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第一段
1 .Listen to part of a lecture in an astronomy class.
听一段在天文学课程上的演讲
第二段
1 .OK. We've been looking at some of the smaller members of our solar system, comets.
我们一直在介绍我们太阳系的小成员们，即彗星。
2 .You already know about the structure of comets.
你们已经了解了彗星的结构。
3 .Let's continue our discussion now by talking about orbits, especially those of the so-called periodic-orbit comets.
让我们继续我们关于彗星的讨论，讨论轨道，尤其是那些所谓的周期性轨道彗星。
4 .These are the comets that circle around the Sun pretty regularly.
这些彗星相当规律的围绕太阳运动。
5 .They return again and again, predictably, after a certain period of time.
它们一次又一次回来，可预测的，在一段时间之后。
6 .That's why we say their orbits are periodic.
这是为什么我们说它们的轨道式可预测的。
7 .Probably the most famous and brightest of these is Halley's comet.
大概彗星中，最著名最亮的是哈雷彗星。
第三段
1 .Halley's comet comes from far out in the solar system, goes in close to the Sun, and then out again.
哈雷彗星来自遥远的太阳系，靠近太阳，然后再次远离。
2 .At its closest approach to the Sun, Halley's comet is about twice as close to the Sun as Earth is.
。在哈雷彗星最接近太阳的时候，哈雷彗星是日地距离的两倍。
3 .And at it's farthest, it's about thirty-five times farther from the Sun than we are, which puts it out beyond Neptune.
在其离太阳最远的时候，其到太阳的距离比地球远35倍，比海王星还要远。
4 .Basically, the idea here is that a periodic comet, with its very elongated orbit, just keeps coming back around again and again.
基本上，这里的想法是：一个周期彗星，具有非常细长的轨道，不断往复。
5 .With Halley's comet, well, it returns every 75 years, roughly.
哈雷彗星大致每75年往复一次。
第四段
1 .But where is Halley's comet during most of this time?
但是，哈雷彗星大多数时间位于哪里？
2 .Well, like all orbiting bodies, a comet moves faster when it's closer to the Sun.
像所有的轨道运行天体，彗星运动加快，当它接近太阳的时候。
3 .So it only spends about a year or two in our neighborhood, inside the orbit of Jupiter.
所以它只能在我们的附近即木星的运行轨道内运行一或两年。
4 .Most of its time is spent way out beyond Jupiter's orbit, poking along near the farther reaches of its own orbit.
它的大多数时间都花在了木星轨道以外，在它轨道远端漫无目的的缓缓行进。
5 .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.
因此，我们只能每75年看到哈雷彗星几个月，首次在其朝向太阳运动，然后在它离开的时候
第五段
1 .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, and as it approaches the sun, it starts to heat up.
还记得我们之前的介绍，彗核，彗星的核心，是由冰和尘埃构成，像冰冻的雪球。当它接近太阳时，它开始升温。
2 .And some of the ice vaporize into gas and spreads out from the nucleus.
一些冰的蒸发成气体并从彗核散发出去。
3 .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，彗星从不收集回来，所以在每个轨道上，彗星都留下了自己的一部分。
第六段
1 .OK, how old is this solar system?
太阳系多少岁了？
2 .Four and a half billion years, remember?
四十五亿年，还记得吗？
3 .And Halley's is going around the sun once every 75 years and losing stuff each time.
哈雷会每75年围绕太阳一次，并且每次都失去一些东西。
4 .So the comet should be long gone by now, right?
所以哈雷彗星到现在为止应该早已不复存在了吧，对不对？
5 .I mean, how, come Halley's is still there?
我的意思是，哈雷为什么仍然存在？
6 .After four and a half billion years, how could it be?
经过四十五亿年，怎么会还存在？
7 .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.
答案是，这颗彗星并非一直在这么短的周期轨道上，因为一旦一颗彗星进入一个轨道，轨道会使彗星频繁的靠近太阳运动，
8 .Well, that comet's probably not going to be around too much longer.
哈雷彗星可能不会存在很长时间了。
9 .So this kind of periodic orbit is only a phase in a comet's life.
所以这种周期轨道的是一颗彗星的生命的一个阶段。
10 .A phase that just precedes its final breakup.
一个在他最终解体之前的阶段。
11 .We've seen comets do that, going toward the sun and come back around, torn into pieces.
我们见过彗星这个情况，朝太阳运动，然后远离，四分五裂。
第七段
1 .But lots of comets aren't like that.
但大量的彗星并不像那样。
2 .They come in, pass behind the sun, and then travel back out.
他们靠近，在太阳背面通过，然后运动远离
3 .But with an orbit so large, and its farthest point so far away from the sun that we just don't know how far out it goes.
但是具有这么大的轨道，并且它轨道最远端离太阳非常远我们不知道它运动到了多远的地方。
4 .We just can't determine that very accurately from the close in part of the orbit that we do see.
我们不能精确的确定远端只从我们看到的轨道接近中心的部分。
5 .So these are often called parabolic-orbit comets.
所以，这些通常被称为抛物线轨道彗星。
6 .Parabolic means the orbit is open at the far end.
抛物线是指轨道在远端是开放的，
7 .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，开放轨道可能确实闭合并最后使彗星回归到太阳附近，但这个时期可能要好几万年。
8 .And basically, we can't determine it.
基本上，我们无法确定。
9 .So we just, we refer to them as open-end parabolic-orbit comets.
所以我们只称它们为开放式抛物线轨道彗星。
第八段
1 .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，什么可以改变一个有长轨道，偶尔运动到太阳附近的彗星成一个更频繁的周期性的访客？
2 .Well, gravitational interaction with planets, right?
行星的引力相互作用，对不对？
3 .If a comet on one of these long period orbit 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.
如果在一个长轨道上的彗星，在某一时刻接近木星或土星和其他行星之一，那么该行星的引力可能改变轨道，可能会让那个轨道变短。
4 .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.
所以这颗彗星，如果它凑巧以合适的方式经过一个行星，它可能会被拉入一个新的轨道，一个会抓住它的轨道并保持它更频繁的绕太阳运动。