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.