development suggest that nerve cells, and possibly every cell in our bodies, may need gravity cues to grow and function properly. Profound changes were noted when the space fetuses were compared with carefully matched counterparts on Earth. Cell death, a normal aspect of development, slowed down in space, as did cell proliferation. Tiny structures that have to move about within the cells and that normally travel at high speed, slowed to a crawl. Without gravity to guide migration of nerve cells that form the outer layer of the cortex, the space-grown brains wound up smaller, and although they appeared to be normal otherwise, they turned out to have fewer nerve cells than normal mouse brains. Just what functional importance this would have in an adult animal awaits further study. But it appears as if the brain struggled to adapt as it developed, only to mask what could be deleterious changes. Women are already forbidden to head into orbit if they are pregnant. These new findings suggest that suggesting that children born, or even conceived in space, might suffer permanent nervous-system damage unless exposed to Earthlike gravity at key points during their early development. At the very least a child who grew up under zero- or low-gravity (for example, on Mars) might have trouble walking on Earth because their nervous systems would be permanently wired for a nonterrestrial environment.
Another concern on longer-duration spaceflights is radiation exposure. Short orbital flights result in exposures about equal to one medical X-ray. The crews on the longer Skylab flights sustained many times this dose. During deep-space exploration missions that may last 18 months or more, astronauts would receive radiation doses in excess of career maximums set by current medical standards unless preventative measures are taken. See radiation protection in space.
Long space missions will not only have physical consequences. There will be psychological effects arising from the close confinement of a few individuals with limited activity. No great problems have been encountered to date, perhaps because most astronauts are chosen for emotional stability and high motivation and because they are assigned enough tasks to keep them almost constantly busy. Even so, there have been some signs of strain – as revealed for example in the diary of cosmonaut Valentin Lebedev.