Themisto (moon)
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| Discovery | |
|---|---|
| Discovered by: | Charles Kowal (1975) Elizabeth Roemer (1975) |
| 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 radius of orbit: | 7,391,650 km (0.04941 AU) |
| Orbital circumference: | 45,972,400 km (0.307 AU) |
| Eccentricity: | 0.2006 |
| Orbital period: | 129.82761 d (0.3554 a) |
| Avg. orbital speed: | 4.098 km/s |
| Max. orbital speed: | 5.074 km/s |
| Min. orbital speed: | 3.379 km/s |
| Inclination: | 45.81° (to the ecliptic) 47.48° (to Jupiter's equator) |
| Satellite of: | Jupiter |
| Physical characteristics | |
| Mean radius: | 4 km[1] |
| Equatorial circumference: | ~25 km |
| Surface area: | ~200 km² |
| Volume: | ~270 km³ |
| Mass: | 6.89×1014 kg |
| Mean density: | 2.6 g/cm3 assumed[2] |
| Equatorial surface gravity: | ~0.0029 m/s2 (0.0003 g) |
| Escape velocity: | ~0.0048 km/s |
| Albedo: | 0.04 assumed[1] |
| Temperature: | ~124 K |
Themisto (thə-mis'-toe, IPA: [θəˈmɪstoʊ]; Greek Θεμιστώ), or Jupiter XVIII, is a prograde irregular satellite of Jupiter. It was first discovered by Charles T. Kowal and Elizabeth Roemer on September 30, 1975, reported on October 3, 1975 (IAUC 2845) and designated S/1975 J 1, but not enough observations were made to establish an orbit and it was subsequently lost, rendering it hypothetical.
It 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 (IAUC 7525, November 25, 2000) 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).
In 2002 it was officially named after Themisto, a lover of Zeus (Jupiter) in Greek mythology.
Themisto's orbit is unusual, as it orbits about midway between the Galilean moons and the first group of prograde irregulars. The diagram illustrates its orbit in relation to 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 eccentricity.
Themisto is about 8 kilometers in diameter (assuming an albedo of 0.04)[1]
[edit] See also
[edit] External links
[edit] References
- ^ a b c Scott S. Sheppard, David C. Jewitt An abundant population of small irregular satellites around Jupiter, Nature, 423 (May 2003), pp.261-263 (pdf).
- ^ Physical parameters from JPL
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Listed in increasing distance from Jupiter. Italics indicate ungrouped moons.
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| Amalthea group | 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 | Ananke · Praxidike · Harpalyke · Iocaste · Euanthe · Thyone (core) Euporie · S/2003 J 3 · S/2003 J 18 · Thelxinoe · Helike · Orthosie · S/2003 J 16 · Hermippe · Mneme · S/2003 J 15 (peripheral) |
| 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 | Eurydome · S/2003 J 23 · Hegemone · Pasiphaë · Sponde · Cyllene · Megaclite · S/2003 J 4 · Callirrhoe · Sinope · Autonoe · Aoede · Kore |
| S/2003 J 2 | |
