I have a whole backlog of questions here and my answers to many of them are far too snappy to justify a full post, so I’m going to economise and do a three-in-one. HOLD ON TO YOUR BUTTS.

Gerry asks

I’ve noticed that when I have a string of some sort (be it a usb cord or an audio cable that I use to plug in my phone to my aux in my car stereo, or any other string-like item) it has a tendency to tangle. This happens all the time (depending on the environment) is there a branch of physics or science/math that investigates this phenomenon? (Chaos maybe?) Not to be confused with the Quantum physical term of entanglement or spooky action)

There absolutely is (well, kind of). It’s called knot theory, which is one of those things bored mathematicians do in their spare time. You can keep a string straight and unknotted if you don’t allow it to cross itself, or cross with other strings (usually by tying it up in a coil). As soon as it crosses, though, you put it in a mode where it can enter a knot state, and since there are vastly more knot states where the string is tangled (trillions) than there are knot states where it is straight (one), the odds of your string remaining unknotted are trillions to one against. All it takes is the slightest movement to instigate a transition through various knot states, and since we’re talking about stuff like tremors transmitted through floorboards, changes in the temperature of the surrounding air causing the string to expand/contract, your cat sleeping on them etc. etc. it’s impossible to avoid this without physically containing the string as in the coil example. Basically, as soon as your string/wire/cable/whatever starts moving it’ll start shifting through knot states. Strings are always moving. Strings always knot.

Joshomon asks

This is a map of meteor craters: http://www.scienceclarified.com/landforms/images/ueol_02_img0082 Two features spring to the eye: the clustering in Europe, America and Australia, and the lack of impacts around the equator. The first should be easier to explain: more densely inhabited, developed countries. But it’s not entirely satisfying. Meteor impacts are measured on a long timescale, and the period in which America and Australia in particular have been developed is short. Presence of geologists and astrophysicists is also interesting, but frankly impact craters are quite big, so I can’t see a reason why eg Chinese impact craters would be obscure. The lack of equatorial impacts is also interesting, There’s only one impact crater between Khartoum and Rio de Janeiro, and even accounting for the lack of population and scientific community that seems very unlikely. So what gives? Are meteor impacts that clustered, or is there an obvious factor in the myopic recording that I’m missing? In either case, why?

 It’s a combination of geographic bias, crater erosion and camouflage. This is a better map of known impact craters. Zoom in on South America. You’ll notice that the south of the continent has plenty of craters, but the north, covered in rainforest, does not. That’s not to say that the craters aren’t there – they almost certainly are – but out chances of finding them under that dense jungle canopy are practically nil. We know of few impact craters in Russia despite it being the largest country on Earth because a large portion of its land area is made up of sparsely-populated taiga; even when we know the rough location of an asteroid impact we can’t find the precise spot where it struck because the place is so sodding huge. I’m less up on my Chinese geography but I imagine there are similar reasons why there are few known impact craters there as well.

 Meanwhile, the clusters in America and Australia are easily explained away by the fact that these are the geological Meccas of the world and are positively teeming with archaeologists, geologists and other scientists¹ who are combing the landscape for interesting features. Impact craters aren’t always easy to find despite their size, either; classical crater-shaped craters are incredibly rare, and the passage of time (and the size of the impact) is more likely to make your crater look like this – i.e., nothing like an impact crater. They take time and effort to identify, which is another reason why we’ve only found the low-hanging fruit so far. That impact craters are clustered over certain areas of the Earth’s surface merely reflects the immense difficulty in finding them rather than a real phenomenon, and our current data isn’t even remotely representative of the full picture.

Taz asks

Last night I was watching ‘Contact’ (http://www.imdb.com/title/tt0118884/) and was wondering what your thoughts were on this film. I might suck at search because I’ve found no article about this on your blog and I am curious. Both in the film being film and how many errors they have made. And the philosophical question behind the film. I hope you’ll write an entry about that film!

