Comets for The 99 BY: Monique Fraser

Background Info on Comets - Welcome to the wonderful world of comets! A comet is a celestial body that resembles a fuzzy star with a glowing tail. Comets are apart of the solar system. They contain frozen gases and dust that are loosely held together. These in turn are kept together by gravitational attraction. It is believed by many astronomers, that comets were formed from interstellar matter that came from outer planets.

A comet can be seen from the Earth when the comet's path goes around the sun. For the rest its period it is very far from the sun. Gravitational attraction of a nearby planet can change the comet's orbit. Only a few comets appear each year. Most are too light to see without a telescope. Comets that return at regular intervals are periodic. Some are so big that they can not be seen from Earth for more than a century.

Comets can be very bright. They can stay visible for many months during the night and day. For as long as 2,000 years, comets can be seen. People were scared that a comet might collide with the Earth and cause and explosion. In reality when a comet comes within the Earth's atmosphere it breaks up into small meteorites.

A comet is made up of frozen ice, dust, and other material. This is called the nucleus. When the comet approaches the sun, the material changes into gases. Ultraviolet radiation comes from the gas. This is called the coma. When comets approach the sun they develop tails. They go away from the comet's head.

The size of the different parts of a comet can vary. The diameter of thenucleus is only a few km. The coma has a very large diameter, but the mass and density are very small. The tail of a comet can vary. It can range from 1 - 100 million km. There are two types of tails on a comet. A straight tail with ionized gases is called a type one tail. Type two are curved and hazy because it is mostly dust and gas. Some can have both.

Halley's comet was the first comet to be recorded as periodic. Some comets can be seen every few years or every century. Edmond Halley discovered that the comets that were seen in 1537, 1607, and 1682 were almost the same. Halley found that they were all the same comet. He predicted that it would return in 1758. It did return right on schedule That is how it became Halley's comet. The last time Halley's comet was seen was in 1985.

Comets rarely come too close to Earth. The closest a comet has come to Earth was 4.7 million km of Earth. It was called the IRAS-Araki-Alcock. Since comets are so far away a visual telescope that can photograph far away areas of the sky is needed. Ten comets are discovered each year. Out of the ten only one can be seen without a telescope.

Since comets are elliptical they can have either short periods or long periods. Short period comets are less than 200 years. The comet with the shortest period is Encke's. It is 3.3 years. Long period comets are several thousand.

In conclusion comets are part of the universe and are wonderful spectra that are nice to look at. Many theories have been made about where comets originated from. The one that most people are comfortable with right now is that comets were formed at the same time as the rest of the solar system. Either way look out for them, they really cool to look at.