Millions of people have watched Echo, the U.S. balloon satellite, as it crosses the sky. And most of them have noticed that it twinkles like a star and also brightens and dims slowly in a way that no star does. Why does it perform in this odd fashion? Last week the explanation came from Dr. Leonard Jaffe of the National Aeronautics and Space Administration. Echo's quick twinkling, said Jaffe, is caused by the same atmospheric irregularities that make stars wink. Some of its slower dimming may be due to thin patches of clouds, invisible...
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