Steven Clark, age 13, of Chatham, New Brunswick, Canada, for his question:

Do comets ever burn out?

Countless comets have been observed and reported through the ages. But there is no evidence that one has ever burned out. However, this may be remotely possible. It could happen if a comet crashed into the sun    which is not likely. It could happen if a comet struck a planet and crashed down through its dense atmosphere. This is even less likely.

During its brief period of golden glory, the head of a major comet outshines the brightest star and its glowing tail spreads like a searchlight over the sky. It may look somewhat like a blazing fire and no doubt its surface, at least, is quite hot. But actually its golden glow is merely reflected glory from the blazing sun.

However, though a comet is not likely to burn up, it is quite prone to other accidents. The Great Comet of 1882 passed right through the sun's outer corona    and shattered asunder. Its nucleus broke into four or five pieces which are expected to return as separate comets in a few centuries. Four major comets of the 1$OOs are thought to be parts of a stupendous comet that shattered, perhaps after its visit to the sun in 166$.

Sometimes the tail of a comet splits apart or its nucleus shows signs of disruption. These upsets may be caused by unstable conditions in the nucleus or by a near collision with some object out in the Solar System    perhaps a swarm of tiny meteors. Several small comets have burst into mysterious flares when far from the sun.

Certainly a comet can have a serious accident, but fire is not a major hazard. Troubles occur because its head is bulky and fragile. It is thought to be an assortment of stones, cemented together in frozen gases. Most of the time it chugs around a huge orbit, where the frigid temperatures of interplanetary space keep the mass frozen together.

Naturally it warms up as it nears the sun. At present, astronomers suspect that the sun causes a comet's surface ice to sublimate, or pass directly from the frozen to the gaseous state. This, plus intense solar pressure, is likely to loosen dusty particles of the comet's stony solid materials. Its fantastic speed around the sun saves it from a fiery end.

Eventually a large comet may dwindle to dusty fragments that sometimes create meteor showers in our skies. It is estimated that Halley's Comet loses about three meters of surface material each time it swings around the sun.