What is a comet?

Comets are sometimes called dirty snowballs or "icy mudballs". They are a mixture of ices (both water and frozen gases) and dust that for some reason didn't get incorporated into planets when the solar system was formed. This makes them very interesting as samples of the early history of the solar system.

Distinct parts of a comet

When they are near the Sun and active, comets have several distinct parts:

1. nucleus: relatively solid and stable, mostly ice and gas with a small amount of dust and other solids;

2. coma: dense cloud of water, carbon dioxide and other neutral gases sublimed from the nucleus;

3. hydrogen cloud: huge (millions of km in diameter) but very sparse envelope of neutral hydrogen;

4. dust tail: up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by escaping gases; this is the most prominent part of a comet to the unaided eye;

5. ion tail: as much as several hundred million km long composed of plasma and laced with rays and streamers caused by interactions with the solar wind.