How large would a moon impact event have to be to affect the Earth?

[Skoll]

How big would an asteroid/comet have to be to impact the moon enough to have a significant effect on the Earth's ecosystem? Dust, tidal forces, etc.

Thanks,

-Mike

 

[gtring]

Hiya Mike. Define "significant effect on the Earth's ecosystem".

Humankind cannot agree on whether global warming is an issue that has a significant effect - heck, we all can't agree on the phrase "significant effect".

If dust were to fall from the Moon to the Earth, it would mainly burn up in the atmosphere. This has probably happens to the Earth every once in a while without us knowing. Of course, if dust obscured the moon so we couldn't see it anymore, we would all go crazy and torch the townsfolk. See Nightfall by Asimov.

A more significant concern would be a change in tidal forces, which could flood low-lying populated areas. This calculation is beyond my expertise.

Cheers,
--Jake

 

[Nabeshin]

I agree with gtring that the most worrisome thing would be the impact throwing off the orbit of the moon, thereby affecting tides and such.

I did a quick calculation and in order for a direct hit to effect a change of 1% in the orbital velocity of the moon (about 1m/s), the asteroid would have to be on the order of 10^18 kg, which corresponds to a radius of around 70-90km. For reference, the average radius of Ceres is about 450km. So, this would have to be one of the larger objects in our solar system.

Looking at this, the impact would release about 10^25J of energy, or about 10^16 tons of TNT. We're looking at about 12 million gigatons of TNT here, and the largest recorded energy release for an earthquake on Earth was 60,000 gigatons of TNT. This makes me think that orbital perturbation isn't the biggest worry here. This kind of damage would likely be catastrophic for the moon.