The space environment is pretty psychologically stressful for astronauts. They may have difficulty adjusting to microgravity, suffer fatigue and struggle with sleeplessness from disruption of their circadian rhythm. There's also the intense, high-pressure workload, and the aggravations that inevitably result when a bunch of people are confined in a small space for days or weeks at a time.
NASA tries to select astronauts who'll be able to function well mentally in the stressful environment of space, and the agency carefully monitors astronauts' behavior while they're there for signs of trouble. While nobody's had a major psychological crisis on a flight so far, astronauts have suffered from mood and anxiety disturbances, and the data suggests that the longer a mission lasts, the greater the risk to astronauts' mental health. When astronauts start making multiyear trips to Mars or even more distant destinations, this could be a big worry [source: Slack et al.].
Author's Note: 10 Ways Space Is Trying to Kill You
When I watched the television broadcast of Neil Armstrong setting foot on the moon in 1969, I have to admit that I was so caught up in the excitement of the moment, and had so much faith in the space program's technology, that it never occurred to me how much of a risk that he was taking. It didn't sink in until years later, when I learned that if Armstrong and Buzz Aldrin had been stranded on the moon, then-President Richard Nixon had a contingency plan in place. The telecommunications link would have been shut down, so that the public would be spared the trauma of watching the astronauts die, and instead Nixon would have given a speech in which he praised them for their bravery. Fortunately for all of us, he didn't have to give that speech.
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The exoplanet Barnard's Star b, or GJ 699 b, is 3.3 times the mass of Earth and orbits its star once every 233 days. Read more about this super-Earth.