One day, a stray asteroid or comet may smash into the Earth, when we shall go the way of the dinosaurs. Pessimists advocate a close watch on asteroidal neighbours, and the building of a vast nuclear missile to disrupt the incomer before it hits the Earth. Daedalus has a neater scheme.

He wants to put an asteroidal ‘anti-missile’ in a high Earth orbit. If a potential invader were sighted, we would arrange for the guard asteroid to collide with it. An asteroid already in orbit would patrol near-Earth space very densely. Even a slight nudge, if well calculated, could guarantee a successful interception, shattering both asteroids.

But how to capture a guard asteroid in the first place? Many asteroids, comets and planetesimals contain hydrogen, water, ammonia or methane ices, which can deliver useful thrust by direct evaporation. Indeed, it has been suggested that an asteroid could be steered by flying a big concave mirror alongside it, to focus sunlight on the icy surface. The resulting plume of hot gas would deflect its orbit. Daedalus goes even further. Most asteroids, he claims, are not loose rubble piles of the sort discussed by E. Asphaug on page 127. They have a compressed centre, in which the ices are converted to energetic hydroxonium, ammonium or methylammonium ‘alloys’. Release the over-pressure, and these compounds will decompose to hot pressurized gas. A space probe that landed on such an asteroid and drilled down to its energetic core would release a gas-jet that would turn the asteroid into a natural rocket. With crafty enough steering, it could be detached from its solar orbit and put into one round the Earth. The drill-hole would then be plugged, restoring the stabilizing pressure. When the asteroid was called on to make the supreme sacrifice for its new primary, the hole would be unplugged again to steer it to its final, fatal encounter.

This elegant scheme can be put into effect long in advance of any threat. With the guard asteroid safely in orbit, the Earth is well defended. Of course, if the deadly incomer turns out to another ice-cored asteroid, it might still be feasible to send a spacecraft to intercept it on the old plan. But instead of blowing it up, it could be steered into Earth orbit as an additional defender.

The further Inventions of Daedalus (Oxford University Press), 148 past Daedalus columns expanded and illustrated, is now on sale. Special Nature offer: m.curtis@nature.com