Is there a way to crash the Moon into Earth? I want to eventually invent a device that does so, but where do I begin with physics?

[Answered on Quora.com by Mats Andersson]

Well ... The Moon’s kinetic energy is in the region of 3.8×10^28 J. If you make it come to a dead stop in its orbit, it’ll fall down on Earth. You’d probably get away with less, but we’re doing back-of-a-napkin calculations here.

To stop it, you simply apply 3.8×10^28 J in the opposite direction. You could do it as in the 70s TV show Space: 1999, and have nuclear explosions throw the Moon out of orbit (of course, in the show, they went away from Earth, amazingly arriving at a new solar system every week, but hey, it’s pretty clear from the plots that the writers were smoking things most of us aren’t… am I off-topic yet?).

So. Nukes. One megaton is equivalent to 4.18 × 10^15 J. So you’d need 10^13 nukes, give or take. That’s ten trillion nukes, and the world supply is in the region of 10,000, which falls a bit short.

You’ll need to invent an energy source with the output of ten trillion nukes.

Also, you’ll need to be careful not to apply it in one go; the gravitational binding energy of the Moon is 1.2 x 10^29 J, meaning that if you overdo it only slightly, the Moon will be vapourised by all the imparted energy, and even if you get it exactly right, you’re probably making a rather large dent in it. (I think I’m going to mention this to Randall Munroe, he’d make a lovely What If? story out of this.)

I think the easiest way would be to slam a few asteroids into the Moon, actually. A 400km asteroid, like asteroid 10 Hygiea, would do the trick. Now, all you have to do is to find a way to accelerate Hygiea to that it collides with the Moon… which is, admittedly, not actually easier than just depositing the energy on the Moon in a less drastic fashion.

Of course, it’s a lot simpler if you aren’t in a hurry.

Say you construct a solar panel array that you use to drive a rail gun, shooting off pieces of the Moon itself to slow its orbital motion. Say that you can shoot 2kg every second at a velocity of 10km/s; that’s better than existing rail guns but it doesn’t seem wildly implausible. That’s 10^8 J per second, so you’ll need to keep it up for 3.8 x10^20 seconds.

That’s 12 trillion years. 1,000 times as long as the Universe has existed.

OK 2 tons every second at a velocity of 100km/s.

You’ll need to wait for 120 million years.

Sorry. I can’t figure out a reasonable way for you even only considering the basic energy requirements.

Also ... I think that if you go ahead with this, someone might actually try to stop you.