In our travels through the Solar System, we may be lucky enough to encounter what appear to be giant balls of ice. These are the comets. Some astronomers have referred to comets as "dirty snowballs" or "icy mud balls" because they are composed mainly of ice with dust and fragments of rock. The ice can be both water ice and frozen gases. Astronomers believe that comets may be composed of the very material that formed the Solar System. Although most of the smaller objects in our solar system represent very recent discoveries, comets have been well known since ancient times. The Chinese have records of comets that date back to 260 B.C. This is because comets are the only small bodies in the Solar System that can be seen with the naked eye. Comets that are in orbit around the Sun can become quite a breathtaking sight.
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Stardust spacecraft close-up image of comet Wild 2
(NASA/JPL-Caltech) |
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ESA Giotto image of comet Haley nucleus showing gas jets
(NASA/NSSDC) |
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Comet Tempel 1 as seen by NASA's Deep Impact probe
(NASA/JPL-Caltech/UMD) |
A Comet's "Tail"
Comets are actually invisible until they begin to get close to the Sun. As they begin to heat up, an amazing transformation takes place. The dust and gases frozen within the comet begin to expand and burst forth at explosive velocities. The solid part of the comet is called the nucleus, while the envelope of dust and gas around it is known as the coma. Solar winds cause the material in the coma to trail behind the comet for a much as a million miles. As the Sun illuminates this material, it begins to glow brightly. This forms the famous tail of the comet. Comets and their tails can usually be seen from Earth and can be quite bright if conditions are right. Some comets may have as many as three separate tails. One will be composed mainly of hydrogen gas, and is invisible to the eye. Another tail of dust glows bright white, while a third tail of plasma usually will take on a thin, blue glow. As the Earth passes through these dust trails left behind by comets, the dust enters the atmosphere and creates meteor showers. Some comets are in an orbit that brings them near the Sun at regular intervals. These are known as periodic comets. Periodic comets lose much of their material every time they near the Sun. Eventually, after all of this material is lost, they will cease to become active and wander the Solar System as a dark ball of rock and dust. Comet Halley is probably the most famous example of a Periodic Comet. Halley makes its appearance every 76 years.
Visitors from the Abyss
The sudden appearance of these mysterious objects in ancient times was often viewed as a bad omen and a warning of disaster to come. We know that most of the comets are found in a dense layer at the very edge of our solar system. Astronomers call this the Oort cloud. They believe that gravity from the occasional passing of stars or other objects can knock some of the comets out of the Oort cloud and send them on a journey to the inner solar system. Comets can, and have, struck the Earth. In June of 1908, something exploded high in the atmosphere over Tunguska in Siberia. The explosion had the force of 1,000 Hiroshima bombs and leveled trees for hundreds of miles. The lack of any traces of meteorite fragments have led some scientists to believe that this may have been a small comet that exploded on impact with the atmosphere. A comet impact may also have been responsible for the extinction of the dinosaurs, and many astronomers now believe that ancient comet impacts may have brought most of the water to our planet. Although the possibility exists that the Earth could be hit by a large comet again in the future, the odds of this event happening in our lifetime is more than a million to one. For now, comets will simply continue to be objects of wonder and amazement in the night skies.
Famous Comets
