| MadSci Network: Astronomy |
> Can we tell how old a comet is? How long does a comet last before it dies out?
The short answer to both questions is: No, not exactly. As much as we know about comets, there is still a great deal that we do not. If it makes you feel better, there are scientists all over the world asking exactly the same questions.
The long answer is:
There are many things that we would have to know about the comet before we
could tell how old it is, like:
How big is it? (What is it's mass?)
What is it made of? (How much is frozen gas, liquid, rock...)
How fast is it losing material in it's tail?
If the comet is very big, but a lot of stuff gets melted off of it when
it gets close to the sun, it may not last very long and may not be very
old.
If the comet is small, but doesn't lose very much material quickly, it may last a lot longer and may be a lot older. away.
If the comet is made of mostly ice and frozen gas, it may eventually melt away.
If the comet is made of mostly rock, it may not melt away at all.
Astronomers are good guessers though. A good astronomer can guess about how big a comet it by just looking at it. It may not be a very exact guess, but it's better than nothing.
Also, the astronomer can guess about the kind of stuff the comet is made of just by looking at the shape of the tail. Gases and light weight material that come off of the comet make a tail that is easily pushed away from the sun by the solar radiation. This is called a 'plasma tail'. Heavy particles coming off of the comet trail behind the comet as it moves, making a 'dust tail'. If the plasma tail is very bright, and the dust tail isn't, then the comet is not losing very much material at all. If the dust tail is very bright, then the comet is losing a LOT of material, and it will only last a long time if it is very large.
Either way, remember that comets can take years or thousands of years or even hundreds of thousands of years to travel though just ONE orbit, and may live through hundreds of orbits before they disintegrate or run into something.
If you are interested in comets, you might read the comets webpage at: The Nine Planets
Thank you for your question!