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In the book The Hitchhiker鈥檚 Guide to the Galaxy, the Vogons demolished Earth. They said it would take approximately 2 minutes, so how much energy would they have needed to obliterate our planet? Would kicking the planet out of its orbit have been more energy efficient than simple destruction?

鈥� In a word, yes. If the Vogons wanted total obliteration within 2 minutes, the 鈥渂eams鈥� they use would have to work very fast, at least at about 50 kilometres per second. This should disperse everything, but would require a massive energy input delivered at several times the speed of sound in solid rock.

Shooting an antimatter asteroid the size of Ceres into Earth鈥檚 core at about 200 kilometres per second should do the trick, but would entail serious technical difficulties.

An Earth-sized planet falling into the sun鈥檚 corona would take longer than 2 minutes to be obliterated. If the book鈥檚 author Douglas Adams merely meant the destruction of the planetary surface, then that would be much easier, but it is not what he wrote. And his specification of 鈥渂eams鈥� was too vague to be helpful.

鈥淚f the Vogons wanted total obliteration within 2 minutes, their beams would have to work very fast鈥�

Anyway, as the questioner suggests, altering Earth鈥檚 trajectory to pass close to the sun would be quick, conclusive and would take a tiny fraction of the energy necessary for 2-minute obliteration in space.

It would still take far more energy than humanity could ever use foreseeably, and would take months or years rather than minutes, but frankly, given their long familiarity with government cost-plus contracts, I doubt that the typical Vogon jobsworth could care less either way.

Jon Richfield, Somerset West, South Africa

There are lots of suggestions about how to calculate the energy needed 鈥� Ed

鈥� I would refer your correspondent to qntm.org/destroy, an excellent study by Ed Macpherson that includes numerous detailed estimates of the amount of energy required to destroy Earth.

It is not clear what method of destruction the Vogons had in mind, but we can at least exclude some of the slower ones. For example, had they elected to blow the planet apart with explosives, they would have needed at least 2.24 脳 10³² joules to overcome its gravitational binding energy.

Richard Miller, London, UK

鈥� The 鈥渄emolition beams鈥� used by the Vogons to destroy Earth are of an unspecified nature. Such a disintegration ray, much loved by science-fiction writers, has no basis in known physics, and so it is impossible to estimate how much energy it would take to disintegrate the planet.

A rough estimate using Einstein鈥檚 famous E = mc² formula and the speed of light suggests that the energy needed to make Earth 鈥渄isappear鈥� would actually liberate a massive 5.4 脳 10⁴¹ joules.

鈥淭o destroy Earth would take far more energy than humanity could ever use foreseeably鈥�

More conventional demolition involves breaking an object up into small pieces. The theoretical minimum energy required to do this is given by the amount of new surface area created multiplied by the surface tension (surface energy) of that new surface.

For instance, to split Earth into two equal hemispheres, assuming for simplicity the planet is molten iron, would require about 5 脳 10¹⁴ joules (about 2-minutes worth of Earth鈥檚 currently installed power generating capacity of approximately 6 terrawatts, which is coincidentally how long the Vogon demolition process took).

The more pieces and area created, the more energy is required. However, in practice, the energy used to break things is orders of magnitude larger than this, because all known breakage processes also waste energy as heat and sound. But then again, maybe the Vogons know some techniques we don鈥檛.

I agree with the questioner that moving the planet to another orbit would be a much more sensible idea. The energy required to completely remove Earth from the sun鈥檚 gravitational attraction is given by the integration to infinity of the gravitational attractive force, approximately 5.3 脳 10³³ joules. Of course, depending on the exact route of the 鈥渉yperspacial express route鈥� that the Vogons were building, it might not be necessary to move the planet quite that far.

Simon Iveson, Chemical Engineering Discipline, University of Newcastle, Callaghan, New South Wales, Australia

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