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Pun not (originally) intended.

In my story, a fairly advanced, newly space-faring civilisation manages to land on the moon and establish a colony, where they do some industrial work e.g. mining. However, there is a massive industrial disaster in which part of the moon gets blown up enough to send fragments of it to Earth below as meteors.

This causes massive damage to the planet, but not so much that all life was wiped out. After a few hundred years, people have been able to somewhat rebuild, and have formed tribal societies.

Is this possible? What would the impact be of this happening? How much would the moon have to be destroyed for large enough fragments to come to Earth and cause significant (but not planet-ending) damage? Could human life make it through, enough to rebuild? How long would it take before this could happen?

Currently some rough effects I've got are:

  • The side of the planet facing the moon when the disaster happened largely got destroyed; there's basically nothing left but craters
  • The rest of the world was struck with tsunamis, dust clouds, earthquakes and a collapse of infrastructure leading to famine and possibly war
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  • $\begingroup$ What's the reason they would set up on the Moon ? They're placing an apparently very dangerous facility close to a populated planet and ignoring a whole raft of perfectly good moons and planets (Mars anyone ?) to use the Moon, which has no particular value I can see. They'd have also ignored the Oort Cloud and the Kuiper Belt objects to get to Earth's Moon, and then ignored Mars, Venus and Mercury as well as Jupiter and Saturn's large moons. $\endgroup$
    – StephenG
    May 11 '20 at 14:33
  • $\begingroup$ Some back-of-the-envelope calculations suggest you're looking at an accidental explosion comparable to multiple atomic bombs - it's tough to do that even with specifically designed equipment. They must have some serious OSHA violations on this moon base to be able to make a boom that big by accident. $\endgroup$ May 11 '20 at 14:41
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    $\begingroup$ @NuclearWang My back-of-the-envelope calculations have something magnitudes higher than an atomic bomb. It's got to jump orbits from the Moon to Earth through a single starting blast. $\endgroup$
    – Halfthawed
    May 11 '20 at 14:44
  • $\begingroup$ How exactly this happened isn't really the focus of the story; it's more about rebuilding society in an effective way, learning from past mistakes etc. The disaster is just an enabler for that. I'd still like it to be plausible though; and tbh I'm only saying it was a moon explosion because I think that's cool. e.g. the characters can look up and see the moon with a big chunk missing, and also to discuss the effects on the planet. $\endgroup$
    – Touchdown
    May 11 '20 at 14:46
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    $\begingroup$ You might be interested in The Moon is a Harsh Mistress en.wikipedia.org/wiki/The_Moon_Is_a_Harsh_Mistress $\endgroup$ May 11 '20 at 15:01
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The Earth and the Moon are pretty far apart. In order for something to go from the Moon to Earth, it needs to escape from the Moon's gravity and then have an orbit which brings it into Earth's field of influence and then crash into Earth. Normally, the way we send ships to the Moon involves said ships boosting in orbit around Earth in order to achieve this feat. What you're doing is hoping to achieve this all through a single blast. Let's step back and look at what you want to do.

  1. Get a rock big enough to cause cataclysmic impact off the moon through an explosion

  2. The rock jumps orbits from the Moon to the Earth and impacts it hard enough to cause significant destruction.

I'm going to stop you right here and explain this is impossible - the regolith simply isn't strong enough. The regolith, which is the moon's surface, is composed of a lot of different materials fused together, take a look, and any explosion on the Moon's surface with the power necessary to launch a chunk of the moon that size will also destroy the chunk completely and turn it into powder such that even if it did make it to Earth, it would incinerate within the atmosphere.

Let's say you dug deeper. The moon's spaceside crust is 40 miles thick, after all. Let's say you dug that deep and had the explosion all the way down there. Still not exactly possible - the explosion you would need would either be too strong and destroy everything or too weak and be mitigated by the miles and miles of rock with would dampen the blast. The explosions we're talking about here aren't exactly your typical Hollywood explosions - these are the equivalent of nuclear explosions which have a tendency of turning everything to ash.

Fret not, because there's one final solution: Blowing up the whole moon, or at least detonating enough of the core to eject a massive amount of it into space and have that crash into Earth. Not only will that cause cataclysmic damage, but the damage to the Moon will destroy the world's weather. And that would be the real problem.

