What Is #6: A Comet?

Our last look in this What Is series focused on asteroids, the rocky and metallic leftover planetesimals in the Solar System. Today we take a look at comets. Comets are most often described as dirty snowballs composed of water ice, rock, dust, carbon-type materials, and other icy, hydrogen compounds. Given they are less densely packed together and are composed of lower density materials, they are less dense overall than asteroids. Comets are the more icy leftover planetesimals in the Solar System.

Comet Hale Bopp taken by Auvo Korpi from Huhtamo, Finland

These dirty snowballs flare up and have an atmosphere, called a coma, when they approach the Sun. As they move away from the Sun, they receive less solar energy and the atmosphere flares back down for most of its trip around the Sun. When flared up, comets have two tails. Yes, that is right, TWO tails. There is the dust tail and ion tail. Both always point away from the Sun, as shown in the image below. 

Comet 1P/Halley as taken March 8, 1986 by W. Liller, Easter Island, part of the International Halley Watch (IHW) Large Scale Phenomena Network.

The white tail above is the dust tail and the blue tail is the ion table. The dust tail is composed of slightly heavier particles, so this tail bends just a bit, whereas the ion tail points directly away from the Sun. Thus even when the comet is moving away from the Sun, the tails point away, leading the comet. The tail is not always trailing the nucleus of the comet.

Comets orbit the Sun repeatedly, although each passing of the Sun results in a bit of mass loss. Eventually a comet, after a few thousand years and several orbits of the Sun, will break apart. Some comets come from the Kuiper Belt, out near Neptune and Pluto's orbit. These are called short period comets. A famous short period comet is Halley's comet, with an orbital period of 76 years. Long period comets come from the Oort cloud, and several tens of thousands of astronomical units from the Sun. These comets have orbital periods of several hundred to possibly several tens of thousands of years.

Most comets require a telescope to observe, but occasionally comets are visible to the naked eye. The last really good naked eye comet I remember was Comet McNaught, which flew by Earth in 2007. I took my astronomy class out a couple of times to view this comet. Comet McNaught has an orbital period estimated to be about 93,000 years, so I won't be seeing that one again. LOL!

Comets are beautiful objects in the sky, so if you ever have a chance to view one, definitely take advantage.