Callisto - one of Jupiter's Moons. False colour image taken by Voyager 2. Callisto is the second largest of Jupiter's four main moons, which were discovered by Galileo, and is approximately the same size as the planet Mercury. The outermost of the four, Callisto takes 16. 7 Earth days orbit Jupiter (NASA Media Archives). Buy this as a superb wall poster, a canvas print or a framed print.