But researchers are also coming up against another more surprising physical risk for future long-haul space travelers -- their immune systems appear to become less capable in space, leaving them more susceptible to stowaway bacteria and viruses.
At the same time, researchers studying the microbes that could infect astronauts recently found that at least one, salmonella, becomes significantly more virulent in weightlessness.
"The question of immunity is a potentially big problem for astronauts on long trips and those who may be living on the moon in the future," said Millie Hughes-Fulford, a former astronaut who is researching the effects of "microgravity" on immunity. Her NASA-supported research has led her to conclude that weightlessness itself is a major cause of the problem.
"Human beings evolved in gravity, and it makes perfect sense that some systems -- especially the immune and skeletal systems -- might not do well without it," she said.
Duane Pierson, senior microbiologist at NASA's Johnson Space Center, said he also fears immunity problems in the future, although they have been contained so far.
"Even though astronauts are not now getting sick on their missions, we see very clearly statistically significant and reproducible change in immune functioning after two weeks in space," he said. "We don't need a crash program or anything like that, but many of us feel this issue is very much on the table because of long-duration flight."