Themisto (moon)

Themisto (/θəˈmɪstoʊ/), also known as Jupiter XVIII, is a small prograde irregular satellite of Jupiter. It was discovered in 1975, subsequently lost, and rediscovered in 2000.

Themisto

Rediscovery images of Themisto taken by the UH88 telescope in November 2000
Discovery[1][2]
Discovered by	Charles T. Kowal (1975) Elizabeth P. Roemer (1975) Scott S. Sheppard (2000) David C. Jewitt (2000) Yanga R. Fernández (2000) Eugene A. Magnier (2000)
Discovery site	Palomar Observatory Mauna Kea Observatory (rediscovery)
Discovery date	30 September 1975 21 November 2000 (rediscovery)
Designations
Designation	Jupiter XVIII
Pronunciation	/θəˈmɪstoʊ/[3]
Named after	Θεμιστώ Themistō
Alternative names	S/2000 J 1 S/1975 J 1
Adjectives	Themistoan /θɛməˈstoʊ.ən/[4] Themistoian /θɛməˈstoʊ.iən/
Orbital characteristics[5]
Epoch 23 March 2018 (JD 2458200.5)
Observation arc	42.54 yr (15,536 days)
Semi-major axis	0.0494401 AU (7,396,130 km)
Eccentricity	0.2522112
Orbital period (sidereal)	+129.95 d
Mean anomaly	2.39396°
Mean motion	2° 46m 13.369s / day
Inclination	45.28121° (to ecliptic)
Longitude of ascending node	192.64162°
Argument of perihelion	241.25168°
Satellite of	Jupiter
Group	(own group)
Physical characteristics
Mean diameter	9 km[6] 16.4 km[7]
Albedo	0.04 (assumed)
Apparent magnitude	21.0[6]
Absolute magnitude (H)	12.9[5] 13.06±0.34 (V)[7]

Discovery and naming

Themisto observed by the Canada–France–Hawaii Telescope on 6 August 2000, several months before its rediscovery in November 2000

Themisto was first discovered by Charles T. Kowal and Elizabeth Roemer on 30 September 1975, reported on 3 October 1975,^[1] 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.^[2] 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).^[8]

In October 2002, it was officially named after Themisto,^[9] daughter of the river god Inachus and lover of Zeus (Jupiter) in Greek mythology.

Characteristics

Diagram illustrating Themisto's orbit (top left) among those of the other irregular satellites of Jupiter. The satellites above the horizontal axis are prograde, the satellites beneath it are retrograde. The yellow segments extend from the pericentre to the apocentre, showing the orbital eccentricity.

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.

Themisto is about 9 km (5.6 mi) in diameter (assuming an albedo of 0.04).^[6] While its true albedo could not be measured by NEOWISE due to poor timing of observations,^[10] it is known to have color index B−V=0.83, V−R=0.46, and V−I=0.94.^[11]