Asteroid families (Hirayama families and Kirkwood gaps)

For the gravitational influences of Mars and Jupiter, the orbits of main belt asteroids are not uniformly distributed over the main belt.The regions which seem to be empty of objects are called Kirkwood gaps and can be easily explained by the near-by presence of Jupiter and the mechanism of resonance.

Moreover, asteroids of the main belt seem to be grouped in what are called Hirayama or dynamical families.These families are groups of asteroids that travel in a cluster along the same orbit. They are probably remnants of a single, large body that was broken into a group of smaller asteroids. The principal Hirayama families are Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles and Hildas (which are named after the main asteroid in the group).
n this graph it is possible to see the distribution of the main belt asteroids as a function of their major semiaxis a and of the inclination of their orbit. Some very evident concentratons of asteroids are visible in the graph. In particular the three most evident concentrations on the right of the plot are the Themis, Eos and Koronis families.

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