Asteroids are both rocky and metallic bodies occupying what is known as the Main Belt, between the orbits of Mars and Jupiter. Because they are so small their gravity is far too small to hold onto any atmosphere. Asteroids larger than around 300 km are approximately spherical, because their gravity (still relatively small) is strong enough to overwhelm the strength of their interior material. The smaller ones don’t have enough gravity to do even this. As a result, they often have an irregular shape, such as the potato shape that is so common.
A typical asteroid reflects about 15% of the sunlight that hits it and through a telescope they appear as sun-like stars. The obvious difference between an asteroid and a star is that a typical asteroid moves about a quarter of a degree per day against the distant starry background. It is this movement that can be detected if the same area of the sky is observed night after night.
The brightness of an asteroid is proportional to its cross-sectional area so as larger and larger telescopes are used, smaller and smaller asteroids can be found. Ceres, the first asteroid to be discovered, is also the largest, having a diameter of about 940 km. Asteroids have been produced by the break-up of larger bodies as they collide and this has let to a size distribution such that there vastly more small asteroids than there are large ones. For example, there are about 1,000 asteroids bigger than 94 km, 1,000,000 bigger than 9.4 km, 1,000,000,000 bigger than 0.94 km and so on. As the irregular asteroids spin they reflect different amounts of light and their brightness varies. Monitoring this change has shown us that most asteroid have spin periods between 6 and 13 hours.
In 1918, Kiyotsugu Hirayama realised that some asteroids had very similar orbits. These asteroidal ‘families’ had been produced by the relatively recent break-up of a larger member of the main belt and so hadn’t yet had time to become more evenly distributed throughout the asteroid belt.
In the early 1970s astronomers started measuring the brightness of asteroids at different wavelengths. This led to a classification system based on colour. The commonest three classes were labelled C, S and M. Carbonaceous asteroids (C type) represent about 75% of all the asteroids and only reflected about 7% of the light that falls on them. Stony - silicateous asteroids (S type) reflected about three times more light and make up about 15% of the total. There were also rare metallic asteroids (M type), having compositions similar to the nickel-iron meteorites picked up from the Earth’s surface from time to time. The different types of asteroid were produced by the break up of both differentiated and undifferentiated members of the belt. Composition also varies as a function of heliocentric distance, with the S and M types predominating in the inner belt and the C types in the outer belt.
As a result of these, and other, observations we’ve learnt that at the dawn of the solar system the material in what is now the asteroid belt was well on the way to forming a rocky/metallic planet about four times more massive than Earth. Gravitational perturbation by the massive planet Jupiter interfered with the process of coalescing and stopped the growth process of this potential planet. As a result this material began to collide and fragment. There is now only about 1 part in 2000 of the original mass remaining, the rest having hit the planets and the Sun. Despite such a small part of the original material still being in orbit, this still means that there is about 1019 (that’s 1 followed by 19 zeros – to put that into context, a million is only 106!) tons of asteroids out there - about a fifth of the Moon’s mass.
We have learnt a huge amount since the days of Giuseppe Piazzi. Asteroids are the only astronomical body that actually comes to see us, either (relatively) benignly in the form of the meteorites that fall to the surface of Earth and then fill our museums, or much more explosively as the progenitors of the large impact craters that litter the face of our planet.