Moon hit by boulder-size meteoroid, causing 'explosion' visible from Earth

If you had been looking up at the moon at the right moment on March 17, you could have seen a one-second burst of heat caused by the impact of a large meteoroid.

NASA/Handout/REUTERS
Hundreds of meteoroid impacts on the moon, detected by NASA's lunar monitoring program, are pictured in this undated NASA handout photo. The brightest, detected on March 17, 2013, in Mare Imbrium, is marked by the red square.

If you had been looking up at the moon at the right moment on March 17, you could have seen an unusual flash of light – a one-second burst of heat caused by the impact of a large meteoroid.

No telescope required.

“For about one second, the impact site was glowing like a 4th magnitude star,” NASA said in reporting the news Friday.

This meteoroid was the size of a small boulder, and was travelling very fast. NASA estimates the size at 0.3 to 0.4 meters wide, and the speed at 56,000 miles per hour. 

The resulting explosion  delivered a force equal to 5 tons of TNT.

NASA puts a footnote on the word “explosion.” The bright light wasn’t combustion, since the moon has no oxygen atmosphere. Rather, it was the glow of molten rock and hot vapors after an impact of large kinetic force.

That said, this was the biggest such “explosion” in eight years of close monitoring of the moon’s surface.

And it’s not that meteoroids on the lunar surface are rare.

The moon lacks a protective atmosphere like Earth’s, in which meteoroids typically burn up. Lunar meteor showers have turned out to be more common than expected, with hundreds of detectable impacts per year.

On March 17, the pyrotechnics on the moon coincided with an active night for meteors in Earth’s atmosphere as well.

NASA’s Space Exploration Policy eventually calls for extended astronaut stays on the moon, so tracking meteor activity has long-term relevance.

“Identifying the sources of lunar meteors and measuring their impact rates gives future lunar explorers an idea of what to expect,” the space agency said in announcing the bright explosion Friday. “Is it safe to go on a moonwalk, or not?  The middle of March might be a good time to stay inside.” 

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