Being the science minister can't be much fun. No one expects economic forecasts to be accurate. But in science exactitude is demanded. And scientists, being a pedantic bunch, are apt to seize on any laxity in official statements.
Take the asteroid and comet impact hazard, and Lord Sainsbury's recent announcement of plans for a near-Earth object (NEO) public information centre. "There are currently no known large NEOs whose orbits puts them on collision course with Earth," he said. Unfortunately, that is not quite right.
We now know of more than a thousand asteroids, of various dimensions, having orbits around the sun that cross the path of the Earth. About one-third will end their lives by running into our planet. That is the bad news. The good news is that the typical interval before each does is more than a million years (although some will hammer into us much sooner).
At the time of writing, there are 14 known NEOs that may strike before the year 2100. Within a period of 48 hours, at least one will be removed from the list, because tracking after its discovery will enable a sufficiently precise definition of its orbit to prove it is not dangerous. Indeed, one large NEO discovered in mid-August caused alarm when the initial data made feasible an impact in either 2005 or 2007, but with a few more days' tracking that possibility disappeared. So, it was quickly deleted.
The remaining 13 members of the list are of longer standing. Not all are really hazardous. Take 1991 BA, discovered more than a decade ago. It may hit in January 2003, or in 2010. The likelihood of a collision is small, around one in a million based on our knowledge of its trajectory. This projectile only measures about 10 metres across, so would almost certainly blow up on crashing into the atmosphere, with spectacular fireworks and shaken china on the ground, but nothing more serious.
There are several similar small NEOs in the data banks. One has a greater than one-in-10,000 chance of hitting us in 2039. Another that caused a furore late last year, 2000 SG344, is now thought not to be an asteroid, but an empty upper-stage rocket body that the Apollo moon landings left in space. There is a slim chance of it returning home in 2071, but it is no more a threat than the Mir space station that fell into the Pacific earlier this year.
Going up in size we come to 2001 BA16, spotted last January. It is 30 to 40 metres in size, and has a better than one-in-10,000 chance of an impact in 2041. We know nothing about its composition. Most asteroids are rocky bodies; if this one is too, then it would disintegrate in the upper atmosphere in an explosion releasing energy equivalent to a few megatonnes of TNT, more than 100 times the power of the Hiroshima bomb. But this would occur at a sufficient altitude for there to be little damage on the surface, unless it was over a city.
But many meteorites are made of nickel-iron mixtures and are very strong, so they are able to punch through the atmosphere at hypervelocity. A small metallic asteroid like this excavated the famous Meteor Crater in Arizona, three quarters of a mile wide.
A more recent discovery (2001 GP2, in April) is slightly smaller. Fourteen separate dates have been identified over the next century when it may slam into our planet, but all are long odds and most likely it will miss us each time. We worry far more about the myriad unseen others, meaning we can expect no warning at all.
Consider the larger NEOs capable of widespread damage, perhaps on a global scale. The granddaddy of them all is Hermes, a half-mile-wide rock seen to whiz past the Earth barely more distant than the moon in 1937. Hermes was observed for only a handful of days.It could come back and hit us at any time.
Much more recently, several potential Earth-impactors have been discovered and then lost due to insufficient tracking data. We can count two in 1994 (sizes around 60 and 300 metres), one in 1995 (30-40 metres across), one in 1997 (about 50 metres), and another in 1998 (250 metres in size). Only the last - 1998 OX4 - gained media attention. It has three opportunities to strike Earth, but not until after 2038, and each has a fairly small probability. Similarly 2001 AV43, a 60-metre NEO spotted last January, has a slim chance of hitting us in 2066.
Another discovery late in 1994 is peculiar. It was tracked for 35 days, and took only two-thirds of a year to circuit the sun, spending most of its life closer to our star than the Earth but coming far enough outwards every orbit to have a chance of colliding with us. Luckily it cannot do so before 2059.
But over the next four decades, 34 separate close approaches rendering opportunities for impact have been identified, with individual probabilities of up to one-in-10,000. One such asteroid, 1994 WR12, is around 200 metres in size, and so could cause damage on a continental scale should it hit. Calculations rendering the impact probabilities mentioned are highly complicated, and Britain does not have any group with the expertise to perform them.
The Czech Republic delivers much more vital NEO tracking than does the UK. The leader in Europe is Italy: not only does that nation host the headquarters of the international Spaceguard Foundation, supported by the European Space Agency, but there are several university and observatory groups dedicated to research on the NEO hazard. The probability calculations discussed above were made by a group at the University of Pisa, headed by Dr Andrea Milani (see http://newton.dm. unipi.it/cgi-bin/neodys/neoibo).
Such calculations are verified at the Helsinki Observatory in Finland, and at institutions in the United States, including Nasa's Jet Propulsion Laboratory in California. Japan is also involved.
Britain has some catching up to do. And preferably well before some NEO is discovered whose impact probability is unity, rather than some small fraction as at present. One day it will happen.
• Dr Duncan Steel is at the University of Salford. His most recent book is Target Earth . Today he will address the British Association science festival in Glasgow on the comet and asteroid impact hazard.
