The Gas Giants (cont.)

Uranus

The ancients knew of five planets in the sky. In less than a generation, all five have been brought close to us by spacecraft. Further out in space, three more planets await our inspection. Uranus, the first outward beyond Saturn, is nearly 3 billion kilometers (nearly 2 billion miles) from the Sun and takes 84 years to make one orbit around it. Pioneer 10 passed the orbit of Uranus, but did not come near the planet. Uranus is another gas giant, smaller than Saturn, a cold world that is a distant greenish disk even in the largest telescopes. From Earth, we can see five moons circling the planet. Uranus has one unique property: its axis of rotation lies in the plane of its orbit rather than nearly vertical to it as is the case with the other planets. Because of this curious orientation, Uranus moves around the Sun, not so much like a top spinning on its end, but like a barrel rolling along on its side.

Dark rings of Uranus.
Discovered by telescopic observations from a NASA aircraft in 1977, the rings around the planet Uranus are painted here as they might appear from a spacecraft approaching the planet. The thin narrow rings, composed of dark particles, are invisible from Earth. They were discovered when they blocked the light of a distant star on a night when Uranus passed in front of it. The rotation axis of Uranus lies almost in the plane of its orbit around the Sun, so the ring system (in the plane of the planet's equator), appears like a gigantic bulls eye as Uranus rolls around the Sun. Uranus is so distant that no spacecraft has yet reached it. Voyager 2 may go on to Uranus after passing Saturn in 1981; it would get there in 1986.

The greatest Space Age discovery about Uranus was made in 1977, not from a spacecraft but rather with a telescope mounted in a high-altitude jet aircraft, the Kuiper Airborne Observatory. Like many great discoveries, it was entirely unexpected, and the scientists were looking at Uranus for something entirely different. They intended to study Uranus' atmosphere by measuring the light from a distant star as the planet passed slowly in front of it. What happened was that the star seemed to flicker on and off several times, long before and long after the planet had passed in front.

The only explanation was that Uranus - like Saturn and now Jupiter - has rings! The planet is surrounded by at least five and perhaps nine rings. The rings are dark, thin, narrow, and invisible from Earth, but they were thick enough to block out the light from the star.

Uranus may soon have its first visitor from Earth. The spacecraft Voyager 2 is now following a path that will take it past Uranus in 1986.