Giant space rocks are falling from the sky more than they used to, but do not worry yet.
In the last 290 million years, large asteroids have fallen to Earth more than twice as much as 700 million years ago, according to a new study published on Thursday. Science.
But there is no need to cast a cautious look up. Large asteroids still only reach the Earth on average every million or a few million years, even with the increased rate of accidents. NASA's list of major space rock collapses shows no major pending threats. The biggest known risk is a 1.3 km wide asteroid, with a 99.988% chance of losing Earth when it is very near here in 861 years.
Tell that to the dinosaurs, though. Most scientists think dinosaurs and many other species have been extinct after a huge space rock fell in Central America about 65 million years ago.
"It's just a game of odds," said lead author of the study, Sara Mazrouei, a planetary scientist at the University of Toronto. "These events are still rare and far between, and I'm not too worried about that."
Mazrouei and his colleagues in the United Kingdom and the United States compiled a list of impact craters on Earth and Moon over 20 km wide and found their dates. It takes a space rock 800 meters wide to create holes of this size.
The team had 29 craters that were no more than 290 million years old and nine between 291 million years and 650 million years ago.
But we can see relatively few large craters on Earth because the planet has more than 70 percent of the ocean and the glaciers of the past softened a few holes, said University of Toronto planetary scientist Rebecca Ghent, co-author of the study.
Extrapolating to what can not be seen raises the total to about 260 space accidents on Earth in the last 290 million years. Adding to other factors, the team of scientists determined that the current rate of collision space is 2.6 times higher than in the previous 700 million years.
Craters older than 650 million years are most often removed from Earth by glacial forces, so scientists used impact craters on the nearby moon as a substitute for holes between 650 million and 1 billion years old. The moon is a good guide for estimating Earth's fall because it is close enough to be on the same bombing path and its craters last longer.
So, what happened almost 300 million years ago?
"Maybe a family of asteroids has been broken in the asteroid belt," speculated Dr. Mazrouei. Space rocks then headed for Earth and the Moon, and the planet got a little bit longer because it is a bigger target and has more gravity, according to Dr. Ghent.
Scientists are divided on the research. Purdue's Jay Melosh said he found the crater number too small to come to a reasonable conclusion, but Avi Loeb of Harvard said the case was convincing.
Humans may not have emerged without mass extinctions of space rocks some 250 million and 65 million years ago, Professor Loeb said in an e-mail.
"But this increased impact rate poses a threat to the next mass extinction event, which we should observe and try to avoid with the help of technology," he said.
"It shows how arbitrary and fragile human life is."