The distinctive feature of Callisto is its heavily cratered surface; it was discovered in 1610 by Galileo and Marius. The article includes a list of the seven largest moons in the solar system.
As far as our current knowledge extends, the following are the largest moons in the Solar System. The largest is a Jovian moon called Ganymede. Titan, the largest satellite of Saturn, occupies second place. Callisto and Io, two more moons of Jupiter, occupy third and fourth place, respectively. Our own Moon is the fifth largest satellite in the Solar System, followed by Europa, another moon of Jupiter. The seventh largest is Triton, which revolves around Neptune. (It is possible that there are yet undiscovered planets with large satellites far beyond Neptune.)
Only seven moons of Jupiter are closer to the mother planet than Callisto, unless there are further moons that have not yet been discovered. In order of increasing distance from Jupiter, they are Metis, Adrastea, Amalthea, Thebe, Io, Europa, and Ganymede. Many other moons are farther away from Jupiter than Callisto.
Orbit of Callisto
According to NASA, the distance of Callisto from the center of Jupiter is 1,868,768 kilometers when closest to the mother planet and 1,896,632 kilometers when it is farthest away. This means that the orbit is slightly elliptical, but its eccentricity is small.
As it revolves around Jupiter, it rises slightly above the plane of Jupiter’s equator and then dips slightly below this plane, but its angle of inclination is small – less than two degrees.
It takes a fairly long time for Callisto to complete a single circuit. More than 16 of our Earth days elapse before Callisto revolves around Jupiter once. Like our Moon, the same side of Callisto always faces the mother planet.
Visits from Earth
No spacecraft has landed on the surface of Callisto. However, several spacecrafts have visited the general area: Pioneer 10 and 11, Voyager 1 and 2, and the Galileo spacecraft. The last three furnished valuable information concerning this Jovian satellite. They sent to earth excellent pictures that showed the surface features of Callisto, and the Galileo spacecraft provided evidence that Callisto had an ionosphere and an extremely thin atmosphere composed mainly of carbon dioxide.
Callisto has a lot of craters. According to NASA, “Callisto is the most heavily cratered object in the solar system.” Wikipedia claims that the surface of Callisto is so saturated with craters that any new crater would probably erase on older one. This may be an exaggeration, but it gives you a good idea what kind of surface Callisto has.