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

Two views of Larissa by Voyager 2

Larissa is the fourth largest satellite of Neptune and the 28th largest moon in the Solar System currently known.

Larissa is irregular (non-spherical) in shape and appears to be heavily cratered, with no sign of any geological modification.

Discovery

Larissa was first discovered by Harold J. Reitsema, William B. Hubbard, Larry A. Lebofsky and David J. Tholen, based on fortuitous ground-based stellar occultation observations on May 24, 1981.

Larissa was given the temporary designation S/1981 N 1 and announced on 29 May 1981. The moon was recovered and confirmed to be the only object in its orbit during the Voyager 2 flyby in 1989 after which it received the additional designation S/1989 N 2 on August 2, 1989.

Naming

Larissa the moon is named after Larissa, a lover of Poseidon (Neptune) in Greek mythology and eponymous nymph of the city in Thessaly.

Stats

Diameter (mean): 194 km

Semi-major axis: 73,548 km

Orbital Period: 0.555 days

Orbit

Larissa's orbit is circular but not perfect and 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.

Larissa 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.

Physical characteristics

Little else is known about Larissa. Larissa 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.