Themisto (moon)
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 | |
|---|---|
| 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 orbit radius | 7,391,650 km (0.04941 AU) |
| Eccentricity | 0.2006 |
| 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[1] |
| Circumference | ~25 km |
| ~200 km² | |
| Volume | ~270 km³ |
| Mass | 6.89×1014 kg |
Mean density | 2.6 g/cm3 assumed[2] |
| ~0.0029 m/s2 (0.0003 g) | |
| ~0.0048 km/s | |
| Albedo | 0.04 assumed[1] |
| Temperature | ~124 K |
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.
Characteristics
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]
Themisto (moon) Media
Themisto observed by the Canada-France-Hawaii Telescope on 6 August 2000, several months before its rediscovery in November 2000
References
- ↑ 1.0 1.1 1.2 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