This post stems from a conversation I was having with Josh about the possibility of colonising Mars. I was saying how the major stumbling block would be finding a solution to the solar radiation problem since Mars is geologically dead. We’re protected from all the harmful stuff on Earth by the magnetosphere, which is generated by a spinning liquid iron core that basically acts as a giant electromagnet, but Mars’ internal dynamo – if it ever had one – ceased activity a long time ago. Josh then asked the following question:

“Could we restart it?”

Ahahaha. Ahahahahaha. Ahaha. No.

Subsequent questioning revealed that he hadn’t actually seen The Core and my blood pressure started to return to normal, but Hollywood and the media are responsible for a very large-scale misunderstanding about what science can do. Specifically, they have led people to believe that if you throw enough nukes at a problem it’ll go away: after all, to a person a nuclear explosion looks pretty big, right? Everyone’s seen the pictures of what was left of Hiroshima and Nagasaki and the spectre of nuclear war has been hanging over the world ever since the damn things were invented, and just because they can cause the extinction of the human race people have gotten the idea that nukes are the final solution to anything.

But the thing is, nukes aren’t actually that powerful. The fireball will burn and blind anyone standing within a couple of kilometres of the blast zone, but anyone standing outside that would survive the first second or so after a nuke went off. Nukes do most of their damage via explosive overpressure; that is, a shockwave of compressed air moving so fast that when it reaches you it’s like being hit bya concrete wall at several hundred miles per hour (this is why ICBMs were set to explode several hundred metres above a city in an airburst so that the shockwave would travel downwards and flatten everything beneath it with little loss of destructive energy). In other words nuclear weapons are destructive because we detonate them inside an atmosphere. If you take them out of that atmosphere they become far less effective, and this is what Hollywood doesn’t seem to understand as it continues to chuck nukes at all of its problems. Asteroid going to wipe out all life on Earth? Throw a nuke at it. Sun dying out? Throw a nuke at it. Core of the world stopped spinning? Throw a nuke at it. Let’s run through a few of the more high-profile scenarios where Nuclear Weapons Saved The World.

The premise of The Core is that the Earth’s iron core has stopped rotating, collapsing portions of the magnetosphere and causing certain people unlucky enough to step out into direct sunlight to quickly take on the appearance of an overcooked chicken. A team of plucky scientists is tasked to venture deep into the Earth’s interior and restart the core by means of detonating a chain of nukes placed around it in strategic locations.

Now, the amount of bad science in The Core is by no means limited to its abuse of nuclear weapons (and no, I don’t care that the movie is somewhat tongue-in-cheek about it) but let’s just think about that one idea for a second. The Earth’s core:

a)      is just a bit smaller than the Moon.

b)      is made of very dense iron-nickel alloy, giving it a mass roughly twenty-five times larger than that of the Moon.

If you stuck twenty-five Moons together and detonated a few nukes on the surface, do you think you could get them to spin? I don’t. Just for the comedy value I’ll crunch some numbers.

One megaton of TNT releases 4.184 10¹⁵ joules of energy. Let’s be generous and assume they have thirty 50 megaton bombs (this being equivalent to the largest nuclear weapon ever detonated, the Tsar Bomba), which gives them a total explosive power of 6.276 × 10¹⁸ joules. Let’s further assume that they can somehow direct all of this explosive force so that it’s translated into rotational moment on the Earth’s core with no loss. The Earth’s core has about 30% of the Earth’s mass, or 1.79 × 10²⁴ kg. The explosion of all thirty bombs will therefore impart 0.00000356 joules of energy per kilogram – in other words, it’ll be enough to accelerate the core up to a whopping two millimetres per second velocity. And remember, this is assuming some things regarding the efficient transmission of the explosive energy that aren’t physically possible.

The point to take away from this is that when you compare nuclear explosions to people, they’re pretty big. When you compare them to planets they’re the equivalent of a gnat farting. But what about stuff that is smaller than planets? Surely if an asteroid were about to hit the Earth, for example, we could just send up some nukes to blow it out of the sky? Uh, no.

