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Solar System Exploration
Thalassa: Overview
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Grainy black and white image showing three moons at Neptune.
A Voyager 2 image of Thalassa (1989 N5), Naiad (1989 N5) and Despina (1989 N3) taken in 1989 at a range of 5.9 million kilometers (3.6 million miles).

Thalassa, like Naiad, most likely formed from fragments of Neptune's original moons, which were smashed by the disturbances caused when the ice giant Neptune captured Triton. Thalassa is unusual for an irregular moon because it is roughly disk-shaped.

Also like Naiad, Thalassa circles the planet in the same direction as Neptune rotates, and remains close to Neptune's equatorial plane. Thalassa's orbit is slowly decaying due to tidal deceleration and may eventually crash into Neptune's atmosphere or be torn apart and form a planetary ring.

Thalassa was discovered in August 1989 in images taken by Voyager 2.

How Thalassa Got its Name:
Moons of Neptune are named for characters from Greek or Roman mythology associated with Neptune or Poseidon, or the oceans. Irregular satellites are named for the Nereids, daughters of Nereus and Doris and the attendants of Neptune.

Thalassa was named after a daughter of Aether and Hemera from Greek mythology. Thalassa is also the Greek word for sea. The moon was originally designated as S/1989 N 5.

Just the Facts
Orbit Size (semi-major axis):  50,074 km
Mean Radius:  41.0 km
Volume:  288,696 km3
Mass:  374,667,108,274,298,000 kg
22 Jan 2003: 
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Last Updated: 12 Aug 2013