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An asteroid that orbits the Sun through the central regions of the Solar System's Asteroid Belt, Io is an irregular body formed from carbon compounds measuring some 170 km from side to side. It rotates on its axis in a little under seven hours in a retrograde direction, against the typical rotation of the system as a whole, and its orbit through the Asteroid Belt carries it around the Sun in a period of some four years and four months.
A significant family of asteroids within the Belt are the stony bodies of the Eunomia group, named for the largest of their number. These represent the remains of an ancient planetoid that broke apart long ago, with its core surviving as Eunomia itself, and its crust shattering into thousands of smaller bodies.
Io follows an orbit that seems to associate it with this busy Eunomia family, but it is formed from carbonates rather than solid rock, and was not part of the original body that formed the family. It is in fact the largest of a group known as 'Eunomian interlopers', bodies that follow the Eunomian orbital patterns, but do not share a common origin with the true members of the family.
The name Io is also used for a moon of Jupiter, the innermost of that planet's four Galilean satellites. The two bodies named Io, asteroid and moon, are entirely unconnected to one another apart from sharing a name derived from the same mythological origins.
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