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As Voyager 2 approached Neptune, scientists had been working on theories of how partial rings, or "ring arcs," could exist. Most settled for the concept of shepherding satellites that "herd" ring particles between them, keeping the particles from either escaping to space or falling into the planet's atmosphere. This theory had explained some new phenomena observed in the rings of Jupiter, Saturn and Uranus.

When Voyager 2 was close enough, its cameras photographed three bright patches that looked like ring arcs. But closer approach, higher resolution and more computer enhancement of the images showed that the rings do, in fact, go all the way around the planet.

Neptune's rings
Neptune's rings (Click on the image for a larger view)

The rings are so diffuse, and the material in them so fine, that Earthbound astronomers simply hadn't been able to detect the full rings. (Based on Voyager's findings, one Earth- based observation of the ring arcs is now attributed to the passage of a small satellite through the ring area.)

Late in the encounter, the scientists were able to sort out the number of rings and a preliminary nomenclature:

  • The "Main Ring" (officially known as 1989N1R, following the IAU convention) orbits Neptune about 38,100 kilometers (23,700 miles) above the cloud tops. The main ring contains three separate regions where the material is brighter and denser, and explains most of the sightings or "ring arcs." Several Voyager photographs show what appear to be clumps embedded in the rings. Scientists are not sure what causes the material to clump.
  • The "Inner Ring" (1989N2R) -- about 28,400 kilometers (17,700 miles) above the cloud tops.
  • An "Inside Diffuse Ring" (1989N3R) -- a complete ring located about 17,100 kilometers (10,600 miles) from Neptune's cloud tops. Some scientists suspect that this ring may extend all the way down to Neptune's cloud tops.
  • An area called "the Plateau," a broad, diffuse sheet of fine material just outside the so-called "Inner Ring."

The material varies considerably in size from ring to ring. The largest proportion of fine material -- approximately the size of smoke particles, is in the Plateau. All other rings contain a greater proportion of larger material.

Both Voyagers have now completed all of the planetary encounters on their itinerary, but both still have work to do. Voyager 1 is heading out of the solar system, climbing above the ecliptic plane in which the planets orbit the Sun. Voyager 2 is also outbound, traveling below that plane. Both are searching for the heliopause, a boundary that marks the end of the solar wind and the beginning of interstellar space. Assuming both spacecraft remain healthy, flight controllers expect to be able to operate the spacecraft for another 20 years, investigating magnetic fields and particles in interplanetary and interstellar space, and observing ultraviolet sources among the stars.

The Voyager Project is managed by the Jet Propulsion Laboratory for NASA's Office of Space Science and Applications.

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