A mysterious flash on the Moon caught on camera 50 years ago is still provoking disagreements about its origin. Astronomer Bonnie Buratti says her new results show that the flash was caused by a 20-metre asteroid hitting the Moon.
If Buratti is right, such impacts may be more frequent than thought-about once every 30 years on the Earth, and every 500 years on the Moon. But other asteroid watchers think the flash was due to a small meteor burning up in Earth's atmosphere.
Amateur astronomer Leon Stuart's 1953 photograph of the Moon shows a light spot near the centre of the Moon's visible surface. It would take a half-megaton explosion to produce such a flash, says Buratti, of NASA's Jet Propulsion Laboratory at Pasadena.
The resulting crater would not be visible from Earth, but it should appear on close-ups taken by lunar probes. In a future issue of the journal Icarus, Buratti reports a fresh impact scar at the site of the 1953 flash on images collected by the Clementine spacecraft as it orbited the Moon in 1994. A bright blanket of ejected material covers an area that is about 1.5 kilometres across, and the colour of the debris indicates that the crater is relatively new.
Point meteors
But the odds against such a big lunar impact are too long for Peter Brown of the University of Western Ontario, who has used US military satellite data to estimate meteor impact rates on the Earth.
"I think they are going too far," he told New Scientist. Instead, he believes that the flash was a meteor falling to Earth, which appeared as a bright spot because it was moving directly towards the observer.
Although such "point meteors" are rare, they are much more common than lunar impacts. He is not impressed by the crater's appearance. "We have no absolute criteria" for the age of lunar craters, he says. "'Fresh'' could be 20 million years old."
Buratti calls Brown's claim "preposterous". She says Stuart was an experienced observer, his half-second exposure shows no trace of motion, and the flash was near a point on the leading edge of the Moon where impacts are most likely. "Our identification of the crater validates Stuart's claim that it was an impact."
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Sun Feb 20, 2011 8:36 pm