A Meteoroid Hits the Moon
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June 13, 2006: There's a new crater on the Moon. It's about 14 meters wide, 3 meters deep and precisely one month, eleven days old.
NASA astronomers watched it form: "On May 2, 2006, a meteoroid hit the Moon's Sea of Clouds (Mare Nubium) with 17 billion joules of kinetic energyâthat's about the same as 4 tons of TNT," says Bill Cooke, the head of NASA's Meteoroid Environment Office in Huntsville, AL. "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," notes Cooke--but this is the best-ever recording of an explosion in progress:
Above: A meteoroid hits the Moon, May 2, 2006; video-recorded by MSFC engineers Heather McNamara and Danielle Moser.  [ ]
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," says Cooke. "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 mph (38 km/s)," he says.
If a rock like that hit Earth, it would never reach the ground. "Earth's atmosphere protects us," Cooke explains. "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.
According to the Vision for Space Exploration, NASA is sending astronauts back to the Moon. Are these meteoroids going to cause a problem?
"That's what we're trying to find out," says Cooke. "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 of the Marshall Space Flight Center, Cooke's group is monitoring the night side of the Moon "as often as ten times a month, whenever the lunar phase is between 15% and 50%."
Right: The light curve of the May 2nd explosion in Mare Nubium. 
Now that regular monitoring has begun, Cooke's group has already found a second impact, the May 2nd event, in only 20 hours of watching. This time, they believe, the impactor was a random meteoroid, "a sporadic," from no particular comet or asteroid.
"We've made a good beginning," says Cooke, but 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]."
Is it safe to go moon walking during a meteor shower? How much shielding does a lunar habitat need? Does the Moon have its own meteor showers, unknown on Earth?
Expect the answers in a flash.
The Sky is Falling -- (Science@NASA) NASA researchers are mining old Apollo seismic data for clues to lunar meteoroid impacts
An Explosion on the Moon -- (Science@NASA) A piece of Comet Encke hit Mare Imbrium; the explosion was visible from Earth
A.L.P.O. Lunar Section: Meteoritics Impact Search -- Lunar meteorite flashes are visible through backyard telescopes. The Association of Lunar and Planetary Observers is helping amateur astronomers get involved in the hunt.