Risk of asteroids crashing studied

WASHINGTON — For the past 290 million years, large asteroids have been crashing into Earth more than twice as often as they did in the previous 700 million years, according to a new study in Thursday’s journal Science.

But asteroids still only hit Earth on average every million or few million years, even with the increased crash rate. NASA’s list of potential big space rock crashes shows no pending major threats. The biggest known risk is a 4,200-foot-wide asteroid with a 99.988 percent chance of missing Earth when it whizzes very near in 861 years.

Tell that to the dinosaurs. Most scientists think dinosaurs and other species went extinct after a space rock crashed into Central America about 65 million years ago.

“It’s just a game of probabilities,” said study lead author Sara Mazrouei, a University of Toronto planetary scientist. “These events are still rare and far between that I’m not too worried about it.”

Mazrouei and colleagues in the United Kingdom and United States compiled a list of impact craters on Earth and the moon that were larger than 12 miles wide and came up with the dates of them. It takes a space rock half a mile wide to leave holes that big.

The team counted 29 craters that were no older than 290 million years and nine between 291 million years and 650 million years old.

But we can see relatively few big craters on Earth because the planet is more than 70 percent ocean and past glaciers smoothed out some holes, said University of Toronto planetary scientist Rebecca Ghent, a study co-author.

Extrapolating for what can’t be seen brings the total to about 260 space crashes on Earth in the past 290 million years. Adding in other factors, the science team determined that the current space crash rate is 2.6 times more than the previous 700 million years.