The "Sydeways" Planet:

Every planet that we have studied so far was known to ancient astronomers, because they are all visible to the unaided eye. Beyond Saturn however, Planets are only visible with telescopes.

History:

It was discovered by Sir Frederick William Herschel. Originally born as Friedrich Wilhelm Herschel in Hanover, Germany, on November 15, 1738. In 1757 Friedrich was sent to England to earn his living as a musician. He learned English quickly and, at age nineteen, he changed his name to Frederick William Herschel.

Unable to obtain a good telescope, Herschel decided to construct his own and, in 1774 had the satisfaction of viewing the heavens with a 6-foot telescope made by his own hands. He then set out on a mission to survey the entire heavens, and, if possible, to ascertain the plan of their general structure. For this he required adequate instruments, and he, his brother and his sister spent hundreds of hours grinding and polishing hundreds of specula. He used these instruments to observe the Moon, to measure the heights of lunar mountains, and to begin a catalog of double stars.

In the hands of William Herschel, the telescope became a powerful scientific tool. Herschel had a talent for telescope making that matched his passion for studying the heavens. He began building them in the 1770s. Soon he was creating the most powerful telescopes in the world and looking deeper into the Universe than ever before. The telescope displayed here was Herschel's favorite: his 20-foot reflector.

On March 13, 1781, Herschel made a discovery which would allow him to continue his astronomical studies as a profession. While conducting one of his methodical reviews of the heavens -- using a 7-foot reflector with an aperture of 6½ inches -- he happened across an object moving outside the orbit of Saturn. Ascertaining that the object was a "new" planet, he dubbed it Georgium Sidus; today we know the object as the planet Uranus. Between 1781 and 1815 he would communicate seven memoirs on the planet to the Royal Society, including two in which he reported his discovery of two Uranian satellites (subsequently named Titania and Oberon). For his discovery Herschel was awarded the Copley Medal of the Royal Society, elected a fellow, and, in 1782, became private astronomer to King George III. Although he was now able to concentrate on his astronomical studies, he had to supplement his income by making and selling telescopes. The necessity for this interruption in his observations was overcome in 1788, when he married the wealthy widow of a London merchant.

During the course of his career, Herschel constructed more than four hundred telescopes. The largest and most famous of these was a reflecting with a 40-foot focal length and 4-foot aperture, which he completed at his home in Slough, England, on August 28, 1789. The first sight he focused on that night was the Saturnian system. Five of Saturn's moons had been known for quite some time, but Herschel had discovered a sixth (Mimas) two years before, using a little telescope of 6½ inch aperture. Now, however, he was able to see that moon in unmistakable brightness. On September 17, using the same telescope, he discovered a seventh moon (Enceladus). The huge telescope proved very cumbersome, however, and most of his subsequent observations were made with a smaller, 20-foot focal length, telescope.

About 1800, Herschel discovered infrared radiation by passing sunlight through a prism and holding a thermometer just beyond the red end of the visible spectrum. The thermometer indicated a temperature increase and this led to Herschel's conclusion that there must be an invisible form of light.

In 1802, Herschel coined the word "asteroid" to describe the star-like appearance of the small moons of the giant planets and of the minor planets.

The Planet:

Uranus is the seventh planet from the Sun and is the third largest in the solar system. It has an equatorial diameter of 51,800 kilometers (1.9 Earth radii) and orbits the Sun once every 84.01 Earth years.

Uranus has a mean distance from the Sun of 19.1 A.U.'s. The length of a day on Uranus is 17 hours 14 minutes. Uranus masses 14.5 Earth masses.

Uranus is distinguished by the fact that it is tipped on its side with a rotational axis that is inclined to the orbit by 98 degrees.

Its unusual position is thought to be the result of a collision with a planet-sized body early in the solar system's history (also note the moons show evidence of a violent event in the past).

