Mice, dogs and monkeys all became astronauts before humans did. On April 12th 1961, Russian cosmonaut Yuri Gagarin became the first man in space, when little was known about the effects spaceflight would have on him.
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These days, astronauts spend a lot longer in space than Gagarin did, and the effects add up over time. How does a trip to space impact the human body?
The first problem for spacefarers is keeping lunch down. Gravity's pull on the inner ear gives people on Earth a definite sense of "up" and "down". Without it astronauts can suffer from disorientation and nausea, or even lose the sense of where their arms and legs are. To prevent that, trainees take practice trips in planes nicknamed "vomit comets". These fly on parabolic trajectories, which means that for short periods, the plane and its occupants are falling at the same rate. That gives the impression of weightlessness.
In space, the body quickly gets used to not having to work against gravity. Astronauts' bone density can drop by as much as 2% a month. Muscle mass can drop by 5% a week. The International Space Station is fitted with lots of exercise equipment to keep astronauts in shape, and they even wear stretchy suits to mimic gravity's compression of their bodies.
Fluids are subject to gravity's pull too, and without it, the distribution of water and blood in the body can change radically. That can give space travellers a puffy appearance, but can also put more pressure on the backs of the eyeballs and compress the optic nerve. NASA scientists are concerned that long periods of weightlessness could put space travellers' vision at risk - a serious concern as talk turns to lengthy flights to other planets.
Spinning part of a spacecraft to provide artificial gravity could solve many of these problems, but spaceflight has other physical and mental impacts. Irregular periods of sunlight and darkness disturb sleep patterns, and spacefarers are constantly bombarded with elevated levels of radiation and subatomic particles called cosmic rays. Longer missions carry a greater risk of stress or even depression. But none of this was enough to put off the 6,000 candidates who applied to enter NASA’s 2013 astronaut class.
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