Asteroid defence: The real star war In 1908 a rock the size of a city block hit the Earth's atmosphere at 15km (9 miles) a second. The explosion flattened an area the size of London. But the land in question was in Siberia, so few people noticed and those who did had little influence. Suppose, though, it had devastated a city in Europe or North America. The history of the 20th century would have been different, as the best scientific and engineering brains were brought to bear on the question of how to stop it happening again. Well, it has happened again, albeit less spectacularly. By chance, Siberia bore the brunt once more, when a meteor crashed in the Urals on February 15th, injuring more than 1,000 people. It could just as easily have hit Germany or Guangdong. Moreover, on the same day another, larger rock called 2012 DA14 passed within 27,000km of Earth. By astronomical standards, that is a hair's breadth. It is time to think seriously about stopping such incidents by building a system that can detect space rocks with sufficient warning, and then either blast them or push them out of the way. It would be costly, of course, and would require the development of new technology. But, as luck would have it, there is a tool lying around that has both the money and the nous to do it, and which is currently underemployed and in need of a new mission. Zap! NASA, America's space agency, has become a curious hybrid. Part of it is one of the world's leading scientific research organisations. This NASA sends robot probes to the planets, runs space telescopes and has already sponsored projects devoted to looking for large asteroids - the ones that would blow humanity to kingdom come if their orbits ever intersected that of the Earth. If such a large, "planet-killing" asteroid were discovered, though, the chances are that earthlings would have decades, or centuries, to act; a small nudge, judiciously applied by rocket motor or nuclear explosion (see article), would be enough to send it off course. The real problem is "city-killers" - things too small for existing surveys to see, but large enough to do serious damage. And it is here that the other NASA might be brought into play. The non-scientific bit of the agency, the bit that brought you the Apollo project, has been looking for a proper job since 1972, when Apollo was cancelled. It thought it had found it in the Space Shuttle, but building a cheap, reliable orbital truck proved impossible. It thought it had found it in the International Space Station, but that has turned into a scientifically useless tin can in the sky. The latest wheeze is to build a rocket that might one day, many administrations hence, go to Mars. In a well-ordered world, this bit of NASA would have been closed down years ago. That it has not been is due, in large measure, to the lobbying power of aerospace companies which see the agency as a way to divert money from taxpayers" pockets into those of their shareholders. This pocket-picking would be less irksome if something useful came of it. Why not, therefore, change this part of NASA's remit to protecting the planet from external attack, not by evil aliens but by an uncaring universe? Two things would be needed. One is a bigger system of telescopes, either on the ground or in orbit, to give notice of a threat. The other is a way to counter the threat. That might be done with lasers, or with controlled explosions that would shift the incoming object's orbit sufficiently to make it miss altogether, or (if that is not possible) hit an unpopulated area. Developing all this would be a technological challenge worthy of NASA's engineers. It would keep the agency's bureaucrats in their jobs. It would keep the money flowing to the aerospace companies. It would probably cost no more than the space station (about $100 billion). And, if it worked, it would provide something that benefited not just America, but the world - precisely the sort of thing a rich country which often claims the moral high ground ought to be doing. When Apollo 11 took off from the Moon on July 21st 1969, its crew left behind a plaque that read, "They came in peace, for all mankind." What an opportunity both America and NASA now have to prove that they meant it.