Neptune's Moon Proteus
(Neptune VIII - 1989N1)

This image of Proteus was acquired by the Voyager 2 spacecraft on August 25, 1989.

Discovered by: Stephen Synnott
Date of discovery: 1989
Mass (kg): ?
Equatorial radius (km): 200
Equatorial radius (Earth = 1): 3.1358e-02
Mean density (gm/cm^3): ?
Mean distance from Neptune (km): 117,600
Rotational period (days): ?
Orbital period (days): 1.122315
Mean orbital velocity (km/sec): 7.63
Orbital eccentricity: 0.0004
Orbital inclination (degrees): 0.04
Visual geometric albedo: 0.06
Magnitude (Vo): 20.3

Proteus [PROH-tee-us], like all six of Neptune's newly discovered small satellites, is one of the darkest objects in the solar system -- "as dark as soot" is not too strong of a description. Like Saturn's satellite, Phoebe, it reflects only 6 percent of the sunlight that strikes it. Proteus is about 400 kilometers (250 miles) in diameter, larger than Nereid. It wasn't discovered from Earth because it is so close to Neptune that it is lost in the glare of reflected sunlight. Proteus circles Neptune at a distance of about 92,800 kilometers (57,700 miles) above the cloud tops, and completes one orbit in 26 hours, 54 minutes. Scientists say it is about as large as a satellite can be without being pulled into a spherical shape by its own gravity. Proteus is irregularly shaped and shows no sign of any geological modification. It circles the planet in the same direction as Neptune rotates, and remains close to Neptune's equatorial plane.

(Credit: Calvin J. Hamilton)