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Buratti and Johnson calculated the energy released at impact was about .5 megatons (35 times more powerful than the Hiroshima atomic bomb). They estimate such events occur on the lunar surface once every half-century.

Really? That would be a quite noticeable impact. If this is thought to occur once every 50 years on the moon, how often ought it to happen on earth? Even given that most asteroids would land in the ocean, I would think we'd have more of a record of observed collisions in historical times if something of this size is coming in every 50 years or so.

Really? That would be a quite noticeable impact.

IF it made it to the surface. I'm not completely up on the details, but according to the story...

"Using Stuart's photograph of the lunar flash, we estimated the object that hit the Moon was approximately 20 meters (65.6 feet) across, and the resulting crater would be in the range of one to two kilometers (.62 to 1.24 miles) across. We were looking for fresh craters with a non-eroded appearance," Buratti said.

I'm fairly sure a 20 m diameter object would be burned to a cinder long before it hit Earth's surface. Having an atmosphere is a nice thing.

> If this is thought to occur once every 50 years on
> the moon, how often ought it to happen on earth?

Five per year in the 1 KT range.
One per century in the 1 MT range.
See:
http://www.eurekalert.org/pub_releases/2003-02/danl-lar021203.php

> Even given that most asteroids would land in the ocean

Or explode in the upper atmosphere.

how often ought it to happen on earth?

About every five years. Most detonate in the atmosphere rather than reach the ground.

Really? That would be a quite noticeable impact. If this is thought to occur once every 50 years on the moon, how often ought it to happen on earth? Even given that most asteroids would land in the ocean, I would think we'd have more of a record of observed collisions in historical times if something of this size is coming in every 50 years or so.

They do. I wrote a FR editorial proposing that NASA should take up the SpaceGuard mission proposed by Arthur C. Clarke in Rendesvous with Rama, to become an international space organization with the capability to protect the Earth from asteroid impacts.

On June 30, 1908, the Tunguska impact caused a major event in Siberia.

The Tunguska Event (with pictures of the aftermath)

On August 10, 1972, a large asteroid or comet fragment passed over Wyoming and other western states before "skipping" back out of the Earth's atmosphere. Below is a picture of the fireball over the Tetons.

There was also a large fireball and possible impact on the Greenland ice sheet in the 1990s.

I should be able to find my editorial with a FR search; I'll post the link in a subsequent reply if I do.

Now is the time for "SpaceGuard"