Every now and then from Earth we can observe a bright spot in the sky that looks like a star with a long tail. These are not stars at all, but instead comets that pass through our solar system that are made of up ice and rock. The tail that we see is actually piece of the rock and ice that break off the head of the comet and burn up do to its incredible speed.
There are a few comets that regularly orbit in our solar system, the most famous of which is Halley's comet. Many of these comets are either too small or too far away to see from Earth, but about every ten years one comes close enough to be seen. Besides Halley's comet another regular visitor is the Hale-Bopp comet that reappeared just a few years ago in 1997.
A huge cloud exists in the outer reaches of our solar system that is home to large quantities of dust that are frozen and covered with ice. This area is known as the Oortcloud and serves as the birth place of comets. Every so often the Oorcloud comes under the influence of a passing star's magnetic field. The magnetic field accelerated one of the frozen dust ball and it starts on its journey through the galaxy.
Anatomy of a Comet
As a comet approaches a star it begins to melt and pieces of it break off giving it a very defined head and two observable tails. The head of the comet is known as the coma, which gives off large quantities of gases as it is slowly vaporized by the star. Normally when traveling through space the coma is between one 1 and 10 kilometers. But these gases cause the coma to swell and it can become as large as millions of kilometers in some rare instances.
The debris that once formed the tail is blown away by solar wind. Comet's tails seem to glow because they reflect sunlight which is why we see them so well from Earth. As the comet approaches the Sun its tail is more visible behind the comet, but as it is leaving our solar system the tail can be seen in front of the coma. The tail can even become as long as 200 kilometers. Another tail is formed by the gases and water particles on the head of the comet. These tails are much smaller than the other and are known as ion tails. They can be identified by their blue colored tail. One interesting thing is that the ion tails always point away from the Sun.
The Orbit of a Comet
Just like the planets, comets follow an elliptical orbit around the Sun. Their orbits range from a few years to several hundred thousand years. According to the laws of Kepler, the speed of a comet is highest near the Sun and decreases as it drifts away. Comets are therefore only inside our solar system for a short period of time and are very far away for the large majority of their lifetime.
The comet heats up when it comes close to the to the Sun and this is when its coma and tails are the largest. As it moves away from the Sun its size decreases again, until it is so cold that only the icy nucleus remains. Since the comets regularly orbit the Sun, it loses matter with every pass and eventually it will be completely dissolved. Halley's comet, for instance, may pass only about two to three thousand times, before it is completely evaporated. As the comet passes it leaves behind small dust particles that can not be seen from Earth. However, when Earth crosses their path they become visible as meteors and even meteorites.