Comets are both strange and interesting objects. They vary in size and brightness, with some being so dim they can barely be seen with a telescope and others that are so bright that they can be easily seen with the naked eye. One of the things that make them so interesting, though, is that comets have tails when they are near the sun. It is small wonder that people may wonder why they have tails and asteroids or similar objects don’t.
A key to understanding the tail of a comet is in understanding the composition of the object. These small masses have been likened to a dirty ice ball or snowball. That is to say that it is believed that most comets are not solid rock but rather a combination of rocks and minerals, dust, dirt, gases and ices. When these bodies are far from the sun where temperatures are extremely low, the ices don’t get warm enough to melt, usually, though the comet may collect more dust and ice that exists in the solar system.
Not all comets orbit the sun. Of those that do, not all of them return a second time and some may be flung out by the sun or consumed by it rather than orbiting. However, most of comets that are detected and observed do orbit in an elliptical path. This brings them close enough to the sun that the comet is assailed by the heat and solar wind that the sun produces. As this happens, the comet begins to heat up and the closer it gets to the sun, the hotter it becomes.
At the point where the nucleus of the comet heats up, the ices begin to melt and vaporize. This, and the gases the comet contains, are pushed away from the sun by the solar wind, resulting in the appearance of the tail. The tail is visible because of reflected sunlight on the particles that are escaping the nucleus of the comet. This means that even though the actual amount of gases that are released may be small, the visible tail can be many millions of miles in length.
Comets also have two tails. This is because the out-gassing forces dust particles away from the body of the nucleus. Though the rays of the sun ionizes the gas atoms, it doesn’t do so for the dust. Dust particles are, then, heavier and larger, so they form a second tail, pushed away from the sun by sunlight rather than the solar wind.
Tail preceding the comet
The solar wind is additionally responsible for the fact that the tail of a comet points away from the sun. If the comet swings all the way around the sun and heads back out toward the outer reaches of the solar system, the tail precedes the comet, gradually becoming smaller as the comet moves farther away and into colder regions of space.
As strange and wondrous as comets are, the mechanism for how and why the tails form is understood fairly well. Doubtlessly, though, there is far more to learn about comets than what science has found out thus far.