On September 11, 2013, an object with the mass of a small car hit the lunar surface in Mare Nubium, an ancient lava-filled basin with a darker appearance than its surroundings. The impact, the biggest seen to date, produced a bright flash and would have been easy to spot from the Earth.
An image of the flash resulting from the impact of a large meteorite on the lunar surface on 11 September 2013, obtained with the 0.36-m-telescope in Sevilla, Spain. Image credit: Jose M. Madiedo / MIDAS.
The flash was briefly almost as bright as the familiar Pole Star, meaning that anyone on Earth who was lucky enough to be looking at the Moon at that moment would have been able to see it.
It was produced by an impactor of around 400 kg with a width of between 0.6 and 1.4 m.
The rock hit Mare Nubium at about 61,000 km per hour and created a new crater with a diameter of around 40 m.
The impact energy was equivalent to an explosion of about 15.6 tons of TNT.
In the video recording made by Prof Jose Madiedo from the University of Sevilla and the University of Huelva, an afterglow of this extraordinary flash remained visible for a further eight seconds.
Prof Madiedo, who is the lead author of a paper published in the Monthly Notices of the Royal Astronomical Society, recalls how impressed he was: “at that moment I realized that I had seen a very rare and extraordinary event.”
Since such impacts take place at huge speeds, the rocks become molten and are vaporized at the impact site instantaneously, and this produces a thermal glow that can be detected from Earth as short-duration flashes through telescopes.
Mosaic of zoomed images showing the flash evolution with time during the first 2 seconds; time increases from left to right in each row, starting from the upper left; the interval between two consecutive images in the same row is 0.1 s. Image credit: Jose M. Madiedo et al.
Generally, these flashes last just a fraction of a second. But the flash detected on September 11, 2013 was much more intense and longer than anything observed before.
“Our telescopes will continue observing the Moon as our meteor cameras monitor the Earth’s atmosphere. In this way we expect to identify clusters of rocks that could give rise to common impact events on both planetary bodies. We also want to find out where the impacting bodies come from,” Prof Madiedo said.
Observing impacts on the Moon gives astronomers an insight into the risk of similar objects hitting the Earth.
One of the conclusions of the Prof Madiedo’s team is that these 1-m-sized objects may strike our planet about 10 times as often as scientist previously thought.
Fortunately, the Earth’s atmosphere shields us from rocks as small as the one that hit Mare Nubium, but they can lead to spectacular ‘fireball’ meteors.
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José M. Madiedo et al. A large lunar impact blast on 2013 September 11. MNRAS, published online February 23, 2014; doi: 10.1093/mnras/stu083
