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Space Topics

Neptune


Update February 7, 2007: The four moons of Neptune discovered in 2002 have now received names: Halimede, Sao, Laomedeia, and Neso.

Neptune, the eighth planet from the Sun, is a colder twin to Uranus, a blue ball composed largely of hydrogen, methane, ammonia, and water. But when Voyager 2 flew by Neptune, viewers back on Earth found a more interesting planet than Uranus had been, with deep blue atmospheric bands, a storm called the Great Dark Spot half the size of Jupiter's Great Red Spot, and little bright "scooters" of clouds, all traveling within the fastest wind speeds clocked in the solar system. Those cloud features seem to have disappeared since Voyager 2; Neptune is a surprisingly active planet, given its great distance from the Sun.

Neptune has fewer moons than one might expect, given the numbers at Jupiter, Saturn, and Uranus. Only one moon, Triton, is large enough to have its own internal geology. Triton travels around Neptune backwards and could be a captured body from the Kuiper Belt. In fact, Triton may look a lot like Pluto. Neptune has rings, but, for reasons that no one understands, they are clumpy and irregular.

Neptune has only been visited by one spacecraft, Voyager 2, in 1989. More recently, it has been observed by the Hubble Space Telescope and the Adaptive Optics-equipped Keck II telescope. No future missions are currently planned to visit Neptune.

Neptune Numbers
Size: 4th largest planet - 49,528 kilometers - 3.883 Earths
Calendar: 1 Neptune year = 163.73 Earth years; 1 Neptune day = 0.671 Earth days
Orbit: 4,495,060,000 kilometers - 30.047 Earth orbits
Axial tilt: 28.32 degrees (4.87 degrees more than Earth's)