If you follow UK news at all you’ll know that recently there was a bit of a brouhaha over the Royal Navy’s failed Trident II missile test just off the coast of Florida. Some of the more hysterical accounts of the incident have the missile veering towards the US mainland before self-destructing; these sound a little dubious, but there’s at least a sense of irony to the idea as Trident is a US-developed weapons system. The UK abandoned its own nuclear weapons development program back in 1958 in favour of simply buying the technology from the Americans, and there are some very good reasons why this is so. One of them is Violet Club.

The UK’s first serious attempts to develop a nuclear weapon actually predate the Manhattan Project, but after it became apparent just how much industrial capacity the US was willing to devote to producing a working fission bomb the British sensibly dropped their parallel development and instead swung their resources behind the American program. The British contribution consisted of both materials and scientists and significantly accelerated the completion of the bomb, but unfortunately one of the scientists in question was the spy Klaus Fuchs, who handed over a vast quantity of atomic data to the Soviet Union. Once this came out it really turned the Americans off of the idea of post-war collaboration on nukes, and after Roosevelt died and the only American copy of the agreement detailing post-war cooperation between the UK and US was lost (the British copy was sent to them, but certain suspicious figures in the US government weren’t convinced it was authentic) the US unilaterally broke off all collaboration, forcing the British to resume independent development of nuclear weapons.

British efforts culminated in the shipborne Operation Hurricane test detonation in 1952 and the first operational nuclear bomb — Blue Danube, which was essentially a copy of the Manhattan Project Fat Man — being tested in 1953. Unfortunately by this point both the US and the Soviets had tested their first thermonuclear weapons – if you want an explanation of the difference between nuclear and thermonuclear weapons you can read either the Wikipedia article or my own blog post on the subject, but a quick primer would be that a thermonuclear weapon uses a regular fission bomb as a first stage to kick-start nuclear fusion in hydrogen isotopes, which releases several times more energy than nuclear fission and creates a commensurately bigger bang.  Thermonuclear weapons effectively rendered pure fission bombs obsolete, as there were hard limits on the amount of fissile fuel that could be converted to energy by the chain reaction in the tiny fraction of a second before that energy was released and the bomb destroyed itself – much of the nuclear material inside a fission bomb therefore ends up being wasted, and cramming more uranium or plutonium into your weapon results in diminishing returns in terms of yield because it ends up being scattered into the shockwave as fallout material rather than being released as explosive power.

This was something of a problem for the British military establishment, as it was additionally becoming very clear that the delivery system of choice for a nuclear weapon was going to be a missile rather than a bomber. A missile has a far smaller payload capability than a bomber, which made the small, efficient thermonuclear devices ideal in comparison to large wasteful fission bombs. Unfortunately for them they’d expected fission bombs to have a longer lifespan and were stockpiling large amounts of highly-enriched uranium to build them. Not only would there be a gap in capability while they developed their own thermonuclear weapons, but said weapons would only require small amounts of this fissile material to trigger the second stage. As they were unwilling to see this expensive nuclear material go to waste, and using a smaller (yet saner) amount of plutonium would have been even more expensive, it was eventually proposed that an “Interim Megaton Device” be constructed using uranium to fill the perceived need for a megaton-grade weapon until the UK could build its own hydrogen bombs. Hence Violet Club.

Even on paper Violet Club sounds like a tremendously stupid idea. It’s a classic implosion design: a sphere of highly-enriched uranium surrounded by a set of high explosive lenses. When triggered the lenses explode inwards, compressing the uranium core so that it achieves critical mass; the uncompressed uranium core on its own is sub-critical and can’t go nuclear unless the explosive lenses trigger, and the explosive lenses are very difficult to trigger accidentally, making this a relatively safe design for fission weapons up until this point. The problem with Violet Club was that the target yield of one megaton was so much larger than a typical fission yield (which at this point was on the order of a hundred kilotons or so) that they had to cram in a truly ridiculous amount of uranium to achieve it. So much uranium, in fact, that the uncompressed mass of the uranium sphere was actually greater than one critical mass. The reason it wasn’t instantly going nuclear was because the Violet Club core was spread out into a thin, hollow uranium shell – critical mass is a bit of a misleading term and should be thought of as more like critical density as it relies on having a lot of fissile atoms in very close proximity so that they can bounce neutrons off one another — and the big gap in the middle was enough to stop the chain reaction from running out of control.

