Thalassa (moon)

Thalassa (tha-las'-a, Greek Θάλασσα) is the second moon of Neptune. Thalassa was named after a daughter of Aether and Hemera from Greek mythology. "Thalassa" is also the Greek word for "sea".

Thalassa was discovered in August, 1989 from the images taken by the Voyager 2 probe. It was given the temporary designation S/1989 N 5. It is also designated Neptune IV.

It is irregularly shaped and shows no sign of any geological modification. Since its orbit is below Neptune's synchronous orbit radius it is slowly decaying due to tidal forces and will one day break up into a planetary ring or impact on Neptune's surface.

• Orbital radius: 50,075 km
• Radius: 54 x 50 x 26 km
• Mass: 3.7×10¹⁷ kg
• Estimated Density: 1.3 g/cm³
• Orbital period: 0.311 d (7 h 30 min)
• Orbital inclination: 0.209°