Before they crash to earth these stony or metallic objects are exposed to cosmic rays. Helium produced by this bombardment provides a new clue to the planetary catastrophes of the past
S. FRED SINGER is director of the Science and Environmental Policy Project in Fairfax, Va., and professor at George Mason University. A pioneer in the use of rockets (captured German V-2s) to investigate the upper atmosphere and near-Earth space, he was the first director of the National Weather Satellite Center. He devised the cosmic-ray method of dating meteorites and was among the first to study the origin and evolution of the Martian moons.