Eunomia

15 Eunomia is the giant of the stony asteroids. Discovered on 29 July 1851 by the Italian astronomer Annibale de Gasparis at Naples, it is the largest S-type — silicate-rich — body in the Solar System: a rocky world roughly 270 km across drifting through the middle of the main asteroid belt. Where the belt's true heavyweights Ceres and Vesta get the attention, Eunomia is the biggest of the far more common stony class.

It is also the namesake and largest member of one of the belt's great asteroid families — thousands of fragments thought to share a single shattered ancestor. De Gasparis named it for Eunomia, one of the Horae of Greek myth and the personification of good order and lawful conduct.

The largest of the stony asteroids

Eunomia is an elongated, irregular body measuring roughly 270 km at its longest, holding on the order of one percent of the entire asteroid belt's mass. It is dense for an asteroid, near 3 g/cm³, consistent with a rocky silicate-and-metal make-up, and it turns quickly — a full rotation every six hours and five minutes. That fast spin and elongated shape make its brightness flicker measurably over a single night.

A family of fragments

Eunomia is the parent of the Eunomia family, one of the largest known asteroid families, with well over ten thousand identified members. Families like this are the debris of ancient collisions: long ago, something struck the parent body hard enough to blast off a cloud of fragments that still travel on similar orbits today. Eunomia is the largest survivor, and most of its family are stony S-types like it — a natural laboratory for studying how asteroids break apart.

Observing Eunomia

At a favourable opposition Eunomia can brighten to about magnitude 8, an easy target for a small telescope or good binoculars for observers who know precisely where to look. As with other asteroids, its true size and shape have been refined by watching it pass in front of background stars.