Proteus 

Proteus was discovered by Stephen Synnott in 1989 and is known as one o the darkest objects in the solar system. Its low reflectivity (6 percent of the sunlight it receives) and close proximity to Neptune (92,800 kilometers : 57,700 miles from the surface) made it virtually invisible from Earth and thus it was only discovered after analyzing satellite photographs.

Proteus orbits Neptune once every 26 hours, 54 minutes. If Proteus were any larger, it could actually be classified as a planet in some definitions. This is because Proteus is as large as it can be without its own gravitational force pulling it into a sphere. This issue was the subject of debate when the planet Pluto was argued to be a very large asteroid but kept its current classification of a planet due in part to its historical background as well as its ability to pull itself into a sphere. Like many other moons of Neptune, Proteus orbits Neptune in the same direction that the planet rotates and lies close to the equatorial plane.

Physical Data Key Mass Mass Mass Mass compared to that of Earth. Radius Radius Radius Radius compared to that of Earth. Mean Density Average Density. Distance Distance to the planet's center. Magnitude The brightness of a star or object.

Orbital Data Key Rotational Period Number of days to make one complete rotation. Orbital Period Number of days to obital the planet. Mean Orbital Velocity Average obital speed. Orbital Eccentricity Eccentricity. Orbital Inclination The tilt of the moon or planet. Escape Escape Velocity. Albedo Visual geometric albedo.