As cold as ice

Distant, mysterious Uranus poses myriad questions for astronomers. Nasa has been charged with finding answers

- Robin McKie

On the night of 13 March 1781, William Herschel was peering through his telescope in his back garden in New King Street, Bath, when he noticed an unusual faint object near the star Zeta Tauri. He observed it for several nights and noted that it was moving slowly against background stars. The astronomer first thought he had found a comet but later identified it, correctly, as a distant planet. Subsequently named Uranus, it was the first planet to be discovered since antiquity. The achievement earned Herschel membership of the Royal Society, a knighthood and enduring astronomical fame.

Studies have since shown Uranus to be an odd world. While the rest of the planets within our solar system spin like tops, Uranus lies on its side. And, although it is not the farthest planet from the sun, it is the solar system's coldest.

Uranus also endures seasons of extraordinary magnitude. Each pole spends decades bathed in non-stop sunlight, then decades of total darkness. For good measure, Uranus is the only planet to be named after a Greek rather than a Roman god. (Uranus was the grandfather of Zeus.)

Surprisingly little effort has been made to get up close to Uranus. Only one robot probe has visited - in 1986when the US Voyager 2 craft swept past on its grand tour of the solar system. It revealed a massive, featureless, pale blue world with an atmosphere of hydrogen, helium and methane, a rich family of moons and a powerful magnetic field. And that has been that.

Such slight regard is about to change. The US National Academy of Sciences this year published a report that urged Nasa to launch a Uranus probe as its highest-priority flagship mission for the next decade. The academy publishes a report on US priorities in planetary exploration every 10 years and each decadal survey carries enormous clout - which means Nasa is now under huge pressure to design and fund such a mission.