During the Bush administration, NASA began monitoring the Moon for explosions - they have turned out to be more common than previously believed, happening hundreds of times each year. 

Smart Science 2.0 readers are already wondering how there can be an 'explosion' when the Moon has no oxygen atmosphere. Lunar meteors hit the ground with so much kinetic energy that even a pebble can make a crater several feet wide and so the flash of light comes not from combustion but rather from the thermal glow of molten rock and hot vapors at the impact site. No oxygen or combustion needed.

On Friday, observers witnessed the biggest explosive event in the history of the program. So big you may have seen it without the aid of a telescope. A 40 kg meteoroid measuring 0.3 to 0.4 meters wide hit the Moon traveling around 56,000 mph and the resulting "explosion" packed as much punch as 5 tons of TNT.

For about a second afterward, the impact site was glowing like a 4th magnitude star.

False-color frames extracted from the original black and white video show the explosion in progress. At its peak, the flash was as bright as a 4th magnitude star. Credit: NASA

"On March 17, 2013, an object about the size of a small boulder hit the lunar surface in Mare Imbrium," says Bill Cooke of NASA's Meteoroid Environment Office. "It exploded in a flash nearly 10 times as bright as anything we've ever seen before."

Cooke believes the lunar impact might have been part of a much larger event.

"On the night of March 17, NASA and University of Western Ontario all-sky cameras picked up an unusual number of deep-penetrating meteors right here on Earth," he says. "These fireballs were traveling along nearly identical orbits between Earth and the asteroid belt."

This means Earth and the Moon were pelted by meteoroids at about the same time.

“My working hypothesis is that the two events are related, and that this constitutes a short duration cluster of material encountered by the Earth-Moon system," says Cooke.

One of the goals of the lunar monitoring program is to identify new streams of space debris that pose a potential threat to the Earth-Moon system. The March 17th event seems to be a good candidate.

The brightest impact in the history of the lunar monitoring program, detected on March 17, 2013 in Mare Imbrium, is marked by the red square. Credit: NASA

Controllers of NASA's Lunar Reconnaissance Orbiter have been notified of the strike. The crater could be as wide as 20 meters, which would make it an easy target for LRO the next time the spacecraft passes over the impact site. Comparing the size of the crater to the brightness of the flash would give researchers a valuable "ground truth" measurement to validate lunar impact models.

Unlike Earth, which has an atmosphere to protect it, the Moon is airless and exposed. "Lunar meteors" crash into the ground with fair frequency. Since the monitoring program began in 2005, NASA’s lunar impact team has detected more than 300 strikes, most orders of magnitude fainter than the March 17th event. Statistically speaking, more than half of all lunar meteors come from known meteoroid streams such as the Perseids and Leonids. The rest are sporadic meteors -random bits of comet and asteroid debris of unknown parentage. 

Credit: NASA

U.S. Space Exploration Policy eventually calls for extended astronaut stays on the lunar surface. Identifying the sources of lunar meteors and measuring their impact rates gives future lunar explorers an idea of what to expect. Is it safe to go on a moonwalk, or not? The middle of March might be a good time to stay inside.

"We'll be keeping an eye out for signs of a repeat performance next year when the Earth-Moon system passes through the same region of space," says Cooke. “Meanwhile, our analysis of the March 17th event continues.”

Source: Dr. Tony Philips at NASA