Iocaste was discovered Nov. 23, 2000 by Scott S. Sheppard, David C. Jewitt, Yanga R. Fernandez, and Eugene Magnier at the Mauna Kea Observatory in Hawaii.


Iocaste is a member of the Ananke group, a family of Jovian satellites which have similar orbits and are therefore thought to have a common origin. The group probably began as an asteroid that was captured by Jupiter's gravity and then suffered a collision, which broke off a number of pieces. The largest remaining chunk was named "Ananke," and the smaller pieces became the other 15 moons in the Ananke group.

All of the Ananke moons are retrograde, which means that they orbit Jupiter in the opposite direction from the planet's rotation. Their orbits are also eccentric (elliptical rather than circular) and rather highly inclined with respect to Jupiter's equatorial plane. All of these characteristics support the idea that the Ananke satellites began as a captured asteroid, rather than forming as part of the original Jupiter system. None of the Ananke members is massive enough to pull itself into a sphere, so they are probably all irregularly shaped.

Iocaste has a mean radius of about 1.6 miles (2.6 kilometers), assuming an albedo of 0.04. It is colored a similar gray to two other moons in the Ananke family: Praxidike and Harpalyke. At a mean distance of about 13.2 million miles (21.3 million kilometers) from Jupiter, Iocaste takes about 632 Earth days to complete one orbit.

How Iocaste Got its Name:

Originally called S/2000 J3, Iocaste was named for the mother of Agamedes by the Roman god, Jupiter.

A name ending in "e" was chosen in accordance with the International Astronomical Union's policy for designating outer moons with retrograde orbits.

Keep Exploring

Discover More Topics From NASA