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Astronomers spot record-breaking lunar impact
Mongo
post Feb 24 2014, 06:25 PM
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Astronomers spot record-breaking lunar impact

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A meteorite with the mass of a small car crashed into the Moon last September, according to Spanish astronomers. The impact, the biggest seen to date, produced a bright flash and would have been easy to spot from Earth.


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On 11 September 2013, Prof Jose M. Madiedo was operating two telescopes in the south of Spain that were searching for these impact events. At 2007 GMT he witnessed an unusually long and bright flash in Mare Nubium, an ancient lava-filled basin with a darker appearance than its surroundings.

The flash was the result of a rock crashing into the lunar surface and 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. In the video recording made by Prof Madiedo, an afterglow remained visible for a further eight seconds.

The October event is the longest and brightest confirmed impact flash ever observed on the Moon. Prof Madiedo recalls how impressed he was: "At that moment I realised that I had seen a very rare and extraordinary event."


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Prof Madiedo and Dr Ortiz think that the flash was produced by an impactor of around 400 kg with a width of between 0.6 and 1.4 metres. The rock hit Mare Nubium at about 61,000 kilometres per hour and created a new crater with a diameter of around 40 metres. The impact energy was equivalent to an explosion of roughly 15 tons of TNT, at least three times higher than the largest previously seen event observed by NASA in March last year.


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Observing impacts on the Moon gives astronomers an insight into the risk of similar (but larger) objects hitting Earth. One of the conclusions of the Spanish team is that these one metre sized objects may strike our planet about ten times as often as scientist previously thought. Fortunately, Earth's atmosphere shields us from rocks as small as the one that hit Mare Nubium, but they can lead to spectacular 'fireball' meteors.


I was not sure if this should go here or in "Telescopic Observations", but this seemed a somewhat more topic-specific location.
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stevesliva
post Feb 24 2014, 07:07 PM
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I wonder why the afterglow lasts so long? (Video in linked article above)
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Hungry4info
post Feb 24 2014, 07:21 PM
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When you generate that much heat, it takes a while to dissipate it.


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charborob
post Feb 24 2014, 08:02 PM
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Has LRO imaged that area since the impact occurred? Do we know the exact coordinates? Otherwise, it would be like searching for the proverbial needle in a haystack.
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Phil Stooke
post Feb 24 2014, 08:03 PM
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Check out this abstract from LPSC:

http://www.hou.usra.edu/meetings/lpsc2014/pdf/2164.pdf

This new crater should be found by LROC pretty soon, and could be very photogenic!

Phil


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Mongo
post Feb 25 2014, 03:23 AM
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The paper now is up on arXiv:

A large lunar impact blast on September 11th 2013

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On 2013 September 11 at 20h07m28.68 ± 0.01 s UTC, two telescopes operated in the framework of our lunar impact flashes monitoring project recorded an extraordinary flash produced by the impact on the Moon of a large meteoroid at selenographic coordinates 17.2 ± 0.2 ∘ S, 20.5 ± 0.2 ∘ W. The peak brightness of this flash reached 2.9 ± 0.2 mag in V and it lasted over 8 seconds. The estimated energy released during the impact of the meteoroid was 15.6 ± 2.5 tons of TNT under the assumption of a luminous efficiency of 0.002. This event, which is the longest and brightest confirmed impact flash recorded on the Moon thus far, is analyzed here. The likely origin of the impactor is discussed. Considerations in relation to the impact flux on Earth are also made.
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charborob
post Feb 25 2014, 01:23 PM
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This recent lunar impact led me to wondering: what are the probabilities of LROC actually photographing an impact. Given the rate of impacts on the Moon and the rate of mapping by LROC, I suppose the chances of catching an impact in the act (so to speak) could be calculated.
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Gerald
post Feb 25 2014, 01:59 PM
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I've been preparing this post - before charborob has been posting - , because the paper leaves space for mis-interpretations.

An adjustment of the impact probability by a factor of 10, as might be interpreted from subsection 4.4 of the the paper, due to a single observation, is not valid. A single observation doesn't allow the calculation of the standard deviation, meaning error bounds are infinite.
The lack of impacts allows inferring statistical statements about upper bounds of impact probability.

So I'd suggest, not to overinterprete the paper in this point.
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0101Morpheus
post Feb 25 2014, 02:02 PM
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In response to charborob:

The thing with impacts is that they are random unless you are lucky enough to find the meteor traveling in advance and have enough prep time to calculate its orbit. I only need to remind to remind you of Chelyabinsk last year to show that is not always possible.

LROC is orbiting 50km above the lunar surface. I think it would be difficult to aim it to observe an impact even if we knew in advance and happened to be in a fortunate orbit. Even if we ignore the probabilities, there is still the risk of the camera getting oversaturated. It wasn't built to look at impacts.
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charborob
post Feb 25 2014, 02:35 PM
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I wasn't thinking about aiming LROC to observe predicted impacts. I was just wondering what would be the probability, during normal operations, of serendipitously observing an impact. What would be the faintest impact flash that could be detected?
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djellison
post Feb 25 2014, 02:42 PM
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QUOTE (charborob @ Feb 25 2014, 05:23 AM) *
This recent lunar impact led me to wondering: what are the probabilities of LROC actually photographing an impact. Given the rate of impacts on the Moon and the rate of mapping by LROC, I suppose the chances of catching an impact in the act (so to speak) could be calculated.


Given that it's cameras all work in a push-broom fashion.....astonishingly low.
LROC NA's field of view is (very roughly) a 5,000m by 0.5m strip.
LROC WA - approx (the filters make this a bit fuzzy) 75,000 x 1,000 m strip.
That 75sqkm of LROC WA account for about 0.000002% of the lunar surface, and of course it isn't imaging 24/7.
So -take any statistical chance of a particular area having an impact, and then factor in the likelihood of LRO seing that area at that time....pretty close to zero.
PLUS....these impacts tend to be seen on the night side of moon.....when the cameras will rarely be turned on anyway.

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monty python
post Feb 26 2014, 07:32 AM
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I was struck by how long the after glow lasted. My first thought was that it had to be a metal rich body to retain such heat ,but is that true? my second thought was that it had to be an acute angle impact to retain heat for less smearing of material. Any guesses? MRO will tell us soon enough.
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john_s
post Feb 26 2014, 03:07 PM
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By the time of the afterglow the impactor will be mostly vaporized, so its composition will not be very relevant. I suspect we're seeing glowing blobs of impact melt in and around the crater, which take a few seconds to cool and freeze.

John
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