Background

Astronomers have studied Neptune since September 23, 1846, when Johann Gottfried Galle, of the Berlin Observatory, and Louis d'Arrest, an astronomy student, discovered the eighth planet on the basis of mathematical predictions by Urbain Jean Joseph Le Verrier. Similar predictions were made independently by John Couch Adams. (Galileo Galilei had seen Neptune during several nights of observing Jupiter, in January 1613, but didn't realize he was seeing a new planet.) Still, any knowledge and understanding of Neptune was limited by the astronomer's abilityto see the distant object, almost 4.5 billion kilometers (2.8 billion miles) from Earth.

Scarcely a month after Galle and d'Arrest first saw Neptune, British astronomer William Lassell spotted a satellite orbiting the planet and named it Triton. Triton, almost the size of Earth's Moon, is the only large satellite in the solar system to circle a planet in a retrograde direction -- in a direction opposite to the rotation of the planet. That phenomenon led some astronomers to surmise that Neptune had captured Triton as it traveled through space several billion years ago.

In 1949, astronomer Gerard Kuiper discovered Nereid, the second of Neptune's escorts. Nereid is only about 340 kilometers (210 miles) in diameter and is so far from Neptune that it requires 360 days to make one orbit -- only five days less than Earth takes to travel once around the Sun.

In 1981, astronomers leaped at an infrequent opportunity: A star would pass behind Neptune so that observers could measure the starlight and how it changed as it passed through the upper layer of Neptune's atmosphere. That would provide clues to its structure.

But the star's light winked off and on before Neptune passed in front of it. Similar measurements were obtained during the mid-1980s. Astronomers concluded that some material (perhaps like that of the rings of Saturn) orbits Neptune, and was responsible for occasional blockage of the star's light. In each observed event, astronomers saw that the ring or rings did not appear to completely encircle the planet -- rather, each appeared to be an arc segment of a ring.

The laws of physics say that, with nothing else acting upon them, rings must orbit a planet at about the same distance from the center all the way around. Ring material, if unrestrained, will tend to disperse uniformly around the planet. In order to have "ring arcs," scientists thought that some objects -- perhaps small satellites -- must shepherd the arcs, keeping them in their place by gravity.

Earth-based telescopic observations of Neptune over the last few years showed tantalizing hints of dynamic cloud structures on the distant planet, from which scientists could estimate the speed of winds circling the planet.

Against that background, Voyager's scientists prepared for the first encounter of Neptune, perhaps the only close-up look at Neptune in the lifetime of many of us. What they found will force scholars to rewrite the astronomy textbooks, and scientists to adjust their views of the solar system's other giant planets.