Ganymede (moon)

Ganymede (; Greek Γανυμήδης) is Jupiter's largest natural satellite, and the largest natural satellite in the Solar System; it is larger in diameter than Mercury but has only about half its mass. Ganymede is one of four planetary satellites of Jupiter that could be seen with the naked eye under favorable conditions, if not for the brighter light of nearby Jupiter. Other than possibly the other Galilean Satellites, the only planetary satellite visible with the naked eye is our own Moon.
See more at Wikipedia.org...