Jupiter's satellites(Jovian satellites) A system of diverse satellites orbiting Jupiter at distances between 128 000 km and 24 million km and ranging in size from the planet-like Galilean satellites to tiny worlds of rock and ice measuring barely 1 kilometer across. At the end of 2004, 63 Jovian satellites were known, most of which were discovered after 2000 through analysis of photographs taken either by spacecraft such as the Voyager probes or Galileo mission or by high-quality groundbased instruments using large-format CCDs. Their physical and orbital properties are given in Table 2, backmatter.
The satellites can be divided into three main groups. The largest is an outer group of at least 48 small bodies moving in loosely bound retrograde orbits (see direct motion) that are highly eccentric and are inclined to Jupiter's equatorial plane by an angle of about 150°–160°; most of them orbit the planet in periods of about 700 days and at mean distances of roughly 21 to 24 million km. Chief among them are Ananke, Carme, Pasiphae, and Sinope. The most remote one of this group so far known, designated S/2003 J2, in fact orbits Jupiter in more than 900 days at a mean distance of more than 28 million km. An intermediate group of five small satellites (Leda, Himalia, Lysithea, Elara, and S/2000 J11) move in approximately 250-day direct orbits at mean orbital distances between 11 and 12 million km; the orbits are generally less eccentric than those of the outer group and are inclined at an angle of about 28° to Jupiter's equatorial plane. Two other satellites follow direct orbits that do not fit neatly into either the outer or intermediate groups. These are Themisto, which orbits Jupiter in 130 days at a mean distance of about 7 million km and with an inclination of 43°, and S/2003 J20, which completes an orbit every 456 days at a mean distance of 17 million km and an inclination of 51°. The inner group is comprised of the four large Galilean satellites – Io, Europa, Ganymede, and Callisto – together with Amalthea and the three satellites Thebe, Adrastea, and Metis; these all move in near-circular direct orbits that lie close to the plane of Jupiter's equator.
The members of the outer and intermediate groups are probably captured asteroids, although the capturing process that would place objects into one group rather than the other has not been defined. Possibly some of them were former members of the Trojan group of asteroids. All eight inner satellites are believed to have formed out of the dust particles that surrounded the disk of gas and dust from which Jupiter formed. These inner satellites all lie within Jupiter's magnetosphere and are effective in sweeping up the charged particles found there, becoming intensely radioactive in the process.
Metis, Jupiter's innermost satellite, was unknown before Voyager 2 photographed it in 1979. So too were Thebe and Adrastea. The irregularly shaped Amalthea had been discovered in 1892, but few details of it were known before the visit of the Voyager probes. The Galilean satellites, however, have been studied throughout the telescopic era but came under much more intensive scrutiny from Pioneer 10 and 11, as well as the Voyager and Galileo missions.