What are Comets?

As one comedian said, comets are not the descendants of one of Santa’s reindeer. Actually, comets are made up primarily of ice (frozen water). There is some odd mixture of dust and small particles, which is picked up along the way. Not only that, but comets usually contain a small amount of basic gases, such as methane and ammonia. The dust and gases keep comets from being pure water. Thus, they are described as being “dirty.”

Some scientists have proposed that comets appeared when some of the larger planets were being formed. The theory states that the ice chunks were cast off of came off of the surface of these celestial giants. However, comets are being formed all the time, with millions traveling throughout the universe.

A comet is composed of four distinct parts, according to scientists – nucleus, coma, plasma tail and dust tail. We can’t really see the nucleus, because it is obscured by the dust and debris surrounding all comets. A comet’s bright appearance comes from this nucleus, astronomers say. However, due to pollution in the Earth’s atmosphere we are probably seeing fewer comets than we might have a few decades ago.

Visibility

Astronomers state that we see comets best when light from the Sun illuminates the coma. This temporary “atmosphere” around the comet’s nucleus helps make these celestial bodies visible to people on Earth. A comet’s size also has something to do with its visibility. Some comets measure only a few hundred feet across while others might be several miles across.

Comets do have recognizable orbits and observers have been able to identify orbiting periods lasting a few years and theorize that some comets take thousands of years to complete their path. Astronomers have even come up with a few places in the universe where comets originate, with their direction and trajectory determined by the giant outer planets such as Jupiter and Saturn.

While some people tend to confuse comets with asteroids, there are a couple of significant differences. Comets have the coma described above, as well as the tail cause by radiation and solar wind. Asteroids may be formed in a slightly different way and may originate inside the orbit of one of the larger planets. However, the distinction between asteroids and comets is still fuzzy.

Scientists have reportedly identified nearly 4,000 comets, with about 500 categorized as short-period comets. Astronomers believe that the number of comets is increased on a consistent basis. But this relatively small number (4,000) is only a tiny percentage of the comets that are probably moving somewhere in the universe. Some studies indicate there may be as many as a trillion comets in existence at this time. Even conservative estimates number the comets in the billions. Yet we can only see one or two comets with the naked eye each year. Most of the time the comet’s appearance is not very exciting. If you see one that is particularly bright you have seen a Great Comet.