The Ins and Outs of Comets

Comet

Throughout history comets have been much more interesting than asteroids. This is mainly because they can be seen with the naked eye because of their immense tail. They have been the stuff of legends the most famous being Halley’s comet as mentioned before.

When the planets were formed they didn’t use up all the material of the solar nebula and some other bits left over created comets. The comets formed in the region of the Kuiper belt and giant planets. They then underwent some changes in their orbits due to the gravity of the giant planets (called orbital evolution if you are interested). This would make some big changes to the comets orbits throwing some comets into the inner solar system causing havoc and colliding with some terrestrial planets but also putting comets in stable orbits that miss the terrestrial planets.

Some comets are just thrown out of the solar system and given the opportunity to explore the Galaxy and perhaps create havoc in other solar systems. If we turn that around though it means that there are comets flying around that could just fly through our solar system at any time creating mayhem. Some of the comets that were thrown out created the Oort cloud which is a big sphere of comets around the solar system.

close-up of a comet

Let’s take a bit more of a detailed look at comets though. If we break down the comet into its different parts we have the head of the comet which is the coma, the nucleus and the tail.

The nucleus is made out of Rocky particles with ices. The ices include water ice, frozen ammonia, carbon dioxide and carbon monoxide. It is not exactly known how a comet is made up but comets appear to have the Rocky particles on the outside and then perhaps on the inside are the ices. It is probably the case that the ices are mixed with the dust particles.

The nucleus is full of holes, i.e. porous and therefore the comet has a very low density. In the last post I posted on asteroids I mentioned a C type asteroid and a comet is very similar to that type of asteroid.

A comets tale is formed when the comet gets closer to the Sun. The closest point to the Sun is the perihelion and this is where the ices turn directly to gas not bothering with the liquid stage. If you want to impress people this is called sublimation. The gas escapes into space carrying dust particles with it.

The gas and the dust form separate tails. The tails point in slightly different directions the dust tail leaving dust behind the comets orbit forming what is called a dust tail. The gas tail produced at the same time always points directly away from the Sun. This is because the tail of gas is ionised and wants to follow the solar wind much more than the dust tail did.

The Two Tails of a Comet

It is possible to classify comets in respect of the orbits that they take through the solar system.

Long period comets come from the Oort cloud and their orbit has a radius of hundreds or even thousands of AU (AU stands for astronomical unit and is the distance from the Sun to the Earth) from the Sun. These comets can come in at any angle relative to the ecliptic plane (the plane of the planets to the plane of the comet). These comets can go either way around the Sun. As these comets have orbits around the Sun of hundreds or even thousands of years we cannot predict their appearance and so they appear unannounced.

Short period comets have an orbit that stays within the orbit of Neptune (the most distant planet from the Sun) and mostly have orbits that are in the same direction as the Sun’s rotation. These comets are from the Kuiper belt but Jupiter’s gravity turns some comets into short period comets that were once long period comets because of its enormous gravity. These comets are much more predictable than long period comets.

Comets can be beautiful but they can also be deadly. Comets can light up the night sky but they can also sneak around when they are not producing a tail and cause havoc. There is a theory that comets may have brought life to Earth. So should we look at comets as good or as bad for us as humans on planet Earth? I think it’s a combination of the two depending on your luck!