The intense heat caused by friction is also a problem in space travel. If you rub your hand hard on your forehead, you will feel this kind of heat. Once a spaceship is in outer space, there is no friction because there is no air to press against. But when the spaceship returns to the earth, it must go through air again. At first the air is very thin . But the closer the ship comes to the earth, the denser the air it meets. A spaceship entering the earth's atmosphere at full speed would get so hot that it would burn up completely and disappear.

由于磨擦作用而引起的高热也是空间旅行中的一个问题。用手在前额部使劲擦，会感到这种磨擦产生的热。当宇宙飞船在外层空间飞行时，因为没有空气紧贴着它，所以不会碰到磨擦的问题。但是，当飞船返回地面时，它必须再穿过大气层。一开始，空气非常稀薄；飞船越飞近地面，遇到的空气就越稠密。以全速飞入地球大气层的宇宙飞船会变得如此灼热，以致使它完全烧毁而消失掉。

Today, scientists are working harder than ever to solve the problems of space travel. In 1971, the Soviet Union sent into orbit the first experimental manned space station, Salyut. (On May 14. 1973, the United States sent its first space station, Skylab 1, into orbit 270 miles above the earth. This laboratory in the sky, larger than a three-bedroom house, provided living quarters for three successive teams of three men each. These teams stayed in space for weeks at a time to perform various scientific experiments. One project was to survey some of the earth's surface in order to discover unknown natural resources, such as deposits of coal and oil. Another project was to map fault lines so that earthquakes can be predicted more accurately. Still another involved the observation of the atmospheric disturbances of the sun by telescope. These disturbances have an effect on the earth's weather.

But the main purpose of Skylab was to find out how people could live and work in space for long periods of time. The effects of prolonged weightlessness on the human body must be determined before people can make long space flights. 

目前，科学家们为了解决空间旅行中的这些问题，而比以往更加努力地工作着。1971年，苏联把第一个实验载人空间站礼炮号送入了轨道。1973年5月14日，美国将其第一个空间站天空实验室1号送入了地球上空270英里的轨道。这个在天空中飞行的实验室，比一幢三居室的房子还要大，它为每组三人，相继三组字航员提供了生活空间。这三组宇航员一次在空间逗留了数周，进行了各种科学实验。一项计划是对地球的表面进行观测，以发现未探明的自然资源，例如煤田油田。另一项计划是对地层断裂线进行测给，以便更准确地预测地震。再一项计划是用望远镜对太阳引起的大气扰动进行现测，这种扰动对地面的气象有影响。

不过，天空实验室的主要目的是找到人类长时期在空间生活与工作的办法。因为人类首先必须弄清持久的失重状态对于人体的影响，然后才能进行长时期的空间飞行。

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