Not by sending up Bruce Willis in an space shuttle with a crew of veteran astronauts and oil drillers, that's for sure. The shuttle can only get to low Earth orbit and anyone who wants to deflect an oncoming asteroid will have to launch years ahead of any impact. So it's a task best left to an unmanned vehicle, built specially for the job, which is more or less what the Apollo astronaut Rusty Schweickart and Edward Lu - recently back from the International Space Station - told a US Senate committee last week.
Consider the problem: an asteroid on a collision course with the Earth could be detected 10 years ahead. Suppose it to have a diameter of 1km. Suppose it to be travelling at a relatively sedate 39,000kph. And suppose it to weigh about 1bn tonnes. Anything that size hitting the Earth at an angle of, say, 45 degrees would, according to a University of Arizona website (lpl.arizona.edu/impacteffects/) generate the equivalent of a thermonuclear explosion of 50,000 megatonnes, enough to wipe out civilisation as we know it. Such collisions do occur, on average every half a million years. It would be a bad idea just to try to hit the thing with a nuclear warhead: even if the asteroid broke up, it would have time to reform and smash into the Earth anyway. But, experts have been pointing out for the last decade, provided the Earth had sufficient warning, this nemesis could be gently deflected. Last September, Imperial College London's asteroid expert Matt Genge calculated that something with the mass, acceleration and thrust of a Robin Reliant could push into a billion-tonne asteroid, with an acceleration of a billionth of a metre per second per second. If it did so for 75 days, it would change the asteroid's velocity by 0.7 cm per second, enough to make it miss its date with the Earth.
Schweickart and Lu told the Senate that they recommended a nuclear-powered spacecraft with ion drive for the job; Nasa is already looking at such a spacecraft as part of its Project Prometheus. And Lu argued that the space agencies should be practising on non-threatening asteroids by 2015 "because there are no doubt many surprises in store as we learn how to manipulate asteroids".
Observers now monitor the northern skies for threatening asteroids. Australian astronomers have just begun monitoring the southern skies. Right on cue, they spotted three. On March 29, a 100 metre asteroid missed earth by 3m kilometres, and a second asteroid 300m across missed by 20m kilometres. A third, 100m across, will miss Earth by 2.25m kilometres on April 20.
