# (Hypothetical) Asteroid Impacts the Moon - How Big to Be a Threat to Earth?

**URL:** <https://boards.straightdope.com/t/hypothetical-asteroid-impacts-the-moon-how-big-to-be-a-threat-to-earth/698145>\
**Category:** Factual Questions\
**Created:** [September 11, 2014, 4:51am UTC](https://boards.straightdope.com/t/hypothetical-asteroid-impacts-the-moon-how-big-to-be-a-threat-to-earth/698145 "2014-09-11T04:51:27Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Werekoala](https://avatars.discourse-cdn.com/v4/letter/w/7993a0/32.png) [@Werekoala](https://boards.straightdope.com/u/Werekoala)\
**Post date:** [September 11, 2014, 4:51am UTC](https://boards.straightdope.com/t/hypothetical-asteroid-impacts-the-moon-how-big-to-be-a-threat-to-earth/698145/1 "2014-09-11T04:51:27Z")

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So I was sitting on the patio looking at the now-waning Harvest Moon, and I remembered a bit from the original Cosmos TV series about the monks who witnessed a small asteroid impact on the Moon in _mumblemumbleyearsomething_ and I wondered; how big of an asteroid would have to hit the Moon in order to eject enough (or large enough) debris in order to pose a threat of ground impacts on the Earth?

I assume the biggest factor would be generating enough velocity for the debris to escape the Moon’s gravity well, and then the debris being large enough to survive atmospheric re-entry, etc. I also suppose that the debris pattern would be such that we would experience multiple events (similar to standard meteor showers from comets) as the Earth moved through the debris cloud, but on a much shorter timescale. Maybe twice a month? Until the debris was swept clean of local space around Earth.

Anyway, just a thought, and I know we have some pretty sharp tacks when it comes to astronomy/astrophysics/etc. around here, so I thought I’d throw it out for discussion.

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**Author:** ![si\_blakely](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@si\_blakely](https://boards.straightdope.com/u/si_blakely)\
**Post date:** [September 11, 2014, 9:50am UTC](https://boards.straightdope.com/t/hypothetical-asteroid-impacts-the-moon-how-big-to-be-a-threat-to-earth/698145/2 "2014-09-11T09:50:38Z")

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There was a novel by Jack McDevitt called _Moonfall_ posited around that conceit - the moon shattered into fragments and some earth-bound impactors. The impactor was extrasolar, big, and fast - 2 orders of magnitude more massive than an average comet and 1 order of magnitude faster according to one summary on the internet - so we are talking of ~ 10000 times the kinetic energy of the sort of impactor that killed the dinosaurs, or maybe more.

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**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\
**Post date:** [September 11, 2014, 2:42pm UTC](https://boards.straightdope.com/t/hypothetical-asteroid-impacts-the-moon-how-big-to-be-a-threat-to-earth/698145/3 "2014-09-11T14:42:39Z")

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Not as big as you might think. Lunar-originating meteorites are all fairly young (all \<10 Ma), and there aren’t enough large, young craters to account for all of them. Paul Warren wrote [the definitive paper](http://www.sciencedirect.com/science/article/pii/S0019103584711493) in the 90s. [More here](http://meteorites.wustl.edu/lunar/moon_meteorites.htm).

[See here](https://www.sciencemag.org/content/298/5599/1752.abstract?related-urls=yes&legid=sci;298/5599/1752)for how even Martian rocks don’t need that big an impactor.

Oh, and as a side-trek, for anyone who isn’t a geologist, see [this pdf](http://cosmochemists.igpp.ucla.edu/Warren%20et%20al-2013-GCA-NWA%206693%20EA.pdf) to see how pretty moon rocks can look, if you know how. Such colours!