A basketball-sized rock hit the moon during the last lunar eclipse

Thousands of people were watching the total lunar eclipse on January 21 when something suddenly smacked into the moon, creating a flash of light. Now professional and amateur astronomers have used fortuitous photographs of the strike to estimate the object’s size.

Astronomer Jorge Zuluaga and his colleagues gathered images taken by amateurs in Colombia and the Dominican Republic, plus a video that was livestreamed on TimeAndDate.com from an observatory in Morocco, and calculated that the impact probably released the equivalent of about half a ton of TNT in energy.

That much energy could be released by an object of a size between a softball and a basketball, and with the mass of a few cans of paint (7 to 40 kilograms), smacking into the moon at 13.8 kilometers per second, the team reports in a paper posted January 28 at arXiv.org.

Zuluaga, of the University of Antioquia in Medellín, Colombia, missed the eclipse due to clouds. But the next day, he saw reports on Twitter that some observers had seen a bright flash, widely interpreted as a meteorite strike. He contacted amateur astronomers he had worked with before, and found that several had caught the impact on camera.

Using those observations, Zuluaga and his team estimated that the impact probably left a crater between 5 and 10 meters wide. That scar could be spotted with a current or future lunar orbiter.

The fact that this impact happened during a lunar eclipse supports the idea that the moon is hit with meteorites almost constantly, Zuluaga says. If such strikes were rare, spotting one during an hour-long total eclipse would be too much of a coincidence. Previous estimates suggest that such objects hit the moon once every hour, on average.

The event also highlights the potential for discovery when amateur and professional astronomers work together. “A lunar eclipse is not as interesting for professionals as for amateur astronomers,” Zuluaga says. “But when you have these kinds of surprises, it is a blessing to have amateurs as friends.”