### How well would the Moon protect the Earth from an Asteroid?

• Would the Earth fare better if the Moon blocked the meteor, comet, rogue planet, or otherwise rather than a direct impact? At what point would the Moon's debris would be an extinction event?

The limit of the impactor is the size, angle of attack, or composition in which to moon would no longer be of any protection.

Question needs to address the size/weight/speed of the impacting object. Numerous craters exist on the moon from small and large objects impacting it.

So, Seveneves, huh?

Agree with the answers then, everyone dies.

You should be aware that an impactor can be large/fast enough to cause a mass extinction if it hit Earth without being anywhere *near* big enough to blast chunks out the other side of the moon. Life is much more fragile than the physical structure of the moons and planets themselves. Case in point, the crater from the impactor that probably killed the dinosaurs, while very large for a crater, is still small on a global scale, and wasn't even discovered until the 70s

3 years ago

The Moon orbits the Earth from $$\approx$$ 380000 km, but its radius is only $$\approx$$ 3500 km. The sky has 41253 sq degrees, and the Moon covers only $$\approx$$ 0.25 sq degree from it.

Thus, the probability that an incoming meteor is blocked by the Moon, is $$\approx$$ 1:160000. Thus, the Moon is totally unfeasible to protect us from anything.

The debris would work like an "insurance": it will be more likely that some debris will finally end up on the Earth, but their summed damage will be likely negligible, compared to the meteor.

Note also, different meteors regularly cross the orbit of the Moon, but they have still a very small probability to hit us.

If you consider that meteors can come from other trajectories rathen than from the ecliptic plane, then the probability of a meteor being blocked by the Moon is even less. The sky has an area of 41253 square degrees, and the Moon only covers 0.25 sq degrees of them. So roughly 1:160000.

I think @IvanPerez's argument is correct: You want to compute the solid angle that the moon protects, and then the probability must be a ratio of solid angles. What you gave in your answer is a solid angle! Not a probability.

@IvánPérez Right, I improved the answer.

This doesn't answer the question. You answered, "how likely is the moon to block a meteor"; the question is "would the Earth fare better if the moon _did_ block the meteor, compared to a direct impact."

It's not quite as bad as that. *Most* meteors lie roughly in the ecliptic, and so does the moon. Also a meteor whose straight line path comes close to the moon but appears to miss it, will probably be deflected by the moon's gravity into the moon. Overall that probably reduces the odds to about 1:100.

You can cover the moon with your thumb with an outstretched arm. It is pretty small.

I have read that the moon has had a protective effect on the earth and that some scientists[who?] believe[citation needed] that life would not exist without it.

@CJDennis When I've heard that (life wouldn't exist without the moon), it is in reference to the Moon's tidal effect keeping the planet nice and volcanically active. If there was no moon, the theory goes, we never would have had enough mantle activity to escape one of the snowball Earth episodes.