c. 4.5 Billion BCE

Main Asteroid Belt

The terrestrial planets were rather quickly assembled about 4.5 billion years ago from small rocky and metallic building blocks called planetesimals, which condensed from the slowly cooling, warm inner regions of the Solar Nebula. Each of the growing planets “swept up” planetesimals along and near its orbital path, generally clearing out their orbital zones until the lack of new material to sweep up set the limit on the growth of these rocky worlds.

Beyond the orbit of Mars, however, the accretion of planetesimals and growth into larger planets was continually thwarted and disrupted by the strong gravitational influence of nearby Jupiter. Jupiter’s influence made collisions between planetesimals more energetic, minimizing the gentle collisions that would allow them to stick together and grow, and close encounters with Jupiter itself ejected many of the planetesimals in the Mars–Jupiter zone. Thus, instead of a large planet in the region between Mars and Jupiter, we have a rather diffuse disk or belt of small rocky and metallic asteroids: the main asteroid belt.

Astronomers estimate that there might be more than a million asteroids larger than a half mile (about a kilometer) in size in the main belt. To date, the orbits, positions, and general characteristics of more than half a million of these are known, including the most massive two, Ceres and Vesta. These two, plus asteroids Pallas and Juno, account for more than half of the total mass of the entire main belt.

Asteroids aren’t just randomly located, though. Jupiter’s gravitational pull has cleared out many gaps in the main belt (see Kirkwood Gaps), and some groups of asteroids travel together in “families” that may represent the disrupted and slowly scattering remains of once-larger objects. Jupiter’s Trojan asteroids are two large groups of small bodies trapped in special orbits where Jupiter’s gravity and the Sun’s gravity balance each other out.

Small pieces of impact-shattered asteroid interiors fall to Earth all the time—we call them meteorites—and their ages and compositions provide an enormous amount of detailed information about the timing, formation, and evolution of our solar system.

Overhead computer-generated plot of the inner solar system on August 14, 2006, out to the orbit of Jupiter (outer blue circle) with the Sun at center. Asteroids in the main belt are colored white. Orange dots are the Hilda asteroid family, and green dots are Jupiter’s Trojan asteroids.