A new study estimates that 16 percent of asteroids that roam the region of space shared by Earth's orbit are actually double asteroids, called binaries. And researchers say these pairs may have been created by the rending effect of Earth's gravity, thought to tear asteroids apart when they make close approaches to the planet.

Other evidence shows that impacts on Earth sometimes involve a pair of craters.

In research published today in the online version of the journal Science, astronomers say binary asteroids larger than 219 yards (200 meters) appear to be formed extremely close to Earth, within a few times the planet's radius. Five such systems have been spotted by radar telescopes. Asteroids this close to the planet are dubbed Near Earth Asteroids, or NEAs, and are watched closely by astronomers who fear they may one day hit Earth.

"The fact that one out of every six large NEAs is a binary and that they typically survive on the order of 10 million years, implies that these close encounters must happen frequently compared to the lifetime of the binary asteroids," said Jean-Luc Margot, a Caltech researcher who led the study.

The spin

Most asteroids live the main asteroid belt, between Mars and Jupiter. A few get booted to the inner solar system, often by the gravitational effects of Jupiter.

Because at least some asteroid are expected to be less like solid rock and more like rubble piles, the gravity of Earth or Mars might easily split one asteroid into two, Margot and his colleagues say. They figure the asteroid's rate of spin is increased until it simply flies apart.

Each of the five binaries examined in the new study had passed near Earth or Mars in the past. None had made close approaches to Mercury or Venus.

In one of the newly studied binaries, called 2000 DP107, the larger rock is about 874 yards (800 meters) in diameter and the smaller one is roughly the size of three football fields. They are separated by about 1.9 miles (3 kilometers). They orbit each other and can even create eclipses of the Sun's light.

The other binaries had similar size and distance relationships.

Support for the idea of nearby double asteroids appears to be quite close at hand.

Of about 28 known terrestrial impact craters with diameters greater than 12.4 miles (20 kilometers), at least three are thought to be double craters formed by impacts of objects about the same size as the newly discovered binaries.

"The discovery of the existence and substantial abundance of binary asteroids in Earth-crossing orbits is a major one," says Steve Ostro, a Jet Propulsion Laboratory researcher who also worked on the study. "Presumably, binary asteroids have hit Earth in the past, and will do so in the future."

In separate work, Michael Lucas, a geology student at Florida Gulf Coast University, said some past mass extinctions can be blamed on paired impacts. He cites one example of a 73-million-year-old double impact structure referred to as Kara and Ust-Kara in Russia.

"Approximately ten percent of the impact structures on Earth are doublets or twin structures, suggesting a nearly simultaneous impact of binary asteroids or fragmented comets," Lucas said at a regional meeting of the Geological Society of America April 4.