5th Largest Moon of Neptune - Galatea (4th Moon outwards from Neptune)

Galatea inside of a faint ring arc near Neptune

Galatea is the fifth largest satellite of Neptune and the 31th largest moon in the Solar System currently known.

Discovery

Galatea was discovered in late July 1989 from the images taken by the Voyager 2 probe.

Naming

Galatea was given the temporary designation S/1989 N4.

Neptune's irregular satellites are named for the Nereids, daughters of Nereus and Doris and the attendants of Neptune. Galatea the moon was named after Galatea, one of the Nereids of Greek legend.

Stats

Diameter (mean): 176 km

Semi-major axis: 61,953 km

Orbital Period: 0.429 days

Orbit

Galatea's orbit lies below Neptune's synchronous orbit radius, so it is slowly spiralling inward due to tidal deceleration and may eventually impact Neptune's atmosphere, or break up into a planetary ring upon passing its Roche limit due to tidal stretching.

Galatea takes as long to rotate on its axis as it does to make one orbit of Neptune; and therefore always keeps the same hemisphere pointed to Neptune.

Shepherd moon

Galatea appears to be a shepherd moon for the Adams ring that is 1000 km outside its orbit. Resonances with Galatea in the ratio 42:43 are also considered the most likely mechanism for confining the unique ring arcs that exist in this ring.

Physical characteristics

Little else is known about Galatea. Galatea is likely, like the other satellites inward of Triton, a rubble pile re-accreted from fragments of Neptune's original satellites, which were smashed up by perturbations from Triton soon after that moon's capture into a very eccentric initial orbit.