Themisto (moon)

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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
Designations
Adjectives Themistoan, Themistonian
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)
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

Themisto (/θɨˈmɪst/; from 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.

Discovery and naming[edit]

Themisto was first discovered 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. (See also Lost asteroids.)

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[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 and lover of Zeus (Jupiter) in Greek mythology.

Characteristics[edit]

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, called the Himalia group.

Themisto is about 8 kilometers (5 miles) in diameter (assuming an albedo of 0.04).[1] That figure can be used to find a surface area of between 200 and 380 square kilometers.

References[edit]

  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
  2. ^ Physical parameters from JPL
  3. ^ IAUC 2845: Probable New Satellite of Jupiter 1975 October 3 (discovery)
  4. ^ IAUC 7525: S/1975 J 1 = S/2000 J 1 2000 November 25 (recovery)
  5. ^ MPEC 2000-Y16: S/1975 J 1 = S/2000 J 1, S/1999 J 1 2000 December 19 (recovery and ephemeris)
  6. ^ IAUC 7998: Satellites of Jupiter 2002 October 22 (naming the moon)
  1. MPC: Natural Satellites Ephemeris Service
  2. Mean orbital parameters NASA JPL

External links[edit]