英语听力文摘 English Digest 518、地球同步卫星（二）

 　　A Stationary¹ Satellite, part 2

 

　　A geostationary satellite orbitsthe planet at the same rate as the planet rotates, so the satellite is always over the same patch of ground.

 

　　Cool! you say. I’d like to have one of those about twenty feet over my house. Sadly, geos only work easily at one distance: 22,500 miles up.Why is that?

 

　　Other satellites can have different orbits, can’t they?

 

　　Yes, they can. But that’s because they can also move at different speeds. A geo doesn’t have that option — by definition it must orbit at the same speed as the earth turns. And that speed determines how high up it must be!

 

　　A satellite has to move at a certain speed to balance out gravity; otherwise it won’t stay in orbit. Now, a geo must move at the speed the earth turns. If you put it in a very low altitude, that speed wouldn’t be enough to balance gravity.

 

　　Go higher, though, and gravity gets weaker. At 22,500 miles, gravity will be just weak enough so that a satellite moving at the speed of the earth’s rotation² will stay in orbit without help.

 

　　Of course, if you wanted to have a geo at a different altitude, you could always attach rockets to it and have them push continuously. But that’s a pretty expensive option. At 22,500 miles, nothing else is required.

1 stationary   

adj.固定的，静止不动的

参考例句：

• A stationary object is easy to be aimed at.一个静止不动的物体是容易瞄准的。
• Wait until the bus is stationary before you get off.你要等公共汽车停稳了再下车。

2 rotation   

n.旋转；循环，轮流

参考例句：

• Crop rotation helps prevent soil erosion.农作物轮作有助于防止水土流失。
• The workers in this workshop do day and night shifts in weekly rotation.这个车间的工人上白班和上夜班每周轮换一次。