The conclusion comes from a re-evaluation of the proportion of asteroid craters on Earth that were formed from binary impacts. It could also spell bad news for those hoping to protect our world from catastrophic collisions in future, New Scientist reported.
Earth bears the scars of twin-asteroid impacts: the Clearwater Lakes near Hudson Bay in Canada, for instance, are really twin craters that formed about 290 million years ago.
However, examples like Clearwater are rare. Just one in 50 of craters on Earth come in such pairs.
That is a puzzle because counts of the rocks zooming around in the vicinity of Earth suggest binaries are far more common.
"It's been known for 15 years that 15% of near-Earth asteroids are binary," said Katarina Miljkovic at the Institute of Earth Physics in Paris. All else being equal, 15% of Earth's craters should be the result of twin impacts.
Miljkovic and her colleagues found an explanation as to why the real figure appears so much lower. They ran computer simulations of binary asteroids hitting Earth and found they often form a single crater.
The team found that only unusual cases involving two small, widely separated asteroids are guaranteed to form a pair of distinct craters. The researchers' simulations confirmed that such binary asteroids are rare enough to explain why paired craters account for only 2% of all Earth craters.