Small Asteroid Caused Mysterious 1908 Blast, Study Says

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The fireball of expanding gas produced strong blast waves that caused damage on the ground, while still leaving some trees standing at "ground zero," he added.

"That's exactly why I don't believe that Tunguska was as big as 15 megatons," Boslough said.

In models of the event that assume such a large explosion, he said, "the fireball gets driven all the way to the surface and ground zero is incinerated. There's no evidence for that at Tunguska."

In November a team of Italian scientists arrived at a different theory, after they found what they believe is a crater formed by the impact of a fragment of the exploding celestial object.

Earth a Target

Extraterrestrial objects of various sizes regularly enter Earth's atmosphere.

"[U.S.] Department of Defense satellites detect these things blowing up in the atmosphere all the time, but they are very small compared to Tunguska," Boslough said.

William Hartmann, an astronomer at the Planetary Science Institute in Tucson, Arizona, agreed.

"On a common night if you wait an hour you'll see a meteor or two. Those are [tiny] fragments," he said.

Tunguska-scale events are thought to occur only once every several centuries, but most of them likely take place over oceans or unpopulated areas, he said.

"The data on objects of White House-size are pretty lousy, so it is very crude to estimate how often they hit," said Hartmann, who is not affiliated with Boslough's research.

"We have the empirical evidence that they don't hit populated areas every 50 or 100 years, and that doesn't change."

However, such events may be more common than scientists know, if Boslough's study is correct, he added.

"If [the new models] are right [that] small objects can cause explosions of that size, it might suggest that these Tunguska events are a little more frequent than people thought."

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