Contact is a bit tricky to talk about because (wisely) the film is mainly dealing with the philosophical question of talking to aliens rather than the scientific question of how we’re going to achieve this. It’s based on a book by Carl Sagan so most of the core concepts are (annoyingly) fairly sound, and the only actual mistakes it makes are nit-picky ones that can be put down to artistic licence.The one thing I might draw the film up on is the idea that the television broadcast of Hitler addressing the 1936 Olympics is going to make it all the way out to Vega, 26 light years away. You could get a coherent signal out that far if you focused it into a directed beam (probably), but the thing about television broadcasts is that they are broadcasts – i.e., they are omnidirectional and spread out in a sphere from the point of origin. This means that the strength of the signal is going to be fading exponentially the further away it gets from Earth and at 26 light years distance it is going to be very faint indeed, to the point where you’d need absurdly sensitive detection equipment to distinguish it from normal background radiation. I don’t think it’s very likely it’d survive in a form where an extraterrestrial species could find it, make sense of it, and beam it back to us.

Otherwise most of the film is out of my purview – the wormhole device is alien Star Trek technology so I can’t deconstruct that, and aside from a woeful misunderstanding of the numbers involved in the Drake equation the bits and pieces of astronomy that make it through into the movie are reasonably solid, which gives me little to talk about. Sorry!

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After spending not a few words talking about Armageddon and fake space rocks last Wednesday I was slightly surprised when an actual real-life asteroid tore through the skies above Russia and disintegrated/detonated in midair somewhere above Chelyabinsk. Thanks to the asteroid’s passage over populated areas and the modern ubiquity of smartphones with some kind of video capture capability – not to mention the uniquely Russian preponderance of car dashboard cameras to provide some protection against the now-famous driving standards in the country, as well as the notoriously corrupt traffic police – this has been by far the most well-documented asteroid “strike” in history, so I thought I’d take a little while to talk about it, and the reaction to it.

First, there’s the question of nomenclature. I’m going to be calling the Russian object an asteroid in this piece, but you should know that the term “asteroid” is rather ill-defined (in fact all the terminology used to refer to things smaller than dwarf planets is rather ill-defined), and the best I can come up given currently-existing IAU guidelines is “a lump of rock that is smaller than a dwarf planet but larger than a meteoroid”. Meteoroids are currently defined as “a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom”, but that definition dates from 1968 and we’ve since determined that meteoroids and asteroids are basically the same population of bodies, with the term “meteoroid” being used to refer to any small-scale asteroid (10m or below) that manages to enter Earth’s atmosphere. Most meteoroids burn up in the atmosphere; the ones that make it through to hit the ground are called meteorites, which is what the majority of the press is calling the Russian asteroid.

Are they correct? Well, yes and no. There are two problems with calling the Russian asteroid a meteorite:

1)      It was actually quite large – NASA says around 17 metres – which puts it out of the generally accepted size range for a meteorite.

2)      It didn’t actually hit the ground. While it didn’t burn up completely, the Russian asteroid broke up just above ground level thanks to the immense heat and pressure of entering the atmosphere.This will have scattered chunks of asteroid everywhere and there’s already been several fragments and craters reported found. Astronomers have a specific term for asteroids that create fireballs and break up in the atmosphere like this: we call them bolides.

However, if the press went around referring to the “Russian bolide” in their news reports nobody would have any idea what the hell they were talking about, so in the absence of any better term calling it a meteorite is fine. This is the IAU’s fault for not ever bothering to classify small Solar System objects properly and relying on fuzzy definitions that are decades-old, not bad reporting.

Now, let’s take a moment to watch a video of the asteroid’s passage.

(I have desperately tried to avoid lifting the video links straight from Bad Astronomy guy’s excellent rundown of what was going on at the time, but the originals I saw have since been buried by recycled clips from news agencies and I didn’t save them for future reference.)

