In addition to the Sun and the major planets with their satellites, the solar system is populated with numerous comets and asteroids. These small bodies are the remains from the formation of the solar system some 5 billion years ago. Contrary the terrestrial planets, which are more or less geologically evolved, many comets and asteroids are to a large extent preserved in their pristine condition.
Most asteroids are found in the main-belt between the orbits of Mars and Jupiter, and are most probably the remains of a planet that failed to form, due to the perturbations of Jupiter. An almost equally large population is the Trojan asteroids, which are trapped in the two Lagrangian points 60 degrees ahead of and behind Jupiter in its orbit. Another important group is the near-Earth asteroids, where several are on Earth-crossing orbits and may pose a potential threat upon the Earth.
The study of asteroids is one important key to the understanding of the early solar system. The evolution of the asteroids and their interactions in the present-day solar system are also important issues. There are many open questions related to the distribution of mass, angular momentum, and chemical composition in the main-belt. The few images available of asteroids show highly irregular shapes, indicating violent histories of impacts and collisions. The traditional distinction between comets and asteroids seems to become increasingly more vague with new discoveries of borderline and transitional objects.
The Uppsala planetary system group