Now, a key feature of nuclear weapons, even back then, was that they should be fail-safe. If the weapon is not armed then it should be impossible for it to go off, even if e.g. the bomber it’s being carried on gets shot down and crashes, or if there’s a fire on-base and the building that it’s being stored in collapses. Violet Club was very emphatically not fail-safe, however; if the uranium core was crushed or damaged in any way that led to that hollow gap in the middle being squeezed out of the sphere, the chain reaction would start and the bomb would detonate. It wouldn’t be as destructive as if it were triggered intentionally as without the explosive lenses to compress the core the reaction would be even more inefficient than it already was, but it would certainly have been enough to ruin the day of anyone caught within a mile or two of the epicentre. This is why the bomb designers included a rather dubious safety feature: a small hole was bored through the uranium shell through to the hollow gap inside through which 133,000 steel ball-bearings were inserted. The ball-bearings were then sealed inside with a plastic bung. The theory went that as long as the ball-bearings were present inside the weapon, that hollow gap that prevented it from going nuclear couldn’t be crushed out of the core and the bomb would remain safe.

That was the theory, anyway. In practice the ball-bearings were even dumber than the bomb itself:

• Having to remove 133,000 pieces of steel from the interior of your weapon before it can be considered armed turned out to take some time – a minimum of half an hour, in fact (and up to 90 minutes during bad weather), which was far too slow considering the increasingly truncated warning time that was expected to precede any hostile nuclear action.

• The ball-bearings increased the weight of the Violet Club bomb assembly by half a tonne. The V-bomber force of the time (so-called as the UK’s nuclear deterrent consisted of Valiant, Vulcan and Victor bombers) could not carry a bomb that heavy, so the ball-bearings had to be removed from the bomb before takeoff. This ensured that Violet Club would always be armed while it was in the air, and this made it far too dangerous to be flown on exercises or even sent to a dispersal base to mitigate the effects of an expected nuclear strike.

• The bombs had to be stored inverted when not in use, as otherwise there was a risk that the plastic bung would fall out and the ball-bearings would exit the core, arming the bomb.

• By their nature nuclear bombers spent a lot of time sitting around on exposed airstrips waiting for an alert, and so did the bombs they carried. If it was a particularly cold day the ball-bearings would freeze solid inside the bomb, rendering it useless.

And so on. The Violet Club bombs were quite possibly the most dangerous and impractical nuclear weapons ever made, and the RAF knew it: 12 were ordered, but only 5 were delivered, and the ground crew who had to handle them were absolutely terrified of them. To add insult to injury, it was estimated (we’ll never know for sure as Violet Club was deemed too dangerous to even test) that thanks to the inherent inefficiency of pure fission bombs Violet Club would have a yield of around 400 kilotons – far short of the one megaton target yield.  In short, it was not an ideal outcome for the UK’s homegrown nuclear weapons development program, and god knows what else we might have come up with if we’d been allowed to continue bodging together the safety features that kept our nuclear deterrent from prematurely exploding.

Fortunately for us the Operation Grapple tests of 1958 proved to the US that we had the capability to build thermonuclear warheads of our own, and at that point the Americans figured that if we were going to get them anyway, they might as well make a few bucks by giving us their warhead blueprints and then selling us the delivery systems to go with them. The 1958 Mutual Defense Agreement swiftly followed and ensured Violet Club was the last purely British nuclear weapon ever deployed; Red Beard (the more prevalent — and conventional — successor to Blue Danube) remained in service until the switch to a primarily submarine-based deterrent force and the first Polaris patrol in 1968. And whatever else you can say about the American designs, at least they aren’t armed by physically yanking out a plastic bung from the warhead casing.

