From June until October of 1989, the Voyager 2 spacecraft stared at Neptune and its moons. Voyager, the only spacecraft ever to pass by this world, made a spectacular flyby, passing fewer than 5,000 kilometers (3,100 miles) over Neptune's north pole.

Voyager revealed that Neptune has a more active atmosphere than Uranus, showing intricate cloud patterns. Most striking among these was a feature scientists quickly took to calling the Great Dark Spot, a storm similar to Jupiter's Great Red Spot.

But when the Hubble Space Telescope photographed Neptune in 1994, this spot had disappeared. Just why Neptune is this active—so cold and so far from the sun—is still a mystery.

Voyager 2 also discovered that Neptune is encircled by a system of thin, dark rings made up of carbon compounds and these rings are much less reflective than Saturn's ice rings.