... Neptune is smaller in diameter but larger in mass than Uranus . ... N eptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. ... A t the time of the Voyager encounter, Neptune's most prominent feature was the Great Dark Spot (left) in the southern hemisphere. ... Neptune's winds blew the Great Dark Spot westward at 300 meters/second (700 mph). ... L ike Uranus and Jupiter, Neptune's rings are very dark but their composition is unknown. ... Neptune's Ring System . ... Uranus .. ...