Note that this unusual axial tilt leads to a peculiar seasonal and diurnal motion as viewed from the "surface". For example, during summer in the northern hemisphere an observer would see the Sun making circles in the sky every 17 hours. As summer wanes, the Sun would gradually move south. Eventually, the Sun would rise and set to the autumnal equinox of equal day and night 21 years after the summer solstice. Then the nights would grow longer until one day the Sun would not rise and a long 21 year night would begin.


Uranus' atmosphere:

Since Uranus lies more than 19 AU from the Sun, it receives 360 times less light and heat from the Sun than Earth. As a result, its atmosphere is extremely cold, with a temperature of about -214C at the 1 bar pressure level (equivalent to the average air pressure at sea level on Earth).

The atmosphere of Uranus is composed of 83% hydrogen, 15% helium, 2% methane and small amounts of acetylene and other hydrocarbons. Methane in the upper atmosphere absorbs red light, giving Uranus its blue-green color. In the inner Jovian worlds (Jupiter and Saturn), ammonium hydrosulfide dominates the coloration of atmospheres with its red's and yellows. But as the temperature drops below 70 K, ammonia gas freezes into ice crystals and drops out of the atmosphere. Methane becomes more dominate and, being a blue gas, the outer Jovian worlds (Uranus and Neptune) go from blue-green to deep blue in their coloration. Note also that methane, CH4 is a greenhouse gas.

Other minor constituents include hydrogen sulfide (H2S), deuterated hydrogen (HD), ethane (C2H6) and acetylene (C2H2). The last two are created by photochemistry the action of sunlight on methane gas. Despite their scarcity, these minor ingredients do have a subtle influence on their surroundings: they condense to form high level hazes which absorb and reflect sunlight, raising the temperature of the upper atmosphere while lowering the temperature of the deeper atmosphere.

The atmosphere of Uranus is arranged into clouds running at constant latitudes, similar to the orientation of the more vivid latitudinal bands seen on Jupiter and Saturn, although these clouds are only visible in the infrared. Winds at mid-latitudes on Uranus move in the direction of the planet's rotation. These winds blow at velocities between 90 and 360 miles per hour.

Uranus lacks an internal energy source such as Jupiter and Saturn, and thus its atmosphere energy system is much less active, resulting in fewer features (i.e. storms, eddies, etc.). Cloud patterns are only seen at the warmer, lower levels deep below the atmospheric haze. In addition, the tilted axis of Uranus produces uneven warming in the two hemisphere which produce long-term North-South flows across the latitude zones. The combination of these effects means that the atmospheric features are washed out much like Saturn.


Uranus' interior:

The interior of Uranus have rocky cores like Jupiter and Saturn. But at that point the similarity ends. The pressures are never sufficient to convert molecular hydrogen to metallic hydrogen in the interior of Uranus. Instead, a large mantle of icy water and ammonia forms about 20,000 km below the surface.


Uranus' magnetic field:

The magnetic fields for Uranus is unusual and is not well understood at this time. As the diagram below shows, the magnetic fields of the strongest three worlds, Jupiter, Saturn and the Earth, are all roughly aligned with the rotational axis of the planets. The generation of these magnetic fields occurs in liquid mantles around solid cores (liquid rock for the Earth, metallic hydrogen for Jupiter and Saturn).

Uranus, on the other hand, have radically different magnetic fields. Not only are they not aligned with the rotational axis of the planet, but neither are they located at the center of the planet either. The magnetic fields are probably be generated by local events in the icy mantles of both planets and may be unstable.

The best theory for the origin of these magnetic fields involves the high concentration of ammonia, NH3 in the planet's interiors. Ammonia, in solution, is high electrically conductive. This is due to a high amount of free ions (atoms missing electrons so that they have net positive charge). These free ions could form a conducting ionic layer in the mantle which would then produce a magnetic field with Uranus high rotation rates.


Summary:

The principal Uranus' characteristcs are:

Uranus has been visited by only one spacecraft, Voyager 2 on Jan 24 1986.