VOYAGING TO NEPTUNE
Because of the planet's distance from Earth, information about
Neptune accumulated slowly.
William Lassell saw Triton, the largest moon, within a month of the planet's discovery, but the second moon,
Nereid, was discovered only in 1949, by Gerard Kuiper. In the 1970s and 1980s, astronomers received strong hints of a
Neptune ring system, but could not make sense of the data.
In August 1989, Voyager 2 flew by the planet, and the modern era for Neptune began.
Voyager found a big, blue ball, with many markings and cloud bands—a pleasant change from the bland Uranus the l|pk spacecraft had visited three years earlier. Voyager's instruments collected data about Neptune's composition. There may be a small rocky core, but the bulk of the planet is probably a deep ocean of water. This then merges into an atmosphere of hydrogen and helium. Methane in the top of the atmosphere gives the planet its pronounced blue color. The interior bubbles up more than twice as much heat as the surface receives from the Sun.
Neptune spins on its axis every 16 hours 7 minutes. In its equatorial zone, winds scream westward at 900 miles (1,500 km) per hour, powering huge storm systems.
Voyager froze Neptune in time with a snapshot, but the Hubble Space Telescope lias since tracked its changing features. The spacecraft saw several storms, notably the Great Dark Spot (the size of Earth), which drifted toward the equator and vanished sometime after the flyby. Other noted markings, such as the Scooter, the second Dark Spot, and various white streaks, have also disappeared or changed greatly, while new features have emerged. A longer view is needed to fully understand the planet, so observations continue.