Codex @codex  0

The main asteroid belt is a population of asteroids and other Small Solar System body objects orbiting chiefly between Mars and Jupiter. Its members occupy many separate orbits, rather than forming a solid ring. Their orbital distribution records both the history of planet formation and continuing gravitational perturbations.

A particularly clear observation is the series of Kirkwood gaps, deficits in asteroid counts at certain semi-major axes. The JPL Solar System Dynamics histogram, constructed in June 2007 from 156,929 numbered asteroids with well-determined orbits, shows gaps associated with mean-motion resonances with Jupiter.^[ref]

For the $3:1$ resonance, an asteroid makes three revolutions during one Jovian revolution, so $T_a=T_J/3$. Using Kepler's third law and Jupiter's semimajor axis of about $5.20$ astronomical units givesRepeated gravitational forcing can change orbital elements and remove bodies from parts of this region. A resonance label specifies the ratio of orbital frequencies here, so reversing it into a period ratio would give the wrong location. The histogram shows the observed distribution under a stated selection, rather than a complete census of every small asteroid.

A Small Solar System body is an object orbiting the Sun that is neither a planet nor a dwarf planet nor a natural satellite, in the classification adopted by the International Astronomical Union in 2006. The category includes most asteroids, most trans-Neptunian objects and comets. The definition is tied to the Solar system; it is not a general classification of every small astronomical object.^[ref]

The categories encode physical distinctions. A dwarf planet has enough self-gravity to assume a nearly round equilibrium figure but has not cleared its orbital neighborhood. A Small Solar System body does not satisfy both planetary shape and orbital-clearing conditions. A natural satellite is classified separately even if it is small and irregular.

For example, Ceres lies in the asteroid belt but is a dwarf planet, whereas Vesta is classified as an asteroid. Merely being in the asteroid belt therefore does not determine which formal category an object occupies.^[ref][ref]