Before Voyager

Nearly 3 billion kilometers (1.8 billion miles) from Earth, Uranus is the most distant object yet visited by a spacecraft. Uranus is so far away that scientists knew comparatively little about it before Voyager 2 undertook its historic first-ever encounter with the planet.

Indeed, since its discovery by William Herschel in 1781, Uranus had remained largely a mystery throughout the ensuing two centuries. Five moons -- the first discovered in 1787, the last in 1948 -- were visible only as tiny points of light. A system of nine narrow rings went undetected until 1977. The planet's rate of rotation could be estimated only roughly and was believed to be anywhere from 16 to 24 hours. Before Voyager, there were indirect indications of a magnetic field at Uranus, although the evidence was not conclusive.

Scientists were not sure what to expect from Uranus's strange orientation. The planet is tipped on its side, with its orbiting moons and rings forming a giant celestial bull's-eye. As a result, the northern and southern polar regions are alternatively exposed to sunlight or to the dark of space during the planet's 84-year orbit around the Sun.