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 precedes 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 comet.
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 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.
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.