Uranus

Uranus is the seventh planet of the Solar System. It has a diameter of about 52400 km, a mass 14.6 times that of the Earth and orbits the Sun every 84 years. Its distance from the Sun varies between 18.3 and 20.1 astronomical units. In January 1986, four and a half years after visiting Saturn, Voyager 2 completed the first close-up survey of the Uranian system. The brief flyby revealed more information about Uranus and its retinue of icy moons than had been gleaned from ground observations since the planet's discovery over two centuries ago by the English astronomer William Herschel. Uranus, third largest of the planets, is an oddball of the solar system. Unlike the other planets (with the exception of Pluto), this giant lies tipped on its side with its north and south poles alternately facing the sun during an 84-year swing around the solar system. During Voyager 2's flyby, the south pole faced the Sun. Uranus might have been knocked over when an Earth sized object collided with it early in the life of the solar system.

From the Earth Uranus can be seen with a telescope. But even with large telescopes it can only be seen as a small greenish disk with no surface details visible. Our knowledge of the outer planets in the solar system has been revolutionised by the amazing results from the close fly-bys of each by Voyager 2 which was at its closest to Uranus on January 24 1986. The most striking observation was that the planet Uranus shows almost no features! It appears as a large green-blue ball from Earth and from the Voyager pictures. Astronomers already knew that Uranus rotated about an axis that is tilted at 98 to the pole of its orbital plane. This means that Uranus is `lying on its side' and rotating in the opposite sense to that of the other planets. The effect on the surface is that the seasons on Uranus are linked to its orbital period about the Sun, so near the poles winter will last 42 earth-years!

Uranus was the first planet to be discovered that was not known to the ancients. William Herschel announced its discovery in 1781 although initially he thought that it was a comet. After he had realised that it was a planet he wanted to call it Georgium Sidus, after his patron King George III. It was not until 1850 when John Couch Adams suggested a change of name that Uranus was universally adopted.

Uranus has 15 known satellites. Unlike the other bodies in the solar system which have names from classical mythology, Uranus's moons take their names from the writings of Shakespeare and Pope. They form two distinct classes: the 10 small very dark inner ones discovered by Voyager 2 and the 5 large outer ones. They all have nearly circular orbits in the plane of Uranus's equator (and hence at a large angle to the plane of the ecliptic).