First you have the Armageddon example, which features an asteroid “the size of Texas” that’s on a collision course with the Earth. A team of zany oil drillers led by Bruce Willis is sent up to drill a big hole into the asteroid and drop a nuke down onto a fault line with the hope of splitting the asteroid in half and deflecting the bits so that they miss the Earth. This premise is less stupid than it could have been, while still being pretty stupid. By embedding the nuke into the asteroid they’re at least ensuring that all of the explosive energy will be absorbed by it rather than having most of it dissipate into space in a surface detonation, and they actually get Jason Isaacs to emphasise this in the film with a firecracker analogy that’s not completely terrible. That’s about all the credit I give it, though, because Armageddon really is one of the dumbest movies ever made.

Okay so — leaving aside why exactly it’s easier to train oil drillers to be astronauts than it is astronauts to dig a hole, leaving aside the space shuttles that handle like fighter jets and the magic space station that can spin fast enough to produce 1g of gravity without coming apart at the seams, leaving aside that godawful scene with the minigun (seriously what the hell) – this asteroid is apparently about the size of Texas. Texas is 1200 km across and the asteroid as shown in the film appears to be roughly elongated so we’ll assume that’s the long axis, with the short axes being half that. This gives the asteroid a volume of about 6.53 × 10⁸ km³, which is apparently a mix of rock and iron. Assuming a generous 90-10 split in the quantities there, the asteroid has a mass of 1.75 × 10²¹ kg. They’ve only got one bomb but I’ll assume it’s really, really big at 100 megatons, and they’re trying to split the asteroid in half so each potential half will have a mass of 8.53 × 10²⁰ kg. Detonating the bomb inside the asteroid will, in theory, give half the bomb’s energy to each half of the asteroid, so that’s an energy density of 0.000254 J kg^-1 which, if entirely converted to kinetic energy, will give each half a lateral velocity of three centimetres per second. Earth has a radius of 6,400 km, so in order for the two pieces to just miss it (and this is saying nothing of the leeway we’d need if we didn’t want the Earth’s gravity to pull them back in in a massive loop and slam into it anyway) this bomb would have to be set off seven years before the impact date.

(And that is assuming the plan would work as advertised. More likely — since the hole Bruce Willis digs is 800 feet deep, or 0.08% of the asteroid’s radius; this is like detonating a bomb in a particularly deep mine on Earth and expecting it to split the planet in half — what it’ll do is blow off a big chunk of the surface material and leave the majority of the asteroid completely intact.)

And then there’s Deep Impact, which is basically the same as Armageddon except instead of an asteroid it’s a comet. It’s a bit unfair to include it here since Deep Impact has science which, on the whole, is pretty solid – the governments of Earth see the comet coming several years in advance and build a magic spaceship (handwaved away in the film as using experimental Russian tech so it’s less offensively stupid) to take a team of granite-jawed astronauts up there to do something about it. Unfortunately that something, rather predictably, is burying a bunch of nukes to blow the comet to smithereens, which is pretty much the polar opposite of what we’d actually do if we saw a space rock/comet coming for us and had some time to prepare. Comets are not solid objects; they are instead loose agglomerations of ice with some silicate content thrown in, which makes setting nukes off inside them a bad idea for the following reasons:

1. Porous objects like comets have been shown to soak up impact energy (somewhat like a sponge) in a startlingly effective manner, as it all goes towards crushing out the pores rather than fracturing the object itself. The effectiveness of any nuclear explosion is going to be significantly dampened by this.
2. It is, however, much easier to knock fragments off of them because of their loose structure, so not only would a nuke fail to destroy the comet but it’d turn one large incoming object into several radioactive ones.

Point 2) is at least somewhat covered in the film, but the fact remains that this is not what we would do with that sort of lead time. The only way to avoid being hit by an incoming object is to deflect it somehow, not to destroy it, and nukes are a really, really bad way of doing this. If we had, say, five years warning (and we’d be astoundingly lucky to see if coming that far off) we could attach some rocket motors to the side of the asteroid and move it to a safe trajectory over a period of several years. If we only had a few months we’d be kind of boned; comets and asteroids are massive on a scale most people have trouble comprehending, and that mass makes them very, very hard to deflect.

Finally there’s Sunshine, which I’m not even going to dignify with a rebuttal except to say that we could crash Jupiter into the Sun and it would swallow it up with barely a burp. A nuke ain’t going to do squat.

So yeah, nukes: not as useful as Hollywood would have you believe. They are devices designed to be used on Earth to kill thousands of people at a time, not as a catch-all solution to space rocks, solar dimming, geological upheaval and alien invasion. We’d do well to remember that.

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