That gets across just how bright the asteroid’s entry into the atmosphere was; it’s a large object travelling at around 18 kilometres per second (i.e. far, far faster than a supersonic jet aircraft, which would be lucky to manage a kilometre and a half per second) so it’s generating an awful lot of ram pressure, which heats up both the asteroid and the air flowing around it. If you direct that amount of force and energy against a lump of rock and metal it is rather understandably going to start falling to pieces, which is exactly what the Russian asteroid did; it lasted barely thirty seconds in the Earth’s atmosphere and broke into bits over the Urals near Chelyabinsk. But did it explode?

This video shows that there was a very large bang about thirty seconds after the asteroid’s passage through the atmosphere. This is caused by a sonic shockwave, which is what broke all the windows, smashed up that zinc factory and injured a thousand people, but the shockwave is not the product of the asteroid exploding or hitting the ground. Instead it’s been caused by the simple passage of the asteroid through the air at such a ridiculous speed. You’ve heard aircraft make sonic booms as they break the sound barrier? This is that, except magnified about a hundred times. The shockwave of a sonic boom is created by air piling up in front of the aircraft, and as the aircraft goes faster and faster the air simply cannot get out of the way fast enough and becomes compressed into a single shock front moving at the speed of sound. Since the aircraft is moving faster than the speed of sound you’ll see a supersonic jet a few seconds before you hear its sonic boom. Scale up from an aircraft to a large chunk of rock 17 metres on a side, and make the time lag thirty seconds (because it was around 30-50 kilometres up, an altitude much higher than aircraft fly at) and you get the shockwave caused by the Russian asteroid.

So the bang wasn’t caused by any explosion in the conventional sense; it’s not like the asteroid reached the end of its trajectory and abruptly exploded like a nuclear weapon. Instead, as bits and pieces sheared off of the asteroid it would have released energy with the same effect as a trail of smaller explosions. NASA puts the total energy released during the asteroid’s passage through the atmosphere at around 500 kilotons, which is roughly equivalent to a mid-sized nuclear bomb, but this energy was released gradually instead of all in one go which is why the property damage was mostly limited to a lot of smashed glass.

That’s what we think happened, but sadly there’s a couple of myths that have already grown up around the Russian asteroid:

• That it was something to do with the asteroid 2012 DA14, which was scheduled to make a very close approach to the Earth on the same day. This is stupid because the two objects were on completely different trajectories as demonstrated by this video; the Chelyabinsk asteroid grazed the Earth’s atmosphere on an east-west trajectory, while DA14 passed it by north-south. They are completely different bodies that just happened to get noticed by the human race on the same day; thousands of meteoroids/meteorites enter the Earth’s atmosphere every year and the vast majority of them are completely unknown to us because they burn up too high, or they come in during the day, or they descend over the ocean. The only things that make the Russian one special are its size and that it happened to come down over a populated area.

• That it was intercepted by Russian air defence forces and destroyed. This is insane for a number of reasons but I’m going to pick the most salient one: the best air defence technology on earth currently cannot make a reliably successful interception on an ICBM moving 7-8 kilometres per second. It would have no chance of hitting an asteroid moving at 18 kilometres per second.

Finally there’s the media reaction to it, which if you look past the usual idiotic misreporting is basically one of total ignorance. Despite reporting on 2012 DA14 for a week or two the fact that an asteroid could actually make it through (most of) the atmosphere and cause some damage to a populated area seemed to take them by surprise. I lost count of the number of hastily-commissioned opinion pieces I saw which basically said “Whoa, maybe we should be a bit worried about this!” like they previously thought it was something that only happened in movies. The good news is that even if 2012 DA14 had hit it would have caused only localised damage (roughly equivalent to one city, and that’s only if it had managed to score a direct hit), and that really threatening impacts only occur over geologic timescales. The bad news is that there is literally nothing we could do to stop either kind of impact without a decade of lead time to prepare – and as the Russian asteroid showed, often we don’t even see them coming at all.

