Discussions about the importance of asteroids and comets tend to emphasize the threat of their occasional collisions with Earth. It is certainly true that occasional catastrophic impacts have altered the history of life on Earth. However, asteroids may also be part of a future solution to some of the environmental problems of Earth. In any case, interplanetary bodies offer some of our best clues to the ancient history and formation of the Solar System.
The four main groupings of interplanetary bodies are the Oort cloud of comets, the Kuiper belt of comets, the main belt of asteroids, and the Trojan asteroids. The Oort cloud consists of icy bodies that were perturbed or deviated from their regular orbits. Dusty debris from these comets creates meteors and meteor showers. Rocky and metallic debris from the asteroid belt is also occasionally perturbed toward the terrestrial planets, producing meteorites and meteorite craters.
Meteorites are extremely valuable because they represent samples from inside different kinds of ancient parent bodies. They show that many parent asteroids — at least many of those larger than a few hundred kilometers across — melted soon after they formed, differentiating to form iron cores and rocky mantles. Other asteroids apparently never experienced strong heating and thus preserved their original composition. The existence of water-bearing minerals on some of these asteroids also proves that they never underwent high temperatures.
The Solar System's small bodies — comets, asteroids, meteors, and meteorites — all began as planetesimals, pre-planetary bodies that formed in the Solar System 4.6 billion years ago. This occurred during a relatively short period that only lasted about 20 million years. All the interplanetary bodies add up to only a small fraction of the mass of the Solar System, but they are important because they provide clues to its history and formation. For example, meteorites of many different types, together with the Moon and Earth, all share a common age of 4.6 billion years. This is one of the most important observational facts of solar system astronomy. It reveals that the Solar System itself formed about 4.6 billion years ago.
About 4.6 billion years ago, the Sun formed within a contracting interstellar cloud composed mostly of hydrogen and helium gas, with a few percent of heavier elements. The Sun formed in the center of this cloud, incorporating over 99% of the gas. As outer parts of the cloud cooled, solid grains of various minerals and ices condensed from the heavier elements and accumulated into small rocky fragments called planetesimals. These planetesimals collided, countless small bodies aggregated into a few larger bodies, and eventually they became planets. All this happened during a relatively brief interval of roughly 50 million years.
Simple physical ideas describe the early evolution of the Solar System. In its gravitational collapse, the solar nebula contracted by a large factor, and gravitational potential energy became heat energy of the newly formed Sun. The conservation of angular momentum governed the transition from a large gas cloud with a small degree of rotation to a small and dense disk of gas that was rapidly rotating. The moon systems of the giant planets formed by a similar process, on a smaller scale. Some moons were added as a result of the capture of interplanetary bodies, and the moons of Earth and Pluto may have formed due to giant collisions. Since none of these origin processes require any special circumstances, it seems likely that they also operated around many other stars. Astronomers have detected many planets around other stars - proof that the processes that led to our Solar System are not unique.