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Comet Hale-Bopp.
Evidence indicates that it is highly unlikely that a comet crash would result in Earth's demise, researchers at the University of Washington said on Thursday.

Writing in Science Express, the online edition of the journal Science, researchers acknowledged that while most scientists agree that an asteroid collision 65 million years ago caused the mass extinction of the dinosaurs, they tend to differ in opinion on how many other mass extinctions have resulted from similar events.

Researchers used computer models to simulate the formation of comet clouds in the solar system for 1.2 billion years. They pinpointed a body called the Oort Cloud as the source for many long-period comets that find their way into Earth's path.

Formed 4.5 billion years ago from the nebula that formed our solar system, the Oort Cloud spans from about 93 billion miles from the sun to about three light years away. Scientist said the Oort Cloud could contain literally billions of comets, many of which are so small and distant to be seen.

"It was thought the long-period comets we see just tell us about the outer Oort Cloud, but they really give us a murky picture of the entire Oort Cloud," said Nathan Kaib, a University of Washington doctoral student in astronomy and lead author.

Kaib worked alongside Thomas Quinn, astronomy professor at the University of Washington. They found that only two or three significant comet collisions are likely to have occurred on Earth for 500 million years.

They based their calculations on the assumption that the inner Oort Cloud was the primary source of long-period comets, although they admit that the actual number of comets is indefinable.

"For the past 25 years, the inner Oort Cloud has been considered a mysterious, unobserved region of the solar system capable of providing bursts of bodies that occasionally wipe out life on Earth," Quinn said.

"We have shown that comets already discovered can actually be used to estimate an upper limit on the number of bodies in this reservoir."

Their findings could explain a minor event known as the late Eocene extinction 40 million years ago, although Kaib and Quinn noted: "if that relatively minor extinction event was caused by a comet shower, then that was probably the most-intense comet shower since the fossil record began."

"That tells you that the most powerful comet showers caused minor extinctions and other showers should have been less severe, so comet showers are probably not likely causes of mass extinction events," Kaib said.

He added that the Earth has benefited from the gravitational pulls of Jupiter and Saturn, which act as comet deflectors.

"We show that Jupiter and Saturn are not perfect and some of the comets from the inner Oort Cloud are able to leak through. But most don't," Kaib said.

The study was funded by NASA and the National Science Foundation.