The post Violet Club appeared first on The Scientific Gamer.

I said I’d talk about thermonuclear weapons at some point. We have now reached it.

So you’ve got your fission weapon designs which are coming up on seventy years old now. They make a pretty loud bang, can seriously mess up a city if detonated over one, and refinement of the designs over the years have allowed us to squeeze ever larger booms out of the same amount of fissile material. Eventually, though, you run up against hard physical limits enforced by the fission reaction itself. Past a certain point you’re converting the vast majority of fissile material inside the bomb into explosive energy and further attempts to increase the bomb’s efficiency have drastically diminishing returns. However, modern nuclear weapon delivery systems – ICBMs with MIRV warheads – mean warhead weight is at a premium. It’s all very well to physically lug your fission bomb over to the target city in a WW2 bomber, but if you want to strike your enemies with genocidal death weapons launched from the other side of the planet you need something even lighter than even the most efficient fission weapon designs.

This is where thermonuclear weapons come in. Thermonuclear weapons are also known as fusion bombs or hydrogen bombs, and that’s because they rely on fusing together isotopes of hydrogen to produce most of their energy instead of a simple fission reaction to split atoms in a fissile material. You may be familiar with the nuclear fusion process as the thing that goes on inside the heart of a star to give us the heat and light we need to survive on Earth, as well as something that alternative energy researchers are very interested in as the possible future of large-scale power generation on this planet. Fusion can liberate a tremendous amount of energy from a very small quantity of fuel. Unfortunately for the energy researchers the pressures and temperatures required to naturally induce fusion are basically those found inside the core of a star, and are understandably difficult to reproduce in a stable form inside a fusion reactor. This is why fusion power was just around the corner twenty years ago, is just around the corner today, and will be just around the corner in twenty years time; achieving steady-state fusion requires materials and technologies that haven’t been invented/perfected yet like room-temperature superconductors, and while these technologies always seem tantalisingly within reach they never quite seem to materialise.

I’m digressing here, though. We’re interested in how fusion bombs work, not fusion reactors, and the cunning thing about the fusion bomb is that while you cannot yet reliably reproduce a star-like environment over a period of time appreciable to a human being (say five minutes or so) you can use a fission bomb as a first-stage to recreate the pressures and temperatures required for nuclear fusion for a very, very brief moment of time. As we saw in the first nuclear weapons post, given the runaway nature of nuclear reactions that one moment is all you need to make a very big bang indeed.

A simple thermonuclear weapon design looks like this (thanks, Wikipedia):

You have a fission bomb primary which works exactly as described in the original nuclear weapons post, so there’s no mystery there. What we’re interested in is the fusion secondary, although the interstage separating the two deserves a brief mention; that has to correctly direct the energy and radiation released by the explosion of the fission primary towards the fusion secondary in such a way that it successfully triggers fusion. If you don’t build your interstage correctly you get a “fissile fizzle”, which is just a small fission explosion and nothing else since the secondary stage didn’t trigger. Sadly I can’t tell you exactly how an interstage works because – as you’d expect – the details are highly classified.

The secondary itself is a column of fusion fuel contained within a tamper. The tamper works just like the tamper in a fission bomb: it is a very dense material such as uranium which serves to contain the expanding energies of the fusion explosion for just a few microseconds longer than they otherwise would, but which nevertheless makes all the difference in amplifying the power of the nuclear explosion. Inside the fusion fuel is a uranium rod which acts as a fission sparkplug; because it’s rod-shaped it does not constitute a critical mass until compressed by the detonation of the fission primary, at which point it itself will go nuclear and further heat and compress the fusion fuel to the point where it starts to fuse.

The fusion fuel is a heady mix of deuterium and tritum formed by lithium deuteride.  The flood of neutrons created by the chain reaction in the fission primary bombards this lithium deuteride, splitting the lithium component into helium-3 and tritum. Tritum is one of the isotopes of hydrogen which can undergo nuclear fusion, the other being the deuterium component of the lithium deuteride. The explosion of the primary bookended with the now-nuclear fission sparkplug compresses the tritum and deuterium together, forcing them to fuse together into helium-4. Incidental byproducts of this fusion reaction happen to include a neutron, which can then go off and split another lithium atom, and 17.59 MeV of energy. The heat provided by the fissionable bomb, tamper and sparkplug is key to the nuclear fusion reaction, hence the term “thermonuclear bomb”.

