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Astronomers have captured the moment a lump of rock slammed into the moon with so much force that the bright flash could be seen from earth with the naked eye.
The 400kg meteorite, travelling at 61,000km/h, punched a fresh crater into the moon's surface some 40 metres wide in what is thought to be the largest lunar impact ever recorded.
The rock, around a metre in diameter, ploughed into an ancient lava-filled basin called the Mare Nubium five months ago. The impact produced a flash almost as intense as the Pole Star and took more than eight seconds to fade.
The impact energy was equivalent to 15 tonnes of TNT - at least three times as great as that from the previous record-holding lunar impact observed by Nasa in March last year.
The event was recorded by Spanish telescopes that monitor the moon under a project called Midas (Moon Impacts Detection and Analysis System). The flash was picked up on September 11, 2013 by two telescopes in Seville.
Unlike earth, the moon has no atmosphere to protect it from incoming meteorites, so the surface is pocked with craters.
Details are published in the latest Monthly Notices of the Royal Astronomical Society.
Astronomer Jose Madiedo, who leads the Midas project at the University of Huelva, saw footage of the strike soon after the telescopes' software had processed the impact. "When I saw it on the screen I realised I had witnessed a rare and unusual event. It was really huge. I couldn't imagine such a bright event," Madiedo said.
"We image a lot of impacts on the moon, but they're caused by very small rocks. But this event was really impressive and very rare," he said.
The telescopes capture scores of much smaller lunar impacts every day. The smallest rocks the telescopes can see weigh only a few grammes and hit the lunar surface every three hours or so.
The telescopes spot impacts from the tiny flashes of light produced as the rocks are vapourised in the intense temperature of the collision. The flashes usually last a fraction of a second, but the flash from the September impact lasted longer than any seen before.
By observing the moon, Madiedo hopes to learn more about threats to earth. "We are very close neighbours. What happens on the moon can also happen on the earth," he said. "This impact shows that the rate of impacts on our planet for rocks of this size, around one metre in diameter, is about 10 times greater than we thought."
Few rocks of this size would be a danger to earth, because most would be destroyed as they burned up in the atmosphere. Though parts might survive the intense heat of entry and reach the ground as small meteorites, they would not pose a serious threat, Madiedo said.


Astronomers capture huge meteorite smashing into moon

The flash was picked up by two telescopes in Spain. Photo: Midas

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Astronomers have captured the moment a lump of rock slammed into the moon with so much force that the bright flash could be seen from earth with the naked eye.
