On the International Space Station, astronauts constantly "fall" around the earth as they orbit, so they don't feel the effects of gravity. While floating around for six months or a year in space sounds like a lot of fun, there is a serious side effect; once they get back to the earth, they're basically cripples (瘸子).If the worst happens, astronauts walking out of their spacecraft for the first time could slip and break a bone the moment they get back to the earth, because their bones and muscles have weakened over time in space.
NASA has made a lot of progress in this regard. It is clearly required that its astronauts spend about 90 minutes per day exercising in space, using a resistance machine as well as either a treadmill (跑步机)or an exercise bike. Although astronauts are burdened with a lot of things to do in space, that exercise time is sacred ——everyone makes sure the astronauts are not pressured to abandon their exercise time in favor of some experiment.
The doctors say that this time sweating it out is extremely important. "On muscles and bones, you have de — loading effects which can be reduced by extensive exercise," says NASA's Peter Norsk.
NASA is especially concerned with how a lack of gravity and increased space radiation(辐射)could hurt explorers on the way to the moon or Mars. On the earth, we are protected from most forms of radiation due to the atmosphere. While in space, explorers in the low-earth orbit still get a little less radiation than in deep space due to the earth's magnetic field (磁场)，which attracts some radiation from space. But on the way to Mars, that's not the case.
However, doctors still don't understand how radiation can affect astronauts'bones and muscles. It's hard to simulate (模仿)deep-space radiation on the earth, and we only sent a handful of astronauts to the moon back in the 1960s and 1970s.