One of the interesting things about the fusion reaction inside a hydrogen bomb is that it creates very few harmful radioactive byproducts, with the main result of the reaction being a crapload of ionising radiation that is reflected back into the bomb core to increase the power of the explosion. If this radiation was released it would certainly kill anything unlucky enough to be in the blast radius but it wouldn’t persist, making a fusion bomb far cleaner in theory than an equivalent fission bomb. Unfortunately the two-stage design of fusion weapons means that they include fission bombs by default, and furthermore the fissionable material providing the heat and pressure for the fusion reaction means that it’s still going to scatter radioactive fission fragments over a wide area, making the typical hydrogen bomb just as bad as any fission weapon in terms of irradiating the blast zone. Designs which replace the fissionable uranium tamper with another dense, non-fissionable material such as lead do exist, and while they reduce the yield of the bomb by about half they’re also far “cleaner”. These are the fabled neutron bombs, which are supposed to kill people but leave infrastructure intact¹, and they do this via the spray of neutron radiation released by the fusion reaction described above.

How much more efficient are thermonuclear weapons compared to common-or-garden fission bombs? As a rough guide, the explosive power of a fission bomb ranges from a few kilotons for the most basic, primitive types (Fat Man and Little Boy) to about half a megaton for a modern fusion-boosted design – that is, a fission bomb with a small amount of fusion material mixed in to boost the yield. By contrast thermonuclear weapons start at about half a megaton and work their way up from there. The largest nuclear weapon ever detonated was the fifty-megaton Tsar Bomba, which actually added a third fusion stage to the fission primary and fusion secondary described here to achieve its obscene yield, but Tsar Bomba was a product of Cold War willy-waving and was subsequently deemed overkill for the practical use of nuclear weapons: destroying cities. Most modern thermonuclear warheads have a yield range anywhere up to about 1.5 megatons, this being more than sufficient to do the job as long as your ICBM targeting is good enough. Nuclear warheads are designed to explode above cities, not on them; this allows the heat and the shockwave to flatten a wide areas below them rather than having their explosive energy soaked up by having to travel through buildings, and if you do it like this it turns out you simply don’t need a very big bomb to devastate a city. Also many small explosions are more efficient in terms of distributing energy than one big one², so scattering a series of smaller MIRV warheads across a large area is a much better way of doing the job.

Well, that was educational. I don’t know about you, but I’d managed to go through 28 years on this planet without knowing just why these nightmare inventions were called hydrogen bombs. And now I do. Isn’t learning fun? The actual effects of the explosions on a civilian population are somewhat beyond my purview, but if you really want to be depressed for the rest of the day you could do worse than watch the War Game, a sixties mockumentary that was banned from broadcast for twenty years because it was “too horrifying for the medium of broadcasting”, and also Threads, the 80s equivalent.

 —————————————————————————-

1. Although they’d still do a fair bit of damage thanks to having a fission bomb jammed in the top.

2. The power of the explosion diminishes exponentially with distance in accordance with the inverse square law, which is another reason why Tsar Bomba was a stupid sabre-rattling exercise.

The post Global Thermonuclear War. appeared first on The Scientific Gamer.

In which I’m going to do my damndest to get the West’s security services interested in my blog. Hi guys!

Making a nuclear bomb is harder than you might think. When you stuff nuclear fuel into a reactor the fission chain reaction is carefully controlled via neutron moderators and absorbers so that it occurs in a stable, steady-state fashion. This nevertheless liberates a tremendous amount of energy every second, so much so that we need dedicated cooling systems just to deal with the aftermath of the chain reaction when the reactor core is shut down. However, if you took away all the control rods and failsafes and dumped all of the coolant out of the reactor and basically tried to make the thing explode, you’d run into two problems.

