GEOLOGIST DISCOVERS KEY TO RIDDLE OF METEORITES' ORIGIN.

Byline: Randy Bradbury Knight-Ridder Tribune News Wire

Somewhere out between Mars and Jupiter, there's an asteroid with Earth's name on it.

Until recently, the writing has been hard to decipher.

Mike Gaffey found the key, and in so doing appears to have found the source for the great majority of meteorites that turn up on Earth, including possibly the one that some scientists believe contributed to the extinction of the dinosaurs.

Gaffey is a geologist at Rensselaer Polytechnic Institute in New York who specializes in the geology of asteroids and meteorites.

He said most meteorites that hit Earth have a similar composition, and scientists have long thought that they originated in the asteroid belt that circles the sun between Mars and Jupiter.

But there's been a problem. Using devices that analyze the light waves reflected back to Earth, astronomers can determine the chemical composition of distant bodies in space. None of the asteroids matched the composition of meteorites.

Using new analyses, Gaffey has determined that one particular asteroid probably has a thin coating of metal, caused by millions of years of melt craters that result from bombardment by other space debris. That coating has reflected the wrong compositional signal back to Earth, but the interior of the asteroid probably is made of the right stuff.

That asteroid is a good candidate, Gaffey said, because it is located between two ``escape hatches'' - areas of the asteroid belt orbit where debris can escape that orbit with help from the gravity effects of Jupiter and Saturn.

Here's what happens, Gaffey says: A chunk of space rock collides with Earth's favorite asteroid, knocking off other pieces of rock, some of which have enough speed to fall into one of the escape hatches and fly free of the asteroid belt.

Because of this asteroid's location in space, those pieces of rock get sent into an eccentric orbit of the sun that puts them on a collision course with Earth. Many of them burn up in the atmosphere before they hit, but perhaps a few thousand a year actually land on Earth.

Gaffey hasn't done any research to confirm it, but he's been told by other scientists that the huge meteorite that hit Earth about 65 million years ago, possibly helping to usher dinosaurs out the evolutionary door, is made of the same material.

At least a few scientists have speculated that other periodic mass extinctions also were caused or aided by massive meteorite strikes. So it's at least possible that Gaffey's cold chunk of sand and iron floating out past Mars has had a profound impact (no pun intended) on the history of the Earth.