Behold the Weird Physics of Double-Impact Asteroids

When an asteroid strikes a planet, it can pack a powerful punch—as the dinosaurs discovered to their detriment 66 million years ago. But what if two asteroids strike at the same time and in the same location?

A first-of-its-kind study published in the journal Icarus investigates this phenomenon on Mars. Looking at the planet, researchers have discovered hundreds of craters that likely resulted from the impacts of a binary system, where one asteroid orbits another, like the moon orbits Earth.

“They’re really difficult to find,” says Dmitrii Vavilov at the Côte d’Azur University in France, the study’s lead author. But the findings show these binary craters are there, he says.

The first discovery of a binary asteroid was made by NASA’s Galileo spacecraft as it journeyed to Jupiter in 1993. While capturing images of an asteroid called Ida on the way, mission scientists were shocked to find a second asteroid orbiting nearby. “They were so confused,” says Harrison Agrusa, an astronomer at the University of Maryland who was not involved in this new study. “People were debating if something was wrong with the camera.”

It wasn’t. Instead, Ida was the first confirmation that asteroids could orbit in pairs, and in some cases even more. Ida’s companion, later named Dactyl, was incredibly small, yet proof of their existence. “It set off a big shock wave in the community,” says Agrusa.

Based on observations of the other millions of asteroids in the solar system, scientists today estimate that about 1 in every 6 asteroids—around 16 percent—is part of a binary system. We can see these orbiting around the solar system, particularly in the asteroid belt between Mars and Jupiter, with one of the most famous pairs—Didymos and its small companion, Dimorphos—the target of a NASA and European Space Agency (ESA) asteroid defense mission later this year.

Asteroids regularly strike planets and moons, so it would be expected that binary asteroids would too. Finding binary craters can be difficult though, especially among the myriad other craters on places like our moon. On Earth it is harder still, as geological processes quickly erase evidence of impacts.

The best candidate for a binary crater on Earth today is the Lockne crater in Sweden and a smaller crater nearby called Målingen. “We dated these structures very exactly and saw that they formed at exactly the same age,” about 450 million years ago, says Jens Ormö from the Astrobiology Center in Spain, who led analysis of the craters published in 2014. One other promising candidate pair is known, the Kamensk and Gusev craters, but their location—on the border between Russia and Ukraine—makes them difficult to study in the current global climate.

On Mars, craters can remain visible for billions of years. So using high-resolution images of the surface taken by Mars orbiters, Vavilov and his colleagues examined nearly 32,000 craters larger than 4 kilometers across to hunt for crater pairs.

Their results showed that 150 pairs appeared to be the result of binary impacts, totaling 300 individual craters. These estimates come from looking for pairs of crater shapes that would be expected following a binary asteroid collision. These include tear-drop craters, where the two craters overlap; peanut craters, where they are connected at their edges; and doublet craters, where there is a gap between the two. The orientation of the two craters depends on the position of the two asteroids at the time of impact.

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