Io

Io was the innermost moon of Jupiter. It was the headquarters of the Galactic Federation during its reign. (PROSE: Legacy) Lord Izlyr planned on taking Princess Lixgaar of New Mars to Federation headquarters on Io until New Mars was safe enough for her to return. (AUDIO: The Prisoner of Peladon)

The terraformed Io was one of the 700 Wonders of the Universe. (PROSE: Legacy)

Behind the scenes
In the real world, it is not the innermost moon, as more inner moons have been discovered.