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  • $\begingroup$ Destroying the entire moon sounds like a cool idea - would it even be possible for human life to continue in such a situation? I think if this were to happen, we'd have to say that any fragments went away from Earth, because otherwise they'd be so huge that if they impacted the planet they'd completely destroy it. So I think what may be plausible is the moon ends up basically being gone, with some small fragments possibly impacting the Earth, and the more devastating effects are, as you say, the weather being messed up. $\endgroup$
    – Touchdown
    May 11 '20 at 15:01
  • $\begingroup$ It's survivable. Most catastrophes are survivable by the human race, the question is whether it's going to be 99.99% who survive or only 0.01% who survive. $\endgroup$
    – Halfthawed
    May 11 '20 at 15:02
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    $\begingroup$ Neal Stephenson describes in Seveneves what might happen if you blow up all of the moon. $\endgroup$
    – Henning M.
    May 11 '20 at 17:20
  • $\begingroup$ What matters when impacting the Earth is total mass, not just the size of the meteorite. If an amount of dust as massive as the dino-killing asteroid hits our atmosphere all at once, we all die just the same. $\endgroup$ May 11 '20 at 19:25
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    $\begingroup$ Even then. The Earth gets the energy from the impact one way or another. You may not have a big crater on the ground, but the atmosphere will warm upfaster than we would be able to handle. In fact, the most recent models for how the dino killing asteroid actually killed dinos has that as the immediate cause of death for most land life - as debris from the impact flew around, it heated the atmosphere and cooked everything alive. It was mostly marine life and burrowing animals in hibernation that survived the impact, but then they died when flora collapsed due to lack of light. $\endgroup$ May 11 '20 at 20:06
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You cannot do this with "an explosion" for the reasons illustrated by @HalfThawed.

You would need the equivalent of a fizzle cannon shot - dig a very deep pit, have a very large "boulder" (a mountain actually) inside, have some reaction create a sufficient overpressure in the pit that the mountain is launched towards Earth but not so quick and violent that it disintegrates. It must also not rotate, since centrifugal forces would quickly disintegrate the chunk anyway.

Then, there is no easy ballistic trajectory from the Moon to Earth: everything on the Moon has an orbital velocity of roughly 6.28*384 thousand km per 28 days. I'd say about one kilometer per second radial. We need to neutralize the Moon's attraction, and the orbital velocity. We need 1.4 km/s upwards and 1.0 km/s counter-orbitwards ("widdershins" maybe, since orbit-wards is sunwise?), so the chunk must be ejected at about 1.7 km/s at an angle of 55° from the horizon (35° from vertical) and at the appropriate point of the Moon's orbit.

To reach a terminal speed of 1.7 km/s (1700 m/s) with an acceleration of 1G, supposing constant acceleration, it takes 170 seconds, so the mountain has to cover a distance (i.e. the depth of the pit) of s = 1/2*g*t^2 = approx 0.5*10*170*170 = 144 kilometers. With a 10 times higher acceleration, pit depth decreases by the same factor, so about 15 km. I am totally not sure of this, but it seems to me that such a large chunk of space-sintered regolith would probably crack under a stress of 10G, turning the "gun" into a space shotgun.

If it did not, we would have a mountain in L1 with zero speed, starting to fall towards Earth from a distance of a bit more than one light-second. The kinetic energy it acquires is the same as the energy required to leave Earth and attain L1, which should not be too different from the energy required to reach infinity, which is Earth's escape velocity and known to be about 11.2 km/s. So, our mountain would gain the same velocity falling down, which means an impact at about 10 km/s after a fall on the order of one day.

If we reduce the horizontal component a bit (allowing for the pit to be more vertical), the chunk would (from the Earth's standpoint) assume a more and more curved trajectory until it missed Earth completely, entering a different orbit (much lower than the Moon's) with a higher and higher perigee. When perigee is below some 400 km, the chunk would experience a significant aerobraking, making the orbit decay and impact after some difficult-to-predict time.

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A single large meteorite would be enough to end civilisation by means of a 'nuclear winter' dust cloud. Crop failures, famine, societal collapse, cancellation of important Netflix series etc.

As an order-of-magnitude estimate, something a mile across would be a global catastrophe. Anything much bigger and you could cause a major extinction event and not just disruption of us Hom Sap for a few centuries. Either way would not have any effect on the moon, which is all craters anyway.

You would need more than an 'industrial accident' to generate something that big, depending on what civilisation looks like in your milieu.

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    $\begingroup$ "Industrial accident" is very broad and loosely defined; it could be something like they were messing around with teleportation and accidentally teleported a chunk of the moon (say 0.5 km radius) far enough towards Earth it fell down as a meteor. I know that's massively incompetent and ridiculous but it could theoretically happen. Anyway what I get from this answer is that it really takes surprisingly little to cause massive devastation to the Earth's surface via a meteor, so there probably wouldn't be any giant moon chunks involved if life is to have any hope of continuing. $\endgroup$
    – Touchdown
    May 11 '20 at 14:53
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The dino-killing asteroid has a size estimate between 11km and 81km. Checkout this video with some asteroids placed upon the city of New York, to give you an idea of size. The Dino killing one would be somewhere between Eros and Shaposhnikov. In other words - in the lower estimate big enough to cover the city in shadow, in the larger one bigger than the city.