1)      The percentage of nuclear fuel that is fissile u-235 (3%) is far too small to create much of a bang.

2)      While nuclear reactors are deliberately built to be inefficient in terms of the rate of neutron reactions (after all, we don’t really want melted down reactor cores dotting the planet) just taking the neutrons off the leash isn’t particularly efficient either.

The efficiency (or rate) of your nuclear reaction largely on how densely your critical mass is packed together; if you have a lot of u-235 atoms crammed into a small volume you dramatically increase your chances of scoring a hit with a fast¹ neutron, as well as increasing the chances of child neutrons inducing subsequent fission events. A critical mass that is unintentionally triggered into undergoing a chain reaction goes on the big list of criticality accidents. Wikipedia says there’s been about sixty of these since nuclear power first became a thing, but you’ll notice there isn’t a corresponding big list of sixty accidental nuclear explosions. This is because criticality accidents are inherently self-correcting in the same way that the light water moderator I talked about in the PWR post is; inducing a chain reaction in a critical mass heats it up, which causes it to expand, which spreads out the fissile atoms, which reduces the critical mass below the density required to sustain a chain reaction.  As a result of this most criticality accidents are like the ones caused by the demon core; they release a burst of hard neutrons that usually kill whoever was fiddling with the critical mass, but they don’t proceed any further than that. In order to get our critical mass to actually explode we’re going to have to get creative.

The first step is in packing as much fissile material into the critical mass as possible. That 3% u-235 is far too weedy and the other 97% u-238 is effectively useless for our purposes; if we want a really big bang we’re going to have to enrich our uranium to the point where 90%+ of it is u-235. To do this we need industrial scale isotope separation facilities like the Manhattan Project’s three plants at Oak Ridge, which work in a sequential cascade process. The way the separation process works is that after you “enrich” raw uranium that you get out of the ground you end up with two separate collections of material: one with a slightly higher concentration of u-235, and one with a slightly lower concentration of u-235. The material with more u-235 is sent up to the next step in the chain which separates it again, producing another two sets of material with more and less u-235 respectively. The material with more u-235 is sent further up the production chain where it progressively has more and more of its u-238 content squeezed out of it. The leftover material with less u-235 isn’t wasted either; instead it’s sent back down the chain to start the process again and get as much pure u-235 out of the raw uranium as possible.

The separation process takes a lot of time and effort and is pretty much the reason why only nation-states have the resources to make nuclear weapons from scratch. Iran keeps trying to establish the industrial base from which they can perform sustained isotope separation, which is why the US and Israel are investing quite a lot of time and effort in fucking with the Iranian centrifuges. Assuming we can build the facilities and that the US doesn’t come in and drop a JDAM on it, though, we’ll eventually end up with a lump of weapons-grade uranium. Or rather, several small lumps none of which are large enough to constitute a critical mass. The next step is to find some way of making them go critical on demand.

The simplest way of doing this is the gun-type fission bomb, which is what was dropped on Hiroshima. Here you have two sub-critical pieces of fissile material separated by a short distance, and when you want your bomb to explode you detonate an explosive inside the bomb which “fires” one piece of material at the other, smooshing them together and making them go critical. Cue one big bang.  This design works, but it’s rather inefficient in terms of both the total amount of fissile material used to make the bang and the amount of that fissile material which is consumed before the bomb explodes. A bomb which uses 100% of its fissile material in the chain reaction before releasing its energy and exploding is one which is operating at maximum efficiency; however this is very much the ideal scenario, whereas in reality the chain reaction inside a simple critical mass doesn’t occur fast enough to ensure all of the atoms in the fissile material get to react with a neutron. If the reaction is so slow that the fissile material explodes before all of it has had a chance to react, then you’ve effectively wasted a certain percentage of your fissile material, and this is bad for two reasons. One: the bang the bomb will make isn’t as big as it could be. Two: given the trouble you went through to produce the fissile material in the first place, you really don’t want to be wasting it.

