Triton (Neptune I) is Neptune's first satellite, by far the largest, discovered by Lassell in 1846.
The Voyager probe observed some geological activity at its surface and phenomena similar to geysers.
Semi-major axis in km: 354,759
Period of revolution: 5.877 days
Period of rotation: 5.877 days
Orbital velocity: 4 km/s
Inclination to Neptune's equator: 156.3°
Eccentricity of the orbit: 0.00002
Equatorial diameter (Earth=1): 0.21
Equatorial diameter: 2705 km
Visual magnitude at opposition: 13.7
Mass (Neptune =1): 2/10,000
Density (water=1): 2.1
Escape velocity: 1450 m/s
Reflectivity (geometric albedo): 0.7
Very tenuous atmosphere: nitrogen, traces of methane