ASTEROIDS are smacking Earth twice as often as before. Giant rocks are falling from the sky more than they use to, but don't worry.

For the past 200 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 no need to cast a weary glance up. Asteroids still only Earth on the 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 [1.3 km] wide asteroid with a 99,988 percent chance that it will miss Earth when it whizzes very near here in 861 years.

Tell that to the dinosaurs. Most scientists think that dinosaurs and a lot of other species went extinct after a huge 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. ''Those 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 [20 km]  wide and came up with the dates of them. It takes a space rock that's half a mile [800 meters] wide to create 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. [Agencies]

The honor and serving of the latest Global Operational Research on Asteroids and Earth, continues.


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