That’s No Moon: Deconstructing the Death Star
Dec. 14, 2012 — In the original Star Wars movie Luke Skywalker, Han Solo and Obi-Wan Kenobi begin to ask thought-provoking questions about the amount of energy needed to vaporize an entire planet, and what sort of laser could achieve this. Maybe an hour into the movie, the three protagonists drop out of hyperspace and encounter a field of debris where they expected to find the planet Alderaan. This leads to the following exchange:
Han: Our position’s correct, but there’s no Alderaan.
Luke: What do you mean? Where is it?
Han: That’s what I’m trying to tell you, kid. It ain’t there. It’s been totally blown away.
Luke: What? How?
Obi-Wan: Destroyed … by the Empire!
Han: The entire star fleet couldn’t destroy the whole planet. It’d take a thousand ships with more firepower than I’ve …
Before he can finish the thought, Han spots another craft — an Imperial fighter — heading toward what appears to be a small moon. He decides to pursue it. This creates a whole other set of problems for our heroes, and as a result they never get a chance to continue the debate as to whether and how the Empire would be able to blow up Alderaan — or any planet, for that matter.
I’ve always been kind of curious about this. So a couple of weeks ago I touched base with Renaud Deguen, a geophysicist at Johns Hopkins University who has written extensively about the dynamics of the Earth’s inner core among other topics.
“It depends on how exactly you define ‘blow up a planet,’ ” he told me, when I asked what it would take. “But assuming this means vaporizing the whole planet, then the energy needed is the energy required to heat the planet up to its melting temperature and then its vapor temperature, plus the latent heat of melting of vaporization.”
In Star Wars, the space station known as the Death Star is armed with a laser powerful enough to destroy an entire planet. ©LucasFilm/Walt Disney Co.
Dr. Deguen very helpfully provided some back-of-the-envelope calculations for the Earth — simply as an illustration; neither of us has any plans to destroy it. These involved the mass of the Earth (~ 6 1024 kg), its specific heat capacity (~ 1000J/kg/K on average), the latent heat of melting (~ 106 J/kg) and the latent heat of vaporization (~ 107 J/kg). Multiply the mass of the Earth by the latent heat of vaporization, he said, carry the one (not really) and, voila, you have the answer.
So what is it? How much energy would we need to destroy the Earth, and what sort of laser could provide it? “The total would not be very far from 1032 J,” he said, “which would require a pretty big laser.”
Pretty big laser, indeed. Consider: The National Ignition Facility at the Lawrence Livermore National Laboratory delivers nearly 2 million joules of ultraviolet laser energy to a target in billionth-of-a-second pulses. The largest laser in the world today, it can produce more than 60 times the energy of any previous laser system. Yet its 2 million joules is a proverbial drop in the ocean compared to the amount of energy we would need to vaporize a whole planet: somewhere north of a nonillion, or one thousand billion billion billion joules.
Building a laser capable of generating that kind of energy is theoretically possible, but it would require resources beyond what any government could provide. And even then there are other, practical concerns. Maintaining the laser would be an almost insurmountable challenge in itself, for example. Every experimental shot of the National Ignition Facility involves up to 60,000 control points, with a path length of more than a kilometer for each of the 192 beams. If you scale this up to the type of laser needed to produce the amount of energy we’re talking about, ensuring precision and simply keeping everything clean start to look like a rather daunting task.
Also: thermal management. Researchers are already exploring ways to address this problem with laser weapons of the near future — for example, by growing thermoelectric coolers in the form of thin films, implementing massive microchannel cooling, or developing cryogenic laser systems (See: A Brave New World of Optics). But these aren’t necessarily meant for lasers that produce half a septillion times more energy than the world’s largest laser today.
In Star Wars, the Empire offers another solution: Building a system of vents leading to a thermal exhaust port on the surface of the Death Star, thus enabling dissipation of the heat generated by energy reactors — and presumably the giant planet-killing laser — at its core.
Of course, we all know how that goes for the Empire. After stealing the plans for the space station, the Rebel Alliance identifies the exhaust port as a weakness and launches an attack in which Luke Skywalker (who earlier posed the question: What sort of weapon could vaporize an entire planet?) fires a single photon torpedo into it. This triggers an explosive chain reaction that ends, somewhat ironically, in the destruction of the Death Star.
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