(Just as an aside, every news article under the sun mentioned the Tunguska event as another meteorite event which happened in Russia, but nobody brought up the Sikhote-Alin meteorite which came down over what was then the Soviet Union in 1947. Unlike Tunguska this was witnessed by people who understood what they were looking at, including an artist who immediately drew this. It’s strikingly similar.)

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Saying that I have a Ph.D elicits a fairly predictable reaction from most people. They will, in an attempt to appear interested, ask “What subject?” and then when informed that I did Astrophysics – one of the simpler branches of physics if you don’t tangle with cosmology or relativity but which appears to have a fearsome reputation in the eyes of the layman – their eyes glaze over and they either stop talking to me altogether, or else they desperately try to change the subject before I can get a chance to pounce on them, knock them to the ground and inject pure Science into their brains via their ear canal¹. There’s a second type of person out there, however; the freakish sort who are genuinely interested in science, and this second type will, after some circumspect small talk, eventually get around to asking me what my thesis was about. And this is a question to which I have gradually evolved a tried-and-tested one-sentence reply:

“I am trying to find out how much energy you need to blow up Pluto.”

As with all one-sentence descriptions it doesn’t even come close to summing up the totality of my work, but it is accurate and it does its job of seizing their interest in an unbreakable choke-hold so that they ask the follow-up question:“Why?” Today, you’re all going to find out. You lucky, lucky people.

What does it mean to destroy something? Gormless techno-fetishist Michael Crichton inadvertently touched on why this is a bit of a tricky question in one of his godawful dinosaur novels, in which the shameless author self-insert of Ian Malcolm tells another character that his statement that nuclear weapons could destroy the world is really, really goddamn stupid. The reasons for this should be fairly obvious given the nuclear weapons post – while nuclear weapons pose a significant threat to humans, they could do very, very little to imperil the Earth. They’d scorch some parts of the surface, irradiate others, screw up the atmosphere for years – things that would have dire consequences for the future survival of the human race — but all this would amount to nothing more than a mild skin rash as far as the Earth is concerned. It’d just keep on truckin’ quite happily while we killed ourselves off.

Clearly you need a different magnitude of threat altogether in order to stand a decent chance of destroying the Earth. The sort of exotic cosmic catastrophe so enamoured of the Hollywood disaster movie notwithstanding (solar flares, quasars, supernovae, mini-black holes etc.), stuff smashing into other stuff is a thing that happens fairly often in our Solar System². What happens if we hit the Earth with a really big space rock? Well, that’s actually happened. The Earth was hit by a Mars-sized object very early on in its lifetime. Mars-sized.  The impact was so destructive we have a constant reminder that it happened in the form of the Moon. The impact was so destructive it blew off a majority of the Earth’s surface material. Yet even after this colossal impact event which would have fit most people’s criteria for “destruction” the Earth got better.

Key to this is that while the physical structure of the Earth was shattered by the impact, with bits and pieces flying in all directions, the heavy iron core was left mostly intact which meant the gravitational centre of mass of the Earth remained largely unchanged. Unless the impact fragments had been boosted up to escape velocity they were gradually and inexorably pulled back towards that centre of mass, with the result that the Earth slowly reformed much like Robert Patrick in Terminator 2. The Mars-sized impactor also left a remnant that accumulated bits and pieces of ejected surface material from the Earth; this went on to form the Moon, and it’s why the Moon is mostly made up of elements we’d expect to find in the Earth’s crust and mantle.

So even if you hit a planet-sized object really, really hard you’re not guaranteed to “destroy” it in any real sense of the term; you will disrupt its physical structure temporarily, but it may eventually reform into a single homogenous body again over millions of years. This destruction thing is a tricky business when applied to planets, and even when dealing with smaller stuff it’s difficult to draw a line. How much of an object do you have to destroy in order to say that the object itself is destroyed? A third? Two thirds? Total destruction? This is the first fundamental question that faces scientists who want to model impact events, and the answer they’ve come up with may seem arbitrary but it does have logic behind it.

