Asteroids collide with Earth twice as many times as before

WASHINGTON- There are more asteroids colliding with the Earth than before, but this is not an alarm signal.

In the last 290 million years, large asteroids have collided with Earth at a rate more than double that 700 million years ago, according to a new study released Thursday by the journal Science.

But it is not like looking at the sky with fear. On average, asteroids collide with the Earth every million or every few million years, even when the increase in the collision rate is included. NASA's list of large steroids that could collide shows that there are no major threats to the eye. The largest known risk is an asteroid 1.3 kilometers (4,200 feet) long with 99.98% chance that it will not touch Earth when it passes through here in 861 years.

Tell that to the dinosaurs. Most scientists believe that dinosaurs and many other species became extinct after a giant asteroid fell in Central America about 65 million years ago.

"It's a game of probabilities," said study leader Sara Mazrouei, a planetary scientist at the University of Toronto. "These events are still rare and do not happen continuously."

Mazrouei and some of his colleagues from the United Kingdom and the United States made a list of craters derived from falling asteroids on Earth and the Moon that were more than 20 kilometers (12 miles) in diameter and found out their formation date. Holes of this size are formed by asteroids 800 meters (half a mile) wide.

The team counted 29 craters that were less than 290 million years old and nine with between 291 and 650 million years old.

But we can see relatively fewer large craters on Earth because the oceans make up more than 70% of the planet and the glaciers of yesteryear flattened some holes, said planetary scientist Rebecca Ghent of the University of Toronto, co-author of the study.

By extrapolating what can not be seen, there are a total of about 260 collisions with asteroids in the last 290 million years. Adding other factors, the scientific team determined that the current rate of asteroid falls is 2.6 times higher than in the previous 700 million years.

Craters more than 650 million years old have virtually disappeared due to glacial forces, so scientists used the number of craters formed by asteroids on the Moon as a substitute for craters formed between 650 million and 1,000 million years ago. AP