VOA慢速英語：巨大太空巖石撞擊地球的頻率是以前的兩倍

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Huge rocks from outer space, or asteroids, are falling from the sky more than they have in the past. But there is no need to worry.

A new study has shown that for the past 290 million years, large asteroids have been crashing into Earth more than twice as often as they did in the 700 million years before that. The journal Science published the study in January.

But, even with the increased crash rate, asteroids still only hit Earth every million or few million years.

The United States space agency NASA lists its observation of large space rocks that could crash into Earth. It shows no major threats coming soon.

The biggest known risk is a 1.3 kilometer wide asteroid with a 99.988 percent chance that it will miss Earth when it passes by in 861 years.

University of Toronto scientist Sara Mazrouei led the study. She told the Associated Press she is not worried about asteroids hitting Earth because they are such rare events.

“It’s just a game of probabilities,” she said.

Mazrouei worked with researchers in Britain and the U.S. on the study. Together they gathered a list of the large marks that asteroids leave on planets and other objects, also known as impact craters.

Researchers looked at impact craters on Earth and its moon that were larger than 20 kilometers wide. It takes an 800-meter wide space rock to create a hole that large.

They also estimated the dates of the craters. The team counted 29 craters that were less than 290 million years old. And they found nine from between 291 million years and 650 million years ago.

Rebecca Ghent, another University of Toronto scientist, also worked on the study. She noted that we can see relatively few big craters on the Earth’s surface because the planet is more than 70 percent ocean. Also, the movement of large mountains of ice, otherwise known as glaciers, covered some of them.

The team estimated that there were 260 space crashes on Earth in the last 290 million years. And that crash rate is 2.6 times more than the 700 million years before then.

Since glaciers mostly covered older craters, the scientists studied craters on the moon to estimate the age of those between 650 million and 1 billion years old on Earth. The moon is close enough to the Earth to be in the same path of the asteroids and its craters last longer.

So what happened nearly 300 million years ago?

“Perhaps an asteroid family was broken up in the asteroid belt,” Mazrouei suggested. The space rocks then headed toward the Earth and moon, Ghent added. The planet then got a few more hits because it is a bigger target and has higher gravity.

Other scientists are divided about the team’s research. Earth scientist Jay Melosh at Purdue University said he found the number of craters too small to prove any major point. But Harvard University’s Avi Loeb agreed with the team’s findings.

Modern humans might not have existed without major extinctions from space rocks, Loeb wrote in an email. Most scientists think dinosaurs and a lot of other animals went extinct after a huge space rock crashed into Central America about 65 million years ago.

“This demonstrates how arbitrary and fragile human life is,” he wrote.

I’m ­Pete Musto.

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Words in This Story

twice – adv. doubled in amount or degree

journal – n. a magazine that reports on things of special interest to a particular group of people

probabilities – n. the chances that something will happen

belt – n. a region that has a lot of a particular thing

extinction(s) – n. the state or situation that results when something, such as a plant or animal species, has died out completely

arbitrary – adj. not planned or chosen for a particular reason

fragile – adj. easily broken or damaged