Sara Snyder, age 11, of Algona, Iowa, for tier question:
What makes a comet's tail turn away from the sun?
It's time to start brushing up on our comets. And the more we know about comets, the better tae will he able to appreciate it. Visits from major comets are very rare and this one is not on the list of scheduled appearances. Astronomers spotted it way outside the Solar System. If their figuring is accurate, and if nothing pulls it off course, it will arrive just in time to hang a stupendous Christmas decoration in the sky.
A bid comet outshines the brightest star and may be seen in the sky when the brilliant sun is at high noon. But without a doubt, its most fabulous feature is a long golden tail, which may spread across half the sky. One would think that this spectacular feature must be made of something pretty substantial. But astronomers tell us that a comet's tail is no more than a stream of infinitesimal particles.
The nucleus of a comet is thought to be a bulky ball of chunky stones held together in an icy mixture of frozen gases. For most of its life, this is all there is. The dark, frozen nucleus spends decades or even centuries dawdling around an orbit that may take it to the edge of the Solar System or even beyond. But dramatic changes occur as the far voyaging comet approaches the end of its orbit which loops around the sun.
By the time it reaches the orbit of Jupiter, it begins to feel the surging forces of the sun. For one thing, its speed accelerates. Sunshine brightens up its dull cold face. As it speeds nearer to the U turn, it feels growing pressure from radiation pouring from the starry sun. Some of its icy material begins to melt and a first breath of solar wind promises mighty pressures to come.
A big comet becomes visible in our skies when or before it crosses the orbit of "tars. Material evaporated from its nucleus surrounds it with a huge glowing halo. This golden ball is the comet's head, reflecting the brilliant glory of the sun.
By this time, the comet is traveling very, very fast. As its icing evaporates, dusty fragments of solid material are left behind along its path. These form the trailing tail and solar radiation pushes it behind the approaching comet. The sunlit tail fans out and may be longer than the distance between us and the sun.
Solar radiation continues its outward pressure as the comet makes its dramatic U turn. Around the curve, the tail swings in a half circle, always pointing away from the sun. As the comet leaves, the tailstreams on ahead.
Astronomers boggle at the speed with which the tail reverses itself as the comet swings around the sun. This is hard to explain and some of the details are not understood. Certainly the pressure of solar radiation can push the filmy tail away from the sun. But some astronomers suspect that other forces may lend a helping hand. Perhaps the future comet will reveal these subtle secrets.