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The oddness of life in space never quite goes away. Here are some examples.
First consider something as simple as sleep. Its position presents its own challenges. The main question is whether you want your arms inside or outside the sleeping bag. If you leave your arms out, they float free in zero gravity, often giving a sleeping astronaut the look of a funny balled （芭蕾）dancer. "I'm an inside guy," Mike Hopkins says, who returned from a six-month tour on the International Space Station. "I like to be wrapped up."
On the station, the ordinary becomes strange. The exercise bike for the American astronauts has no handlebars. It also has no seat. With no gravity, it's just as easy to pedal violently. You can watch a movie while you pedal by floating a microcomputer anywhere you want. But station residents have to be careful about staying in one place too long. Without gravity to help circulate air, the carbon dioxide you exhale (呼气) has a tendency to form an invisible （隐形的）cloud around you head. You can end up with what astronauts call a carbon-dioxide headache.
Leroy Chiao, 54, an American retired astronaut after four flights, describes what happens even before you float out of your seat,"Your inner ear thinks your're falling . Meanwhile your eyes are telling you you're standing straight. That can be annoying-that's why some people feel sick." Within a couple days -truly terrible days for some -astronauts' brains learn to ignore the panicky signals from the inner ear, and space sickness disappears.
Space travel can be so delightful but at the same time invisibly dangerous. For instance, astronauts lose bone mass. That's why exercise is considered so vital that National Aeronautics and Space Administration (NASA) puts it right on the workday schedule. The focus on fitness is as much about science and the future as it is about keeping any individual astronauts return home, and, more importantly, how to maintain strength and fitness for the two and a half years or more that it would take to make a round-trip to Mars.

What is the major challenge to astronauts when they sleep in space?

Deciding on a proper sleep position Choosing a comfortable sleeping bag Seeking a way to fall asleep quickly Finding a right time to go to sleep.

The astronauts will suffer from a carbon-dioxide headache when _____.

the y circle around on their bikes they use microcomputers without a stop they exercise in one place for a long time they watch a movie while pedaling

Some astronauts feel sick on the station during the first few days because _____.

their senses stop working they have to stand up straight they float out of their seats unexpectedly whether they are able to go back to the station

One of the NASA's major concerns about astronauts is _____.

how much exercise they do on the station how they can remain healthy for long in space whether they can recover after returning home whether they are able to go back to the station