Callisto and Themisto are occasionally confused in Greek mythology. Is that why astronomers gave these two names to neighboring satellites of Jupiter, or was it purely a coincidence?
As far as we currently know, Metis is closer to Jupiter than any other moon. The next closest is Adrastea, followed by Amalthea, Thebe, Io, Europa, Ganymede, Callisto, and Themisto. However, while Callisto and Themisto are next door neighbors, they do not live very close together. At an average, Themisto is well over four million miles from the center of Jupiter. In contrast, the distance between Callisto and the center of Jupiter is a little more than one million miles. Consequently, even when the two heavenly bodies are in conjunction, they are still far apart.
Leda is the Jovian moon immediately beyond the orbit of Themisto. There is normally a lot of space between these two neighbors also. It would be interesting to know how close they can approach one another. Both have rather eccentric orbits, and there is a difference of only 11,709 kilometers between the distance of Leda from Jupiter at its periapsis and the distance of Themisto from Jupiter at its apoapsis, according to figures given by NASA. However, they probably might never come so close together because of other factors. For example, Themisto has a considerably greater angle of inclination than Leda. It is also possible that the NASA figures are not correct. Wikipedia gives a lower figure for the apoapsis of Themisto
At any rate, Themisto is a sort of lone wolf. It is the only moon thus far discovered between the four large Galilean satellites and the so called Himalia group, consisting of Leda, Himalia, Lysithea, and Elara.
As mentioned above, Themisto has an eccentric orbit. According to NASA, its distance from the center of Jupiter ranges from 5,687,303 kilometers to 9,326,697 kilometers. According to NASA, its orbit inclination is 43.068 degrees, but Wikipedia gives a slightly higher figure. According to both sources, Themisto revolves around Jupiter in approximately 130 earth days.
According to NASA, the mean radius of Themisto is 4 kilometers. However, the accuracy of this figure depends on whether they have arrived at the correct figure for the albedo of the satellite. The albedo of a moon or planet is the fraction of the total light falling on its surface that is reflected from it, according to the Oxford Dictionary of Astronomy.
NASA gives 0.04 as the albedo of Themisto. If this is correct, Themisto is a rather dark body. In contrast, the albedo of bright portions of the Jovian satellite Io ranges from 0.6 to 0.7, according to NASA.