It is the stuff of nightmares and, until now, Hollywood thrillers. A huge asteroid is on a catastrophic collision course with Earth and mankind is poised to go the way of the dinosaurs.

To save the day, Nasa now plans to go where only Bruce Willis has gone before. The US space agency is drawing up plans to land an astronaut on an asteroid hurtling through space at more than 30,000 mph. It wants to know whether humans could master techniques needed to deflect such a doomsday object when it is eventually identified. The proposals are at an early stage, and a spacecraft needed just to send an astronaut that far into space exists only on the drawing board, but they are deadly serious. A smallish asteroid called Apophis has already been identified as a possible threat to Earth in 2036.

Chris McKay of the Nasa Johnson Space Centre in Houston told the website Space.com: "There's a lot of public resonance with the notion that Nasa ought to be doing something about killer asteroids ... to be able to send serious equipment to an asteroid.

"The public wants us to have mastered the problem of dealing with asteroids. So being able to have astronauts go out there and sort of poke one with a stick would be scientifically valuable as well as demonstrate human capabilities."

A 1bn tonne asteroid just 1km across striking the Earth at a 45 degree angle could generate the equivalent of a 50,000 megatonne thermonuclear explosion. Attempting to break it up with an atomic warhead might only generate thousands of smaller objects on a similar course, which could have time to reform. Scientists agree the best approach, given enough warning, would be to gently nudge the object into a safer orbit.

"A human mission to a near Earth asteroid would be scientifically worthwhile," Dr McKay said. "There could be testing of various approaches. We don't know enough about asteroids right now to know the best strategy for mitigation."

Matt Genge, a space researcher at Imperial College, London, has calculated that something with the mass, acceleration and thrust of a small car could push an asteroid weighing a billion tonnes out of the path of Earth in just 75 days.

Gianmarco Radice, an asteroid expert at Glasgow University, said the best approach would be to land a device to dig into the object. "You could place something on the surface to eject material that would push the asteroid in the other direction."

Mirrors, lights and even paint could change the way the object absorbed light and heat enough to shift its direction over 20 years or so. With less notice, mankind could be forced to take more drastic measures, such as setting off a massive explosion on or near the object to change its course. In 2005, Nasa's Deep Impact mission tested a different technique when it placed an object into the path of a comet.

Dr Radice said robots could do the job just as well, doing away with the need for a risky and expensive manned mission. Last year Japan showed with its Hayabusa probe that a remote spacecraft can land on an asteroid.

But with manned missions to the moon and possibly Mars on its to-do list again, Nasa is keen to extend the reach of its astronauts.

Dan Durda, a senior research scientist in the Department of Space Studies at the Southwest Research Institute in Boulder, Colorado said an asteroid landing mission would be a good way test the new Constellation programme spacecraft, the Apollo-style planned replacements for the space shuttle with which Nasa hopes to return to the moon.

He told Space.com: "A very natural, early extension of the exploration capabilities of this new vehicle's architecture would be a "quick-dash" near-Earth asteroid rendezvous mission."

Tom Jones, a former shuttle astronaut, said: "After a lunar visit, we face a long interval in Earth-Moon space while we build up experience and technology for a Mars mission. An asteroid mission could take us immediately into deep space, sustaining programme momentum, adding public excitement and reducing the risk of a later Mars mission."

Europe has its own efforts to tackle asteroids. Its planned Don Quijote mission will launch two robot spacecraft, one to tilt at a harmless passing space rock, and a second to film the collision and watch for any deviation in the asteroid's path.



'Not if, but when...' Hits and near misses



At Nasa's Jet Propulsion Laboratory, in California, scientists monitor all "potentially hazardous asteroids" that might one day end up on a collision course with Earth. So far they number 831. The next close-ish shave - at a mere 17 times the distance from the Sun to the Earth - will be asteroid 2004QD14 on November 29.

The Earth has a long history of asteroid strikes. Thirty five million years ago, a 5km-wide asteroid ploughed into what is now Chesapeake Bay, in the US, leaving an 80km crater. In 1908, an asteroid devastated swaths of Siberia when it exploded mid-air with the force of 1,000 Hiroshimas. The theory that the dinosaurs were wiped out by a huge asteroid striking Mexico 65m years ago is controversial since scientists uncovered rocks from the crater predating the extinction of the dinosaurs by 300,000 years.

A near miss, when asteroid QW7 came within 4m km of Earth in September 2000, led Liberal Democrat MP Lembit Opik to declare: "It's not a case of if we will be hit, it is a question of when. Each of us is 750 times more likely to be killed by an asteroid than to win this weekend's lottery."

In January 2002, the former science minister, David Sainsbury, announced the government's response to the threat from hurtling asteroids: a new information centre based in Leicester.



theguardian.com/space