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. Jul 30, 2019 at 14:13
• So, Seveneves, huh? Jul 30, 2019 at 14:52
• Agree with the answers then, everyone dies. Jul 30, 2019 at 15:40
• 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
– Ray
Jul 30, 2019 at 19:04

It would be much better for Earth if the impactor hit the moon...

In this Worldbuilding answer, I used a paper on ejecta kinematics to do calculations for ejecta velocity upon impact. Without going into too much detail here, much of the ejecta from a large impactor would not exceed the moon's escape velocity of 2.38 km/s. You can examine Figure 7 from the linked paper which shows the logarithmic relationship between ejecta velocity and the edge of the crater. Only material within a few tens of meters of the edge of the formed crater can attain the velocity needed to escape the Moon's gravity.

The mass of the asteroid increases with the cube of its radius; while the mass of material within $$n$$ meters of the edge of the impact crater increases with radius; so it is evident that the the larger the meteor itself gets, the lower the danger of the any potential ejecta relative to the original impactor.

Furthermore, material ejected from the Moon has the potential to land back on the Moon, enter a stable orbit of Earth, or be ejected from the Earth-Moon system; so only a (probably small) fraction of Moon ejecta would threaten Earth.

Overall, fewer, smaller rocks are much better than large rocks when it comes to getting hit by things (except maybe for your car's windshield).

...unless the impactor knocked the Moon into Earth.

Of course, there is always the possibility that a very, very large impactor would knock the moon into a different orbit, potentially one that eventually impacts Earth. That would, obviously, be the worst case scenario. Significant changes to the orbit of the moon, even if these changes do not cause a collision with Earth, have the potential to do significant damage in terms of tides; not just ocean tides but also the poorly understood effect of tidal forces on Earth's mantle.

In any case; it is probably much better for a large impactor to hit the Moon instead of the Earth, but possibly much, much worse.

• If the impactor is large enough to knock the Moon into Earth, a direct collision would most likely not be preferable, either. Jul 30, 2019 at 14:48
• If it knocked the moon into the Earth it wouldn't matter--anything that could do that would have left Earth permanently uninhabitable anyway (blow off the atmosphere and oceans.) Jul 30, 2019 at 14:48
• There's nothing in the Solar System that is both large enough to knock the Moon out of orbit and on a possible collision trajectory. They've all been either thrown out of the Solar System by Jupiter, or absorbed into the planets.
– Mark
Jul 30, 2019 at 20:01
• Running the numbers... knocking the Moon into Earth means killing roughly 3e28 J of orbital energy (that's 100,000 more energy than Chicxulub). You'd need something the size of Pallas, one of the most massive asteroids, smacking into the Moon head on at 14 km/s. 3e28 J is not far away from its gravitational binding energy of 1e29 J. That close to its binding energy would probably shatter the Moon, the bits raining down on the Earth before it can reform. It's moot to say whether this is "better" than being directly hit by such a beast. Jul 31, 2019 at 1:48

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. Jul 30, 2019 at 11:11
• 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. Jul 30, 2019 at 11:47
• @IvánPérez Right, I improved the answer. Jul 30, 2019 at 11:56
• 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." Jul 30, 2019 at 13:37
• 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. Jul 30, 2019 at 16:30

A meteor is a small, sand grain or pea-sized fragment which burns up in the atmosphere before hitting the ground. Those large enough to reach the ground ae called meteorites. It seems to me the ones you are talking about are large enough to be called asteroids. The moon affords hardly any protection against asteroids, and those that strike the moon do very occasionally cause tektites (small glassy beads) and small pieces of debris to hit the Earth. The next asteroid on a collision course for Earth is most unlikely to be blocked by the moon, but there's no need to worry. Chelyabinsk or Tunguska events are fairly common and occur several times a century, usually striking the sea, but the really big ones like Manicouagan or Chicxulub occur on average about once every 150 - 200 million years. We will probably have already become extinct by the time the next one hits.

• This doesn't answer the question, which is "Would it be better for the Earth if a meteor hit the moon instead". Jul 30, 2019 at 13:55