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.
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.
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.