Exploring the Planets

The Moons of Jupiter

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 Galilean Satellites

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

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, 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.