Next week, the Earth's residents can once again play the popular game of "What if?" What if a hunk of cosmic rock is out there on a collision course with Earth? At the moment, an asteroid labeled "(433) Eros" is rushing toward our planet on a course that will bring it relatively close, at least on a cosmic scale. On the one hand, the chunk of rock—measuring 30 by 13 by 13 kilometers (19 by 8 by 8 miles)—is approaching Earth closer than any asteroid of this size has for a long time. But on the other, it is currently...