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Neptune

Before Voyager's flyby, little was known of Neptune. All observations had been Earth-based and were thus limited because Neptune is 30 times farther from the Sun than is Earth. It took Voyager 2, traveling at an average velocity of 42,000 miles per hour, 12 years to reach Neptune. Although Voyager's closest flyby of Neptune was on August 25, 1989, the spacecraft observed Neptune from June to October of that year and was able to shed light on many of the mysteries that had been intriguing astronomers.

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Left: Neptune, with its Great Dark Spot storm system clearly visible

Voyager determined that Neptune has many features in common with Jupiter and Uranus. Although it is the smallest of the giant outer planets, Neptune is also the densest. Its atmosphere, like that of Jupiter, contains methane, lending it a blue hue.

What surprised scientists was that, while Neptune has only 3% as much solar energy to drive a storm system as has Jupiter, westward winds gust by at speeds up to 1,500 miles per hour, higher than observed at any other planet. Similar to Jupiter's Great Red Spot, Neptune has a storm system, dubbed the Great Dark Spot, as large as Earth that rotates counterclockwise in 16 days.

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Voyager 2's last face-on view of the Great Dark Spot, taken 45 hours before closest approach. The wispy, white clouds suggest counterclockwise rotation.

In addition to the Great Dark Spot, Neptune has a Dark Spot 2, an almond-shaped storm system, with a shiny core, that takes 16 hours to revolve eastward around the planet. A bright, irregular cloud, called Scooter also zips eastward around Neptune in 16 hours. It appears that Neptune's winds are less turbulent than Jupiter's and thus require less energy to maintain their high velocities.

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This image provides us with a rare opportunity to view together the Great Dark Spot (left of center), with its bright white companion, small, bright Scooter (below and to the left), and the Dark Spot 2, or D2, with its shiny core (below Scooter). Strong eastward winds, up to 400 mph, cause D2 to overtake and pass the Great Dark Spot every five days. Voyager 2 took this image at 3.8 million miles from Neptune.

Voyager 2 measured periodic radio emissions from Neptune, which indicated that the planet has a magnetic field. On August 24, 1989, Voyager 2 crossed Neptune's bow shock and then entered the magnetosphere. Voyager discovered that, as with Uranus, Neptune's magnetic poles are much closer to the equator than had been expected. The magnetic axis is tilted 47 from the axis of rotation, with its center offset from the center of Neptune by more than half of Neptune's radius.

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While Neptune's magnetic field is not as strong as that of Uranus, the similarities in the two planets' magnetic fields may mean that they contain similar flows of electrically conducting water.

Another interesting aspect of the orientation of Neptune's magnetic axis is that the magnetic polar region almost directly faces the solar wind every 16.11 hours, which permits the solar wind ions to access the magnetic axis and approach closer to Neptune than would normally be possible.

Before Voyager's flyby of Neptune, the best Earth-based estimate of the length of Neptune's rotation period was 18 hours. Voyager's planetary radio astronomy instrument was able to determine that a day on Neptune is 16 hours, 7 minutes.

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Voyager 2 detected three rings encircling Neptune and a broad sheet of fine particles, called the Plateau. The outermost Main Ring is 23,700 miles above the cloud tops and contains three dense, brighter regions. The Inner Ring lies about 17,700 miles above the cloud tops, while the Inside Diffuse Ring lies about 10,600 miles above the cloud tops. Because the rings are so diffuse and contain such fine material, astronomers on Earth had detected only ring arcs, rather than complete rings.

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Image of Neptune's rings, taken as Voyager 2 left the planet

Voyager 2 discovered, in addition to Neptune's two known moons, Triton and Nereid, six smaller satellites, Proteus, Larissa, Despina, Galatea, Thalassa, and Naiad.

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The graininess of this Voyager 2 image of Neptune's moon Proteus is due to the short camera exposure time.

While the smaller moons probably arose from larger moons that were shattered by collisions with comets and meteoroids, Triton, the largest of Neptune's moons, is quite different. Triton's polar ice cap is composed of frozen nitrogen and methane and is so reflective that Triton absorbs very little solar energy.

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Composite of about 12 Voyager 2 images of Triton, taken on August 25, 1989. At the bottom we can see the highly reflective, pinkish south polar cap, that may consist of nitrogen ice, from the previous winter, that is melting slowly. The darker, redder coloring north of the south polar cap may be the result of ultraviolet light and magnetospheric radiation reacting with the methane on the surface and in the atmosphere.

Although Triton's surface temperature of 38 Kelvin, or -409, is the coldest measured in the solar system, Voyager 2 took images of active geyser-like eruptions shooting dark dust particles five miles above the polar cap.

Triton is relatively dense compared with the icy moons of other planets and its orbit is in the opposite direction of Neptune's rotation, so it probably did not originate near Neptune, but was captured.

No spacecraft has yet visited Pluto, the outermost planet in our solar system, but Triton appears similar in size, density, temperature, and chemical composition. In an indirect way, Voyager may thus have given us our best impressions of the last unexplored planet.

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This portrait of Pluto and its companion, Charon, was taken by the Hubble Space Telescope in 1994. Pluto is usually the most distant planet from the Sun but because of its eccentric orbit Pluto crossed inside of Neptune's orbit in 1979. On Thursday, February 11th, it crossed back out, recovering its status as the most distant of nine planets. Pluto is still considered to be a planet, although very little is known about it compared to other planets. Pluto is smaller than any other planet and even smaller than several other planet's moons. Pluto is probably composed of frozen rock and ice, much like Neptune's moon Triton. Credit: R. Albrecht (ESA/ESO), NASA.

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