Mark Reynolds, age 13, of Lake Charles, La., for his question:
WHEN WILL WE NEXT SEE HALLEY'S COMET?
Astronomers estimate that Halley's Comet will a seen by people on earth in 2062. It appears at fairly regular intervals every 76 to 79 years. An English astronomer named Edmund Halle was the first to suggest that comets belonged to the solar system an took definite paths around the sun at regular intervals. Before his time, people believed that comets appeared by chance and traveled through space in no set path.
Halley discovered that paths taken by certain comets in 1456, 1531 and 1607 were identical with the path of a comet observed in 1682. He decided that the same come made all these paths and he correctly predicted that it would appear again 1759.
From earth, Halley's Comet ca be seen only as it nears the sun. It was last near the sun on Apr' 24, 1910, approaching as close as 55 million miles.
Scientists in Egypt actually photographed Halley's Comet in August, 1909. It was then about 300 million miles from the sun, but the astronomers knew it was on its way toward the sun and earth.
As the comet approach d, it became brighter. In early May, 1910, the comet's head was as brilliant as the brightest stars. Its great tail stretched about two thirds the distance from the horizon to directly overhead.
Halley's Comet came as close to the earth as 14 million miles on may 20, 1910. On the next day, the earth is believed to have passed through the comet's tail. It was last observed with a camera on July 1, 1911.
The tail of a comet always points away from the sun. Also, most comets travel around the sun in elliptical or oval shaped paths.
At one time scientists believed that the nucleus of a comet consisted of tiny solid particles held together loosely by gravitation. Now they think it is made of frozen water and gas.
Scientists tell us that the nucleus of a comet 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. The gases include ammonia, carbon dioxide, carbon monoxide and methane.
Gases and water probably make up 70 to 80 percent of the mass of a comet. The remaining 20 to 30 percent may resemble meteoric particles.
As the comet approaches the sun, the heat causes the outer layers of the ice 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 pressure makes a comet's tail point away from the sun. When a convent 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.