Facts About Comets in the Solar System

Comets Grow Tails When Approaching the Sun

• A comet is comprised of rock and ice. In the cold conditions of space, the water and gases remain frozen. As a comet approaches the sun, heat causes the water and gas to melt. The comet's tail is dust and gas trailing behind the comet. The tail glows when the sun illuminates it, making it visible. Solar winds push the tail, so it always points away from the sun. The tail may be millions of miles in length.

Why Astronomers Are Uncertain How Many Comets Exist

• Astronomers believe that there may be trillions of comets orbiting the edges of our solar system in the Oort Cloud. However, these comets may near the sun only once in several hundred or even several thousand years. Astronomers believe some have orbits as long as 30 million years in duration. Mankind's history simply is not long enough to contain a record of every comet that might reappear.

Halley's Comet Has a Short Return Period

• Halley's Comet returns every 75 to 76 years, which, at once or possibly twice in a lifetime, may seem like a long time. However, astronomers consider it a short-period comet, along with other comets in the Kuiper belt, which lies between Neptune and Pluto.

Not All Comets Are Named for Humans

• Naming comets was not too difficult in the early part of the 20th century. Guidelines from the International Astronomical Union specified that the astronomer or other individual discovering the comet would attach his name to the comet. If he discovered another comet, a number attached after his name revealed how many he had discovered previously. With the development of probes, however, the situation became a bit more complicated. Unmanned exploratory spacecraft proved to be quite efficient at finding comets. Now, comets may bear the name of the spacecraft, such as WISE or SOHO, that discovered them.

Comets Do Not Last Forever

• Every time a comet approaches the sun, it loses some of its water and gases. Eventually, no material remains to generate a tail. At that point, only the rock remains, which may continue to follow the comet's orbit as an asteroid. Astronomers think that many asteroids are burned-out comets.

Comets May Be Responsible for Earth's Oceans

• When the earth first formed, its heat was so great that any water would evaporate. Astronomers theorize that during a period of intense bombardment, comets brought ice and water to the planet.