First, impact scientists do not say impacts destroy something. We say they disrupt it, for the very good reason that disrupt is a term that can apply to any quantity of blown-off material, whereas saying something is destroyed runs into any number of problems up to and including the rather large one that matter cannot be created or destroyed, merely converted into energy. We define disruption by how much of the original body is left after we’ve whacked it with an impactor; in the case of my experiments I weighed my targets before they went into the gun, shot the crap out of them, and then weighed the biggest remaining chunk I could find afterwards. If that chunk was less than half the mass of the original body I’d rip open my shirt and beat my chest while let out an Arnie-in-Predator-esque roar of “DISRUPTION!”

The fifty-percent mark delineates disruption from cratering. If an impact removes less than fifty percent of an object’s mass it doesn’t count as disruption since the majority of the original object is still intact. Technically it’s a crater. It might be a really, really, really big crater – a crater that’s almost larger than the remaining mass of the body – but it’s a crater nonetheless. If the impact blows off more than fifty percent, though, it’s a catastrophic disruption outcome. This is as close to the word “destroyed” as you’ll ever get an impact scientist to go; personally I think the work in my thesis showed there’s a more granular range of impact outcomes than the simple binary choice of disruption and cratering described here, but my work was never published so eh.

So we’ve got a technical definition of “destroyed” now. In order to get there, we need to hit the object with enough kinetic energy to permanently disrupt more than 50% of its mass. Kinetic energy is worked out by the equation

where m is the mass of the impacting body and v is its velocity. Using kinetic energy allows us to make useful comparisons between an impact by a very small thing moving very quickly and an impact by a very big thing moving very slowly – i.e. it essentially makes the analysis independent of what the impactor actually is. We can further remove dependence on the size of the thing being hit by dividing the impacting kinetic energy value by the mass of the target; this gives the energy density of the impact, or Q, measured in joules per kilogram.

Q is, in theory, a very, very useful quantity. Say you have one target weighing 1 kilogram and another target weighing 1000 kilograms. By carrying out a series of impact experiments on 1 kilo targets in a lab environment you have determined that the energy density at which the 1 kilo target will lose more than 50% of its mass – the energy density at which it will suffer catastrophic disruption, or the critical energy density Q* — as 50 J kg^-1. In theory, it should then be possible to predict how much energy it’ll take to disrupt the 1000 kilogram target without having to manhandle any one-tonne weights around your laboratory; you just multiply by a thousand and bam, that’s how much energy you need to pump into it to remove 50% of its mass. In theory, we can predict how planet-scale masses will behave under impact using the same measurement we took from the 1 kilo target.

In practice, though, it doesn’t really work like this. You have a number of factors that alter Q* depending on the size of the target. At first, as you make a target larger, it actually becomes easier to disrupt, pound for pound, than a comparable smaller target. This is because cracks propagate far more easily through large objects than they do small ones, and it is crack propagation that breaks off the huge chunks of material required for disruption. So Q* will initially decrease as target size increases. Then, when you’ve increased the mass of the target to the point where its gravity starts to become a significant factor (as explained in the Earth impact example above), this trend reverses itself. From this point onwards the bigger – and therefore heavier – the target, the harder it is to disrupt, since you’ve not only got to blow the thing apart but you’ve also got to do it with enough force that you give the majority of the fragments escape velocity.

This is a graph from a paper published by Willi Benz and Eric Asphaug³ in 1999. Benz is a computer modeller while Asphaug does impact experiments in the laboratory; and they’ve combined their data in an attempt to put some numbers on how exactly Q* will change with target size. At first Q* decreases; the initial value of Q* and the rate at which it decreases will be heavily dependent on the material the target is made of – rock is harder to disrupt than ice, and so on – causing this to be referred to as the material regime. Then at target sizes of around 100 metres – 1 kilometre Q* starts to increase with target size; this is the gravitational regime. What’s really interesting about these graphs is that they’re for two wildly different materials – solid ice has a Q* of about 10 – 40 J kg^-1, while basalt is roughly twenty times that – and yet the shift to the gravitational regime occurs at exactly the same place and in exactly the same way for each. This means that once you hit a target size of 100-1000 m, the material strength of the target has entirely ceased to matter when calculating the result of an impact. It is instead the body’s self-gravity which must be overcome in order to disrupt it

