Asteroids are mostly lumps of rock and iron left orbiting between the Mars and Jupiter. They are left over from the formation of the Solar System, and cannot form a proper planet because of the disruption caused by the gravity of Jupiter. Some asteroids are the remains of comets, which have lost their ice and other volatile materials, and so no longer show tails or other characteristics typical of comets.
Asteroids are numerous (about 40,000 have been identified as of mid 2002 and more are being discovered all the time) but are mostly very small compared to normal planets. The largest, Ceres, is over 1000km across, but most are very much smaller, with no real lower limit - small asteroids just become meteoroids, and burn up as meteors if they hit the Earth's atmosphere, or land as meteorites if they are large enough.
They are normally hard to see and few come even within binocular range. They are interesting and important none the less because some have orbits that intersect that of the Earth, and so may potentially collide with it and cause immense devastation in the future.
A 10km wide object is capable of destroying most life on Earth and rendering it virtually uninhabitable for hundreds of years - an asteroid or comet of about this size contributed to the extinction of the dinosaurs and wiped out about 80% of all species of life 65 million years ago. A 1km object is capable of destroying a typical country and a 100m one would wipe out a city. As a real example, an object of some 50m diameter exploded over Siberia in June 1908, with a force equivalent to a 20MT nuclear bomb and flattened an area of forest roughly equivalent to the ring of the M25 motorway around London.
The hunt is therefore on to identify and track as many of these potentially hazardous "Earth-crossing" asteroids or "Near Earth Objects" as possible - keep up to date with NASA's NEO Program.
|NEO Program Watch on Near Earth Asteroids|