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A Stationary Satellite, part 2
地球同步衛(wèi)星（二）
A geostationary satellite orbitsthe planet at the same rate as the planet rotates, so the satellite is always over the same patch of ground.
同步衛(wèi)星以行星旋轉(zhuǎn)的速度繞著行星運(yùn)動(dòng)，所以衛(wèi)星總是處在地面同一塊地方的上空。
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?
你會(huì)說(shuō)，好酷！我寧愿有一個(gè)離我們房子只有二十英尺的衛(wèi)星。不幸地，同步衛(wèi)星只能在同一個(gè)高度工作，22,500 米的高空。為什么？
Other satellites can have different orbits, can't they?
其他衛(wèi)星有不同的軌道，他們有嗎？
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!
是的，他們有。那是因?yàn)樗麄兡芤圆煌乃俣冗\(yùn)動(dòng)，而同步衛(wèi)星沒(méi)有那樣的選擇——他必須與地球自轉(zhuǎn)同速運(yùn)動(dòng)。運(yùn)動(dòng)速度決定了他必須在的高度。
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.
衛(wèi)星必須以某一確定速度運(yùn)動(dòng)來(lái)平衡我們的重力；否則將不能保持在同一軌道上。現(xiàn)在，同步衛(wèi)星必須以地球自轉(zhuǎn)的速度同速運(yùn)動(dòng)。如果處于很低的高度，衛(wèi)星的速度將不足以平衡重力。
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 rotation will stay in orbit without help.
高度越高，重力越小。在22,500的高空，重力小到可以使衛(wèi)星在沒(méi)有幫助的情況下與地球同速運(yùn)動(dòng)，并且始終在同一軌道上。
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.
當(dāng)然，如果你想有一個(gè)在不同高度的同步衛(wèi)星，可以把衛(wèi)星附在火箭，讓火箭不斷推動(dòng)衛(wèi)星前進(jìn)。但是，這是相當(dāng)昂貴的選擇。在22500的高空，其他什么東西都不需要。
