the Infrared Astronomical Satellite. "Telltale streaks in the images," Cowen writes, "revealed the presence of giant, never-before- seen trails of dust particles associated with three comets that visit the inner solar system every three to seven years." The trails' pebble- sized debris was larger than the extremely tiny particles in the dust tails visible when comets move near the sun. That same year, the Euro- pean Space Agency's Giotto spacecraft flew within 605 kilometers...