The impacts that made the Popigai crater in Siberia and the Chesapeake Bay crater on the US East Coast were two of the largest asteroid strikes since the one that killed the dinosaurs, so big that their dust left thin layers of debris all over the globe.
The depth of debris layers shows that the asteroids struck in quick succession - within 20,000 years of each other, around 36 million years ago. A Popagai-sized impact happens just once every 20 million years. So the thinking was that the two rocks were fragments of one larger asteroid.
But when Birger Schmitz of Lund University in Sweden examined sediment from a site in central Italy containing debris from both impacts, he found two distinct types of meteoritic grains. Those from the part of the deposit associated with Popigai were rich in iron; those from the part associated with Chesapeake Bay were iron-poor. So the pair can't be explained by a single body breakup - they must be from two separate asteroids (Earth and Planetary Science Letters, doi.org/5cb).
But how could that happen? One explanation is that chaos in the inner solar system disturbed asteroids' orbits, sending several Earthwards.
It's even possible that a change in Earth's own orbit sent the asteroids on their paths. Such a shift might explain why Earth entered an ice age around 35 million years ago: a new orbit meant less energy from the sun.
"The analysis of the grains is very good and convincing," says David Nesvorny of the Southwest Research Institute, San Antonio, Texas. But some meteorites are simply made of many kinds of rock, he says.
This article appeared in print under the headline "Dual strike hints at chaos in the asteroid belt"
Comment: Russia's Popigai meteor crash linked to mass extinction