Uranus. This fine crescent is Uranus as seen by the mission Voyager 2 in 1986. Uranus is one gas giant planet that is mainly composed of a small rocky core surrounded by layers of gas. At the difference of Jupiter and Saturn which are mostly hydrogen under diverse forms, Uranus, like Neptune, contains some other compounds like helium and methane. All these gas giants formed in the outer part of the primitive protoplanetary disk where Sun's radiation pressure was not strong enough to blow gas and ice away. Based on a small solid planetesimal the gas giants formed accumulating gas. Until the Voyager mission, Uranus had never been seen like this fine light blue-green world. As seen from Earth, Uranus is at the limit of the visual magnitude and it was discovered by William Herschel -the English astronomer, in 1781 only. NASA Voyager 2 mission was part of two missions originally scheduled for Jupiter and Saturn. Both became extended at the point that both craft are now coasting at the far reaches of the solar system, headed for the stars. Voyager 2 was the first mission ever at Uranus, then at Neptune. Voyager 2 is now journeying at minus 48° under the ecliptic and is 7 billion miles (11.3 billion km) away. picture courtesy JPL