They come from outer space

Worried about an asteroid strike destroying our planet? Well, you should be, says Ian Brown. Scientists believe the danger is all too real

Humanity could one day be caught between a rock and a hard place – and it just could be a very large fast-moving rock. The potentially catastrophic threat posed to humanity by an asteroid colliding with the Earth is to be discussed at a meeting today of the Royal Astronomical Society in London.

This is not the stuff of purely academic debate or Hollywood science fiction. Last year, the Government set up a Task Force on Potentially Hazardous Near Earth Objects to consider the threat. This made 14 recommendations for action, including some thoughts on what are the options for deflecting any object on a collision course with Earth.

Meanwhile, both Europe and America aim to take a much closer look at the problem. Among today's speakers, Marcello Coradini of the European Space Agency will explain Europe's plans to study the physical nature of asteroids and comets. The American National Aeronautics and Space Administration (Nasa) has several spacecraft either already in space or planned for launch within the next few years.

Europe's Rosetta mission, the first probe aiming to orbit a comet, is scheduled for launch in 2003. GAIA, a space telescope due for launch around 2010, will be able to detect asteroids as small as 500 metres wide. Another probe, called BepiColombo, will also be able to search for unknown asteroids between the Sun and Earth although its main destination is Mercury. These Near Earth Objects (NEOs) are particularly dangerous, since they can approach the Earth from the direction of the Sun, and so be masked by its glare.

Today's meeting will also discuss how British astronomers and researchers will work with the international Spaceguard programme, and how the asteroid hazard ranks against other large-scale potential disasters (such as nuclear power station accidents).

One of the meeting's organisers, Duncan Steel, a physicist from Salford University, says that this is astronomy close to home. "Only recently has the importance of comets and asteroids to our own planet been recognised," Steel says. "But quite apart from potential impact catastrophes, NEOs are worlds in their own right. Studying them is becoming a central feature of solar system exploration. The next few years promise a wealth of interesting information on asteroids and comets."

Steel is urging governments around the world to take the issue more seriously. "The UK has the expertise in astronomy. We have the telescope facilities in the Canaries, Hawaii and elsewhere that would be ideal in making a contribution to a wider international effort. This is something to be taken very seriously, but nothing has been done," he says.

Millions of asteroids and comets orbit our Sun, but only a tiny fraction approach the Earth. Don't breath a sigh of relief yet, though. These objects range in size from pebbles to mountains. And because they travel at very high speed even the medium-sized objects pose a potential danger. At present, 291 potentially hazardous asteroids have been firmly detected. But astronomers estimate that there are between 750 and 1,100 near-Earth asteroids bigger than one kilometre, the minimum size thought to pose catastrophic risk.

Such objects, of course, have collided with Earth since its formation. Some brought the carbon and water which made life possible. Others caused widespread changes in the Earth's surface and climate. It is now generally accepted that a 10-kilometre asteroid killed the dinosaurs 65 million years ago.

In 1994, astronomers were treated to a rare – and sobering – direct glimpse of the devastation caused by such impacts. Thankfully it wasn't here on Earth. They were watching fragments of Comet Shoemaker-Levy 9 bombard Jupiter in a cataclysmic display of cosmic violence.

The amount of damage caused by a NEO's impact with Earth would depend on its size. In 1908, an asteroid or comet 50 metres across exploded above Siberia, devastating a vast but uninhabited forest. Such an impact over a city would have instantly levelled it. An asteroid over one kilometre would release energy equivalent to 10 million times the power of the Hiroshima atomic bomb and wreak global catastrophe. The report of the UK's Task Force questions how prepared we are to address the risk. "The threat from Near Earth Objects raises major issues," it says, "among them the inadequacy of current knowledge, confirmation of hazard after initial observation, disaster management (if the worst came to the worst), methods of mitigation including deflection, and reliable communication with the public."

The Task Force, led by Harry Atkinson, a former chairman of the European Space Agency council, was specifically asked to confirm the nature of the hazard; the potential level of risk; and advise on what further action should be taken.

His report concludes: "Discussions of this global problem with the US Department of Defence, Nasa, ESA and the UK Defence Evaluation and Research Agency have thus far provided no clear position on what should be done, although it is clear that the highest priority lies in the provision of improved observation to provide the maximum possible warning time."

There are two main options for deflecting NEOs, the Task Force says. Launch one or more small spacecraft many years in advance of the predicted impact-date to rendezvous with and gently "nudge" the NEO away from its collision course. Or last-minute deflection using high energy explosive devices – "the use of which would need to be very carefully considered," it adds, with very British understatement. "Mitigating any impact by deflection would appear to be a more attractive option than break-up, since the latter might well result in a greater number of smaller NEOs to cope with worldwide," it adds.

The risk from comets is estimated to be between 10 and 30 per cent of that from asteroids – but the advance warning period for a potential impact from a long-period comet may be as short as a year compared to decades or centuries for asteroids.

At present, the British National Space Centre is to take the lead in Whitehall policy on the threat posed by NEOs. But the Government is also looking into the options for a dedicated British Centre for Near Earth Objects. The successful applicant is expected to be announced any day now.

Duncan Steel, however, is not impressed: "The only remit of this centre is to give information, not do any research," he said. "We have to stop talking and start taking action."