Humans evolved with gravity, and gravity affects human physiology and general health. Studies show that weightlessness has a number of negative health effects. Finding ways to mitigate these dangers is essential for long-distance space travel.
All life on Earth evolved to live with the Earth's gravity, and humans are no exception. As a result, human muscles, bones and various systems depend on gravity to function properly. Before the United States and the USSR launched astronauts into space, they tried to study the effects of weightlessness on astronauts. While weightlessness can be simulated with parabolic motion in aircraft, the simulations last only minutes.
Short launches early in the development of space technology showed that people suffered from no ill effects on short trips. When astronauts spent longer periods of time in space, however, some health effects became clear. Humans lose muscle mass and cannot put it back on with exercise.
Experts often propose simulating gravity for lengthy stays on space stations and for long space trips. This is generally accomplished by using a spinning motion. While no existing space technology can accomplish this, it is fairly simple in practice, and these plans will likely be used if people need to spend years at a time in space on missions.