Themisto (), also known as Jupiter XVIII, is the innermost and one of the medium-sized irregular satellites of Jupiter.
Discovery and naming
Themisto was first discovered by Charles T. Kowal and Elizabeth Roemer on 30 September 1975, reported on 3 October 1975, and designated S/1975 J 1. However, not enough observations were made to establish an orbit and it was subsequently lost. Then, on 21 November 2000, a seemingly new satellite was discovered by Scott S. Sheppard, David C. Jewitt, Yanga R. Fernández and Eugene A. Magnier and was designated S/2000 J 1. It was soon confirmed from computing the past trajectory of the satellite that this was the same as the one observed in 1975. This observation was immediately correlated with an earlier observation on 6 August 2000 by the team of Brett J. Gladman, John J. Kavelaars, Jean-Marc Petit, Hans Scholl, Matthew J. Holman, Brian G. Marsden, Philip D. Nicholson and Joseph A. Burns, which was reported to the Minor Planet Center but not published as an IAU Circular (IAUC). In October 2002, it was officially named after Themisto, daughter of the river god Inachus and lover of Zeus (Jupiter) in Greek mythology.
Orbit
Themisto orbits Jupiter at an average distance of 7.4 million km in 130 days, at an inclination of about 45° to the ecliptic, in a prograde direction and with an eccentricity of 0.252. Its orbit is continuously changing due to solar and planetary perturbations. This means that Themisto has the shortest orbital period of all irregular moons, apart from Triton. Themisto's orbit is unusual: unlike most of Jupiter's moons, which orbit in distinct groups, Themisto orbits alone. The moon is located midway between the Galilean moons and the first group of prograde irregular moons, the Himalia group.
Physical characteristics Themisto is about 9 km (5.6 mi) in diameter (assuming an albedo of 0.04). While its true albedo could not be measured by NEOWISE due to poor timing of observations, It is known that Themisto is colored red in the visible spectrum. (B−V=0.83, V−R=0.46, and V−I=0.94) similar to D-type asteroids.
Origin Themisto probably did not form near Jupiter but was captured by Jupiter later. Themisto is probably the remnant of a broken, captured heliocentric asteroid.
References
External links David Jewitt's pages Jupiter's Known Satellites Archived 18 March 2015 at the Wayback Machine (by Scott S. Sheppard) "MPC: Natural Satellites Ephemeris Service". International Astronomical Union Minor Planet Center. "Planetary Satellite Mean Orbital Parameters". JPL.
