Discovery
Proteus is the second largest Neptunian moon, and Neptune's largest inner satellite. Proteus is the 19th largest moon of the solar system.
Proteus was discovered from the images taken by Voyager 2 space probe two months before its Neptune flyby in August 1989. It received the temporary designation S/1989 N 1.
Naming
On 16 September 1991 S/1989 N 1 was named after Proteus, the shape-changing sea god of Greek mythology.
Stats
Diameter (mean): 420 km
Semi-major axis: 117,646 km
Orbital Period: 1.12 days
Orbit
Proteus orbits Neptune at the distance approximately equal to 4.75 equatorial radii of the planet. Its orbit has a small eccentricity and is inclined by about 0.5° to the planet's equator.
Proteus is the largest of the regular prograde satellites of Neptune. It rotates synchronously with the orbital motion, which means that one face always points to the planet.
Origin
Proteus, like the other inner satellites of Neptune, is unlikely to be an original body that formed with it, more probably having accreted from the wreak rubble that remained after Triton's capture.
Triton's orbit upon capture would have been highly eccentric, and would have caused chaotic perturbations in the orbits of the original inner Neptunian satellites, causing them to collide and reduce to a disc of rubble. Only after Triton's orbit became circularised did some of the rubble disc re-accrete into the present-day satellites.
Physical characteristics
Proteus, although about 420 km in diameter, is not spherical in shape. The shape of Proteus is close to a sphere with the radius of about 210 km, although deviations from the spherical shape are large—up to 20 km.
Scientists believe it is about as large as a body of its density can be without being pulled into a perfect spherical shape by its own gravity.
Proteus is slightly elongated in the direction of Neptune, although its overall the shape is closer to an irregular polyhedron than to a triaxial ellipsoid.
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