My, that all got a little bit technical. If you didn’t understand the fine detail don’t worry; I didn’t explain it that well and it’s not necessary to grasp all of it to understand what I was doing with Pluto. All you need to take away from it is this:

• We can do impact experiments in the laboratory to get a value for the amount of energy we need to hit a target with to blow it up.
• We can then take that energy value and, allowing for variations in Q* with target size as per the above graphs, scale it up to larger bodies to predict how much energy we’d need to hit them with to blow them up.

So that’s how I was trying to find out how much energy we’d need to blow up Pluto. But why Pluto? And why is it important? Unfortunately I’ve already rambled on a little more than I really intended to, and thinking about it I probably need to explain some additional background before I get on to that. So it will have to wait a week or two. Sorry!

1. Presumably.
2. Over a timescale of hundreds of millions of years, that is.
3. Fun fact: I met Eric Asphaug at a conference in Spain and stole all his water.

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This is not what the interior of a light gas gun looks like.

 Time for something a little more hands-on. There’s all sorts of crap hurtling around the solar system — satellites, asteroids, comets and so on – and most of it is travelling at velocities that are, to put it mildly, completely insane. Supersonic jets and rifle bullets travel at about 1 km/s. Ramjets go at around 3 km/s. The average Earth-crossing asteroid, on the other hand, is hurtling through space at 20 km/s. This is Very Fast, and the reason Earth-crossing asteroids (so called because they cross the Earth’s orbit) worry people so much is because the kinetic energy of an object is proportional to the square of its velocity as per the equation

It doesn’t help that asteroids also tend to be rather heavy, and so they tend to possess a rather staggering amount of kinetic energy. Kinetic energy that would, if released on Earth somehow – say, by the asteroid hitting it – just ruin everyone’s day.

 So, that shit be fast and potentially very destructive. This makes studying asteroid and comet impacts an area of interest to scientists, since not only would we quite like to know what would happen if the Earth ever were hit by an asteroid, but it turns out stuff hitting other stuff is actually fairly intrinsic to the way the Solar System has developed and will continue to develop over time. Unfortunately there’s one small problem with this particular avenue of research: the human race has, in the entirety of its history, directly observed and recorded exactly one naturally-occuring impact in the Solar System. This was the Shoemaker-Levy 9 comet impact on Jupiter in 1994, which was funny because despite the fact that comets are sodding huge a lot of people didn’t think the impact would be that big. I’d have paid a fair amount of of money to see their faces when this happened.

 Why have we only observed one impact? Well, it’s actually very, very tricky to predict when and where a particular comet or asteroid’s orbit will intersect with that of a planet. You may notice every so often the news will report on an asteroid that astronomers think has a small chance of hitting the Earth, but they’re not really sure and need some time to figure out exactly where it’s going — and remember, those are the ones you’d think we’d particularly care about, what with them having the potential to wipe out vast swathes of the human race. Instead of spotting asteroids before they hit us (or something else) we normally only find out about the impact after it’s happened. For example, another asteroid/comet hit Jupiter in 2009 and the first we knew of it was when somebody spotted a dark patch on the surface that wasn’t there before. Even big impacts on Earth tend to go unnoticed; the famous one is the Tunguska event*, which exploded with the force of a 15 megaton bomb but which was only observed by the few Siberian natives unlucky enough to be standing nearby at the time.

