- guardian.co.uk, Thursday December 28 2000 01.27 GMT
There is another reason to celebrate this January 1. It is the bicentenary of the discovery of the first asteroid.
Astronomers keep a master list of asteroids with well-determined orbits. As new bodies are added, they get a number, and a name. So far almost 20,000 are on the list, such as 1815 Beethoven, 2001 Einstein, and 2985 Shakespeare. (Oh, and 4713 Stell.) But the first is 1 Ceres.
Ceres was found on January 1 1801, by an Italian astronomer named Giuseppe Piazzi, from Palermo in Sicily. It was not an unexpected discovery.
Late in the 18th century, astronomers had become convinced that there must be another planet, yet to be spotted, between the orbits of Mars and Jupiter. Their belief was based on a numerical relationship known as Bode's Law, which seemed to govern the distances from the sun of the planets Mercury, Venus, Earth, Mars, Jupiter and Saturn, but with a gap between the fourth and fifth. When Uranus was discovered - in 1781, by William Herschel, from his observatory in Bath - in a location consistent with an extrapolation of Bode's Law, astronomers were convinced that the intermediate planet must soon be found.
Soon after Piazzi made his discovery, however, it was realised that Ceres was hardly big enough to deserve the accolade of "planet". It is only about 600 miles across, much smaller than our moon. Indeed, the proper astronomical term for an asteroid like Ceres is a "minor planet."
The situation became more confused in 1802, when another minor planet was found between Mars and Jupiter, and shortly thereafter two more. These three are named 2 Pallas, 3 Juno and 4 Vesta.
Out of that confusion, another hypothesis was developed: perhaps these asteroids are simply the fragments of a large planet that, for some reason, had exploded? This idea held sway for some time, until it was eventually realised that the myriad bodies orbiting between Mars and Jupiter (in what is now called the main belt) are simply primordial debris that never managed to accumulate into the planet predicted by Bode's Law. The reason is the gravitational stirring imposed by massive Jupiter, which keeps these objects dynamically agitated so that collisions between them result in their erosion and destruction, rather than the gradual agglomeration of a single large body.
The majority of the 20,000 asteroids in our data banks remain on well-behaved paths forever circuiting the sun in the main belt. In 1873, however, one was found on a trajectory that brought it in to cross the orbit of Mars.
It was obvious that an asteroid-planet collision was possible. Fundamentally this was not a new idea, because comets had been known to have planet-crossing orbits for some time. Even as far back as 1694, Edmond Halley (for whom the famous comet is named) had suggested that impact-induced calamities and upheavals must have occurred in the past. But there are few comets, whereas 19th-century astronomers were finding a host of asteroids. Comets are easy to find because they are bright, being surrounded by vast clouds of water vapour. The recognition of dark asteroids required the development of better telescopes, and then photography led to a blossoming of the discovery rate.
Astronomers' fears were confirmed when in 1898 the first earth-approaching asteroid was spotted. This is 433 Eros. (For the past year, a Nasa spacecraft has orbited around Eros, sending us back detailed images of its surface and other characteristics.)
Then, in 1932, the first possible Earth-impacting asteroids were found. Over subsequent decades several more were discovered, essentially by accident on celestial photographs obtained with powerful telescopes. It has only been in the past few years that dedicated search projects in the US have reaped a harvest of the skies.
Almost 1,000 small asteroids crossing our orbit are now known, 300 of them found in the year 2000 alone. Many keep their distance, and we know - from numerical experiments in which their paths are followed forward by computer for decades, while they loop around the sun - that they cannot hit our planetary home soon. By "soon" I mean the next century: the period of interest to you, your children, and your grandchildren.
We are all familiar with shooting stars. Tiny rocks from space, the size of a pea or a grape, cause these. Many of the known asteroids are small enough to blow up on meeting the atmosphere. They could still make a mess. The rocky asteroid that entered the atmosphere above Siberia in 1908 was only about 60 yards across. When it exploded at a height of around five miles it released energy equivalent to about 15 megatons of TNT, burning and flattening the largely uninhabited forest over an area of almost 1,000 square miles. Such an event over London would destroy most buildings out to the M25. About once per century is our best guess of how often a cosmic calamity like that must occur somewhere on Earth. Fingers crossed, then.
The major hazard posed by asteroids comes from larger objects. A one-mile asteroid would release energy equivalent to about a million megatons of TNT in a terrestrial impact. That is 80m times the Hiroshima bomb, enough to cause a global catastrophe. No matter where it hit, we would all be severely affected. There is about a one in half-million chance that such a disaster will occur next year without any warning (we know that as of yet we've found only a fraction of these cosmic projectiles). Compare that with your one in 14 million chance of winning the Lottery.
Given the deadly potential of asteroids, should we celebrate the anniversary of the first discovery? An optimist could point out that whereas an impact is unlikely, our exploitation of asteroids and comets as sources of the raw materials (rock, metal, water and oxygen) we will need for our future expansion into space makes them valuable discoveries. As we map their orbits, we not only safeguard our future on planet earth, but also make feasible our conquest of the final frontier. And that is worth toasting as January 1 properly begins the new millennium.
• Duncan Steel works at the University of Salford. His new book Target Earth is published next week by Time Life (£14.99). To order a copy for £11.99 freephone 0800 3166 102 or, send your order with a UK cheque, payable to The Guardian CultureShop, to FREEPOST Books, LON3590, London, W3 6BR. Please add 99p UK p&p.