To make the bomb as efficient as possible involves taking advantage of a few of the quirks of the critical mass. The critical mass isn’t set in stone; it’s simply the mass of fissile material required to sustain a chain reaction, and so it can change depending on the presence of various conditions that make things more or less favourable for the neutron pinball going on inside the bomb core. The three main ones we’re concerned with are:

Density. We’ve saturated our bomb core with as many fissile atoms as possible by enriching the uranium to 90% u-235, and this makes the core a target-rich environment for any neutrons looking to split an atomic nucleus. However, we can go further than this. The density of u-235 at room temperature is comparatively low. If we were to somehow artificially compress the bomb core we could jam the u-235 atoms into a much smaller space, increasing the efficiency of the chain reaction yet further and ensuring most of them are used to power the explosion.

Presence of a neutron reflector. The most efficient shape for a critical mass is a sphere, as this has the lowest surface area to volume ratio of all possible shapes and ensures the majority of u-235 atoms are surrounded on all sides by other u-235 atoms. This makes child neutrons produced as part of the chain reaction far more likely to strike and react with other u-235 atoms. However, a sphere of fissile material will still lose neutrons from the outer layers of the sphere, decreasing the efficiency of the reaction. This can be solved by surrounding the bomb core with a neutron reflecting material such as beryllium, which bounces escaping neutrons back into the core and ensures as many of them react with u-235 atoms as possible.

Presence of a tamper. When a critical mass undergoes a chain reaction, it will get hot. When things get hot, they expand. When things expand, their density decreases. This is undesirable when we’re trying to make the bomb core as dense as possible during the detonation process, so bomb cores are also surrounded by a very dense material called a tamper that contains this expansion to a degree. It cannot stop the expansion because no material can withstand that stresses and forces contained with an exploding nuclear weapon, but it will slow it down. Early fission weapon designs combined the tamper and the neutron reflector into one component – usually u-238 since this had the added bonus of occasionally reacting with the fast neutrons emitted by the chain reaction and boosting the yield of the explosion even more.

This is why the second type of nuclear weapon design – the implosion design – looks like this:

It’s a core of subcritical fissile material surrounded by a tamper/reflector and a series of high-explosive lenses. When you want to detonate the bomb you trigger the high explosive. This creates a shockwave focused inwards towards the core which compresses it very very quickly, increasing its density and causing it to go critical. The chain reaction starts, and escaping neutrons are reflected back into the core by the tamper. The tamper itself only delays the explosion itself by a fraction of a millisecond, but this is enough time for several additional steps in the chain reaction to take place. Given the exponential nature of the chain reaction (1, 2, 4, 8, 16, 32 etc.) that additional fraction of a millisecond is enough time for the energy of the explosion to increase tenfold².

Implosion weapons are more complex and advanced than gun-type nuclear bombs, but they’re also far more efficient. The Little Boy bomb dropped on Hiroshima was a gun-type fission weapon containing 141 pounds of 80% enriched uranium, and had a yield of 16 kilotons. The Fat Man bomb dropped on Nagasaki had a 13.6 pound core of plutonium, and yet the nuclear reaction catalysts described above increased its yield to 21 kilotons, producing a bigger bang with less than 10% of the fissile material. If you want economy in your fission weapon, then the implosion design is the way to go.

Fission weapons aren’t the whole story, though. In fact, they’re only the first page. There will be a sequel to this post at some point, and it will talk about the current cutting edge in nuclear weapon design: the thermonuclear bomb.

1. Note: there is no moderator present inside a nuclear weapon to produce low energy thermal neutrons, so we have to rely on fast neutrons to sustain the chain reactions
2. I can’t find a source for the exact number so I’ve made a guesstimate based on the prompt lifetime of a neutron in a fast fission reaction (10^-7 seconds). If anything I’ve lowballed it, though.

The post Our Words Are Backed By Nuclear Weapons! appeared first on The Scientific Gamer.

 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.

The post Nukes: The Swiss Army Knife Of Hollywood. appeared first on The Scientific Gamer.