List of Minor-planet Groups - The Asteroid Belt

The Asteroid Belt

The overwhelming majority of known asteroids have orbits lying between the orbits of Mars and Jupiter, roughly between 2 to 4 AU. These could not form a planet due to the gravitational influence of Jupiter. Jupiter's gravitational influence, through orbital resonance, clears Kirkwood gaps in the asteroid belt, first recognised by Daniel Kirkwood in 1874.

The region with the densest concentration (lying between the Kirkwood gaps at 2.06 and 3.27 AU, with eccentricities below about 0.3, and inclinations smaller than 30°) is called the asteroid belt. It can be further subdivided by the Kirkwood Gaps into the:

• Inner asteroid belt, inside of the strong Kirkwood gap at 2.50 AU due to the 3:1 Jupiter orbital resonance. The largest member is 4 Vesta.
  • It apparently also includes a group called the main-belt I asteroids which have a semi-major axis between 2.3 AU and 2.5 AU and an inclination of less than 18°.
• Middle (or intermediate) asteroid belt, between the 3:1 and 5:2 Jupiter orbital resonances, the latter at 2.82 AU. The largest member is Ceres. This group is apparently split into the:
  • Main-belt IIa asteroids which have a semi-major axis between 2.5 AU and 2.706 AU and an inclination less than 33°.
  • Main-belt IIb asteroids which have a semi-major axis between 2.706 AU and 2.82 AU and an inclination less than 33°.
• Outer asteroid belt between the 5:2 and 2:1 Jupiter orbital resonances. The largest member is 10 Hygiea. This group is apparently split into the:
  • Main-belt IIIa asteroids which have a semi-major axis between 2.82 AU and 3.03 AU, an eccentricity less than .35, and an inclination less than 30°.
  • Main-belt IIIb asteroids which have a semi-major axis between 3.03 AU and 3.27 AU, an eccentricity less than .35, and an inclination less than 30°.