Even if you haven’t seen Star Wars, you likely know the scene – the Empire have captured Princess Leia and taken her to her home planet of Alderaan, threatening to destroy it if she does not reveal the location of the rebel base. Leia lies, but they give the order to fire – the Death Star launches a laser-like ray which blows up the planet. In honour of Rogue One coming to cinemas at the end of the year, let’s take a look at the science behind this weapon.
Alderaan is a similar size to the Earth, so let’s use that as a stand-in and think about how much power we would need to destroy it. You know that the Earth is held together by gravity – to destroy the Earth, we would need to apply more energy than holds it together, which is an astounding 2.24 x 1032 Joules. To put that in perspective, we would need a full week’s worth of the Sun’s total energy output (3.8 x 1026 Watts) delivered in a few seconds to cause the reaction we’re after.
So is it possible? Let’s think of the Sun again – it generates energy via the nuclear fusion of hydrogen into heavier isotopes and elements – and imagine the Death Star is doing something like that, but with a touch more efficiency. If we say that the Death Star generates the energy and expels it in a few seconds, it doesn’t quite work – it would cause a Moon-mass object to accelerate in the other direction at 78 km/s, which clearly doesn’t happen in the film.
The issue here would be more with storing the antimatter, but it’s not unreasonable to assume that a civilisation advanced enough to master faster-than-light travel and such could figure it out.
We can approach this another way, however, using that old sci-fi staple of antimatter. We can assume that Alderaan is made of matter as Princess Leia is, and suggest that the Death Star is firing pure antimatter at the planet – thus, it only needs to supply half the energy, as the planet itself will provide the other half. If we run with that, we only need 1.24 trillion tonnes of antimatter to provide the minimum amount of energy necessary to blow the world apart, and that isn’t very big in the grand scheme of things. In this case, the amount needed is 0.00000002% the mass of the planet – once it hits the planet’s surface, it would have no trouble clearing a path to the core, where the planet’s densities are highest, and destruction would occur right away (it’s nothing more than the energy released according to E=mc2).
The issue here would be more with storing the antimatter, but it’s not unreasonable to assume that a civilisation advanced enough to master faster-than-light travel and such could figure it out. On Earth, we have been able to create and store neutral antimatter, and it’s only our failure to keep normal matter away from it that causes it to annihilate – the Empire’s scientists have no doubt come up with a solution to this, hence their fearsome super-weapon.