About four football fields across, the asteroid YU55 might not be big enough to eliminate civilization, but if it struck Earth it could leave a crater six miles wide, which could turn in to a bubbling cauldron and spew enough dust to cool temperatures for years to come. Astronomers, who've been tracking it for six years, say that won't happen. But tonight, YU55 will pass closer to Earth than the Moon. In any case, its close call will provide a rare opportunity for scientists who study the origin of the Earth. Jon Giorgini is a senior analyst at the Jet Propulsion Laboratory's Solar System Dynamics Group.