Narrator: Listen to part of a lecture in an astronomy class.

旁白：听一段天文学的演讲。

Professor: OK.We have been looking at some of the smaller members of our solar system, comets.

教授：好的。我们已经讨论过我们太阳系中的一些较小的成员，彗星。

You already know about the structure of comets.

你们已经了解了彗星的结构。

Let's continue our discussion now by talking about orbits, especially those of the so-called periodic-orbit comets.

让我们通过讨论轨道来继续我们的讨论，尤其是那些名为周期性轨道彗星的轨道。

These are the comets that circle around the Sun pretty regularly.

有一些彗星非常有规律地环绕太阳运行。

They return again and again, predictably, after a certain period of time.

根据预测，在一定时间之后，它们一次又一次的回来。

That's why we say their orbits are periodic.

这是为什么我们说他们的轨道是周期性的。

Probably the most famous and brightest of these is Halley's comet.

他们之中最有名最明亮的也许就是哈雷彗星。

Halley's comet comes from far out in the solar system, goes in close to the Sun, and then out again.

哈雷彗星来自于太阳系较远的地方，运行到离太阳较近的地方，然后又再一次离开。

At its closest approach to the Sun, Halley's comet is about twice as close to the Sun as Earth is.

在哈雷彗星离太阳最近点的时候，它离开太阳的距离大约是地球到太阳的两倍。

And at its farthest.

而在它最远的地方，

It's about thirty-five times farther from the Sun than we are, which puts it out beyond Neptune.

大约是我们离太阳距离的25倍，这个使它比海王星还要远。

Basically, the idea here is that a periodic comet, with its very elongated orbit, just keeps coming back around again and again.

这里主要的概念就是：一个有着细长轨道的周期彗星会持续一次次地运行回来。

With Halley's comet, well, it returns every 75 years, roughly.

而哈雷彗星大约每75年会回来一次。

But where is Halley's Comet during most of this time?

但是大部分的时间哈雷彗星在哪里呢？

Well, like all orbiting bodies, a comet moves faster when it's closer to the Sun.

就像是所有航道运动的物体一样，当彗星离太阳越近，运行地越快。

So it only spends about a year or two in our neighborhood, inside the orbit of Jupiter.

所以，它在我们附近运行的时间大约只有一两年，在木星轨道之内。

Most of its time is spent way out beyond Jupiter's orbit, poking along near the farther reaches of its own orbit.

它大部分的时间都在木星轨道以外，沿着它自己轨道较远的区域运行着。

Because of this, we can only see Halley's for a few months every 75 years, first on its way in toward the Sun, and then on its way out again.

因为这样，我们只能每75年，看到哈雷彗星几个月。首先，是在它向太阳运行的时候，而然后是在它再一次离开的时候。

Now, you remember from our previous discussion that a comet's nucleus, its core, is made up of ice and dust, like a frozen snowball.

从我们上一次讨论中个，你们应该记得彗星的核是由冰和灰尘组成的，就像是一个雪球。

And as it approaches the Sun, it starts to heat up.

并且当它接近太阳时，它开始升温。

And some of the ice vaporizes into gas and spreads out from the nucleus.

有一些冰从核当中蒸发成气体，然后从核向外扩散。

The gases that vaporize from the comet, the comet never collects them back again, so on every orbit, the comet leaves part of itself behind.

从彗星蒸发的气体，彗星从来不再次将他们收集回来。所以，每一次的绕轨运行，彗星都会将它自己的一部分丢失。

OK. How old is this solar system?

好，太阳系有多少年历史了？

Four and a half billion years,remember?

45亿年，记得吗？

And Halley's is going around the Sun once every 75 years and losing stuff each time.

哈雷彗星一直都围绕太阳运转，并且每一次都丢掉东西。

So the comet should be long gone by now, right?

那么到如今，彗星应该早就不见了，不是吗？

I mean, how come Halley's is still there, after four and a half billion years.

我的意思是，为什么哈雷彗星现在还在呢，过了45亿年之后？

How could it be?

它是怎么能够还在的？

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.

答案就是，这个彗星并没有一直在这么一个短周期轨道上运行，因为，一旦一个彗星运行到一个轨道，这个轨道使它频繁地靠近太阳。

Well, that comet's probably not going to be around too much longer.

那么他们可能也不会在这个轨道上存在太长时间。

So this kind of periodic orbit is only a phase in a comet's life.

所以，这种周期性轨道只是一个彗星生命中的一个阶段。

A phase that just precedes its final breakup.

在它最终解体前的一个阶段。

We've seen comets do that, going toward the Sun and then come back around, torn into pieces.

我们看到过彗星经历过这个阶段，朝着太阳运行，然后又回来，然后撕裂成了碎片。

But lots of comets aren't like that.

但是很多彗星并非如此。

They come in, pass behind the Sun, and then travel back out.

他们过来，移到太阳后面，然后又转回去。

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.

但是，有一个很大的轨道并且轨道最远处离太阳很远的话，我们就不知道它到底运行到多远的地方了。

We just can't determine that very accurately from the close-in part of the orbit that we do see.

我们不能通过离我们较劲的轨道部分来精确地推测。

So these are often called parabolic-orbit comets.

所以这就是我们通常说的抛物线性轨道彗星。

Parabolic means the orbit is open at the far end.

这意味着轨道在遥远的边界是开放的。

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.

事实上，这个轨道可能会让彗星接近太阳，并且将他们最终带回到太阳的附近，但是这个时间可能会是好几万年。

And basically, we can't determine it.

并且，最主要的是，我们无法确定这个时间。

So we just, we refer to them as open-ended parabolic-orbit comets.

所以，我们把他们称作为开放式抛物线轨道彗星。

So, what can change a comet with one of these long orbits where they only come by the Sun occasionally into a much more frequent periodic visitor?

那么，什么可以将这些长轨道卫星，从偶尔从太阳旁边走过，转变成一个更加频繁的周期性的访者呢？

Well, gravitational interaction with planets, right?

和地球之间的重力作用，对不对？

lf a comet on one of these long period orbits at some point comes 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.

如果这些长周期轨道的其中一个彗星，轨道运行至离木星、土星或者其他行星比较近的一个点，那么那个行星重力的拉力可能会改变彗星的轨道，可能会使轨道变短。

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'll capture it and keep it coming back around the Sun much more often.

那么，这个彗星，如果它碰巧以正确的方式路过一个行星，它可以被卷入一个新的轨道，一个可以抓住它，使它更频繁地回到太阳附近。