dipper@utastro.UUCP (Debbie Byrd) (10/21/85)
It's difficult to predict just how bright Comet Halley is going to get. More -- after this. October 21 Comet Halley When Comet Halley came out from behind the glare of the sun this past summer, it was visible only though telescopes. Halley had spent the summer hidden from our view -- traveling on the far side of the sun from the Earth. But all the while it was getting closer to the sun -- and growing brighter. When the comet was recovered, it was about as bright as had astronomers had expected it to be -- based on past experiences with comets. But comets are unpredictable. A comet may or may not get as bright as predicted. Also, comets can vary rapidly in brightness -- sometimes changing over a period of just a few hours. These changes depend in part on its rotation -- and partly on the coma -- the giant cloud of gases and dust surrounding the comet's nucleus or core. The coma grows steadily larger as a comet gets closer to the sun. It's created from the material coming off the nucleus. But these materials aren't evenly distributed -- and different parts of the surface of the nucleus may be more active than others. So as the comet comes in toward the sun, its brightness can vary. Also, sometimes a comet will produce a larger or smaller coma than expected -- so the comet can be brighter or dimmer than we thought it would be. When it came out from behind the sun this summer, Comet Halley was about as bright as anticipated. It is predicted to become visible to observers with binoculars sometime next month. Script by Diana Hadley and Deborah Byrd. (c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin