We start with a detective story, a near certainty of calamity, but a trace of hope.

Some 4,500 m years ago, the solar system was a huge ball of gas ringed by a thin disc of gas and dust. The gas drew together under the force of gravity and the kenetic or ëvelocityí energy translated to escalating, enormous heat, creating a concentrated ësuní radiating energy from thermonuclear reactions, with a gas cloud blown off as far as the outer planets and now a source of comets.

Meanwhile the thin disc was consolidating through gravitational attractions creating the planets, and an asteroid belt between Mars and Jupiter. The belt now contains an estimated 10 asteroids >10 km diameter, 2000 > 1 km, 100,000 > 100 m, 150,000,000 > 10 m, and lots of dust.

Whilst the asteroid belt lies between Mars and Jupiter, and the comet clouds further out still, the different elliptical orbits (and the possibility of deviation caused by gravitational attraction) means regular interaction with the Earth. 50,000 tonnes of rocks arrive as ëshooting starsí and small meteorites every year. In 1908 an asteroid or chunk of comet 60 m across exploded over Siberia, at Tunguska. Delayed 4 hours, it would have obliterated London: indeed the whole area within the M25. In 1994, an asteroid 1.6 km dia. was detected going away from the Earth, having passed between it and the moon.

In short, it is not a question whether or not a huge asteroid will collide with Earth. Collision is certain unless something is done. The question is when.

Any asteroid > 1km dia. is planet threatening. Scientists have tracked all > 10 km asteroids and none is likely to collide with Earth in the next 100 years. However the paths of only 10% of the 1 km asteroids are known, even though the probability of collision is significant - between 0.1 and 1% in the next 100 years. Indeed the risk of death from an asteroid impact is estimated at 1 in 25,000, about the same as dying from a plane crash (1 in 20,000), and 100 times more likely than being killed by BSE or any form of food poisoning.

In electoral terms however, the probability is low - enough to be joked about or ignored. In any case, what can be done?

In truth, ours is the first generation capable of ëdoing somethingí. NASA and the Japanese are carrying out limited investigations and, though an Anglo-Australian initiative fizzled out, the European Space Agency (ESA) are planning probes. The (voluntary) Spaceguard Foundation is seeking support to locate and track all > 1 km asteroids within 10 years. This needs 6 telescopes costing about £36m to set up and £6/year to run - not large sums. In the UK, £0.5m would much extend the work of the renowned Armagh observatory.

Diversion of an asteroid on a collision path is a great deal more tricky and expensive. Given 8 days warning, existing technology could intercept an asteroid and detonate a nuclear bomb nearby, but there are obvious risks from scatter of the remains, mal function of the rocket, and radio activity. A coat of aluminium might divert an asteroid through the effect of the solar wind. Before serious planning can begin however, it is important to determine a key unknown: the material of the asteroid itself.

The Rosetta mission will be launched in 2003. Using very clever programming and engineering, a probe will orbit the Wirtanen comet in 2008, land in 2012, drill for samples, and relay back all relevant information - if all goes to plan.

In summary then, there is a remote but real risk of a calamitous disaster. A strategy exists to ëdo somethingí, at modest expense which, if nothing else, could foster international collaboration. Government ministers Lord Sainsbury and John Battle appear supportive, but greater public awareness would help.

Speakers at this intriguing meeting were: ALDES members Dr Laurence Cox and Dr Gareth Hartwell, and Lembit Opik MP. Laurence also organised the meeting. Our thanks to them all.