Asteroid hits moon, debris hits Earth ?

They say asteroid strikes are very rare. But could we include asteroid strikes on the moon as an Earth hazard, if they knock off a lot of debris into our gravitational field ?

…and as a supplementary question - are there any bits of moon rock in orbit round the Earth ?

Well, sure, it’s possible for bits of Moon to be knocked off and some of them to land on Earth. In fact, examples of just that exist.

Those found have been pretty small. They could hurt you if they hit you on the way down. Otherwise, not large enough to cause significant damage.

COULD a really big one get knocked loose? Yeah, it could happen. Just like any other really large rock from space could happen.

Hmm. What about the second bit - is there orbiting rock ?

Not long term, because their orbits would decay.

You might want to read Neal Stephenson’s “Seveneves” for a really exotic possibility if something large enough to fracture it hit the moon. I have been unable to determine if his science is accurate, but if it is, it is not a good scene.

Related question: If an asteroid is aimed toward the Earth-Moon system, what are the odds it will hit the Moon instead of the Earth?

Well the earth has a diameter of about 7900 miles and the diameter of the moon is about 2200 miles, so the earth has about 12.9 times the surface area (7900/2200)^2.

Without knowing the actual probability of an object striking the moon, I’d say that something headed in our general direction would have about a 12.9 times greater chance of hitting the earth instead of the moon. There are doubtless other factors that will come into it but that’s a good start.

Moonfall by Jack McDevitt covers some of this, too.

There is exactly one big chunk of Moon rock in orbit around the Earth.

Well, it’s a reasonable estimate…

I think you would have to look at the size of their respective gravity wells. The moon is much less dense than the Earth and has only 1/6 the gravity.

Chunks of rock from Mars have been found on the Earth, BTW.

But I suspect a direct asteroid hit of any given size is far, far more likely than a secondary hit.

(The reason for my question is: I dimly recall reading that the Moon serves as a shield for Earth, deflecting asteroids or meteors toward itself. Googling is inconclusive except to learn that the very massive Jupiter is considered to protect Earth from asteroids much better than it would with 1/5 its actual mass.)

Not really a GQ answer, but IMO the idea the moon is a shield is 99.998% BS.

Impactors are in solar orbits and arrive in the area of Earth/Moon going at great speed. The influence of Earth’s and the Moon’s gravity wells on the trajectory is very, very, very marginal.

Yes, there will be some tiny fraction of impactors that would have hit a naked Earth but *juuuust *miss due to the Moon pulling it slightly aside. But there will also be a fraction that would have just missed a naked Earth but due to the Moon on the other side of its orbit pulling it slightly towards the Moon and therefore *juuuust *enough to hit the Earth. Only the net difference of those two scenarios would be of benefit (or harm!) to the Earth.

Yes, from certain angles of approach the Moon is superimposed in front of the Earth from the asteroids’ POV and therefore the Moon acts as a shield for those would-be impactors. But if we do the math on every possible approach angle in spherical space versus those few angles which place the Moon in the way it doesn’t amount to much. A tiny fraction of 1 percent.

I suspect these stories got started in the pre-scientific era and are kept alive today in the anti-scientific section of the public and the many websites and authors eager to cater to that ever-growing market.

A quick look at the Moon even with binoculars shows that it is clearly covered by craters. Whereas Earth is not. The obvious reason is that the Moon has sucked away all the incoming asteroids, so none have ever hit Earth. Simple thinking like that, elaborated by BS artists is alive and well.

Prior to the discovery of Barringer Crater in Arizona (, the idea that meteorites had hit Earth other than in era of the early solar system formation was controversial in learned circles. It certainly isn’t controversial today.

The reason we do not see impact craters on earth (barring really big ones) is twofold:

  1. Earth has an atmosphere which burns up the smaller ones before they hit and mitigate the ones big enough to hit

  2. Erosion. Over time all but the biggest impacts are erased. The moon has no such mechanism (atmosphere or erosion). As such pretty much any impact on the moon surface remains for eons.

Interestingly we see that the "dark side"of the moon (I know it is not “dark”) shows substantially more impacts. The moon by no means intercepts all meteor strikes but it catches a lot.

To the OP we even have Martian rocks on Earth so sure we will get some moon rocks too.

After impact it is just orbital mechanics. Some rocks will be flung free of the planet/moon’s gravitation but most will remain within the gravitational influence of the planet/moon they came from.

Some do get free though and some of those find their way to the Earth.

The far side shows more impacts, it’s true. But that’s not evidence that the Moon is shielding the Earth. Rather, it’s evidence that the Earth is shielding the Moon.

Can it be both?

Given the Earth is bigger I would expect it does more of the shielding but that does not mean the moon hasn’t caught a few on our behalf.

I hope I was clear that I don’t subscribe to that view, but rather was explaining the thought process of the primitives and current ignoramuses who do believe the Moon has been sucking up all the asteroids.

But just in case I wasn’t clear: “I know and understand and agree that erosion (99%) plus atmosphere (1%) explain the lack of obvious craters on Earth. I was explaining how simpler-era or simpler-minded folks arrived at the bogus conclusion at the heart of Senegoid’s Q.” :slight_smile: