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
Rhea (moon) Rhea (moon) Rhea ( ; ) is the second-largest moon of Saturn and the ninth-largest moon in the Solar System. It is the second smallest body in the Solar System for which precise measurements have confirmed a shape consistent with hydrostatic equilibrium, after dwarf planet Ceres. It was discovered in 1672 by Giovanni Domenico Cassini. Rhea was discovered by Giovanni Domenico Cassini on 23 December 1672. It was the second moon of Saturn that Cassini discovered, and the third moon discovered around Saturn overall. Rhea is named after the Titan Rhea of Greek mythology, the "mother of the gods". It