評論
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So the comet should be long gone by now, right?
所以，這顆彗星理應早就消失了，對吧？
I mean, how come Halley's is still there?
我是說，哈雷彗星怎么會還在這兒呢？
After four and a half billion years, how could it be?
在四十五億年后，怎么可能還在呢？
Well, the answer is that this comet hasn't always been in such a short periodic orbit, since once a comet gets into an orbit that keeps it coming in close to the sun quite frequently...well, that comet's probably not going to be around too much longer.
答案是，這顆彗星不總是繞這個短周期性軌道運轉，因為一旦彗星頻繁進入了這種會使自己接近太陽的軌道，那彗星可能不久之后就不復存在了。
So this kind of periodic orbit is only a phase in a comet's life.
所以這種周期性軌道只是彗星生命中的一個階段。
A phase that just precedes its final breakup.
這個階段預告它的最終解體。
We've seen comet do that, going toward the sun and come back around, torn into pieces.
我們已經見過彗星這么做了，靠近太陽，回來的時候已經碎了好多塊了。
But lots of comet aren't like that.
但很多彗星不是這樣的。
They come in, pass behind the sun, and then travel back out.
它們靠近太陽，從太陽后面經過，然后再離開。
But with an orbit so large, and its farthest place so far away from the sun that we just don't know how far out it goes.
但對于軌道范圍這么大，軌道最遠處離太陽這么遠的彗星，我們不知道最遠能到哪里。
We just can't determine that very accurately from the close, in part of the orbit that we do see.
我們在它（離我們）較近時，即它在我們可見范圍的部分軌道內時，我們也無法決定。
So these are often called parabolic-orbit comet.
這種通常稱為拋物線軌道彗星。
Parabolic means the orbit is open at the far end.
拋物線是指軌道遠端是開放的。
Actually the orbit probably does close and return the comet to the vicinity of the sun eventually, but the period might be tens of thousands of years.
事實上，很可能軌道是封閉的，彗星最終會回到太陽附近，但這個周期可能需要上萬年。
And basically, we can't determine it.
基本上我們是無法確定的。
So we just, we refer to them as open-end parabolic-orbit comets.
所以我們稱之為開放性拋物線軌道彗星。
So, what can change a comet with one of these long orbit where they only come by the sun occasionally into a much more frequent periodic visitor?
那么，是什么改變了彗星，使其沿偶爾才會經過太陽的長軌道運行，變為更頻繁的周期性拜訪太陽？
Well, gravitational interaction with planets, right?
行星間的相互引力作用，對吧？
If a comet on one of these long period orbits at some point comet close to Jupiter or Saturn or one of the other planets, then the pull of that planet's gravity might alter the orbit, maybe make it much shorter.
假如，一顆沿這種長周期軌道運行的彗星在某個時候靠近木星或土星或別的行星，這些行星的引力可能會改變彗星的軌道，可能會讓軌道變短。
So this comet, if it happens to pass by a planet just the right way, it can be drawn into a new orbit, one that will capture it and keep it coming back around the sun much more often.
如果這顆彗星恰好以正確的方式經過了一顆行星，彗星可被吸引到一個新的軌道上去，這個軌道會捕捉這顆彗星，讓它更頻繁地回到太陽附近。
