Uranus

Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. Uranus is similar in composition to Neptune, and both have different bulk chemical composition from that of the larger gas giants Jupiter and Saturn. For this reason, scientists often classify Uranus and Neptune as “ice giants” to distinguish them from the gas giants. Uranus’s atmosphere is similar to Jupiter’s and Saturn’s in its primary composition of hydrogen and helium, but it contains more “ices” such as water, ammonia, and methane, along with traces of other hydrocarbons.

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Discovery

Sir William Herschel observed Uranus on March 13, 1781 from the garden of his house at 19 New King Street in Bath, Somerset, England, and initially reported it (on April 26, 1781) as a comet. Herschel “engaged in a series of observations on the parallax of the fixed stars”, using a telescope of his own design.

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Orbit and Rotation

Uranus orbits the Sun once every 84 years. Its average distance from the Sun is roughly 20 AU (2 billion miles). The difference between its minimum and maximum distance from the Sun is 1.8 AU, larger than that of any other planet.

The rotational period of the interior of Uranus is 17 hours, 14 minutes. As on all the giant planets, its upper atmosphere experiences strong winds in the direction of rotation. At some latitudes, such as about 60 degrees south, visible features of the atmosphere move much faster, making a full rotation in as little as 14 hours.

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Exploration

In 1986, NASA’s Voyager 2 interplanetary probe encountered Uranus. This flyby remains the only investigation of Uranus carried out from a short distance and no other visits are planned. Launched in 1977, Voyager 2 made its closest approach to Uranus on January 24, 1986, coming within 50,600 miles of the cloudtops, before continuing its journey to Neptune. The spacecraft studied the structure and chemical composition of Uranus’s atmosphere, including its unique weather, caused by its axial tilt of 97.77°. It made the first detailed investigations of its five largest moons and discovered 10 new ones. It examined all nine of the system’s known rings and discovered two more. It also studied the magnetic field, its irregular structure, its tilt and its unique corkscrew magnetotail caused by Uranus’s sideways orientation.

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