From Asteroid to Meteorite There are two pieces of evidence, which link meteorites to asteroids and allow the inference that meteorites are fragments of asteroids that fall to the Earth. The first is the observation that meteorites and asteroids are made of the same materials: rock and metal. Astronomers use telescopes to look at the spectrum of sunlight reflected from the surface of asteroids. Using laboratory-based equipment, similar measurements can be made on meteorites. The two sets of observations give almost identical results. Different types of asteroids can be matched to different types of meteorite showing that the objects are related. Another link can be made between meteorites and asteroids through observations made as meteorites come through the atmosphere and are seen fireballs. Although it is very difficult to photograph an incoming meteorite, camera networks have been set up to photograph the sky, hoping to capture incoming fireballs on film. This process has been successful on several occasions, allowing the speed and direction of entry to be determined. This, in turn, allows the orbit of the incoming meteoroid to be calculated; in all instances, the orbits originate from within the asteroid belt between the orbits of Mars and Jupiter. Fireballs and meteors An object coming in through the atmosphere is known as a meteoroid, often seen as a fireball as the outermost surface is melted by frictional heating. The molten material is swept away from the meteoroid, exposing a fresh layer of material, which, in its turn, becomes heated and melts. Eventually, however, atmospheric friction slows the meteoroid down, such that it is no longer travelling sufficiently fast for the outer layer to melt. At this stage, the outer layer quenches, forming a fusion crust. Examination of meteorites shows that this crust is very thin and is usually less than ~ 1mm thick. The inside of the meteoroid has not been affected by heating, and meteorites are usually cool to the touch when they land. The fusion crust is a characteristic which aids in the recognition of meteorites. Meteors, or shooting stars, are seen when tiny pieces of dust burn up in the atmosphere; no material is recovered from a meteor. Particularly large numbers of meteors are seen when the Earth passes through meteoroid streams, in most cases cometary particles that are moving in orbits near those of the parent comet. Announcements about possibilities for meteor observations are made by the many national meteor organisations and through the International Meteor Organisation". Meteorites Meteorites are named after the place in which they fall or are found. Information about recent meteorite falls is maintained by the Meteoritical Society (an international organisation) and regularly posted in the Meteoritical Bulletin (http://www.uark.edu/campus-resources/metsoc/metbull.htm).

A kilometre-wide crater on the asteroid Eros, obtained by the NEAR spacecraft on January 22nd, 2001 (NASA) An extremely bright meteor, known as a bolide or fireball, seen during the Leonid meteor shower in November 1998. Credit & Copyright: V. Winter & J. Dudley, ICSTARS Astronomy