The estimate for the dino killer's mass also puts it somewhere between a 10th of a millionth of the Moon's mass, to 100,000th of the Moon's mass.

A single projectile that big and massive detaching from the Moon and getting into a trajectory that impacts the Earth would not require suspension of disbelief. It would require absolute dissolution of disbelief. It is so ridiculous that Hollywood did not pick this idea up neither during the asteroid craze that led to Deep Impact and Armageddon, nor during the end of times streak that had movies such as 2012.

So you need lots and lots of those fragments going up, so that at least a massive one impacts the Earth. Let's say 10,000 fragments. Once they escape the Moon (with at least ~2.38 km/s), they then are orbiting the Earth at a very circular orbit at 1.022 km/s. Since they won't be slowed down by thrusters in order to lower the perigee, the only way to have a trajectory intersecting the Earth is if they have the escape velocity for the Earth-Moon system instead. This means leaving the Moon at 2.42 km/s.

We are talking about giving 1,000th of the Moon's mass (7.3 x 1019 kg) a relative speed of 2.42km/s to it. So approximately 4.2 x 1026 joules just for the speed alone, not counting the energy gone into actually breaking that amount of mass apart from the Moon.

That is about a second and a half of the Sun's total energy output. In other words, it would be like having an object smaller than the Moon outputting more energy than the sun.

This thing would probably melt and vaporize the whole Moon. We are not talking about mere planet-buster weapons here. I think the fragments headed for the Earth would be the least of Earth's concerns. The radiation alone would disassemble the Earth before the rocks could hit us. Far away from the solar system, astronomers will baffle at the mini-nova that made the Sol system more than double its brightness for a short while.

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Actually, my comment on your question may make a good answer.

In The Moon is a Harsh Mistress Heinlein has the moon residents configuring a rail gun materials transporter, used to launch moon minerals into orbit, as a weapon against the earth.

How about a fully automated alien mining venture, unattended, that mines minerals on the moon and automatically launches them into a trajectory back to some harvesting station in our solar system, to be accumulated and sent back to their home planet? The system malfunctions (OOOps!!!!) and starts aiming the packages at earth. The malfunction goes unreported and undetected by the alien engineer remote monitors back on the space station, or even the home planet, for quite some time, and when it is detected and shut down, too late the damage is done.

Would an 'industrial accident' be considered 'murder' in an intergalactic court of law?

If these packages were frequent and regular (say one an hour) and large enough (say ten kT, well within the possibilities of an advanced-civilization rail gun) the effects would be devastating on the earth. It would be a pounding, for sure.

Of course, the manufacturing plant would refine and concentrate the material to save transportation costs, into a solid, dense 'bullet'. And make the mined material extremely toxic to humans (that is why the aliens are interested in it), you now have the danger in the atmosphere, even if the packages never reach earth ground. And if they hit water, better still. Toxic rain for years, as the impact kicks up huge geysers of the stuff and spreads it throughout the oceans.

In a few weeks of this, no more earth society.

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I went put it as comentary only:

Seems the Moon will looks like Earth in Adventure Time. Also, if possible, hit hard in Brasilia, please.

The accident can be some cargo mined in outer solar system no landed properly in the Moon. If there exist a complex industry, its plausible they will need some different minerals that are most easily to find in Ganymede, Titan, Pluto, etc. Lets say they try bring 16Psyche to Moon and the thing give totally wrong. Ruin the operation, the industrial complex, the colonization of solar system and those poor naked apes living in the main rock of this orbit.

Only issue is not so plausible a spheroid body missing a piece. Some dust like rings around the Moon and Earth, remains of the event, can appear too.

Dust of event will eventually fall back in Earth and Moon, as well some dust of a possible previous ring exist as equatorial wall in Iapetus.

A giant problem is expect half of Earth destroyed while the remainments restart from an early stage of civilization. There no way to hit half of Earth surface killing enough hoomans and let the least survive. If you decide hit in East Asia will kill more than half of humankind but rest keep technological advanced. If decide destroy the Western will kill few people and rest of world will just thanks. Most effective way would to destroy whole northern hemisphere due the southern is few inhabited and tons of advanced resources would be lost, but you will need a weird trajetory from the Moon to hit close of north pole (and keep Brasilia intact, for sure the remains will be a lot harmed).

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