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.