Callisto is the outermost of the four Galilean moons of Jupiter and is almost the same size as the planet Mercury (about 4800 kilometers in diameter).
The is the only complete full color image of Callisto obtained by Galileo, which has been orbiting Jupiter since December 1995. Callisto's surface is uniformly cratered but is not uniform in color or brightness. Scientists believe the brighter areas are mainly ice and the darker areas are highly eroded, ice-poor material.
Voyager 2 took this image from over 2 million kilometers away as it approached Jupiter revealing the impact crater sites in much more detail.
Voyager 1 took this image of Callisto from a distance of about 8 million kilometers during its flyby of the Jupiter system. The bright spots are probably impact craters on a surface of mostly rock and ice.
Italian scientist Galileo Galilei has discovered what he calls the Medicean stars (in honor of the Medici family) Jupiter I, II, III and IV orbiting the planet Jupiter when he pointed his telescope to the heavens.
Of course photography did not exist in the 1600's so the picture above is a modern day view through a small telescope that closely represents what Galileo would have seen. Jupiter IV we now know as Callisto. This discovery led to the demise of the long held theory that the Earth was at the center of the universe and a new understanding that the planets (including the Earth) orbit the Sun.