Uranus
DISTANCE FROM SUN: 1.8 billion miles (2.9 billion kilometers).
REVOLUTION AROUND SUN: 84 years.
ROTATION: 17.2 hours.
DIAMETER: 31,770 miles (51,100 kilometers).
DENSITY: 1.3 x that of water.
MASS: 14.5 x that of Earth.
SURFACE TEMPERATURE: -297°F (-183°C) at cloud tops.

Satellites (closest to most distant):
Cordelia
Ophelia
Bianca
Cressida
Desdemona
Juliet
Portia
Rosalind
Belinda
Satellite 1986 U 10
Puck
Miranda
Ariel
Umbriel
Titania
Oberon
Caliban
Sycorax

Third of the gas giants from the sun, Uranus glows blue-green in the dim sunlight as methane in its upper atmosphere absorbs red light waves.

Uranus’s most remarkable feature is its 98° spin axis—the planet’s poles sometimes point directly toward and away from the sun. The odd alignment is thought to be the result of a collision with some other body, probably a planet-sized object, early in its history. For half of its 84-year orbit, each pole’s hemisphere is either plunged into darkness or bathed in continuous sunlight. Uranus also spins from east to west, the opposite of all planets except Venus and perhaps Pluto.
Its 18 known moons orbit roughly on the same plane as the planet’s equator, and its surface is streaked with high-speed clouds that rush across Uranus’s face at speeds topping 400 miles (580 kilometers) per hour. Its atmosphere is 15 percent hydrogen and a little helium, 2 percent methane, and trace amounts of acetylene.
In 1977 observers on a NASA airplane discovered that Uranus had rings. We now know the planet has 11 concentric rings of dark dust. Uranus is tipped so its rotation axis lies in the plane of its orbit so the rings appear (to us) nearly face-on. A Voyager 2 picture of the planet shows a ring system filled with belts of micron-sized dust.

A PLANET ON ITS SIDE Because its unique rotation often causes its poles to face the sun, Uranus gives viewers from Earth a clear look at the dynamics of what goes on at the pole of a gas planet. In an image adjusted to heighten contrast, the planet shows a dark hood over its south pole, surrounded by concentric bands, each progressively lighter in color toward the equator. The feature seems to indicate an upper atmosphere wind flow that concentrates a type of smog at the pole. Extensive bright clouds also frequently appear floating across the bright blue background of Uranus’s disk. They float along on high-speed winds, circumnavigating the planet. measure 2,500 miles (4,000 km) across.

RINGS
Uranus’s rings are extremely thin and charcoal black; only extensive computer processing makes them visible in images taken by the Voyager spacecraft and the Hubble Space Telescope. But because of the planet’s pole-on orientation, Earth observers get a unique topside view of their structure.
There are at least 10 rings around Uranus. Two moons, Cordelia and Ophelia, are called shepherd moons because their gravitational influences help define the shape and extent of the ring structure.

MOONS It seems that every time an in-depth study is made of Uranus, more moons are discovered. William Herschel discovered the two largest, Titania and Oberon, in 1787. Two more were found in the next century, and another was spotted in 1948. That was considered to be the lot of them until Voyager 2 flew by in 1986—and found another 10! Miranda, one of Uranus’s five larger moons, has severely jumbled terrain, like an ill-fitting puzzle. By one theory, it may have been torn apart in a collision or series of collisions with asteroids, comets, or other objects. Then its pieces were drawn back together by mutual gravitational pull. Other moons have their own mysteries. Ariel’s face is grooved, and Umbriel is unusually dark.