Thalassa is the second innermost satellite, the seventh largest satellite of Neptune and the 51th largest moon in the Solar System currently known.
Discovery
Thalassa was discovered sometime before mid-September 1989 from the images taken by the Voyager 2 spacecraft.
Naming
The moon was given the temporary designation S/1989 N5.
Thalassa the moon was named after the sea goddesss Thalassa, a daughter of Aether and Hemera from Greek mythology. "Thalassa" is also the Greek word for "sea".
Stats
Diameter (mean): 82 km
Semi-major axis: 50,075 km
Orbital Period: 0.311 days
Orbit
Thalassa's orbit 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.
Relatively soon after, the spreading debris may impinge upon the third moon Despina's orbit.
Thalassa 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
Thalassa is irregularly shaped and shows no sign of any geological modification.
Unusually for irregular bodies, Thalassa appears to be roughly disk-shaped.
Little else is known about Thalassa. Thalassa 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.
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