【英语时差8,16】地球同步卫星：速度和轨道-下

 A satellite has to move at a certain speed to balance out gravity; otherwise it won’t stay in orbit¹. 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 orbit   

n.轨道；vt.使沿轨道运行；使进入轨道运行；vi.沿轨道运行，环行

参考例句：

• The space rocket was launched and went into orbit.宇宙火箭发射后进入轨道。
• We can draw the earth's orbit round the sun.我们可以画出地球绕太阳运行的轨迹。

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。