Ananke was discovered on Sept. 28, 1951 by Seth Barnes Nicholson on a photograph made with the 100-inch (2.5 m) Hooker telescope at the Mount Wilson Observatory in California.
With a mean radius of 14 km (assuming an albedo of 0.04), Ananke is the largest member of the Ananke group, a family of Jovian satellites which have similar orbits and are therefore thought to have a common origin. Ananke was probably an asteroid that was captured by Jupiter's gravity and then suffered a collision which broke off a number of pieces. Those 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.
Ananke's observed color is somewhere between gray and light red. At a mean distance of about 21.3 million miles km from Jupiter, the satellite takes about 630 Earth days to complete one orbit.
How Ananke Got its Name
Satellites in the Jovian system are named for Zeus/Jupiter's lovers and descendants. Names of outer satellites with a prograde orbit generally end with the letter "a" (although an "o" ending has been reserved for some unusual cases), and names of satellites with a retrograde orbit end with an "e."
Ananke was named for the mother of Adrastea by Zeus, the Greek equivalent of the Roman god Jupiter. In other accounts, Adrastea is described as a nymph of Crete who was one of the nursemaids of the infant Zeus. Ananke is the personification of fate or necessity in ancient Greek literature, who rewards or punishes people for their deeds.