The huge stones in space are falling out of the sky more than they were, but do not worry yet.
In the last 290 million years, large asteroids collapsed in Earth twice as often as in the previous 700 million years, according to a new study in the Thursday journal Science.
But you do not have to take a cautious look up. Large asteroids still tear the Earth, on average, every million or a few million years, even with the increase in the damage rate. NASA's list of potential large-area weather accidents shows that there are no major threats waiting. The biggest known risk is a 1.3 km wide asteroid with 99.988% chance of losing Earth when it will fly very close here in 861 years.
Tell that dinosaurs, though. Most scientists believe that dinosaurs and many other species have disappeared after a large space rock collapsed in Central America 65 million years ago.
"It's just a game of probabilities," said lead author Sara Mazrouei, a planetary scientist at the University of Toronto. "These events are still rare and not too worried."
Mr. Mazrouei and colleagues from the United Kingdom and the US compiled a list of impact craters on the Earth and beyond 20 km and came up with their data. It takes a 800-meter-long rock to create large holes.
The team counted 29 craters that did not have more than 290 million years old and nine of them between 291 million years and 650 million years old.
But we see relatively few large craters on Earth, because the planet is more than 70% of the ocean and the past glaciers have filled some holes, "said Rebecca Ghent, co-author of the study at Toronto University.
By extrapolating for what can not be seen, it brings the total to about 260 space-related casualties on Earth over the last 290 million years. Adding other factors, the scientists have determined that the current accident rate is 2.6 times higher than in the previous 700 million years.
Older 650 million years old craters are largely wiped out of the Earth by the glacial forces, so scientists have used impact craters for the nearest month as a stand-in for holes between 650 million and 1 billion years old. The moon is a good guide to Earth's earthly estimation, because it's close enough to be in the same bombing path, and its craters take longer.
So, what happened about 300 million years ago?
"Perhaps an asteroid family was broken in the asteroid belt," Dr. Mazrouei speculated. The space stones headed toward the Earth and the moon, and the planet reached a little more, because it is a higher target and has a higher gravity, according to Dr. Ghent.
Scientists are divided on research. Purdy's Jay Melosh said he found the number of craters too small to reach a reasonable conclusion, but Avi Loeb of Harvard said the case was convincing.
People may not have gone without massive disappearances from the space rocks with about 250 million and 65 million years ago, Professor Loeb said in an e-mail.
"But this increased impact rate is a threat to the next mass disappearance event that we should follow and try to avoid using technology," he said.
"This demonstrates the arbitrary and fragile human life."