PASADENA, Calif. -- Two drifting clouds of debris, possibly the torn remnants of destroyed moons, were discovered Friday around distant Neptune by NASA's Voyager 2 probe, confirming predictions based on scanty Earth-based observations, scientists said.
Launched nearly 12 years ago from the Cape Canaveral Air Force Station in Florida, Voyager 2 is less than two weeks away from its final planetary encounter in a once-in-a-lifetime grand tour of the outer solar system that has carried the hardy craft past Jupiter, Saturn, Uranus and a host of moons.
All three of those planets have well-developed ring systems, and prior to Voyager 2's arrival in Neptune's neighborhood, there was controversial Earth-based evidence for the existence of partial rings, or 'ring arcs,' around the eighth planet from the sun.
Voyager 2 already has discovered four previously unknown moons and a gargantuan storm system in Neptune's atmosphere, but the discovery of the two ring arcs Friday marked a major pre-encounter milestone.
The arcs were discovered in pictures radioed back from Voyager late Thursday that arrived at the Jet Propulsion Laboratory in Pasadena around 1 a.m. PDT Friday.
'We're certainly excited,' assistant Voyager project scientist Ellis Miner said in an interview. 'You can actually see a curved arc cross the picture and it describes something like an eighth of a complete circle.
'It doesn't look much like what we predicted. We had predicted very short arcs and possibly several of them along any given radius of the planet.'
Both newly discovered ring arcs appear to be associated with two of four new moons that were discovered by Voyager 2 earlier this summer. Neptune already had two known moons, Triton and Nereid.
One arc stretches 30,000 miles and circles Neptune just outside the orbit of a moon temporarily dubbed 1989 N4, which orbits about 23,300 miles above Neptune's cloud tops.
The second arc stretches some 6,000 miles and appears to trail the moon 1989 N3 by about 90 degrees, or 50,000 miles. 1989 N3 orbits Neptune at an altitude of about 17,000 miles.
Miner said it was not clear whether the moons were 'shepherding' the ring arcs along as is the case with rings around Uranus and Saturn, or if their gravity merely serves to collect dust and debris in their wake.
'It's a little hard to tell right now whether they're actually shepherding or whether they are collecting debris around them,' he said. 'The analysis remains to be done to see how close they are to the orbits of these moons.'
Miner said it was too soon to say what might have created the ring arcs, although it is probable they are made up of debris from 'moons that have been broken up and we're seeing debris trails.'
The arcs were discovered in 8-minute exposures by Voyager 2's camera system. More are expected to be discovered as the spacecraft gets closer to Neptune.
Prior to the Voyager encounter, the best way to find such dim swaths of dust and debris was to carefully watch what happens when a planet moves in front of a star as viewed from Earth. If rings are present, the light from the star will appear to twinkle on and off as it filters through the material of the rings.
The light will then go out as the planet moves directly in front of the star -- called an 'occultation' -- and the procedure will be repeated after the star appears on the other side. At least, that is how it worked for the discovery of the rings around Uranus.
Over the past nine years, astronomers have studied 110 occultations of stars by Neptune. In eight cases, the starlight dimmed, but only on one side of the planet. It did not dim on both sides as one would expect if Neptune was circled by a complete ring system.
The best pre-encounter data indicated at least three regions where narrow ring arcs might exist at a distance of 10,500 miles to 26,100 miles from the planet's cloudtops.
The Voyager 2 discovery announced Friday confirmed the existence of at least two such arcs and as the spacecraft gets closer, it should discover more, scientists believe.