Hi all,

I have a bit of a thought exercise for anyone who’s interested. The truth is I’m working on a novel and having trouble with some research.

What I want is speculation on the likely effects of a large (10km+) asteroid impact on Earth’s moon. I want to talk about both immediate and long-term effects.

What would it look like?
Would fragments make it to earth and if so would they be large or small (shooting stars or devastating fireballs)?
Could the moons orbit be effected and if so what would this do to the earth?
How long before things calm down and once they do what would the moon look like?
Any other effects?

Many thanks in advance for any input.

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Dog

The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts. - Bertrand Russell

We can probably get much more specific (for example there are a couple of impact calculation websites that will tell you a bit more accurately what the crater size will be, etc., but I don't have the URLs as handy as I should), but a 10km impactor will probably create something like a 50km crater on the Moon. It would be very spectacular to watch with ejecta flung near and far - large rays would form extending out from the resulting crater and secondary craters would be quite abundant, especially closer to the impact crater that is formed. The crater would be complex possibly with central peaks and slumped terraced walls, probably between about Tycho and Kepler in size and appearance. Fragments would definitely hit Earth and would have a size distribution of some sort such that there will be many more small boulders than big boulders (I'd guess we wouldn't see boulders bigger than about 50 meters in diameter hit Earth and those would be very rare), and there would be many more pebble sized bits of debris than small boulders and there would be a lot of dust. The impact velocity would be several km/sec and it would take a few days for the first debris to reach Earth and we'd have a rain of debris that would be significant and last days to weeks with probably a falloff to much lower, but still enhanced impact rates over the next few years, perhaps longer at much lower enhanced rates from debris that would escape the Earth-Moon system and enter solar orbit that would occasionally return. The Moons orbit would be insignificantly affected, with a change of speed that would be very difficult to measure. The Moon would look pretty much like it does today, but with a new rayed crater once the "dust settles".

Other affects? Yes, we'd have to build lots of new satellites and if humans are out there and can't get back soon enough, bury a lot of dead astronauts. I suspect the ISS would be damaged, so the crew would have to get into the Soyuz and return with some prudence (they'd probably have a couple days to prepare, assuming the Moon was visible (not too close to new) at the time of impact and have less (maybe no!) warning if we saw the affects as the Moon came out of conjunction with the Sun. If the impactor is large enough, there might be enough dust loading in our atmosphere to significantly affect the climate for a short time, but it would take a huge impact before it would cause the affects of a much smaller impact on Earth.

The probability of such an impact is very low compared to the same impact on Earth, BTW. The Moon has about 1/20 or so the impact cross-section as Earth, so the impact of a 10km object which is probably something that happens on Earth about every 20-50 million years or so on Earth would happen about every half to 1 billion years on the Moon.

Also, we already know of all the 10km NEAs (Near Earth Asteroids) out there, so this will not happen anytime in the predictable. We are at around 70% completion in surveying NEAs down to about 1km, so the chances of not knowing about an impactor larger than 1km ahead of time is getting smaller with each new NEA discovery. We'd probably want to deflect a potential Lunar impactor just as we would an Earth impactor considering the kinds of resources we have in space and hopefully will have on and near the Moon.

Don't take these number literally, just as a general idea of magnitude and affect. We should do more detailed estimates to be a bit more precise on the affects.

Jim.

Thanks Jim, that’s really good stuff.

You have led me to believe I have seriously underestimated the size of the impactor I need. Maybe I should just describe the effect I want and see if anybody wants to propose a mechanism.

The novel is an alternative reality type. As a background to the plot I want to investigate the effects of a major global disaster on the development of human civilisation.

I need a hit on the near side big enough to create an easily visible naked eye scar. Basically I want to significantly change the look of the moon. I want a cataclysmic effect on earth - multiple large meteor strikes. Ideally the impact would take place sometime around 8000-10000 BC.

I’m starting to imagine a comet strike, possibly following a very near miss of the earth. What would that look like?

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Dog

The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts. - Bertrand Russell

Halley's is set to pass closer to us in the next go round--and sky-filling the next!

For something that would significantly alter the naked-eye appearance of the moon, you would need an impactor along the lines of the Mare impactors. These were huge (I can't remember the figures off-hand, but I'm guessing >100 km diameter) and would need to be on a very specific trajectory to impact the near side of the moon. As you can imagine, the far side of the moon is far more susceptible to asteroid impacts.

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The quarrelsome oarsmen were rowing,
The great violinist was bowing;
But how is the sage
To tell, from the page:
Was it pigs or seeds that were sowing?

