Huntsville, AL - There's a new crater on the Moon. It's about 14 meters wide, 3 meters deep and precisely one month, eleven days old - and NASA astronomers watched it form. "A meteoroid hit the Moon's Sea of Clouds (Mare Nubium) with 17 billion joules of kinetic energy - that's about the same as 4 tons of TNT," said Bill Cooke, the head of NASA's Meteoroid Environment Office at Marshall Space Flight Center.

"The impact created a bright fireball, which we video-recorded using a 10-inch telescope."

Lunar impacts have been seen before - "stuff hits the Moon all the time," Cooke said - but this was the best-ever recording of an explosion in progress.

The video plays in 7x slow motion; otherwise the explosion would be nearly invisible to the human eye.

"The duration of the fireball was only four-tenths of a second," Cooke said. "A student member of our team, Nick Hollon of Villanova University, spotted the flash."

Taking into account the duration of the flash and its brightness (7th magnitude), Cooke was able to estimate the energy of impact, the dimensions of the crater and the size and speed of the meteoroid.

"It was a space rock about 10 inches (25 cm) wide traveling 85,000 miles per hour (38 kilometers per second)," he said.

If a rock like that hit Earth, it would never reach the ground. "Earth's atmosphere protects us," Cooke explained. "A 10-inch meteoroid would disintegrate in mid-air, making a spectacular fireball in the sky but no crater."

The Moon is different. Having no atmosphere, it is totally exposed to meteoroids. Even small ones can cause spectacular explosions, spraying debris far and wide.

NASA's Vision for Space Exploration is meant to send astronauts back to the Moon. Are these meteoroids going to cause a problem?

"That's what we're trying to find out," Cooke said. "No one knows exactly how many meteoroids hit the Moon every day. By monitoring the flashes, we can learn how often and how hard the Moon gets hit."

The work is underway. Using a computerized telescope built by Rob Suggs and Wesley Swift at Marshall, Cooke's group is monitoring the night side of the Moon "as often as 10 times a month, whenever the lunar phase is between 15 percent and 50 percent."

During a telescope test last November, Suggs and Swift recorded an explosion on their very first night of observing. A piece of debris from Comet Encke struck the plains of Mare Imbrium, making a crater about 3 meters wide.

Now that regular monitoring has begun, Cooke's group already has found a second impact, the May 2 event, in only 20 hours of watching.

This time, they suspect, the impactor was a random meteoroid, "a sporadic," from no particular comet or asteroid.

"We've made a good beginning," Cooke said, adding that much work remains. He would like to observe all year long, watching the Moon as it passes in and out of known meteoroid streams.

"This would establish a good statistical basis for planning (activities on the Moon)," he said.