Themisto (moon)

Themisto
Discovery
Discovered by	Charles Kowal (1975); Elizabeth Roemer (1975); Scott S. Sheppard (2000); David C. Jewitt (2000); Yanga R. Fernández (2000); Eugene A. Magnier (2000)
Discovery date	September 30, 1975; November 21, 2000 rediscovered
Orbital characteristics
Periapsis	5,909,000 km (0.039 AU)
Apoapsis	8,874,300 km (0.059 AU)
Mean orbit radius	7,391,650 km (0.04941 AU)
Eccentricity	0.2006
Orbital period	129.82761 d (0.3554 a)
Average orbital speed	4.098 km/s
Inclination	45.81° (to the ecliptic); 47.48° (to Jupiter's equator)
Satellite of	Jupiter
Physical characteristics
Mean radius	4 km
Circumference	~25 km
Surface area	~200 km²
Volume	~270 km³
Mass	6.89×1014 kg
Mean density	2.6 g/cm3 assumed
Surface gravity	~0.0029 m/s2 (0.0003 g)
Escape velocity	~0.0048 km/s
Albedo	0.04 assumed
Temperature	~124 K

Themisto or Jupiter XVIII, is a small prograde non-spherical moon of Jupiter. It was found in 1975, lost, and then refound in 2000.

Discovery and naming

Themisto was first found by Charles T. Kowal and Elizabeth Roemer on September 30, 1975, reported on October 3, 1975^[3] and designated S/1975 J 1. However, not enough observations were made to establish an orbit and it was subsequently lost.

Themisto appeared as a footnote in astronomy textbooks into the 1980s. Then, in 2000, a seemingly new moon was found 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 that this was the same as the 1975 object. The Sheppard et al. announcement^[4] was immediately correlated with an August 6 2000 observation 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 — an observation that was reported to the Minor Planet Center but not published as an IAU Circular (IAUC).^[5]

In October 2002 it was officially named after Themisto,^[6] daughter of the river god Inachus by Zeus (Jupiter) in Greek mythology.

Themisto's orbit is unusual. Unlike most of Jupiter's moons, which orbit in groups, Themisto orbits alone, midway between the Galilean moons and the first group of prograde irregulars.

Themisto is about 8 kilometers in diameter (assuming an albedo of 0.04)^[1]

v t e Moons of Jupiter
Listed in increasing approximate distance from Jupiter
Inner moons (4)	Metis Adrastea Amalthea Thebe
Galilean moons (4)	Io Europa Ganymede Callisto
Themisto group (1)	Themisto
Himalia group (9)	Leda Ersa S/2018 J 2 Himalia Pandia Lysithea Elara S/2011 J 3 Dia
Carpo group (2)	S/2018 J 4 Carpo
Valetudo group (1)	Valetudo
Ananke group (26)	Euporie Jupiter LV Eupheme S/2021 J 3 Jupiter LII Jupiter LIV Mneme Euanthe S/2003 J 16 Harpalyke Orthosie S/2022 J 3 Helike S/2021 J 2 Praxidike Jupiter LXIV S/2021 J 1 S/2003 J 12 Jupiter LXVIII Thelxinoe Thyone S/2003 J 2 Ananke Iocaste Hermippe Jupiter LXX
Carme group (30)	S/2022 J 1 S/2022 J 2 S/2016 J 3 Pasithee Jupiter LXIX S/2021 J 6 S/2003 J 24 Chaldene Jupiter LXIII Isonoe S/2021 J 4 Kallichore Erinome Kale Eirene Aitne Eukelade Arche Taygete Jupiter LXXII Carme Herse Jupiter LXI Jupiter LI S/2003 J 9 Jupiter LXVI Kalyke S/2018 J 3 S/2021 J 5 S/2003 J 10
Pasiphaë group (18)	Philophrosyne Eurydome Jupiter LVI S/2003 J 4 S/2016 J 4 Jupiter LXVII Hegemone Pasiphaë Sponde Megaclite Cyllene Sinope Jupiter LIX Aoede Autonoe Callirrhoe S/2003 J 23 Kore
Related page	Rings of Jupiter
Category