The most serious concern is the loss of bone calcium that increases with the length of a mission.

The calcium loss from bones subjected to extended microgravity takes place at 10 times the rate of an elderly person suffering from osteoporosis.

Astronauts report an overall feeling of weakness and loss of balance upon return to Earth, though recovery is nearly complete after a week.

Researching the effects of microgravity has been an on-going programme at Nasa for the past 25 years.

There are a number of other ways to create low-gravity environments on Earth for research, but these last for short periods of time.

For instance releasing experiment samples from tall drop towers provides about four seconds of microgravity.

Research aircraft can expose experiments to about 30 seconds of low-gravity while the aircraft approaches the top of a steep climb and begins a sharp descent.

你喜欢整天躺在沙发上看电视吗？愿意坐在一个地方玩上数小时的游戏吗？如果答案是肯定的，那么恭喜你，你可以申请成为美国航空航天局的志愿者了。

据英国《每日邮报》9月16日报道，美国国家航空航天局（NASA）正在招聘卧床70天的志愿者，用以研究微重力对人体的研究。志愿者将获得每个月5000美元左右的报酬。

这项研究在休斯敦的约翰逊航天中心进行，志愿者将在那里待上两周，做一些日常活动，让科学家检测其在正常情况下的身体状态。

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