Neptune's satellites
[ Triton ]
Neptune has 8 satellies, two of which are large bodies. One of these is significantly larger than Pluto, and is called Triton. The table below provides some information on the dimensions and orbital characteristics of Neptune's satellites (Illingworth, 1994).
| Satellite | Year of Discovery |
Diameter (Km) |
Orbital Radius (Km) |
Eccentricity | Orbital Period (days) |
Inclination (°) |
|---|---|---|---|---|---|---|
| Naiad | 1989 | 54 | 48 000 | < 0.001 | 0.29 | 4.74 |
| Thalassa | 1989 | 80 | 50 000 | < 0.001 | 0.31 | 0.21 |
| Despina | 1989 | 150 | 53 000 | < 0.001 | 0.33 | 0.07 |
| Galatea | 1989 | 160 | 62 000 | < 0.001 | 0.43 | 0.05 |
| Larissa | 1989 | 208 x 178 | 74 000 | 0.001 | 0.55 | 0.20 |
| Proteus | 1989 | 436 x 416 x 402 | 118 000 | < 0.001 | 1.12 | 0.55 |
| Triton | 1846 | 2 700 | 355 000 | 0.000 | 5.88 R | 157.35 |
| Nereid | 1949 | 340 | 5 513 000 | 0.751 | 360.14 | 27.6 |
Triton
This satellite became an important target in the Voyager 2 mission, and was the last object passed by the spacecraft before it left the planetary system.
[ NASA / JPL / Caltech ]
Prior to the encounter, Triton was believed to be so cold and distant that nitrogen would exist in various states upon it, as water does on Earth. They believed that Voyager 2 would image and detect nitrogen clouds, oceans and polar caps. When the spacecraft finally arrived, it imaged nitrogen clouds and huge nitrogen-frosted polar caps on a world far colder than previously thought - only 37° above Absolute Zero in equatorial surface temperature, which is -236°C. This is so cold that the nitrogen only existed in solid and gaseous form, as frost at the poles (see the image above, showing Triton's south pole) and clouds of ice particles in the tenuous nitrogen atmosphere.
Nereid
The third largest body orbiting Neptune (after Triton and Proteus) is Nereid, which is in a highly elliptical and inclined (27.6°) orbit, taking 360.14 days to complete one orbit of Neptune.