Themisto ( Jupiter XVIII) is one of the smallest known moons of the planet Jupiter.
Themisto was first discovered on September 30, 1975 by the astronomer Charles Kowal and Elizabeth Roemer and initially received the provisional designation S/1975 J 1 However, it was not enough observations are carried out to determine the path data accurately, so that the heavenly bodies first again lost.
In 2000, Scott S. Sheppard, David C. Jewitt, Yanga R. Fernández and discovered Eugene (Gene ) A. Magnier a supposedly new moon in Jupiter, received the designation S/2000 J 1. However, it soon turned out that it must be at the 1975 discovered object.
Themisto orbits Jupiter at an eccentric orbit at an average distance of 7.507 million km in about 130 days. The track has an eccentricity of 0.2420 and is strongly inclined 43.3 ° relative to the local Laplace plane.
Themistos train is unusual as it moves between the Galilean moons and the moons of Himalia group.
Themisto has an average diameter of only 8 km. Their density is estimated to be 2.6 g / cm ³, which is why they should be constructed primarily of silicate rock. Its surface is very dark. The albedo is 0.04, which means that only 4% of the incident sunlight is reflected.
With an apparent magnitude of 21m, it is extremely faint.