Callirrhoe

Logged as an asteroid

Spacewatch imaged Callirrhoe from Kitt Peak between 6 October and 4 November 1999. At first it was thought to be an ordinary asteroid and received the provisional asteroid designation 1999 UX18.^[1]

Orbit calculations at the Minor Planet Center revealed it was actually circling Jupiter. Tim Spahr established this on 18 July 2000, and it was redesignated S/1999 J 1 — the 17th confirmed moon of Jupiter.^[2][1]

Ending a twenty-year gap

After the Voyager probes found three small inner moons in 1979, no new moons of Jupiter were discovered for two decades. Callirrhoe’s chance discovery in October 1999 broke the drought — just as digital CCD cameras were transforming sky surveys.^[4]

A backwards, tilted orbit

Callirrhoe orbits Jupiter in the retrograde direction, opposite to the planet’s rotation, at an average distance of about 23.8 million km. Its orbit is inclined about 145° and noticeably elongated (eccentricity ≈ 0.29).^[3][1]

Part of a family — maybe

Its orbit resembles that of the larger moon Pasiphae, so it is counted in the Pasiphae group, thought to be fragments of a captured body broken up by a collision.^[2] But its mean inclination and different colour mean it could also be an independent object, captured on its own.^[1]

Orbit

Callirrhoe’s path takes it from about 16.9 to 30.7 million km from Jupiter, completing one lap in roughly 750 days.^[3] For comparison, Callisto — the outermost of the four large Galilean moons — orbits at 1.88 million km.^[3]

Named for a river god’s daughter

In October 2002 the moon was named after Callirrhoe, daughter of the river god Achelous and, in the IAU’s account, one of Zeus’s conquests.^[1] The name means “beautiful flow” in Greek.^[6]

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