According to a survey of asteroid craters at least 6.2 miles wide, the number of asteroids slamming into Earth has nearly tripled since the dinosaurs first roamed. "Researchers worked out the rate of asteroid strikes on the moon and the Earth and found that in the past 290 million years the number of collisions had increased dramatically," reports The Guardian. "Before that time, the planet suffered an asteroid strike about once every 3 million years, but since then the rate has risen to once nearly every 1 million years." From the report: The findings suggest that the dinosaurs may have been unfortunate in evolving 240 million years ago, just as the odds of being wiped out by a stray asteroid were ramping up. It was one of those impacts, on top of other factors, that did for the beasts 66 million years ago. Many scientists had assumed that asteroid strikes were a rare but constant threat in Earth's deep history, but the latest study challenges that belief.

Writing in the journal Science, the researchers describe how they turned to the moon to examine the violent history of Earth. The Earth and moon are hit by asteroids with similar frequency, but impact craters on Earth are often erased or obscured by erosion and the shifting continents which churn up the crust. On the geologically inactive moon, impact craters are preserved almost indefinitely, making them easier to examine. Using images from Nasa's Lunar Reconnaissance Orbiter, the scientists studied the "rockiness" of the debris surrounding craters on the moon. Rocks thrown up by asteroid impacts are steadily ground down by the constant rain of micrometeorites that pours down on the moon. This means the state of the rocks around a crater can be used to date it. The dates revealed that the moon, and by extension the Earth, has suffered more intense asteroid bombardment in the past 290 million years than at any time in the previous billion. On Earth there are hardly any impact craters older than 650 million years, most likely because they were eroded when the planet became encased in ice in an event known as Snowball Earth.