Hentzau in "Not actually building giant space laser" shocker.
Since I have a free hour, I thought I'd clear up a few things about what it is I actually do all day. The internet would have you believe that I am building a giant space laser with which to destroy Pluto and take over the world. Now, while this is certainly a worthy cause, it is not an entirely accurate description of my work. Yes, I am trying to work out how much energy is needed to blow up Pluto. I am not, however, using a giant space laser to do it. I am instead using this:
It's a light gas gun, and it accelerates small projectiles (a couple of millimetres in diameter) up to velocities of seven kilometres per second. For comparison your average sniper rifle fires bullets at just under one kilometre per second, and the cannon on the Leopard 2 tank has a muzzle velocity of 1.75 kilometres per second. In other words, seven kilometres per second is fast. Since most stuff in the solar system (comets, asteroids and whatnot) is flying around with a velocity either comparable to this or even greater, this makes the light gas gun an ideal tool for modelling the sort of collisions that occur out there.
This brings us to Pluto. Pluto is on the inner edge of an area of space known as the Kuiper Belt; a large belt of icy bodies that circle the Sun beyond the orbit of Neptune. We've only managed to find 800 Kuiper Belt objects so far, mainly because the damn things are suspected to have an average albedo of around 2.5% - roughly the same as, say, road asphalt. The ones we've found just happen to be the largest and the brightest. Anyway, due to orbital resonances (way too complicated to explain here) Pluto shares its orbit with 150 other bodies, called Plutinos. While it's a large orbit this does make things rather crowded, and so Pluto is a good body for modelling impacts, both because it's more likely to undergo one and because we know more about Pluto than we do about any other Kuiper Belt object.
So, I'm using the gun to model impacts on Pluto. I'm slightly limited by what we can fire - for example, the gun can't fire ice, which is a shame because that's what Pluto is most likely to be hit by. Nylon has a pretty similar density but is a complete pain in the arse; of the four nylon shots I've done only one has succeeded. Most of the time we use stainless steel projectiles, which is unrealistic but convenient. But what are we shooting at?
The short answer is this:
That is a typical target about the size of a ping-pong ball. It is the size of a ping-pong ball because that's what I used to make it - I fill the ball up with sand and water, freeze it and then cut the shell off. This gives me a ball of ice and sand. Pluto is suspected to have an ice-silicate core with a thin layer of ice covering it, so I then dip the ball into water and liquid nitrogen repeatedly to create an ice shell.
Then I shoot it.
To cut the rest of this long story short, I measure the mass of the largest remaining fragment and compare this with the kinetic energy of the projectile, and that lets us model how much energy it would take to blow up Pluto. As for exactly why I'm doing this, I couldn't begin to tell you. I cannot help but feel that, in the end, the giant space laser would have been more interesting.