Mark Driscol, age 12, of Indianapolis, Indiana, for his question:
Do comets ever burn out?
About 2,000 years ago, at this time of year, the famous Halley's comet appeared overhead in the Holy Land. Modern astronomers suspect that it was the star that led the Wise Men of Bethlehem for that first Christmas. Brilliant comets are rare events and for ages people regarded them as mysterious signs and omens. In the 1600's, astronomers began charting their orbits. Gradually the great comets lost their awesome mystery.
The head of a comet may be a mile to ten miles wide, and perhaps as light as buoyant balsa wood. Astronomers suspect it to be a fragile wad of meteoric dust embedded in ices. Its dust may include fragments of iron and nickel, magnesium and calcium, sodium and silicon. Its ices may include frozen water and carbon dioxide, methane and ammonia. Out in the lonely reaches of space, a cold dark comet might exist indefinitely. It the past few years, astronomers discovered an enormous halo of perhaps ten billion comets, orbiting far outside the farthest planets.
Most likely these outer comets have been there since the birth of the Solar System. Barring accidents, they may continue crawling around their enormous orbits indefinitely. But accidents happen, even in the lonely realms of space. A close brush with Pluto or some other heavenly body may pull an outer comet into a different orbit. It may be swerved toward the center of the Solar System.
Astronomers suspect that our major comets may have been captured from orbits beyond Pluto. Their new orbits loop close around the sun and swing back around*one of the outer planets. These paths are very risky and doomed to disaster. Some may collide with planets and come blazing down through the atmosphere. But most comets do not end in flames. They disintegrate when their ices evaporate and release their solid fragments through space.
The greatest risk is the daring U turn. As the comet approaches, its icy crust evaporates and carries off solid fragments. The sun illumines its fuzzy, swollen head and pushes back the lost material in a bright, gauzy tail. More material is lost forever as the comet accelerates around its fast U turn. Astronomers estimate that it loses 1/200th part of its mass each time it visits the sun.
Perhaps a crust of meteoric dust forms around the comet's head and protects its frozen core for a while. But eventually, all of its material disintegrates. After perhaps 200 visits to the sun, the last ices evaporate and strew the last solid fragments through space.
A comet makes its U turn at terrific speed and sometimes the sun's tidal friction tears it apart. In 1882, a major comet broke into five pieces as it swung around the sun. In 1889, another one broke in half. These broken pieces stay in their old orbits as smaller comets. After a few more visits to the sun, they too disintegrate to dust and gases.