This figure shows a plan view of the orbits of all 20 previously known outer irregular satellites of Jupiter before the latest discovery of new objects in late 2001 . Irregular satellites have large orbits, inclinations and eccentricities. Because of their orbit characteristics and small size they are believed to be objects captured during the early formation of Jupiter.
-Black Dot is Jupiter's location. -Purple dotted line is the orbit of the outer most Galilean satellite Callisto. -Green dotted and dashed line is the innter most irregular prograde satellite S/2000 J1. -Blue dashed lines are the irregular satellites in the prograde group. -Red solid lines are the irregular satellites in the retrograde group.
The Irregular Satellites of Jupiter
Himalia, the largest irregular satellite with a diameter of about 150 km, was the first irregular satellite of Jupiter discovered. It was found by Perrine on photographic plates in 1904. Additional members of the irregular satellites have been slowly added to this group throughout the 1900's. A new wave of discovery has been started in the past couple of years by new modern technology through the use large format charge-coupled devices (CCDs). The recent discovery of such a large number of satellites around Jupiter tells us that the environment around this giant planet is rich in small rocky satellites. From the initial surveys using modern CCD detectors there seems to be hundreds of irregular satellites larger than about 1 km in diameter (1 mile) around Jupiter. Continued observations of the space around Jupiter will help us further unravel the Jupiter environment and add the currently known 20 irregular satellites of Jupiter.
14 irregular satellites have orbital radii about 300 times the radius of Jupiter (20,000,000 km) and inclinations near 150 to 160 degrees, meaning that they are retrograde or orbit the planet opposite of its rotation.
5 irregular satellites have orbital radii near 150 Jupiter radii (11,000,000 km) and inclinations near 30 degrees, meaning they are prograde or orbit the planet in the same direction of Jupiter's rotation.
1 irregular satellite (S/2000 J1) occupies neither of the dynamical clusters. It has a prograde orbit with orbital radius near 100 Jupiter radii (7,000,000 km) and inclination 45 degrees.
Origin of the Irregular Satellites
Irregular satellites are thought to have been captured from heliocentric orbit when Jupiter was young. The best evidence for capture is that the orbits of some of the irregular moons are actually retrograde (meaning, they orbit the planet in a direction opposite to Jupiter's rotation). Essentially the only plausible way to produce irregular satellites is by capture. However, it is not easy for Jupiter (or any other planet) to directly capture passing asteroids from heliocentric orbit. In general, some of the initial energy of the heliocentric objects must be dissipated so that Jupiter can hold on to them. The origin of the dissipation that lead to the capture of Jupiter's irregular satellites is unknown. In fact, at the present time there is no plausible source of dissipation so that capturing satellites is presently almost impossible. It is theorized, however, that the youthful Jupiter sustained a bloated atmosphere that extended far above the cloud tops of the present planet. Friction with this atmosphere could have captured the irregular satellites. In possible support of this hypothesis, we note that most of the satellites belong to dynamical groups or "families" with similar semi-major axes and inclinations. Perhaps these families were formed when initial bodies broke up on capture, due to the pressure exerted by impact with the extended atmosphere.
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