Thalassa is the fourth closest inner satellite of Neptune. It was discovered in 1989 from images taken by the Voyager 2 spacecraft.
Discovery and Naming
Thalassa was discovered by Stephen P. Synnott on images taken by Voyager 2 between August and September 1989, and was given the temporary designation S/1989 N 5. It was officially named Thalassa in 1991, after the Greek sea goddess, Thalassa, a daughter of Aether and Hemera.
Physical Characteristics
Thalassa is irregularly shaped and has a mean radius of about 41 km (25 mi). Its dimensions are approximately 80 km × 50 km × 40 km. It is believed to be composed mainly of ice and rock. Its surface is dark and appears to be heavily cratered, though detailed features are difficult to discern due to the limited resolution of the Voyager 2 images.
Orbit
Thalassa orbits Neptune at a distance of approximately 50,000 km (31,000 mi), well within the planet's rings. It orbits Neptune in a prograde direction (the same direction as Neptune's rotation) and completes one orbit in about 7 hours and 30 minutes. Thalassa orbits below Neptune's synchronous orbit radius, meaning that it is gradually spiraling inward toward the planet due to tidal deceleration and is expected to eventually either break apart into a planetary ring or impact Neptune's atmosphere.
Relationship to Neptune's Rings
Thalassa acts as a shepherd satellite for the Le Verrier ring, helping to confine the ring particles. Its gravitational influence helps to maintain the ring's sharp edges.
Exploration
Thalassa has only been observed closely by the Voyager 2 spacecraft during its flyby of Neptune in 1989. No future missions are currently planned to specifically study Thalassa or the other inner satellites of Neptune. Therefore, much about its composition, surface features, and origin remains unknown.