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Discovered by astronomers in 1845, the planet Neptune has a diameter of 30,690 miles. Mathematical studies of the orbit of Uranus made it possible for the position of Neptune to be predicted before it was discovered. Considered a near twin of Uranus, its atmosphere contains large amounts of methane gas. The planet is small, appearing as a blue green disc in the sky. In 1989 when Voyager 2 passed Neptune, astronomers had the opportunity to study the planet and its moons for the first time. A great dark spot was revealed that appeared to be a spiral storm the size of earth. Neptune seemed to have little atmospheric structure which is believed to be caused by an internal heat source which gives the planet approximately as much energy as it gets from the sun.

Neptune orbits the sun every 165 years and is 4.5 billion km from the sun. Two moons, the largest of which is Triton, were discovered on Neptune. This moon is known to be larger than the planet Pluto and has methane gas in its atmosphere. As the Voyager 2 approached Triton it appeared to shrink. It was learned that Triton was not a large moon, but instead about half the size of earths moon. Nereid, the smaller moon of Neptune, has an orbit that is fifteen times larger than Tritons and very out of round. The name Nereid means sea nymph in Greek. Six more moons were discovered around Neptune when the Voyager 2 spacecraft made its visit.