Discovered by S. Synnott
Date of Discovery 1989
Distance from Neptune 117,650 km
Radius 218 × 208 × 201 km
Mass ???
Orbital Eccentricity 0.0004
Orbital Inclination 0.55°
Orbital Period 1.122315 days
Rotational Period ???
Density (gm/cm3) ???
Proteus is one of the darkest objects in the solar system. It reflects only 6% of the light it receives, about as dark as coal. It was not discovered earlier because of its darkness and its closeness to Neptune. The glare off the planet's surface hid it from view until Voyager 2 happened by.
Irregularly shaped, it is about as large as an object can get without its gravity pulling it into a sphere. Other than a lot of impact craters, its surface has no distinguishing marks.
Sources:
NASA.gov
space.com