Leah Wilbourn, age 13, of Huntsville, Ala., for her question:
WHAT IS A COMET?
An elliptical or oval shaped orbit is followed by most comets as they travel around the sun. The time it takes for a comet to make a complete swing is called its period.
Some comets have short periods of less than seven years while others follow huge paths that pass near the sun only once in a thousand or even a million years.
A comet is an object that resembles a fuzzy star. It travels along a definite path through the solar system, first heading toward the sun and then swinging away from it.
All comets seen by astronomers are considered part of the solar system. No comet appears to have approached the sun from beyond the limits of our solar system.
Some comets develop long, shining tails when they come near the sun. The tail of a comet may stream across space as far as 100 million miles.
A comet has a nucleus or distinct center that measures less than 10 miles in diameter. Surrounding the nucleus is a hazy cloud called a coma. The diameter of the coma and nucleus, called the comet's head, can be 1 million miles in diameter.
The nucleus of a comet, scientists believe, is like a dirty snowball that was formed in the cold, distant regions of the solar system. The nucleus is most likely made up of frozen gases and frozen water mixed with dust particles. The gases include ammonia, carbon dioxide, carbon monoxide and methane. From 70 to 80 percent of the comet's mass is made up of gases and water. The remaining 20 to 30 percent may resemble meteoric particles.
Most comets can't be seen without a telescope. Some are visible to the naked eye but only for several weeks or months when they pass closest to the sun.
As a comet moves toward the sun, the heat causes the outer layers of the icy nucleus to evaporate. The evaporation releases dust and gases, which form the coma.
The pressure of the sun's light may push the smallest dust particles and gas molecules away from the coma, forming one or more tails. This pressure makes a comet's tail point away from the sun. When a comet approaches the sun, its tail brings up the rear but as it moves away, the tail leads.
All of a comet's light comes from the sun. The brightness results partly from sunlight reflected by its nucleus and coma. In addition, when a comet is closest to the sun, gas molecules in the coma release energy absorbed from the sun's rays.
Perhaps the most famous comet is Halley's Comet. It was last seen from earth in 1910 and it appeared in our heavens in about 1986.
Halley's Comet was named for the English astronomer Edmund Halley. Its period of orbit can vary between 76 and 79 years.