Jupiter has numerous moons, which make the Jovian system much like a miniature solar system. The four largest — Io, Europa, Ganymede and Callisto — were discovered by Galileo in 1610 and are known as the Galilean Satellites.
The other, smaller moons have their orbits either between Jupiter and Io, or outside the orbit of Callisto. The small outermost moons may be asteroids captured by the gravitational pull of Jupiter.
| Name | Discoverer | Diameter | Distance from Jupiter | Orbital Period (days) |
| Metis | Synott (V1),1979 | 40 km/25 mi | 127,960 km/79,510 mi | .29 |
| Adrastea | Jewitt,Danielson 1979 | 25 km/16 mi | 128,980 km/80,140 mi | .30 |
| Amalthea | Barnard, 1892 | 170 km/106 mi | 181,300 km/112,660 mi | .50 |
| Thebe | Synott (V1),1979 | 100 km/62 mi | 221,900 km/137,880 mi | .68 |
| Io | Galileo, 1610 | 3,630 km/2,256 mi | 421,600 km/261,970 mi | 1.77 |
| Europa | Galileo, 1610 | 3,140 km/1,951 mi | 670,900 km/416,880 mi | 3.55 |
| Ganymede | Galileo, 1610 | 5,260 km/3,268 mi | 1,070,000 km/664,870 mi | 7.16 |
| Callisto | Galileo, 1610 | 4,800 km/2,983 mi | 1,883,000 km/1,170,000 mi | 16.69 |
| Leda | Kowal, 1974 | 15 km/9 mi | 11,094,000 km/6,893,500 mi | 238.72 |
| Himalia | Perrine, 1904 | 185 km/115 mi | 11,480,000 km/7,133,300 mi | 250.57 |
| Lysithea | Nicholson, 1938 | 35 km/22 mi | 11,720,000 km/7,282,500 mi | 259.22 |
| Elara | Perrine, 1905 | 75 km/47 mi | 11,737,000 km/7,293,000 mi | 259.70 |
| Ananke | Nicholson, 1951 | 30 km/19 mi | 21,200,000 km/13,173,100 mi | 631.00 |
| Carme | Nicholson, 1938 | 49 km/30 mi | 22,600,000 km/14,043,000 mi | 692.00 |
| Pasiphae | Melotte, 1908 | 50 km/31 mi | 23,500,000 km/14,602,000 mi | 735.00 |
| Sinope | Nicholson, 1914 | 35 km/22 mi | 23,700,000 km/14,726,500 mi | 758.00 |
| S/1999 J1 | 1999** | (?)10-15km/6-9mi | 24,000,000 km/15,000,000 mi | ~730.00 |
| As many as 17 more satellites of Jupiter have been found and scientists continue to search for more. | ||||
** International Team: T. Gehrels, A. Gleason, J. Larsen, B. Marsden, R. McMillan, J. Montani, J. Scotti, T. Spahr, G. Williams
The four Galilean satellites of Jupiter were faint dots of light in astronomers' telescopes before the encounters of the Pioneer and Voyager spacecraft. Both missions provided more information on these satellites during a few brief hours of encounter than had been assembled in the 370 years since their discovery.
Montage of Voyager 1 images showing the Galilean satellites
From farthest out, the Galilean satellites are Callisto, Ganymede, Europa, and Io. They are shown here in their correct positions relative to Jupiter, but are not to scale. The largest moon in the solar system, Ganymede, is larger than the planet Mercury.
Amalthea
Amalthea, one of the small Jovian moons, is highly irregular in shape, and keeps its long axis pointed towards Jupiter as it rotates around the planet. The red color may result from a coating of sulfur ejected by the active volcanism of Io, and "swept up" by Amalthea.
