Caught on camera: A small rock hit the moon

On January 21, something smacked into the moon, creating a flash of light. Things hit the moon all the time. But this time one thing was different. Thousands of people were watching. The impact took place during a total lunar eclipse. And there are pictures to prove it.

Astronomers are using the pictures to gauge the size of the rock that hit the moon. Jorge Zuluaga is one of them. He works at the University of Antioquia in Medellín, Colombia. He and his colleagues gathered images from amateur astronomers in Colombia and the Dominican Republic. Those two countries are in South America and the Caribbean. There was also a video livestreamed on TimeAndDate.com from an observatory in Morocco.

With these data, Zuluaga’s team calculated characteristics of the impact. For instance, it likely released the energy equivalent of about half a ton of TNT. That means the rock that hit the moon was somewhere between the size of a softball and a basketball. It probably had the mass of a few cans of paint — 7 to 40 kilograms (15.5 to 88 pounds). The astronomers also calculate that the object smacked into the moon at 13.8 kilometers per second. That’s almost 31,000 miles per hour.

Their results appear in a paper posted January 28 at arXiv.org.

Here’s how Zuluaga remembers the story. He couldn’t see the eclipse because it was cloudy. But the next day, he saw reports on Twitter. Some observers had reported a bright flash. People assumed it was a meteorite strike. Zuluaga contacted amateur astronomers he had worked with before and found several who had photographed the impact.

The researchers then estimated that the impact probably left a crater on the moon between 5 and 10 meters wide (16 to 32 feet). That scar could be spotted with a current or future lunar orbiter.

That the impact happened during a lunar eclipse is important, Zuluaga says. It supports the idea that the moon is hit with meteorites almost constantly. Scientists estimate something hits the moon once an 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.”