When astronauts return from long missions in space, their exit from the spacecraft is usually hidden from public view. This is because after several months in weightless conditions, things we usually take for granted, such as walking and keeping our balance, present great difficulties for the astronauts.

　　Forces Around Us

　　The culprit is gravity, or the virtual lack of it in space. Gravity is the force with which particles of matter attract each other. The greater its mass, the stronger an object’s gravitational force.

　　That’s why we normally stay rooted to the surface of the earth instead of floating off; the earth’s mass is much greater than ours.

　　Medical Problems

　　This gravitational state of affairs might seem rather limiting: wouldn’t it be fun to be able to float around like astronauts?

　　Perhaps, but anyone who spends an extended period of time in space faces serious medical problems due to the near total absence of gravity.

　　For instance, the small bones and fluid of the inner ear that allow us to keep our balance and know which way is up depend on gravity.

　　Weightlessness

　　In weightlessness, astronauts often initially experience intense dizziness, and are sometimes unable to orient themselves to their hands and feet. What’s worse, our bones, ligaments and muscles all maintain strength by working against the weight caused by gravity.

　　Without weight, bones weaken, ligaments deteriorate, and muscles atrophy. That’s why astronauts who have been in space for many months have trouble walking when they return to earth.

　　地心引力为何对我们有好处？

　　当宇航员在太空长时间执行任务后返回地球的时候，他们走出太空飞船的场景往往不曝光于公众。其间的原因便是：在失重的状态下生活多个月以后，一些我们认为理所当然能够做到的事情，如走路或保持平衡等，对宇航员来说却相当的困难。

　　环绕四周的力量

　　这个罪魁祸首便是地心引力，或者说太空缺地心引力。地心引力是指有质量的物体间相互吸引的力量。一个物体质量越大，其地心引力越大。

　　由于地球的质量远大于我们，所以我们总是能够站在地球表面而非漂浮在空中。

　　医疗问题

　　万事都处于引力的状态似乎限制多多：跟宇航员一样能漂浮起来难道不是一件很有意思的事情吗？

　　事实并非如此，由于缺乏引力，长期在太空生活的人将面临严重的医疗问题。

　　例如，处于耳朵内部的小骨与液态流质对保持身体平衡意义重大，它们的健康状态取决于引力。

　　失重

　　在失重状态下，宇航员常常最先经历强烈的眩晕，经常无法控制他们的手脚。更糟糕的是，我们的骨头，韧带以及肌肉需要与引力产生的重量抗衡方能保持强健。

　　在失重的情况下，骨头变得脆弱不堪，韧带情况恶化，肌肉萎缩。这就是为什么宇航员在数月的太空工作返回地球之后，连走路都变得困难起来的原因。