Dinosaur-killing asteroid may have been a double menace!

The space rock that slammed into Earth and wiped it clean of dinosaurs, around 65.5 million years ago, may have been a binary - two asteroids orbiting each other, according to a new study.

The dino-killing asteroid is usually thought of as a single rock with a diameter of 7 to 10 kilometres, but it may really have been two widely separated rocks with that combined diameter, researchers said.

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 about 15 per cent of near-Earth asteroids are binary," said Katarina Miljkovic at the Institute of Earth Physics in Paris, France.

All else being equal, 15 per cent of Earth's impact 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 that 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 per cent of all Earth's craters.

An obvious implication is that binary asteroids hit Earth more often than the crater record appears to suggest - with ramifications for efforts to prevent future impacts.

The simulations also suggested that it is possible to identify which of Earth's single craters had binary origins. These craters should be subtly asymmetrical, and that makes the crater near Chicxulub in Mexico - thought to be the result of an asteroid impact 65.5 million years ago that wiped out the dinosaurs - a strong candidate.

"The Chicxulub crater shows some important asymmetries. It is worth considering that it was formed by a binary asteroid," said Miljkovic.