Nuking Asteroids

A letter I wrote to New Scientist on the 15th of November 2009.

The Editor wrote, on the subject of nuking asteroids, that “A nuclear bomb detonated above the asteroid will vaporize its surface, thereby changing its momentum, and nudging it onto a safer trajectory”. If only it were that simple.

If the asteroid is a single, mechanically robust chunk of rock, this would probably work*. Many asteroids seem to be more like piles of rubble, and the effect of a nuclear bomb “above” (near, surely?) such an asteroid would be very different – the most likely result would be to turn it into an ever-spreading shower of bits and pieces, a significant proportion of which, particularly the bigger pieces, would continue on the original trajectory. Much the same would apply to any less-than-robust chunk of rock.

Then, even for the robust chunk of rock (if we can tell from a huge distance that that’s what it is), there’s the much glossed-over issue of the precision with which orbits can be predicted. In order for a relatively gentle nudge to change an orbit significantly, it has to be applied a long time before the encounter. Unfortunately this means that it has to be applied at a time when there is still considerable uncertainty about whether a collision is actually probable. Artillery experts talk about “circular error probable”, refering to the patch of land around the target where the projectile is most likely to land. An asteroid has a similar “conical error probable”. It’s not actually a cone, however – it’s more like an exponential horn, getting wider more rapidly as the distance increases.

The probability function is so nearly flat near the middle of a very wide cone that a deflection – even one many times the diameter of the Earth – wouldn’t actually make much difference to the probability of a collision. Indeed one couldn’t really be sure whether one was really decreasing it at all; one might be increasing it. The width of the cone increases in exactly the same way that the effect of a small nudge increases with distance, so you don’t gain anything at all by applying the nudge at a very great distance.

Fortunately rocks big enough to cause planet-wide destruction only hit at intervals of many tens of millions of years. Our species is unlikely to receive one – ever in the lifetime of our species, never mind in the foreseeable future. Rocks big enough to cause massive damage to a big city arrive more frequently, but fortunately cities cover only a minuscule percentage of the Earth’s surface, and the chances of one being hit are correspondingly minuscule. Please can we worry about – and spend our resources on – real risks? There are plenty of them.

©Clive K Semmens 2009

* What I didn’t write at this point in my letter: this is only in the sense that the asteroid would be nudged into a different trajectory, not necessarily a particularly safer one.

Folks pushing an asteroid-diverting agenda are like folks pushing fusion power: playing fun games at public expense, but achieving nothing - at least, achieving nothing in the direction they're selling to the public. Some of the fusion power folks have (semi-)covert military objectives (themselves of - to put it charitably – dubious merit) – perhaps the asteroid-diverters do too?