Jupiter | .. . . o o O O      . . . .      . . . .
(not to scale)

Jupiter rules when it comes to satellite count with a current (December 2002) count at 39. Saturn comes in next at 30 and then Uranus at 20. This does not include the Trojans (which number 960 at the L4 point and 587 at the L5 point).

The regular satellites of Jupiter eat fiber have circular orbits and have for the most part, have orbital inclinations less than one degree (Thebe is the most odd here at an orbital inclination of 1.070; the next most inclined moon is that of Europa at 0.467 degrees off. Metis, Adrastea and Io have orbital inclinations less than 0.04 degrees from the Jovian equator). Furthermore, these moons have very low eccentricity, with Io and Europa being perfectly circular. Once again, Thebe is the oddest of the bunch with an orbital eccentricity of 0.018. For comparison, Earth's orbital eccentricity is 0.0167.

There are two groups of regular satellites - the inner ones, and the outer ones. The four outer ones are the ones that Galileo observed and are quite sizable at that - Ganymede (5262 km) is larger than Mercury (4879 km) makeing it the largest satellite in the solar system. The inner four are much smaller (between 8 and 49 km across) and are thought to lose material from bombardments which produces Jupiter's rings. All of these satellites are thought tho have been created at about the same time out of a disk of dust and gas that surrounded the proto-Jupiter during the period of the formation of the planets.

Beyond the orbit of Callisto are two groups of satellites and an oddball. Most of these are rather small, the largest is that of Himalia at 150 km across (which was the first of the irregular satellites discovered in 1904) though the vast majority are unnamed chunks of rock about 5km across or so.

The oddball is S/2000 J1 - the first satellite of Jupiter to be discovered in the year 2000. It is 9 km across has an orbital inclination of 45 degrees and an eccentricity of 0.208. It orbits at 7,411,410 km out, or about 4 times the orbit of Callisto.

The first group of irregulars is that of the prograde irregulars. These number five and include Leda, Himalia, Lysithea, and Elara along with the 4km across moon with the name S/2000 J11. These moons orbit between 11,165,000 and 12,656,940 km out, have orbital inclinations between 27.46 and 28.61 degrees. Orbital eccentricities range from 0.112 and 0.215.

The second group of irregulars is rather large in count, though there are many many small bodies. The named moons in this group are Ananke, Carme, Pasiphae, and Sinope. The largest of this set is Pasiphae at 58 miles across. There are 21 other moons with boring names. All of these moons have a retrograde orbit (they orbit the 'wrong way') and very very high orbital inclination - ranging from 145.9 and 165.7 degrees. This means that these moons orbit Jupiter in almost a polar orbit rather than about the equator. There is also a wide range of inclination - from 0.096 to 0.601. The orbits range from 20,478,570 km to 24,240,750 km with one at 18,978,000 km.

It is believed that the irregular satellites are captured bodies early on that have then broken up with a larger atmosphere of an early Jupiter. The best evidence for this capture is the retrograde moons which can't be accounted for with the disk of dust and gas that formed the regular moons. Similar density and albedo of these bodies seems to indicate that they were part of a larger object before.

Most of these moons were discovered in 1999, 2000 and 2001 with large CCDs attached to telescopes. However, at some point, someone said thats too small to be a moon (as opposed to saying thats too big to be a space station) - initial surveys listed hundreds of irregular satellites.

The names of the named moons of Jupiter comes from a tradition established by by Simon Marius (who also discovered the Galilean moons of Jupiter) based upon a suggestion of Kepler:

Jupiter is much blamed by the poets on account of his irregular loves. Three maidens are especially mentioned as having been clandestinely courted by Jupiter with success. Io, daughter of the River, Inachus, Callisto of Lycaon, Europa of Agenor. Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell .... I think, therefore, that I shall not have done amiss if the First is called by me Io, the Second Europa, the Third, on account of its majesty of light, Ganymede, the Fourth Callisto ...
Here is the list of moons spelled as a table:
           Distance  Radius    Mass
Satellite  (000 km)   (km)     (kg)   Discoverer   Date
---------  --------  ------  -------  ----------  -----
Metis           128      20  9.56e16  Synnott      1979
Adrastea        129      10  1.91e16  Jewitt       1979
Amalthea        181      98  7.17e18  Barnard      1892
Thebe           222      50  7.77e17  Synnott      1979
Io              422    1815  8.94e22  Galileo      1610
Europa          671    1569  4.80e22  Galileo      1610
Ganymede       1070    2631  1.48e23  Galileo      1610
Callisto       1883    2400  1.08e23  Galileo      1610
Leda          11094       8  5.68e15  Kowal        1974
Himalia       11480      93  9.56e18  Perrine      1904
Lysithea      11720      18  7.77e16  Nicholson    1938
Elara         11737      38  7.77e17  Perrine      1905
Ananke        21200      15  3.82e16  Nicholson    1951
Carme         22600      20  9.56e16  Nicholson    1938
Pasiphae      23500      25  1.91e17  Melotte      1908
Sinope        23700      18  7.77e16  Nicholson    1914
Names for the newer (discovered in the year 2000 or before) moons have been established (Oct 22, 2002):
Jupiter XVII    Callirrhoe  = S/1999 J 1
Jupiter XVIII   Themisto    = S/1975 J 1  = S/2000 J 1
Jupiter XIX     Megaclite   = S/2000 J 8
Jupiter XX      Taygete     = S/2000 J 9
Jupiter XXI     Chaldene    = S/2000 J 10
Jupiter XXII    Harpalyke   = S/2000 J 5
Jupiter XXIII   Kalyke      = S/2000 J 2
Jupiter XXIV    Iocaste     = S/2000 J 3
Jupiter XXV     Erinome     = S/2000 J 4
Jupiter XXVI    Isonoe      = S/2000 J 6
Jupiter XXVII   Praxidike   = S/2000 J 7
Themisto was discovered in 1975 and then lost for 25 years until it was rediscovered in 2000 - thus the two designations.

Please note that I have no intention of making the ASCII art used to include the various irregulars under 10km across - that would make for quite a mess. If you happen to be so inclined, feel free to message me.