Comet Facts
Comets are small icy objects that orbit the Sun in highly elliptical orbits, meaning that sometimes they get close to the Sun and sometimes they're a long way out in the Solar System.
There are four parts to a comet: The nucleus, the coma, the dust tail and the ion tail. Notice that there are two tails, although the second tail may be hard to see in some comets.
The main solid part, the nucleus, can be as small as a few metres or as large as several kilometres wide.
When a comet is a long way from the Sun, it's just the nucleus. It looks like a dirty iceball, in fact that's how many people describe comets because most of them appear to be made of ice and small amounts of dirt and rock. Some comets could have more dirt and rock, making them more like icy dirtballs.
When a comet gets close to the Sun, solar radiation and solar wind blast the nucleus and it forms a coma and tails.
The coma is a fuzzy cloud around the nucleus. If the comets comes close to Earth you may be able to see its coma with binoculars or a telescope. Many comets don't get much past the coma stage and you can't really see a tail.
The tails are long lines of vaporized material blown off the nucleus.
The dust tail is the most visible part of the comet and that's what most people look for when viewing with the naked eye. This tail tends to stream along behind the comet as it moves through its orbit.
The ion tail (aka plasma tail or gas tail) is harder to see but often fairly easy to capture in a long-exposure photograph. It tends to point directly away from the Sun, no matter which direction the comet is travelling.
If the Earth happens to cross the path of a comet's extended debris trail, bits of the comet are seen falling as meteorites (shooting stars).
It is thought that many comets reside in the Kuiper Belt and Oort Cloud, two of the outermost regions of the Solar System. Occasional something perturbs the orbit of one of them, sending it into the inner Solar System.
Comet History
In ancient times comets were seen in many cultures as bad omens.
Edmond Halley never saw the comet that is named after him. He predicted when it would arrive but he died before it happened.
Halley's Comet photographed in 1910