It is extremely important to record and track the positions of asteroids. This ensures that, once they are found, we do not lose them. Some can be dangerous, even catastrophic, for life on Earth - and we need to know which.
Originally asteroids were represented by both names and glyphs (Pallas’ glyph was, for example, a diamond with a cross underneath, and Metis’ was an eye). By 1851 fifteen asteroids were known and the original nomenclature had become too complicated. It was decided to introduce a simple numbering system. For the first 100 years asteroids were given female names which were chosen by their discoverer. The number of appropriate names had seemingly run out by about 1940 and masculine asteroids were then admitted, a maximum length of 16 characters being allowed.
In 1892 it was decided to give asteroids a provisional designation before the actual naming process. This designation depended on the time of discovery. The year was divided into 24 sections. Asteroids discovered between January 1 and January 15 were labelled A, January 16 and 31 B, and so on. The first asteroid to be discovered between January 16 and 31 1971 would then by 1971 BA, the second 1971 BB, etc.
The registering and tracking of asteroids is overseen by the Minor Planet Centre in Cambridge, Massachusetts and the Institute for Theoretical Astronomy in St. Petersburg, Russia. The former, currently under the direction of Dr Brian G. Marsden, keeps the catalogue and registers new finds, and the later publishes an annual ephemeris that gives details of the celestial positions of the observable asteroids in any specific year.
Many asteroids have unusual orbits. 433 Eros was discovered in 1898 and has an orbit which crosses that of Mars. 588 Achilles was discovered in 1906 and is locked in a 1:1 resonance with Jupiter, travelling around the Sun in a Jupiter-like orbit staying about 60 degrees in front of that planet. 944 Hidalgo, found in 1920, has an orbit that takes it beyond the orbit of Saturn, In 1932 the asteroid 1862 Apollo was found and this has an orbit that crosses the Earth’s orbit. 2062 Aten, discovered in 1976, has a semi-major axis (the major axis is the longest distance across its elliptical orbit – the semi-major axis is simply half this length) less than the Earth’s. The later two are clearly Near Earth Asteroids and have a potential for impact.
Many of the craters on Mercury, Venus, Earth, Moon, Mars and the outer solid planetary satellites have been formed by impacting asteroids. Craters are still being produced, but at a much lower rate than in the past. This is because an asteroid has to have a mass of at least 1000 tons (around 10 m across) if it is to have a chance of producing a crater and there are now far fewer asteroids large enough left in the solar system. Most of the large asteroids have, over the life of our solar system, now hit the Sun, the planets, or one another which caused them to break up into smaller fragments. Despite this there are still plenty of asteroids out there with a capacity to cause a crater if they were to hit the Earth, probably at least as many as 100,000,000,000 (one hundred thousand million!). Thankfully the chances of this happening are fairly slim, but in the long term it's bound to happen.
At the present time thousands of asteroids are being found each year. Needless to say, the average size of these ‘new’ asteroids is very much smaller than the size of the early ones. After a new asteroid has been discovered it is vitally important that astronomers keep track of it over the following months, and search old photographic plates to see if it was seen before and had been accidentally overlooked. An accurate measurement of its changing position over time then enables a precise set of orbital parameters to be established, and this ensures that the asteroid is not ‘lost’. This is easier said than done. Fast moving Apollo, Amor and Aten asteroids can often only be seen for a few weeks, while they are close to Earth. At large distances from Earth they are too faint to be detected. 878 Mildred was discovered in 1916. It was then lost, only to unexpectedly reappear in a photographed in April 1991, 75 years later.
After all these observations and measurements to accurately establish an asteroid’s orbit it is finally given an official number and name. Despite this care and attention to detail mistakes are occasionally made. For example, asteroid 525 Adelaide (discovered in 1904 by the German astronomer Max Wolf, Heidelberg) and asteroid 1171 (Rusthawelia, discovered by S. Arend, in October 1930, from Uccle, Belgium) turned out, in 1958, after the orbit of Rusthawelia had been well established, to be one and the same object.
So far (2004) we've discovered around 100,000 asteroids, which sounds impressive until you find out that there are thought to be somewhere between 100 million and 10,000 million asteroids bigger than 1 km that have yet to be discovered – which is pretty worrying when you realise that a 1 km asteroid would produce a crater on Earth with a diameter of around 20 km. Some experts have suggested that a crater this big would be so energetic and destructive that it could lead to the end of civilisation as we know it.
But we are making huge steps forward with many observational projects actively searching for asteroids. Bigger and bigger telescopes are being used which enables smaller and smaller asteroids to be discovered. In fact, some experts predict that by 2010 we will have discovered around a million asteroids. The benefits of modern computer techniques allow these observations to be quickly converted into accurate orbits, so we can easily work out how close these asteroids will get to Earth in the future.
There’s no doubt in my mind that, at some point, we are going to discover one that is going to hit us. Hopefully we will find it in plenty of time and by then, I hope, we will have the rockets, explosives and knowledge of asteroidal structure which will enable us to deflect or destroy this incoming asteroid.