The Asteroid Patrol

ONCE A MONTH, AS THE MOON WANES, GEOLOgist Eugene Shoemaker, 64, and his wife Carolyn, 63, leave their house in Flagstaff, Arizona, load warm clothes into their station wagon and set off to the west on an 800-km (500-mile) trip across the desert. Their destination: Palomar Mountain, site of the mighty Hale telescope, among others. There, using a smaller Schmidt telescope, they begin a seven-night stint of sentry duty.

The intruders they watch for are Earth-crossing asteroids, giant rocks that have strayed from their neighborhood between the orbits of Mars and Jupiter and periodically pass close to Earth -- and sometimes...