Voyager 2 Finds Rings at Neptune (But Not All the Way Around It)
In its first major discovery at Neptune, the Voyager 2 spacecraft yesterday detected two partial rings of debris around the planet, providing the first direct evidence that Neptune has rings and that they are strikingly different from those of any other planet. Mission scientists said the arc-shaped clumps of material orbiting Neptune were found in photographs transmitted by the spacecraft early yesterday morning as it sped at 42,000 miles an hour for its planned encounter with the distant planet on Aug. 24. At the time of the discovery, Voyager was still 13 million miles away from Neptune and almost 2.8 billion miles from Earth. Predicitions Are Confirmed Finding the ring arcs not only confirmed predictions by astronomers but also was a crowning achievement for the Voyager 2 and its earlier twin, Voyager 1, whose cameras have made surprising discoveries of the splendor and variety of rings around all the four giant outer planets: Jupiter, Saturn, Uranus and Neptune. Until 1977, only Saturn was known to be circled by rings. Its rings are of icy particles reflecting sunlight. Officials of the Jet Propulsion Laboratory in Pasadena, Calif., which manages the Voyager program for the National Aeronautics and Space Administration, said analysis of photographs showed that one of the Neptune ring arcs extended about an eighth of the way around the equator, making it about 30,000 miles long. A smaller arc is one-36th of the circumference, or about 6,000 miles long. The partial rings are unlike those of other planets in that they have remained only arcs, rather than gradually spreading until they encircled Neptune. Scientists said the two ring arcs are apparently associated with two of the four small Neptunian moons that were discovered earlier this month. The longer arc was observed just outside one moon, which orbits at an altitude of 23,300 miles from the planet's thick cloud tops. The smaller arc seemed to trail another of the small moons, which orbits about 17,000 miles above the planet's clouds. Moons Act as Shepherds At Saturn and Uranus, Voyager photography disclosed the phenomenon by which tiny moons seem to act as shepherds for orbiting debris, their gravity shaping and containing the material in narrow rings. The implication is that the nearby moons of Neptune may serve the same function. Scientists speculated that the debris creating the partial rings may be the remains of moons that have been broken apart. Dr. Bradford A. Smith, an astronomer at the University of Arizona and leader of the team analyzing Voyager photography, was relieved to be finding some of the arc rings while there was still time to adjust the spacecraft's trajectory to avoid possible collisions. Voyager will pass about 3,000 miles above Neptune's northern hemisphere and at least two course corrections are planned before then. But scientists said they would not be surprised to find many more ring arcs and tiny moons. They cannot tell from the pictures received yesterday whether the material in the arcs is dust, pebbles or boulders. Since some of these hazards may escape Voyager's detection, Dr. Smith said: ''There's some risk of collisions. I'm not predicting doom, but it's going to be an anxious moment when we fly through the equatorial plane.'' Besides the advantages of flying close to get more detailed photographs, Voyager's pass will be near enough to use Neptune's gravitation to bend the spacecraft's flight path for a rendezvous five hours later with Triton, the planet's largest moon. Triton is a primary objective of the mission because it is one of the few moons in the solar system with an atmosphere and the only one that moves around its parent body in the direction opposite the planet's rotation. Starlight Provided a Clue For some years astronomers have suspected the existence of a system of rings around Neptune that formed incomplete circles. Observations from the ground have occasionally produced clues that some material orbiting Neptune was causing the flickering of the starlight seen through it. The announcement by the Jet Propulsion Laboratory said Voyager 2 had provided ''the first photographic evidence that such a ring system exists.'' As the spacecraft moves in closer, scientists said, the new photographs might determine the composition of the rings and perhaps locate others. The unmanned Voyager 2, launched in 1977, has previously flown by Jupiter, Saturn and Uranus in the most far-reaching photographic reconnaissance of the solar system. The visit to Neptune will be the spacecraft's last fly-by before it cruises out of the planetary system and into interstellar space. Scientist still receive signals from Voyager 1, but it is headed out of the solar system. drawing Inside NYTimes.com Health " Too Hot to Handle