Joan Horne, age 12, of Center; Texas, 'for her question:

Can a comet have more than one tail?

In the last century several large comets broke in pietas. Each fragment went on its way with a tail of its own. But as a rule, a comet has only one tail at a time. It streams out as the comet comes in to make a U‑turn around the sun. In 1914, the earth passed right through the tail of Halley's comet, and no one felt it. The experts think that the showy tail is a sunlit stream of rare gases, perhaps thinner than the almost‑vacuum in a light bulb.

The gases may come from frozen methane, ammonia and water in the comet’s head. Near the sun, some of these ices evaporate. The sums radiation exerts pressure on the evaporating gases. They are pushed in a stream away from the sun. As the comet nears the sun, its tail streams behind. As it leaves the sun, its tail streams ahead. The comet cannot reclaim these and some of its ice must b e lost with every trip around the sun.