Themisto was initially discovered on Sept. 30, 1975, by Charles Thomas Kowal and Elizabeth Roemer. It was subsequently lost until 2000, when it was rediscovered by Scott S. Sheppard, David C. Jewitt, Yanga Roland Fernandez and Eugene A. Magnier as part of a systematic search for small irregular Jovian moons. They used two CCD cameras — the largest in the world at the time — one mounted on the 8.3-m Subaru telescope and the other on the 3.6-m Canada-French-Hawaii telescope on Mauna Kea in Hawaii.
Themisto is a small satellite with a mean radius of 2.5 miles (4 kilometers) whose orbit is inclined with respect to Jupiter's equatorial plane. It travels in the same direction as Jupiter's rotation (a prograde orbit).
At a distance of about 4.5 million miles (7.3 million kilometers) from Jupiter, Themisto takes about 130 Earth days to complete one orbit.
How Themisto Got Its Name
Satellites in the Jovian system are named for Zeus/Jupiter's lovers and descendants.
Themisto was originally called S/1975 J1 and then S/2000 J1 when it was rediscovered. Its current name comes from the Greek mythological character who was the mother of Ister by Zeus, the Greek equivalent of the Roman god Jupiter. She was changed into a bear by a jealous Hera, who was Zeus' wife and sister.
Among Jupiter's outer satellites, Themisto is the first to be given a name that ends in "o," denoting those with inclinations of about 40 to 60 degrees. Most of Jupiter's prograde satellites have names ending in "a" and most of its retrograde satellites have names that end in "e.