In the aerospace context, "Callisto" usually refers to one of the moons of the planet Jupiter. Callisto is the fourth-largest moon of Jupiter and the second-largest of the Galilean moons, a group of four large moons that orbit Jupiter and were discovered by the Italian astronomer Galileo Galilei in 1610. Callisto is named after one of the lovers of Zeus in Greek mythology.
Callisto is a large, icy moon with a diameter of about 4,800 kilometers (3,000 miles), making it the third-largest moon in the solar system after Ganymede and Saturn's moon Titan. It is located about 1.2 million kilometers (750,000 miles) from Jupiter, which is farther out than any of the other Galilean moons. Callisto has a very low density and is thought to be composed mostly of water ice, with a small amount of rock and other materials. The surface of Callisto is heavily cratered and is thought to be one of the oldest surfaces in the solar system, with an estimated age of about 4 billion years.
Callisto has been the subject of scientific study by a number of spacecraft, including NASA's Galileo spacecraft, which orbited Jupiter and made a number of flybys of Callisto in the late 1990s and early 2000s. Scientists are interested in Callisto and the other Galilean moons because they provide insight into the early history of the solar system and the conditions that existed in the outer solar system at the time the planets were forming.