Uranus

[General] [History] [Satellites] [Rings] [Unknowns]

PHYSICAL DATA Sidereal period:   84.01 years Rotation period:      17.2 hours Mean orbital velocity: 6.80 km/s Mass (Earth is 1): 14.6 Volume (Earth is 1): 67 Mean Surface Temp: -210°C Distance from Sun: 2,735 to 3,004 million km

    Uranus wasn't discovered until the 18th century. In 1781, William Herschel (1738-1822) discovered Uranus. Six years later, Herschel found two of its largest moons. Uranus is a gas planet, composed mostly of hydrogen (83%) and helium (15%). It gets its blue color because methane (which represents about 2% of the planet) absorbs red light.

General
Uranus is a giant gas planet. It has a large diameter and low density. Like Jupiter and Saturn, Uranus has a layered atmosphere, high winds, and lots of hydrogen. On the other hand, its winds are not as high, its atmospheric bands are not bold and colorful, and it might have more rock in it than either Jupiter and Saturn.

Aside from its color (see true and false colors of Uranus, right), Uranus is best known for lying on its side. It's possible a large planet-sized object collided with it and tipped Uranus over. In any case, it is tilted almost ninety degrees, making it difficult to say which is its north and south poles. Either, its tilted 98 degrees and has a direct rotation pattern rotation and is tilted by 88 degrees. Interestingly, Uranus' original south pole points directly at the Sun but the hottest area on the planet is around its equator. Puzzling. or it has a retrograde

Uranus is about four times bigger than Earth (left). Located 2.9 billion miles from the Sun, it takes Uranus 84 years to complete its circuit. Like the other gas planets, Uranus has rings (but they are dark) and lots of moons (10 small ones and 5 large ones). It also has an extensive magnetic field that sort of corkscrews behind the planet.  

History
Unlike most planets, who were named for Roman gods, Uranus was named from Greek mythology. According to Greek lore, Uranus was the god of the sky and king of the other gods. Originally thought to be a star, Uranus was call 34 Tauri. When the English astronomer William Herschel (1738-1822) discovered the planet, he wanted to name it after King George. But ultimately it was decided to keep the theme of naming planets after mythological figures.

Only one spacecraft has visited Uranus. Voyager 2, on its way to Neptune, passed Uranus in 1986.  

Satellites
Like Jupiter and Saturn, Uranus has many satellites. So far there are 15 named moons and 2 newly discovered ones. Named for literary characters (mostly Shakespeare), the five outer satellites (right) were discovered before Voyager 2. Oberon and Titania were discovered in 1787, and  Umbriel and Ariel were found in 1851. But Miranda wasn't discovered until 1948. The inner moons, which are dark and small, were discovered by Voyager 2 in 1985-86. All the satellites have nearly circular orbits. 

Rings
Uranus has rings (see right), just like the other gas planets, but they are more difficult to detect and in fact weren't discovered until 1977. The outer ring, Epsilon, is made mostly of blocks of ice so it is the brightest ring. The other 10 rings tend to be darker and have more dust and rock material in them. Two of Uranus's moons can be seen straddling the Epsilon ring (below). 

* Distance is from the center of Uranus to the inner edge of the ring.

Unknowns

1. Was Uranus, like Jupiter, on its way to being a star but never grew up?
2. Why does its south pole face toward the Sun?
3. What is its interior like?
4. Why is Uranus at such a strange tilted axis tilted?

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