Meteorites quickly reach Earth.

Fragments from collisions between large bodies in the asteroid belt, which lies between the orbits of Mars and Jupiter, can reach Earth in as little as 100,000 years, chemical analyses of such material suggest.

Excavations at a quarry in southern Sweden have yielded more than 50 meteorites. Those stones fell into a shallow sea about 480 million years ago, when the carbonate-rich sediments that eventually became limestone were accumulating, says Rainer Wieler, an astrochemist at the Swiss Federal Institute of Technology. in Zurich. Chemical analyses of chromite mineral grains from nine of the meteorites, found at various levels in the quarry, mark the stones as pieces of a single extraterrestrial body, says Wieler.

The chromite grains' concentrations of neon-21, an isotope produced when cosmic rays strike minerals, provides a measure of how long the meteorites traveled through space after the collision of asteroids that most likely produced them. Space rocks found highest in the limestone strata fell to Earth about 1.2 million years after the purported collision. The lowest-lying meteorites in the quarry reached our planet earlier, between 100,000 and 200,000 years after the impact. Some computer models suggest that such short transit times are possible, but until now, scientists haven't found evidence to bolster those analyses, say Wieler and his colleagues in the July 15 Nature.--S.P.