Now I can't remember the author of the book but I think the title is "Moonfall." It's about a very large and very fast asteroid that hits the Moon and breaks it (the Moon) up. The U.S. has a permanent Moon base set up so things get interesting.
Of course, I don't have the book anymore, but my loss is the Salvation Army's gain.

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If you can't convince them, confuse them.

Why?

I think I need this one explained to me as well. The earth could block a few, but it seems like that ought to be a small proportion...

Any potential impactor approaching the Earth-Moon system from space would be influenced by the gravity of both the Earth and the Moon. If the impactor approached from the side of the system away from the Moon (i.e. if the moon was hidden from the impactor by the Earth), the impactor would most likely strike the Earth, not the Moon.

If it approaches from the opposite direction, it would strike the far side of the Moon.

Approaching the system from the side (at any one of a range of angles), the impactor is far more likely to strike the Earth than the Moon due to the far greater mass (and hence gravitational influence) of the Earth compared to the Moon. As pointed out in post #2 of this thread, the Earth's impact cross-section is about 20x that of the Moon.

For a potential impactor to strike the near side of the moon, it would need to approach the Earth-Moon system following a specific trajectory. Such a trajectory would have to pass close to the Earth itself (otherwise the impactor is more likely to hit either the far side of the Moon or the Earth). Therefore, the energy of the impactor and its trajectory would need to fall within a narrow range of parameters such that the impactor could pass close to the Earth without falling onto it and yet still be deflected enough to strike the moon.

Of course, I am assuming that the impactor does not itself enter Earth orbit (in which case it could subsequently strike any part of the moon), but I think such an assumption is reasonable, given that we want an impactor with enough energy to cause a significant difference to the naked-eye appearance of the Moon.

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The quarrelsome oarsmen were rowing,
The great violinist was bowing;
But how is the sage
To tell, from the page:
Was it pigs or seeds that were sowing?

Another way of looking at the same thing is to think of the situation in 2 spatial dimensions, with a third dimension showing gravtational potential. The Earth would be at the bottom of a very deep, wide gravity-well, with the moon rolling around somewhere between halfway up and the rim. The moon has its own gravity well, but this is far smaller and shallower than the Earth's. Any object approaching in such a way that it is heading towards the near side of the moon would be far more likely to hit the Earth than the moon. Any object approaching perpendicular to the Earth-Moon axis would similaerly be deflected towards the Earth by the effect of gravity (so a side-on strike to the moon is unlikely). Any object approaching the system from the moon side is more likely to hit the moon, but is most likely to hit the far side, not the near side.

I hope this helps.

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The quarrelsome oarsmen were rowing,
The great violinist was bowing;
But how is the sage
To tell, from the page:
Was it pigs or seeds that were sowing?

Dr. Nigel,

Thanks for your explanations. I get a lot of ideas and don't have time to follow through on all of them, so I'll probably never do this. But it occurs to me that I could do some simulations to see how strong the effect is. So, I would need to begin simulations with an asteroid approaching from a good distance away, so the earth-moon system would be well approximated at that distance by a point mass. I guess they could approach uniformly from all directions - or are some approaches more likely than others? Then the distribution of speeds is needed (and it seems like that might not be uncorrelated with the direction of approach). Finally, does the atmosphere make a difference? If burning up in the atmosphere counts as an impact, does the atmosphere nonetheless matter, in that an asteroid might skim the earth but continue onward with an altered orbit? Or would it be a reasonable approximation to just assume the earth is a big rock with no atmosphere for these purposes?

Again, I'll probably never do this, so certainly don't put yourself out trying to answer these questions, but if you can, I'd appreciate the answers just for curiosity's sake.

Thanks!

What about if the impactor would have missed if it wasn't because earth's gravity effected it? It could be something like when a golfball hits the edge of the hole but fails to falls in and just get it's direction changed by the holes inner curve. If you end up with an object that has to come in from and totally unrealistic angle for an astroid, then just make it an rouge astroid from somewhere outside our solar system. Those objects are extremly uncommon but of should exists. You could let your story start with a supernova somewhere and then sort of follow a single fragments path's out from it's dying start system. This could also account for an unusal speed of the impactor.

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"-UFO detected, interceptor launched"

I like that. Very cool.

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Dog

The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts. - Bertrand Russell

Dr Nigel

Thanks for your input. This sort of stuff is very helpful as well as being just interesting in it's own sake.

I take your point about the likelihood of a nearside impact - fortunately in fiction it doesn't have to be terribly likely, just possible.

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Dog

The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts. - Bertrand Russell