Themisto (moon)

From Wikipedia, the free encyclopedia

Categories: Jupiter's moons | Irregular satellites

**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^[2]
Circumference	~25 km
Surface area	~200 km²
Volume	~270 km³
Mass	6.89×10¹⁴ kg
Mean density	2.6 g/cm³ assumed^[3]
Equatorial surface gravity	~0.0029 m/s² (0.0003 g)
Escape velocity	~0.0048 km/s
Albedo	0.04 assumed^[2]
Temperature	~124 K

Themisto (pronounced /θɨˈmɪstoʊ/ thə-MIS-toe, or as in Greek Θεμιστώ), also known as Jupiter XVIII, is a small prograde irregular satellite of Jupiter. It was discovered in 1975, lost, and then rediscovered in 2000.

1 Discovery and naming
2 Characteristics
3 See also
4 References
5 External links

Discovery and naming

Themisto was first discovered by Charles T. Kowal and Elizabeth Roemer on September 30, 1975, reported on October 3, 1975^[4]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 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 that this was the same as the 1975 object. The Sheppard et al. announcement^[5] 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).^[6]

In October 2002 it was officially named after Themisto^[7], daughter of the river god Inachus by Zeus (Jupiter) in Greek mythology.

Characteristics

Image:TheIrregulars JUPITER GROUPS.svg

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, 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)^[2]

References

^ [1]
^ ^a ^b ^c Sheppard, S. S.; Jewitt, D. C.; An abundant population of small irregular satellites around Jupiter, Nature, 423 (May 2003), pp. 261-263
^ Physical parameters from JPL
^ IAUC 2845: Probable New Satellite of Jupiter 1975 October 3 (discovery)
^ IAUC 7525: S/1975 J 1 = S/2000 J 1 2000 November 25 (recovery)
^ MPEC 2000-Y16: S/1975 J 1 = S/2000 J 1, S/1999 J 1 2000 December 19 (recovery and ephemeris)
^ IAUC 7998: Satellites of Jupiter 2002 October 22 (naming the moon)

Ephemeris IAU-MPC NSES
Mean orbital parameters NASA JPL

External links

v • d • e

Moons of Jupiter

Listed in increasing distance from Jupiter. Temporary names in italics.

Inner moons

Metis · Adrastea · Amalthea · Thebe

Galilean moons

Io · Europa · Ganymede · Callisto

Themisto

Himalia group

Leda · Himalia · Lysithea · Elara · S/2000 J 11

Carpo · S/2003 J 12

Ananke group

core	Ananke · Praxidike · Harpalyke · Iocaste · Euanthe · Thyone
peripheral	Euporie · S/2003 J 3 · S/2003 J 18 · Thelxinoe · Helike · Orthosie · S/2003 J 16 · Hermippe · Mneme · S/2003 J 15

Carme group

S/2003 J 17 · S/2003 J 10 · Pasithee · Chaldene · Arche · Isonoe · Erinome · Kale · Aitne · Taygete · S/2003 J 9 · Carme · S/2003 J 5 · S/2003 J 19 · Kalyke · Eukelade · Kallichore

Pasiphaë group