When Earth is next hit by an asteroid, the impact may well be doubled, since a new study estimates that 16 percent of asteroids in the region of space shared by Earth’s orbit are actually double asteroids, called binaries. Evidence shows that impacts on Earth sometimes involve a pair of craters. Researchers say these pairs may have been created by the effect of Earth’s gravity, which tears asteroids apart when they come too close.

Astronomers say binary asteroids larger than 219 yards appear to be formed extremely close to Earth. Five such systems have been spotted by radar telescopes. Asteroids this close to the planet are called 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,” says Jean-Luc Margot, a Caltech researcher who led the asteroid study.

The first asteroid-moon pair was spotted in 1993, by the Galileo spacecraft. Other pairs have been seen since, but this new study is the first to strongly link asteroid pairs with the potential for terrestrial impacts.

Most asteroids live the main asteroid belt between Mars and Jupiter. Because at least some asteroids are less like solid rock and more like piles of rubble, the gravity of Earth or Mars can easily split one asteroid into two, Margot and his colleagues say. Each of the five binaries examined in the new study had passed near Earth or Mars in the past.

One of the newly studied binaries has a large rock that?s about 874 yards in diameter and the smaller one that?s the size of three football fields. They are separated by about 1.9 miles and orbit each other.

Of about 28 known terrestrial impact craters with diameters greater than 12.4 miles, 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.”

Michael Lucas, a geology student at Florida Gulf Coast University, says some past mass extinctions can be blamed on paired impacts. One example is a 73-million-year-old double impact crater in Russia. He says, “Approximately 10% of the impact structures on Earth are doublets or twin structures, suggesting a nearly simultaneous impact of binary asteroids or fragmented comets.”

For a new look at what?s out in space, read ?Dark Matter, Missing Planets and New Comets? by Tom Van Flandern, click here.

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