Updated May 8, 2019 | Infoplease Staff

Amalthea, Jupiter's innermost satellite, was discovered in 1892. It is so small—165 mi (265 km) long and 90 mi (150 km) wide—that it is extremely difficult to observe from Earth. Amalthea is an elongated, irregularly shaped satellite of reddish color. It orbits the planet every 12 hours and is in synchronous rotation, with its long axis always oriented toward Jupiter.

Originally thought to be heavily cratered, late 2002 data from the Galileo mission indicated that Amalthea may be a loosely-packed pile of rubble. It was discovered that the satellite had an unexpected low density, close to the density of ice. Amalthea may originally have been one piece that was bombarded and broken into chunks now held together by the gravity of the pieces. The gaps between the boulderlike pieces may make up more of the moon's volume than the actual rocks. Amalthea may be mostly rock with some ice, rather than the rock and iron it was previously held to be.

Jupiter's other named moons are Adrasta, Metis, Thebe, Leda, Himalia, Lysithea, Elara, Ananke, Carne, Pasiphae, Sinope; those discovered in 1999 and 2000 are Themisto, Iocaste, Harpalyke, Praxidike, Taygete, Chaldene, Kalyke, Callirrhoe, Megaclite, Isonoe, and Erinome. The 2001 moons are Euporie, Orthosie, Euanthe, Thyone, Hermippa, Pasithee, Kale, Aitne, Eurydome, Autonoe, and Sponde. The names come from members of Jupiter's (or Zeus's) entourage. The 2003 Jovian moons were named after daughters of Zeus: Hegemone, Mneme, Aoede, Thelxinoe, Arche, Kallichore, Helike, Carpo, Eukelade, and Cyllene. Galileo images of Thebe taken in Jan. 2000 found a prominent impact crater that is about 25 mi (40 km) across and named the crater Zethus.

Between 2000 and 2003, 45 moons were found, bringing Jupiter's satellite total to 63, the greatest in the solar system. The new moons were generally small with distant retrograde orbits (orbital movement opposite to the planet's spin). Most of the new moons were sighted using Hawaii's Mauna Kea telescopes. Some astronomers believe that Jupiter's moon count could reach 100.

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