Callirrhoe (moon)

Callirrhoe was imaged by Spacewatch at Kitt Peak National Observatory from 6 October through 4 November 1999.^[8] It was originally believed to be an asteroid, and was thus given the minor planet provisional designation 1999 UX₁₈.^[9][10] It was discovered to be in orbit around Jupiter by Tim Spahr on 18 July 2000, and then given the designation S/1999 J 1.^[1][11] It was the 17th confirmed moon of Jupiter.^[8]

It was named in October 2002 after Callirrhoe, daughter of the river god Achelous, one of Zeus's (Jupiter's) many conquests.^[12]

Callirrhoe orbits Jupiter (at an average distance of 23.3 million km) on a high-eccentricity (0.33) and high-inclination (147° to ecliptic) retrograde orbit.^[5] The orbital elements are continuously changing due to solar and planetary perturbations.

It belongs to the Pasiphae group, a group of retrograde moons jupiters with semi-major axes spread over 22–25 million km, inclinations between 141° and 158°, and higher eccentricities between 0.22 and 0.44.

Callirrhoe has an apparent magnitude of 20.8,^[7] making it even fainter than dwarf planet Eris at magnitude 18.7.^[13] Jupiter is about 2.1 billion times brighter than Callirrhoe.^[b]

Callirrhoe's measured albedo is around 5.2%, which means its diameter is 9.6 kilometers. ^[6]

While Pasiphae belongs to the grey color class (V=17.22 B−V=0.74, V−R=0.38), Callirrhoe falls under the light red color class (V=21.39 B−V=0.72, V−R=0.50), similarly to Megaclite and Sinope.^[14]

Callirrhoe probably did not form near Jupiter but was captured by Jupiter later. Callirrhoe is believed to be a fragment from a captured asteroid along with other Pasiphae group satellites.^[15][16]

However, it falls into a different color class than Pasiphae and could therefore have been captured by Jupiter independently of the Pasiphae group.

The New Horizons spacecraft flew through the Jovian system in early 2007, using Jupiter for a gravity assist to shorten its journey to Pluto. As a navigation exercise, New Horizons imaged Callirrhoe from a distance on 10 January 2007 using its LORRI instrument.^[17]