Spacecraft ExPloration

Although the missions of the twin Voyager 1 and 2 spacecraft originally called for fly-bys of only Jupiter and Saturn, the timing of Voyager 2's launch allowed for a change to its trajectory so that it could be retargeted to Uranus and Neptune for an extended mission, which ultimately was carried out. After more than eight years in space, Voyager 2 sped through the Uranian system on Jan. 24, 1986. Its instruments provided an accurate determination of the masses and radii of the planet and its major moons, detected Uranus's magnetic field and determined its strength and orientation, and measured the planet's interior rotation rate. Images of the Uranian system, which totaled more than 8,000, revealed for the first time the weather patterns in the planet's atmosphere and the surface characteristics of the moons. In addition to Voyager's discoveries of new moons, a ring, and dust bands between the rings, it provided details of ring structure at scales not achievable from Earth.

Yet, despite these achievements, Voyager left many unanswered questions that only another spacecraft mission or a major advance in Earth-based observational technology would be able to address. As of 2009, no missions to Uranus are planned.

Neptune

The third most massive planet of the solar system and the eighth planet from the Sun is Neptune. Because of its great distance from Earth, it cannot be seen with the unaided eye. With a small telescope, it appears as a tiny, faint blue-green disk. It is designated by the symbol W.

Neptune is named for the Roman god of the sea, who is identified with the Greek deity Poseidon, a son of the Titan Cronus (the Roman god Saturn) and a brother of Zeus (the Roman god Jupiter). It is the second planet to have been found by means of a telescope. Its discovery in 1846 was a remarkable combination of the application of solid Newtonian physics and a belief in a numerological scheme that later proved to be scientifically unfounded. Neptune's orbit is almost perfectly circular; as a result, its distance from the Sun varies comparatively little over its nearly 164-year period of revolution. Although the dwarf planet Pluto's mean distance from the Sun is greater than Neptune's, its orbit is so eccentric (elongated) that for about 20 years of each revolution Pluto is actually nearer the Sun than is Neptune.

Neptune is almost four times the size of Earth but slightly smaller than Uranus, which makes it the smallest in diameter of the four giant, or Jovian, planets. It is more massive than Uranus, however, having a density roughly 25 percent higher. Like the other giant planets, Neptune consists primarily of hydrogen, helium, water, and other volatile compounds, along with rocky material, and it has no solid surface. It receives less than half as much sunlight as Uranus, but heat escaping from its interior makes Neptune slightly warmer than Uranus. The heat liberated may also be responsible for the storminess in Neptune's atmosphere, which exhibits the fastest winds seen on any planet in the solar system.

Neptune has 13 moons (natural satellites), only two of which had been discovered before the Voyager 2 spacecraft flew past the planet in 1989, and a system of rings, which had been unconfirmed until Voyager's visit. As is the case for Uranus, most of what astronomers know about Neptune, including its rotation period and the existence and characteristics of its magnetic field and magnetosphere, was learned from a

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