Online Astronomy eText: Satellites (Moons)
The Satellites (Moons) of Neptune: Larissa Link for sharing this page on Facebook

Larissa, as photographed by Voyager 2 in 1989 (NASA, Voyager 2 Team)

     Larissa (la-RISS-ah) has an irregular shape and appears heavily cratered, suggesting that it has an ancient surface with a total or relative lack of geological activity since its formation. It is thought that it, as well as the other satellites between Triton and Neptune, are rubble piles accreted from the debris created by the destruction of Neptune's original satellites as a result of Triton's capture. Like the other moons discovered by Voyager 2 it lies inside the synchronous orbital distance, at which a satellite orbits the planet in the same time the planet takes to rotate. As a result it orbits Neptune faster than the planet rotates, should be slowly spiralling inward as a result of tidal orbital deceleration, and will eventually disintegrate into a ring system (if it is loosely aggregated) or in Neptune's outer atmosphere (if it is more rigidly structured).
     Although not imaged until the Voyager 2 flyby of Neptune in 1989, Larissa's existence was discovered in 1981, when it happened to pass in front of a star being observed by Harold Reitsema (and others), allowing an estimate of its size and orbital characteristics from the stellar occultation data.

Data for Larissa

Discovered by Harold Reitsema et al, May 24, 1981 (as S/1981 N1, due to accidental stellar occultation)
     First actual images on Voyager 2 photos, July 1989 (as S/1989 N2); named in 1991
Named after a lover of Poseidon
Orbital size 73548 km (45,700 miles), about 48,800 km (30,300 miles) from Neptune's surface
Orbital eccentricity 0.1%
Orbital inclination 0.2 degrees
Orbital period 13h 18.7m = 0.55465332 days
Rotational period unknown (but probably synchronous)
Diameter 215 x 204 x 170 km (about 135 x 125 x 105 mi)
Mass estimated at 4 million billion (4 times 10 to the 15th power) tons
Density about 1.2 times that of water
Albedo (reflectivity) 9%
Surface temperature about 50 Kelvins (-365 Fahrenheit)