Uranus

While surveying the skies on a March evening in 1781, English astronomer Sir William Herschel discovered the planet Uranus, at first mistaking it for a comet. After observing Uranus' path among the stars, astronomers determined that Uranus' orbit extends 19 times farther from the Sun than Earth's orbit. Although the diameter of the planet is four times greater than that of Earth, at this distance it appears in the sky as a faint disk spanning one thousandth of a degree, making it barely visible to the unaided eye only on clear, dark nights.

Early astronomers observed that the orbits of the four then-known Uranian moons were tipped 98 degrees relative to the planet's orbit around the Sun. These satellites, as well as Miranda (a Uranian moon discovered in 1948), and 10 small inner moons discovered by Voyager 2 in 1986 (bringing the total number of Uranian moons to 15), all lie in Uranus' equatorial plane. Several more Uranian moons were discovered in 2003 bringing the total number of Uranian mooms to 27.

Tipped Uranus behaves as a giant top as it spins on an axis almost in the plane of the orbit. This motion leads to extreme seasonal variation in what sunlight is available. Over the period of I Uranian year (84 Earth years), the polar regions of the planet go through four seasons, as on Earth, with perpetual sunlight in the summer, and total darkness in the winter. Periods of alternating day and night are interspersed in the spring and fall.

With winters and summers extending for 21 Earth years, it would seem that Uranus should experience drastic temperature changes, but this is not the case. Uranus is so far from the Sun that its energy input per area is 360 times less than that on Earth; thus, little heating occurs during the summer. The rate of heat loss depends on the temperature of the region that is exposed to space; low cloud temperature leads to little heat loss during the winter. Despite Uranus' strange seasons, the temperature of the clouds shrouding the planet remain somewhat constant at -220 ~C.

Only one spacecraft has observed Uranus at close range Voyager 2. The Voyager spacecraft revealed that recurring patterns in radio signals from the planet indicated a rotation period (length of day) of 17.3 hours. Voyager scientists also discovered that, while the strength of Uranus' magnetic field is similar to Earth's, the Uranian poles are an amazing 60 degrees away from the rotational pole.

When Voyager 2 flew by the planet, the spacecraft's cameras revealed an almost featureless atmosphere; however, faint cloud markings between 20 and 50 !S latitude were recorded. The rotation rate of these clouds compared with the rotation of the magnetic field indicated wind speeds of 100-600 km/hr, which, unlike the winds of Jupiter and Saturn, blow westward.

In 1977, Uranus was observed passing in front of a star. During this observation, it was revealed that Uranus possesses a system of at least 11 thin, widely, separated rings. In 1986, Voyager 2 confirmed the rings' existence.

Radio measurements showed the outermost ring, the epsilon, to be composed mostly of ice boulders several feet across. However, a very tenuous distribution of fine dust also seems to be spread throughout the ring system.

Incomplete rings and the varying opacity in several of the main rings leads scientists to believe that the ring system may be relatively young and did not form at the same time as Uranus. The particles that make up the rings may be remnants of a moon that was broken by a high-velocity impact or torn up by gravitational effects.

Today, we know that the dimly lit Uranian system consists of a planet surrounded by a flat system of rings and satellites. Bits of debris are concentrated into thin rings that orbit the planet between 1.4 and 2.0 Uranian radii, with the tiny moon Cordelia orbiting inside the brightest, outermost ring. Nine other small moons orbit from 2.1 to 3.4 Uranian radii. The five outer moons, with diameters ranging from 13 to 15 percent the size of our Moon, revolve around the planet at distances from 4 to 15 radii, or one-third to one-and-a-half times the distance between Earth and our Moon. Voyager 2 revealed that a remarkable variety of surface features mark these larger satellites, including craters, fractures, and frozen water.

Because no missions are currently being planned to return to Uranus, future information will need to be gained using ground-based or Earth-orbiting facilities.