Astronomers spot largest impact on the moon ever recorded

Last March, NASA astronomers spotted a bright explosion of the moon, the brightest they'd ever seen during the eight years they'd been watching for these events. Just six months later, astronomers from Spain spotted another explosion that blew the first one away, becoming the brightest impact ever recorded on the moon.

On the night of September 11th, 2013, when the moon was just shy of half-full, Spanish astronomer Jose Madiedo was watching with two telescopes that are part of MIDAS — the Moon Impacts Detection and Analysis System — when he recorded a flash of light on the dark side of the moon. The bright flash occurred as just after 8 p.m. UTC, and the afterglow from it lasted a full eight seconds.

"At that moment I realized that I had seen a very rare and extraordinary event," Mediedo, one of the authors of a description of the event (published Feb. 23, 2014), said in a press release from the Royal Astronomical Society.

A study of the impact (recorded in the video above), showed that it was likely produced by a rock somewhere between 0.6-1.4 metres wide and tipping the scales at 450 kilograms, smashing into the moon's surface travelling at around 61,000 kilometres per hour. The explosion was the equivalent of around 15 tons of TNT and likely formed a crater around 50 metres wide in the western portion on of the Mare Nubium ("sea of clouds").

By comparison, the rock that hit the moon back in mid-March was estimated at 40-kilograms, between 0.3 to 0.4 metres wide, and travelling at nearly 90,000 kilometres per hour. Even impacting at around one-and-a-half times faster, this earlier explosion only produced about one third as much energy (about 5 tons of TNT equivalent).

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There's still some uncertainty about the rock that produced this explosion, though. Since they didn't see the rock before it hit, the astronomers use the brightness of the explosion to figure out how much energy it produced, and then they can work backwards from there.

If the meteoroid or asteroid was a 'sporadic' — meaning that it's just a lone, random chuck of debris left over from the formation of the solar system — then they have the typical speeds these travel at and they worked out the size and mass, as above. However, sometimes these chunks of rock aren't by themselves. The explosion last March is thought to have been produced by a meteoroid that was part of a cluster of small objects encountered by both Earth and the moon, since several fireballs were seen in our atmosphere that at roughly the same time. If the rock that caused this most recent explosion was part of a larger meteor shower event, such as the September Epsilon Perseids, it would have been smaller and less massive (around 46 kg), but travelling much faster, around 190,000-234,000 km/h.

One of the purposes of the MIDAS program is to help figure out where these objects come from. Their telescopes monitor the moon for impacts, while at the same time a network of all-sky cameras watch our skies for fireball meteors, which can help them trace back to the source if impacts are seen on both bodies. Also, the researchers have concluded from the program that these larger rocks, around one-metre in size, strike our atmosphere about 10 times more often than previously thought.

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