Mark your calendar: On Jan. 29, 2008, one night before a Tunguska-class (50m-wide) asteroid threatens to strike Mars, an even larger asteroid will fly past our own planet.

At closest approach, 2007 TU24 will be 1.4 lunar distances from Earth. There's no danger of a collision, but the 400m-wide space rock will be close enough to photograph through backyard telescopes as it speeds through the constellation Cassiopeia glowing like a 10th magnitude star.