Could We Destroy The Moon? (Not should we. Because we shouldn't...)

It is that time of the week and we have selected an interesting question from a faithful follower about whether we could (not should) destroy the moon.

"Hi ARSE, if we had to or even if we simply wanted to, could we destroy the moon?" - Micah.

Hi Gareth and thank you for inquiring about the current state of Uranus. Uranus is almost always at the forefront of our minds.

Yes. Very morally questionable yes, but yes. We could totally destroy the Moon if we wanted to - but it’s a bad, bad idea, for more than just the super obvious reasons.

And the most potent weapon we have to potentially destroy the moon probably isn’t what you’re thinking.

Not nukes or lasers…

So, how would we do it? How do we go about destroying the Moon?

A shiny ball made of 73 septillion kilograms of silicon, oxygen, and iron. If we want to get rid of all that matter, we're gonna need an equally absurdly large weapon.

We don't need to build that weapon, since we are already standing on it.

Yeah. We're gonna destroy the Moon with the Earth.

Yes, I'm using the word "destroy" a little too loosely. Earth isn't gonna destroy the Moon - it's just gonna rip it apart into tiny little pieces. It's pretty much the same thing.

What are our chances of convincing the Earth to destroy the Moon on our behalf? After all, the Earth isn't ripping the Moon to shreds right now. Astronomers call it the Roche limit, and it's the reason why. And the Roche limit is exactly what we're gonna use to destroy our friendly neighbourhood satellite.

So what is the Roche limit, anyway? Simply put, the Roche limit determines how close an object can orbit its primary body without crumbling to pieces.

It's all orbital mechanics when it comes to the Roche limit. And orbital mechanics is another story we’ll leave for another day. For now, all you need to know is that objects orbiting closer to their primary body move faster than objects orbiting further away.

Why is that relevant?

So, let's think about the moon. There are two sides to the moon - one faces the Earth, and one faces away. There's no doubt that the side facing the Earth is closer to it than the one facing away from it. Who knew?

This is significant because - technically - the side of the Moon facing the Earth wants to move faster than the side facing away from it. So that's why the Roche limit tears orbiting bodies apart - because the closer side wants to move faster than the farther side, the orbiting body just falls apart like a wet sand ball.

Imagine taking your other hand and aggressively rolling the sand ball across your palm. That’s what planetary disassembly via Roche limit looks like!

If the two sides of the Moon are moving at different speeds, why hasn't the Moon already collapsed? Gravity. Since the Moon is so far away from Earth, the difference in velocity isn't as extreme. So the Moon can stay attached to Earth with its gravity.

This is how the Roche limit is defined - it’s the distance where the orbiting body’s own gravity is only just barely strong enough to hold itself together against the variation of velocities, aka tidal forces, of its two halves.

So now that you understand how the Roche limit works, let’s use it to destroy the Moon.

The Roche limit of the Moon: it's 18,470 km above sea level. Problem is, how are we gonna get the Moon that close?

It’s actually simple, but not easy.

Strap an engine to the Moon.

That's how we'd bring the Moon down to its Roche Limit if humanity were set on destroying the Moon.

“What a ridiculous idea!”, and you're not wrong.

These engines would require fuel - and you'd have to resupply it constantly. Well, lucky for us, we don't have to - the Moon has ice, and ice has hydrogen, and hydrogen makes hydrazine (rocket fuel), and oxygen makes oxidizer (what the heck). Unlimited rocket fuel on the Moon (until we destroy it).

The engine has gotta be in the right place. It's good that the Moon is tidally locked to the Earth, so we always see the same side of it. Thank goodness we don't have to worry about things like lunar rotation.

Now, where do we put the engine? You can't just point it at Earth - you have to slow it down, and strangely enough, firing it at Earth also wouldn't make it any closer.

Our Moon-killing engine would be perfect in a direction parallel to lunar Earth, that would look like the engine is on the Moon’s left side. That way, we’d be lowering the Moon’s angular velocity, thereby slowing it down - which is exactly what we want!

So that’s it, right? Just blast that thing continuously?

Nope - that would take wayyyyyy too long to bring the Moon down to the Roche limit. Instead, we would fire up our engine at one specific point of the Moon’s orbit.

Why, exactly? Simple - instead of continuously slowing down the Moon’s velocity, by burning at one specific point in the Moon’s orbit, we would turn the Moon’s orbit into an eccentric ellipse! An eccentric ellipse whose lower point would eventually reach the Moon’s Roche limit!

It works out quite beautifully. We fire our engine at some arbitrary time in the Moon’s orbit for, say, 12 hours, and for the rest of the time, we can let our Moon-mining and fuel-refining robots do their thing. Rinse and repeat!

Also, we’re gonna need a lotta engine, too - the Moon’s getting about an inch further away from the Earth every year, so we gotta oppose that, and start slowing the Moon down. Ah, well - I’ll leave the engineering problems to the idiots who gotta solve ‘em.

The process will take some time, obviously. You're going to have to wait a long time for those engines to make a difference on the Moon. Eventually, the Moon's Roche limit will be reached, and we can rip it apart with the help of the Earth.

Tada! One free-range, fresh-crumbled Moon! Now to worry about the quintillions of tons of lunar debris that’s about to impact the Earth’s surface…

Cheers for the question legend.
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