The point I’m trying to make here is that gathering the data we need from actual impacts in the Solar System is practically impossible because we barely ever see one. If we want to study how impacts work, we have to create them ourselves. We’ve done this a couple of times by crashing probes into various astronomical bodies of interest (see Deep Impact) and learned a lot from it, but that sort of thing is prohibitively expensive since space probes don’t grow on trees. What we really want is a method of carrying out impacts in a laboratory setting – but here we run into the problem I mentioned at the start of this piece, which is that these things move really goddamn fast and even the fastest conventional manmade objects don’t even come close to their velocities.

 Enter the light gas gun (or “space laser”, as certain friends of mine have taken to calling it against my strenuous objections). This is a custom-built piece of laboratory apparatus specifically designed to accelerate very small millimetre-scale projectiles up to the lower bound of the velocities we might expect a typical asteroid to have**. It works more-or-less the same way as a regular gun – the projectiles are encased in a sabot, the barrel is rifled, and it relies on a rapidly expanding gas to drive the projectile – but with one crucial difference which you can probably guess from the name.

 To ram this home I’m going to have to explain how normal guns work. Or to be more precise, normal bullets, since it’s the bullet cartridge which contains all the stuff which makes the bullet go. The cartridge contains gunpowder and a primer; when the trigger of the gun is pulled a firing pin comes down which strikes the primer, igniting the gunpowder. The gunpowder burns and turns into an expanding gas which is confined by the gun barrel; since every other component of the gun is fixed in place the only way it has to expand is by pushing the bullet ahead of it, which is then propelled out of the gun at what is to us a reasonably high speed. So the gun basically fires the bullet by way of a precisely directed explosion; the problem here, though, is that gunpowder has what is called a low brisance. This means it doesn’t explode very fast in comparison to a high explosive like nitroglycerin.

 Now, from the gun manufacturer’s point of view this is a desirable attribute for gunpowder to have, since packing a gun with an explosive that has a higher brisance would rupture the barrel and injure the person shooting it, not to mention creating enough recoil to shatter the shooter’s arm. From the impact scientist’s point of view, though, this is rubbish. Not only does gunpowder expand slowly but it also burns unevenly, producing a sizeable pressure gradient in the column of gas propelling the projectile – in other words the front of the gas column that’s pushing the projectile is expanding faster than the rear of the gas column, meaning all the kinetic energy in the rear of the gas column never gets to the bullet and ends up being wasted. As it turns out, gunpowder is woefully inefficient for the purposes of carrying out impact experiments.

 This is why light gas guns don’t use gunpowder as their primary propellant. Instead they use a light gas such as hydrogen or helium, which have the lowest molecular weights possible. Further, they separate the propellant gas and the projectile into two distinct stages separated by a small metal burst disc. In the first stage, a shotgun shell detonates driving a piston down the gun which compresses the light gas very, very quickly. Once the light gas has been compressed enough it’ll have enough pressure to rupture the burst disc separating it from the projectile. This allows the light gas to expand again, and since there’s no burning of gunpowder going on it expands evenly and transfers all of its kinetic energy to the projectile.

But why use a light gas at all? It’s because gases with a low molecular weight will compress to a greater degree than heavier gases, giving them a higher expansion velocity once the burst disc bursts. Every aspect of the gun’s firing process is designed to wring the maximum possible amount of kinetic energy out of it, allowing the tiny projectiles it fires to reach velocities of up to 7 km/s! This is perfect for simulating impacts that might take place out in the Kuiper belt, with the caveat that you’re unlikely to find that many impactors out there which are perfect spheres composed of pure stainless steel. But hey, the gun only gets you so far. Scaling your lab results which use millimetre-diameter projectiles up to the sort of object sizes you’re likely to encounter out in the Solar System is a whole different bag of cats. But that, again, is a post for another day.

*Technically not an impact since the Tunguska meteorite exploded in the atmosphere before hitting the ground, but it proves the point.

 **It helps that things in the Solar System move slower the further out they are from the Sun’s gravity well. The icy stuff beyond the orbit of Pluto in the Kuiper belt ambles along at a relaxed 3 km/s, so the light gas gun works well for simulating that kind of impact speed.

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