If a large asteroid impacted the moon (I'm thinking of one of the recent ones that are often touted as earth killers, that pass near the earth) what would it mean for us on the Earth? This wiki article seems to say even a small one would have a significant effect on the Earth.

 

http://en.wikipedia.org/wiki/Giordano_Bruno_(crater)

 

Modern theories predict that a (conjectural) asteroid or comet impact on the Moon would cause a plume of molten matter rising up from the surface, which is consistent with the monks' description. In addition, the location recorded fits in well with the crater's location. Additional evidence of Giordano Bruno's youth is its spectacular ray system: because micrometeorites constantly rain down, they kick up enough dust to quickly (in geological terms) erode a ray system.[3] So it can be reasonably hypothesized that Giordano Bruno was formed during the span of human history, perhaps in June 1178.

However, the question of the crater's age is not that simple. The impact creating the 22-km-wide crater would have kicked up enough debris to trigger a week-long, blizzard-like meteor storm on Earth – yet no accounts of such a noteworthy storm of unprecedented intensity are found in any known historical records, including the European, Chinese, Arabic, Japanese and Korean astronomical archives.[4] This discrepancy is a major objection to the theory that Giordano Bruno was formed at that time.[5]

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Moderator Note

Please make an effort to place these discussions in an appropriate area rather than putting them in the Lounge, which is more for chit-chat as opposed to science discussions.

Moved to Astronomy and Cosmology.