= Asteroid belt
{wiki}
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.https://ssd.jpl.nasa.gov/diagrams/mb_hist.html[Asteroid Main-Belt Distribution, data selection and resonance labels]{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 gives
$$
a_a\simeq\frac{5.20}{3^{2/3}}\,\mathrm{au}\simeq2.50\,\mathrm{au}.
$$
Repeated 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.
Back to article page
