Nearly 4.5 billion kilometers from the Sun, Neptune orbits the Sun once every 165 years, and therefore it has not quite made a full circle around the Sun since it was discovered. It is invisible to the naked eye because of its extreme distance from Earth. Interestingly, due to Pluto�s unusual elliptical orbit, Neptune is actually the farthest planet from the Sun for a 20-year period out of every 248 Earth years.
Voyager 2 captured this convergence of atmospheric features on Neptune.
Neptune has the smallest diameter of our solar system�s giant gas planets (including Jupiter, Saturn, and Uranus), so called because they have no solid surfaces. Even so, its volume could hold nearly 60 Earths. Neptune�s atmosphere extends to great depths, gradually merging into water and other "melted ices" over a heavier, approximately Earth-sized liquid core. Neptune�s rotational axis is tilted 30 degrees to the plane of its orbit around the Sun. Its seasons last an incredible 41 years. During the southern summer, the south pole is in constant sunlight for about 41 years, and in northern summer, the north pole is in constant sunlight for about 41 years. Neptune�s atmosphere is made up of hydrogen, helium, and methane, the last of these giving the planet its blue color (because methane absorbs red light). Despite its great distance from the Sun and lower energy input, Neptune�s winds are three times stronger than Jupiter�s and nine times stronger than Earth�s.
In 1989, Voyager 2 tracked a large oval dark storm in Neptune�s southern hemisphere. This hurricane-like "Great Dark Spot" was large enough to contain the entire Earth; spun counterclockwise; and moved westward at almost 1,200 km per hour. Recent images from the Hubble Space Telescope show no sign of the "Great Dark Spot," although a comparable spot appeared in 1997 in Neptune�s northern hemisphere.
The planet has several rings of varying widths, confirmed by Voyager 2�s observations in 1989. The outermost ring, Adams, contains five distinct arcs (incomplete rings) named Libert�, Equalit� 1, Equalit� 2, Fraternit�, and Courage. Next is an unnamed ring coorbital with the moon Galatea, then Le Verrier, Lassell, Arago, and Galle. Neptune�s rings are believed to be relatively young and relatively short-lived.
Neptune has 11 known moons, six of which were discovered by Voyager 2. The largest, Triton, orbits Neptune in a direction opposite to the planet�s rotation direction, and is gradually getting closer until it will collide with the planet in about 10 to 100 million years, forming vast rings around Neptune that will rival or exceed Saturn�s extensive ring system. Triton is the coldest body yet visited in our solar system; temperatures on its surface are about -235 �C. Despite the deep freeze, Voyager 2 discovered great geysers of gaseous nitrogen on Triton.
4,498,252,900 km
30.06896348 A.U.
Mean Equatorial Radius
24,764 km
(3.883 of Earth's radius)
Mass
102.44 � 10-27 g
Natural Satellites
1. Naiad
2. Thalassa
3. Despina
4. Galatea
5. Larissa
6. Proteus
7. Triton
8. Nereid
9. S/2002 N1
10. S/2002 N2
11. S/2002 N3
Evidence for incomplete arcs around Neptune first arose in the mid-1980's, when stellar occultation experiments were found to occasionally show an extra "blink" just before or after the planet occulted the star. Images by Voyager 2 in 1989 settled the issue, when the ring system was found to contain several faint rings, the outermost of which, Adams, contains three prominent arcs now named Liberty, Equality and Fraternity. The existence of arcs is very difficult to understand because the laws of motion would predict that arcs spread out into a uniform ring over very short timescales. The gravitational effects of Galatea, a moon just inward from the ring, are now believed to confine the arcs. Several other rings were detected by the Voyager cameras. In addition to the narrow Adams Ring 63,000 km from the center of Neptune, the Leverrier Ring is at 53,000 km and the broader, fainter Galle Ring is at 42,000 km. A faint outward extension to the Leverrier Ring has been named Lassell; it is bounded at its outer edge by the Arago Ring at 57,000 km.