Bill Turner Jr., age 12, of Rome, N.Y., for his question:
WHAT ARE COMETS MADE OF?
Comets are objects that resemble fuzzy stars. They travel through the solar system along definite paths. Scientists believe the nucleus of a comet is made of tiny solid particles held together loosely by gravitation.
Most astronomers agree that the nucleus resembles a dirty snowball that was formed in the cold, distant regions of the solar system. The nucleus probably is made of frozen gases and frozen water, mixed with dust particles.
A comet has a distinct center called a nucleus that measures less than 10 miles in diameter. A hazy cloud called a coma surrounds the nucleus. Its diameter may be as large as 1 million miles. The coma and nucleus make up the comet's head.
The gases in a comet's nucleus most likely include ammonia, carbon dioxide, carbon monoxide and methane. The gases and water make up from 70 to 80 percent of the mass of a comet. The remaining 20 to 30 percent may resemble meteoric particles made up of fine grains and larger chunks of rocky material.
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 around the nucleus. 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.
The heat of the sun's light makes a comet's tail point away from the sun. When a comet approaches the sun, its tail brings up the rear. But when the comet moves away from the sun, its tail leads.
All of a comet's light comes from the sun. The brightness of a comet 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 ray.
Most comets travel around the sun in oval shaped or elliptical paths. The time it takes a comet to make a complete orbit is called its period. Some comets have short periods of less than seven years while others travel in such huge orbits that they pass near the sun only once in a thousand or even once in a million years.
No comet seems to have approached the sun from beyond the limits of the solar system. Therefore, all comets seen by astronomers are considered part of the solar system.
The tail of a comet may stream across space as far as 100 million miles.
Perhaps the best known comet of all is Halley's comet. It was last seen in 1910 and is expected to appear again about 1986. It crosses the orbits of the earth and other planets near the sun every 76 to 79 years. It was first seen before 86 B.C.
Comet Kohoutek, seen here on earth by many people in 1973, will not come back this way for another 75,000 years. It has a large orbit.