(NASA, Jet Propulsion Laboratory, "Voyager at Neptune: 1989," JPL 400-353, U.S. Government Printing Office, Washington, D.C., March 1989, pp. 9,10.)
* Neptune has eight known satellites, Triton, Nereid and six others. Neither Triton nor Nereid travels in the plane of the planet's equator; Triton's orbital plane is at an angle of about 20 degrees to Neptune's equator, while Nereid's is at an angle of about 30 degrees.
* Triton completes one rotation on its axis in the same amount of time that it takes to circle Neptune, 5.88 Earth days. Because the rotation rate is synchronized with its orbital period, the same hemisphere always faces Neptune. (Similarly, Earth's Moon is also in synchronous rotation, keeping the same face toward Earth.)
* At an average distance of 354,600 kilometers (220,300 miles) from the center of its planet, Triton is nearly as far from Neptune as the Moon is from Earth. Triton is the only large moon in the solar system with a retrograde orbit; that is, it travels in the direction opposite the planet's rotation. Because of its retrograde orbit, Triton is spiraling slowly toward Neptune.
* Triton is roughly the size of Earth's Moon. Estimates of Triton's diameter range from 2,200 to 4,000 kilometers (1,400 to 2,500 miles).
* If Triton is small‹perhaps about 2,200 kilometers (1,400 miles) in diameter‹scientists expect to find only a very thin atmosphere. If, on the other hand, Triton is larger‹perhaps about 4,000 kilometers (2,500 miles) in diameter‹then it is expected to have a lower surface temperature and could have a thicker atmosphere (which otherwise would have escaped to space long ago), and Voyager's cameras may not be able to see the surface.
* Because Triton's size is uncertain, the Voyager flight team has prepared primary and alternative designs for some observations. This strategy will allow a critical decision just a few days before the encounter, based on the latest information available: if Triton is large and has a thick atmosphere, observations will be concentrated on the atmosphere; if Triton is smaller and the atmosphere thin, observations instead will be concentrated on the surface. The best Voyager images of Triton are expected to show features smaller than one mile.
* Scientists hope that Voyager 2's cameras will be able to see through Triton's atmosphere to the surface, where there is methane frost or ice and solid or liquid nitrogen. Small quantities of methane may also be dissolved in ponds of liquid nitrogen.
* On the dark side of Triton, Voyager 2 will look for temperature differences that may indicate the existence of liquid bodies. (Since liquids cool more slowly than fine-grained solids, warmer areas on the dark side might be liquid.)
* Triton's seasonal cycle is complex and extreme because of the combined effects of its orbit and its rotation.
* Each of Triton's poles spends long periods in darkness, where temperatures are extremely low and most molecules are frozen.
* Where the Sun is directly overhead on Triton, the temperature is near the freezing point of liquid nitrogen (63 kelvins or -346°F). The southern hemisphere of Triton is now approaching summer, and sunlight currently strikes directly at about 40 degrees south latitude. As this hemisphere warms, some liquids and solids vaporize quickly. However, as Triton rotates, this hemisphere is plunged alternately into darkness and daylight, and this too affects the freezing, melting, and vaporization of the nitrogen and methane.
* Voyager 2 is slated to pass through a narrow area behind Triton where both the Sun and Earth will be hidden from view for about 4 minutes. During this time, the ultraviolet spectrometer on board the spacecraft will study the satellite's atmosphere by viewing the Sun shining through Triton's atmosphere, while at the same time the spacecraft's radio beams will probe the atmosphere to determine temperature and pressure levels. Just days before closest approach, navigators will instruct Voyager 2 to fine-tune its flight path to target for this area of Triton's shadow; this adjustment will be based on the best estimates of the location and size of Triton and on the gravitational effects of Neptune on the flight path.
* Nereid, which is between 300 and 1,100 kilometers (190 and 680 miles) in diameter, travels around Neptune in a highly elliptical orbit that ranges from 1,390,000 to 9,635,000 kilometers (860,000 to 5,990,000 miles). Voyager 2's closest flyby distance to Nereid will be about 4,655,000 kilometers (2,890,000 miles). Even at that range, Voyager may discern bright and dark areas on Nereid's surface.