Spacecraft Exploration

The first spacecraft to visit Saturn, the U.S. Pioneer 11, was one of a pair of probes launched in the early 1970s to Jupiter. Though a retargeting was not part of the original objective, mission scientists took advantage of Pioneer 11's close encounter with Jupiter's gravitational field to alter the spacecraft's trajectory and send it on to a successful flyby of Saturn. In 1979 Pioneer 11 passed through Saturn's ring plane at a distance of only 38,000 km (24,000 miles) from the A ring and flew within 21,000 km (13, 000 miles) of its atmosphere.

The twin spacecraft that followed, the U.S. Voyagers 1 and 2, were launched initially toward Jupiter in 1977. They carried much more elaborate imaging equipment and were specifically designed for multiple-planet flybys and for accomplishing specific scientific objectives at each destination. Like Pioneer 11, Voyagers 1 and 2 used Jupiter's mass in gravity-assist maneuvers to redirect their trajectories to Saturn, which they encountered in 1980 and '81, respectively. Together the two spacecraft returned tens of thousands of images of Saturn and its rings and moons.

The Cassini-Huygens spacecraft was launched in 1997 as a joint project of the space agencies of the United States, Europe, and Italy. It followed a complicated trajectory involving gravity-assist flybys of Venus (twice), Earth, and Jupiter that brought it to the Saturnian system in mid-2004. Weighing almost six metric tons when loaded with propellants, the interplanetary craft was one of the largest, most expensive, and most complex built to that time. It comprised a Saturn orbiter, Cassini, designed to carry out studies of the planet, rings, and moons for several years, and a probe, Huygens, that descended by parachute through Titan's atmosphere to a solid-surface landing in early 2005. For about three hours during its descent and from the surface, Huygens transmitted measurements and images to Cassini, which relayed them to scientists on Earth.

Uranus

Beyond Saturn is Uranus, the seventh planet in distance from the Sun and the least massive of the solar system's four giant, or Jovian, planets, which also include Jupiter, Saturn, and Neptune. At its brightest, Uranus is just visible to the unaided eye as a blue-green point of light. It is designated by the symbol 1$.

Uranus is named for the personification of heaven and the son and husband of Gaea in Greek mythology. It was discovered in 1781 with the aid of a telescope, the first planet to be found that had not been recognized in prehistoric times. Uranus actually had been seen through the telescope several times over the previous century but dismissed as another star. Its mean distance from the Sun is nearly 2.9 billion km (1.8 billion miles), more than 19 times as far as is Earth, and it never approaches Earth more closely than about 2.7 billion km (1.7 billion miles). Its relatively low density (only about 1.3 times that of water) and large size (four times the radius of Earth) indicate that, like the other giant planets, Uranus is composed primarily of hydrogen, helium, water, and other volatile compounds; also like its kin, Uranus has no solid surface. Methane in the Uranian atmosphere absorbs the red wavelengths of sunlight, giving the planet its blue-green colour.

Most of the planets rotate on an axis that is more or less perpendicular to the plane of their respective orbits around

Uranus

Two views of the southern hemisphere of Uranus, produced from images obtained by Voyager 2 on Jan. 17,1986. Uranus shows the banded cloud structure common to the four giant planets (right). Small ring-shaped features in the right image are artifacts arising from dust in the spacecraft's camera. Jet Propulsion Laboratory/NASA

Two views of the southern hemisphere of Uranus, produced from images obtained by Voyager 2 on Jan. 17,1986. Uranus shows the banded cloud structure common to the four giant planets (right). Small ring-shaped features in the right image are artifacts arising from dust in the spacecraft's camera. Jet Propulsion Laboratory/NASA

the Sun. But Uranus's axis lies almost parallel to its orbital plane, which means that the planet spins nearly on its side, its poles taking turns pointing toward the Sun as the planet travels in its orbit. In addition, the axis of the planet's magnetic field is substantially tipped relative to the rotation axis and offset from the planet's centre. Uranus has more than two dozen moons, five of which are relatively large, and a system of narrow rings.

Uranus has been visited by a spacecraft only once—by the U.S. Voyager 2 probe in 1986. Before then, astronomers had known little about the planet, since its distance from Earth makes the study of its visible surface difficult even with the most powerful telescopes available. Earth-based attempts to measure a property as basic as the planetary rotation period had produced widely differing values, ranging from 24 to 13 hours, until Voyager 2 finally established a 17.24-hour rotation period for the Uranian interior. Since Voyager's encounter, advances in Earth-based observational technology have added to knowledge of the Uranian system.

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