Mapping these clouds and determining their orbits is important to NASA for obvious reasons: the more probes we send to Mars and elsewhere, the more likely they are to encounter uncharted clouds. No one wants their spaceship to be surprised by a meteor shower hundreds of millions of miles from Earth. Much of Cooke's work at NASA involves computer-modeling of cometary debris streams--long rivers of dust shed by comets as they orbit the sun. He studies how clumps form within the streams and how they are deflected by the gravity of planets (especially giant Jupiter). He and his colleagues also watch the sky for meteor outbursts here on Earth. 'It's a good way to test our models and discover new streams,' he says. One such outburst happened on June 27, 1998. Sky watchers were surprised when hundreds of meteors streamed out of the constellation Bootes over a few-hour period. Earth had encountered a dust cloud much as Mariner 4 had done years earlier.