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 time

September 30, 1975

November 21, 2000 rediscovered
Orbit
Shortest distance from what it orbits around 5,909,000 km (0.039 AU)
Longest distance from what it orbits around 8,874,300 km (0.059 AU)
Avgdistance from the center of its orbital path 7,391,650 km (0.04941 AU)
How egg-shaped its orbit is
("eccentricity")
0.2006
How long it takes to complete an orbit 129.82761 d (0.3554 a)
Average speed 4.098 km/s
Angle above the reference plane
("inclination")
45.81° (to the ecliptic)
47.48° (to Jupiter's equator)
What it orbits Jupiter
Size and Other Qualities
Average distance from its center to its surface 4 km[1]
Distance around its equator ~25 km
Area of its surface ~200 km²
Volume inside it ~270 km³
Mass 6.89×1014 kg
Average density 2.6 g/cm3 assumed[2]
Gravity at its surface ~0.0029 m/s2 (0.0003 g)
Slowest speed able to escape into space
("escape velocity")
~0.0048 km/s
How much light it reflects 0.04 assumed[1]
Avg. surface temp. ~124 K

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 and naming[change | edit source]

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[change | edit source]

Diagram illustrating Themisto's orbit (top left) among those of the other non-spherical moons of Jupiter. The moons above the horizontal axis are prograde, the moons 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 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]

References[change | edit source]

  1. Ephemeris IAU-MPC NSES
  2. Mean orbital parameters NASA JPL

Other websites[change | edit source]