Listen to part of a lecture in an astronomy class.

OK. We've been looking at some of the smaller members of our solar system, comets. You already know about the structure of comets. Let's continue our discussion now by talking about orbits, especially those of the so-called periodic-orbit comets. These are the comets that circle around the Sun pretty regularly. They return again and again, predictably, after a certain period of time.That's why we say their orbits are periodic. Probably the most famous and brightest of these is Halley's comet.

Halley's comet comes from far out in the solar system, goes in close to the Sun, and then out again. At its closest approach to the Sun, Halley's comet is about twice as close to the Sun as Earth is. And at its farthest, it's about thirty-five time farther from the Sun than we are, which puts it out beyond Neptune. Basically, the idea here is that a periodic comet, with its very elongated orbit, just keeps coming back around again and again. With Halley's comet, well, it returns every 75 years, roughly.

But where is Halley's comet during most of this time? Well, like all orbiting bodies, a comet moves faster when it's closer to the Sun. So it only spends about a year or two in our neighborhood, inside the orbit of Jupiter. Most of its time is spent way out beyond Jupiter's orbit, poking along near the father reaches of its own orbit. 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.

Now, you remember from our previous discussion that comet's nucleus, its core, is made up of ice and dust, like frozen snowball, and as it approaches the sun, it starts to heat up. And some of the ice vaporize into gas and spreads out from the nucleus.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.

OK, how old is this solar system? Four and a half billion years, remember?And Halley's is going around the sun once every 75 years and losing stuff each time. So the comet should be long gone by now, right?I mean, how, come Halley's is still there? After four and a half billion years, how could it be? 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. So this kind of periodic orbit is only a phase in a comet's life. A phase that just precedent its final breakup. We've seen comet do that, going toward the sun and come back around, torn into pieces.

But lots of comet aren't like that. They come in, pass behind the sun, and then travel back out. 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. We just can't determine that very accurately from the close, in part of the orbit that we do see. So these are often called parabolic-orbit comets. Parabolic means the orbit is open at the far end. 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. And basically, we can't determine it. So we just, we refer to them as open-end parabolic-orbit comets.

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? Well, gravitational interaction with planets, right? If a comet on one fo the these long period orbit 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. 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.