2/13/79 20 million km (12.4 million mi)
Jupiter and two of its planet-sized satellites, Io at left and Europa at right, are visible in this Voyager 1 picture. Jupiter’s four largest satellites—Io, Europa, Ganymede and Callisto—were discovered in 1610 by Galileo Galilei. The two outer Galilean satellites are Ganymede and Callisto, not shown in this picture. All four satellites probably formed about four billion years ago but their surfaces vary in age tremendously. Io and Europa have younger, more active surfaces than Ganymede and Callisto. Like our Moon, the satellites keep the same face toward Jupiter. In this picture, the sides of the satellites that always face away from the planet are visible.
Amalthea, Jupiter’s innermost satellite, was discovered in 1892. It is so small and close to Jupiter that it is extremely difficult to observe from Earth. Amalthea’s surface is dark and red, quite unlike any of the Galilean satellites. The three Voyager 1 pictures and the one Voyager 2 picture following (seen against the disk of Jupiter) reveal a small, elongated object, about 265 kilometers (165 miles) long and 150 kilometers (90 miles) in diameter. Amalthea keeps its long axis pointed toward Jupiter as it orbits around the planet every 12 hours.
3/4/79 1.25 million km (780,000 mi)
3/4/79 695,000 km (430,000 mi)
3/5/79 425,000 km (264,000 mi)
