Callisto (moon) facilitate remote exploration of Europa, or be an ideal location for a Jovian system waystation servicing spacecraft heading farther into the outer Solar System, using a gravity assist from a close flyby of Jupiter after departing Callisto. In December 2003, NASA reported that a manned mission to Callisto might be possible in the 2040s. Callisto (moon) </math>.</ref> Callisto (Jupiter IV) is the second-largest moon of Jupiter, after Ganymede. It is the third-largest moon in the Solar System after Ganymede and Saturn's largest moon Titan, and the largest object in the Solar System not to be properly differentiated. Callisto was discovered
Orbit of the Moon Orbit of the Moon The Moon orbits Earth in the prograde direction and completes one revolution relative to the stars in about 27.32 days (a sidereal month) and one revolution relative to the Sun in about 29.53 days (a synodic month). Earth and the Moon orbit about their barycenter (common center of mass), which lies about from Earth's center (about of the radius of Earth). On average, the distance to the Moon is about from Earth's center, which corresponds to about 60 Earth radii. With a mean orbital velocity of 1.022 km/s (0.635 miles/s), the Moon covers a distance approximately