S/2003 J18


S/2003 J18 was discovered in April 2003 by Brett Joseph Gladman at the Mauna Kea Observatory, Hawaii.


S/2003 J18 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.

S/2003 J18 has a mean radius of less than a mile (about one kilometer), assuming an albedo of 0.04. At a mean distance of about 13 million miles (20.5 million kilometers) from Jupiter, it takes about 598 Earth days to complete one orbit.

How S2003 J18 Got its Name

S/2003 J18 was so designated because it is a satellite (S) that was discovered in 2003, and was the 18th satellite of Jupiter (J) to be found that year.

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