Neptune

Neptune's satellites may be classified into groups according to (1) orbital distances, (2) orbital properties, and (3) composition. By orbital distances, there are (a) intra-ring satellites (note—these are not shepard satellites like those around Saturn) Naiad, Thalassa, Despina, and Galatea and (b) extra-ring satellites (Larissa, Proteur, Triton, and Nereid). By orbital properties, there are (a) inclined, elliptically orbiting, prograde satellites (Nereid and Galatea), (b) equatorial, circular-orbiting, prograde satellites (Naiad, Thalassa, Despina, Larissa, and Proteus), and (c) an inclined, circular-orbiting, retrograde satellite (Triton). By composition, there are (a) irregular, dark soil-covered icy bodies (perhaps captured comets), including Naiad, Thalassa, Despina, Galatea, Larissa, and Proteus, (b) a regular (nearly spherical) methane ice bodies (Triton), and (c) an irregular methane ice body (Nereid).

Voyager 2 discovered six more small satellites (all under 125 mi [200 km] diameter, except Proteus, 250 miles [400 km] diameter), which all have direct orbital motion around Neptune and are much closer to it than Triton. This brings the total number of known satellites of Neptune (excluding the vast number of ring particles) to eight. The four innermost satellites Naiad, Thalassa, Despina, and Galatea are within the ring system and must interact with it. Galatea's possible effect in producing and maintaining the arcs in the Adams ring has already been described; it orbits just 560 mi (900 km) inside that ring. Despina orbits Neptune about 434 mi (700 km) inside the Leverrier ring and may contribute to its stability, although arcs are not obvious in that ring.

All of Neptune's six innermost satellites are very dark, having albedos of 0.065 or less. Proteus, the largest and outermost of the newly discovered satellites, had details on its surface imaged. It is heavily cratered, with a 93 mi (150 km) diameter crater visible there. Proteus turns out to be somewhat larger than Nereid and has a mean radius of 250 mi (400 km); images of Proteus indicate it to be decidedly non-spherical. The images indicate Figure 3. Graph of the latitude of the subsolar point on Triton from 1993 to 3000. This illustrates the complicated nature of the seasons on Triton. Illustration by Hans & Cassidy. Courtesy of Gale Group.
also that Proteus is tidally locked to Neptune (that is, its rotation period equals its period of revolution around Neptune and keeps the same hemisphere always turned towards Neptune), as is Triton.

Larissa also was imaged with sufficient resolution for its surface to be studied in some detail. It is also decidedly non-spherical and dark. The sizes, shapes, albedos, and states of rotation of the four innermost newly discovered satellites are less definite. Nereid, Neptune's outermost satellite, was observed by Voyager 2, which determined its size (105 mi [170 km] radius) and albedo (0.15 to 0.20), which indicate that its surface seems more like those of Uranus's icy satellites Umbriel, Oberon, and the darker areas on Miranda than the surfaces of the six newly discovered satellites. Limited information was obtained from the probe's observations because Nereid was more than 2,800,000 mi (4,600,000 km) from Voyager 2 at closest approach, and it give no definite information about Nereid's rotation period (it is probably not tidally locked to Neptune), pole position, or its mass. Finally, from the discovery observations of Larissa and its period of revolution around Neptune, it seems that Larissa was probably the satellite which produced the 8.1 second decrease of the brightness of a star in 1981 which was mentioned above.