Themisto (moon)

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S 2000 J 1.jpg
Rediscovery images of Themisto taken in November 2000
Discovered byCharles T. Kowal (1975)

Elizabeth P. Roemer (1975)
Scott S. Sheppard (2000)
David C. Jewitt (2000)
Yanga R. Fernández (2000)

Eugene A. Magnier (2000)
Discovery dateSeptember 30, 1975
November 21, 2000 (rediscovered)
Named after
Θεμιστώ Themistō
S/2000 J 1
AdjectivesThemistoan /θɛmɪˈst.ən/[2] Themistoian /θɛmɪˈst.iən/
Orbital characteristics[3]
7507000 km
+130.0 days
Satellite ofJupiter
Group(own group)
Physical characteristics
Mean diameter
9 km
Mass6.89×1014 kg[citation needed]
Mean density
2.6 g/cm3 (assumed)[4]
Albedo0.04 (assumed)[5]
Temperature~124 K

Themisto /θɪˈmɪst/, also known as Jupiter XVIII, is a small prograde irregular satellite of Jupiter. It was discovered in 1975, subsequently lost, and rediscovered in 2000.

Discovery and naming[edit]

Themisto observed by the Canada-France-Hawaii Telescope on August 6, 2000, several months before its rediscovery in November 2000

Themisto was first discovered by Charles T. Kowal and Elizabeth Roemer on September 30, 1975, reported on October 3, 1975,[6] and designated S/1975 J 1. However, not enough observations were made to establish an orbit and it was subsequently lost. (See also lost minor planet.)

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 one observed in 1975.[7] This observation was immediately correlated with an observation on August 6, 2000, 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, which was reported to the Minor Planet Center but not published as an IAU Circular (IAUC).[8]

In October 2002 it was officially named after Themisto,[9] daughter of the river god Inachus and lover of Zeus (Jupiter) in Greek mythology.


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).[5] That figure can be used to find a surface area of between 200 and 380 square kilometers.


  1. ^ Cf. 'Themista' in Noah Webster (1884) A Practical Dictionary of the English Language
  2. ^ 'Themis[t]oan' in James Hall (2015) Moons of the Solar System, p. 82.
  3. ^ S.S. Sheppard (2019), Moons of Jupiter, Carnegie Science, on line
  4. ^ "Planetary Satellite Physical Parameters". JPL.
  5. ^ a b Sheppard, S. S.; Jewitt, D. C.; An abundant population of small irregular satellites around Jupiter, Nature, 423 (May 15, 2003), pp. 261–263
  6. ^ Brian G. Marsden (October 3, 1975). "IAUC 2845: Probable New Satellite of Jupiter". International Astronomical Union Central Bureau for Astronomical Telegrams.
  7. ^ Brian G. Marsden (November 25, 2000). "IAUC 7525: S/1975 J 1 = S/2000 J 1". International Astronomical Union Central Bureau for Astronomical Telegrams.
  8. ^ "MPEC 2000-Y16: S/1975 J 1 = S/2000 J 1, S/1999 J 1". International Astronomical Union Minor Planet Center. December 19, 2000.
  9. ^ Daniel W. E. Green (October 22, 2002). "IAUC 7998: Satellites of Jupiter". International Astronomical Union.

External links[edit]