When looking at the night sky and you observe a comet, it will appear to move very slowly, when in fact it is hurtling through space at several thousands of miles per hour. This is because very few of them come within a few million miles of Earth and so the huge distance makes them appear very slow. Also at times the comet is coming towards us or moving away from so and us might even appear stationary. Comets are generally visible for periods ranging from a few days to several months, and appear to change little in position night-after-night
Comets are sometimes called "dirty snowballs" because they are mixtures of ices and dust. The comets have a core or a nucleus, made up of mainly ice and dust, and is frozen solid. In this state they only reflects light and so are generally invisible in the far outreaches of the solar system, it is only when they approach the sun, do they become luminous. As comet nears the sun, the nucleus begins to warm up and some of the ice begins to evaporate, these gases carry dust particles along with them and create the coma, that envelope the nucleus in a cloud of gas and dust. The dust reflects still more light; while the gases absorb ultraviolet rays and begin to glow. They range from a few miles to tens of miles wide, but as they orbit closer to the Sun, they heat up and spew gases and dust into a glowing head that can be larger than a planet. As they get closer to the sun, the comets develop tails of radiant material that stretches into millions of kilometers. The tail always faces away from the sun and is shaped by the solar winds as well as the radiation emitted by the sun. What is really interesting is that comets can sometimes split their tails; they can even have multiple tails and can even lose the tails at times.
Comets are divided into two types by the period of their orbit: short period comets complete their orbit in 200 or less years and long period comets take more than 200 years to orbit the Sun. Of the two types, short period comets have less elliptical orbits. Short period comets, like asteroids and meteoroids, are left over bits that were never incorporated into a planet during planet formation.
The Kuiper Belt, which is 30-100 AU from the Sun, is a reservoir of these short period comets. During the formation of the Solar System, the Kuiper belt was on the outer part of the pre-planetary disk. Since the part of the disk was less dense than the inner part, only small comets could be formed, not large outer planets.
Long period comets, which have very elliptical orbits, usually originate from the Oort Cloud. Occasionally, a star passes by the Oort Cloud and disturbs the orbits of the comets within it. Some of these comets change course and enter the Solar System.
Comets are composed of dust and ices. The ices contained by comets include water, methane, ammonia, and carbon dioxide. These ices are sublimed off the nucleus when the comet nears the Sun. A dust tail, which is the part of a comet easiest to see, forms from dust particles that are driven off the nucleus by escaping gases and plasma. An ion tail, which can be as long as 2 AU when the comet is near the Sun, forms from plasma that interacts with the solar wind. In addition, a coma forms around the nucleus. A coma is a large, bright cloud of gases and dust ejected from the nucleus.
Meteoroids, Meteors, and Meteorites:
Meteors are small rocks, sometimes as fine as grains of sand. When these meteors enter the earth’s atmosphere, due to friction, they get heated up, and make the air around them glow. They last for only a few seconds before completely disintegrating and are commonly known as shooting stars. Some of them might be big enough to reach the earth’s surface and are called meteorites.
Meteors enter the Earth's atmosphere on a regular basis and on most nights you can see a few meteors per hour. During meteor showers, however, meteors are visible at a much higher rate. Meteor showers are usually associated with comets. Comets cast off debris when they near the Sun and when Earth passes through the debris; its sky displays astonishing meteor showers.
It is also assumed that early collisions between earth and comets resulted in the vast amounts of water that now make up 3/4th of the earth’s surface. It is only because of these waters that life on earth was possible, so in that sense we possibly owe all our waters and therefore our lives to these brilliant comets.