The planet Saturn was described today as a world of moons and rings in continuous collision with one another, grinding away in a process that makes Saturn look like a giant Mixmaster 1 billion miles away.
"We see so many rings circling the planet that it's conceivable the same particles are colliding every five to 10 hours," Voyager Project scientist Edward C. Stone said at a news conference at the Jet Propulsion Laboratory, where scientists are directing the Voyager II spacecraft's flight by Saturn.
"These ring particles don't hit hard but when they do, one particle bumps the other in toward the planet while it gets bumped away from the planet."
Voyager is flying toward the planet at more than 28,000 miles an hour and is scheduled to fly over Saturn's rings late Tuesday night, coming as close as 5,000 miles from the rings at 11:25 p.m. EDT. The silver-and-black spacecraft was almost 1 billion miles from Earth today, and was within 1 million miles from Saturn at 4:30 p.m. EDT today.
Voyager already has sent back more than half of the 18,500 pictures it is expected to take of the striped planet, its 17 known moons and hundreds of dazzling rings.
Among them was a sequence of pictures of the moon called Hyperion that suggested it was wobbling in orbit around Saturn, possible after suffering a collision of some kind hundreds, thousands or even millions of years ago.
The first photograph of Hyperion suggested it was shaped like a peanut, a second made it look like a potato and a third had it looking like a giant peanut again. The moon is almost twice as long (223 miles) as it is wide (130 miles), and scientists believe it could only be shaped the way it is if it had run up against something and lost a big chunk in a collision.
"I think the reason it was changing its shape from picture to picture is that it's wobbling about," Dr. Garry Hunt of University College, London, said here today. "I think the reason it's wobbling is that it's still suffering the ill effects of some ancient collision, it still hasn't damped out the motions it would make after its accident."
Drawn towards Saturn by the sheer strength of the planet's gravity, Voyager II was speeding up by almost a thousand miles an hour every hour it flew on toward its Tuesday rendezvous, the last time for five years that an American spacecraft will have a close encounter of any kind with another planet.
The next encounter will come in 1986 when Voyager II flies by Uranus, the seventh planet out from the sun.
On its way toward the rings of Saturn, Voyager II also will take close-up photographs of the moons Tethys and Eceladus, which were not photographed close-up by Voyager I when it flew by the planet last November.
As it moves in late Tuesday, Voyager will photograph the rings with the sun at its back so that the light will be illuminating the tops of the rings for the spacecraft's cameras.
Voyager I did not have the advantage of the sun at its back, having flown in under the rings while it was facing their dark, or unlit, side. Scientists deliberately arranged different flight paths of the two Voyagers so that their pictures of the rings and moons would not duplicate each other.
On its passage over the rings, Voyager II will aim one of its telescopic cameras at a star named Delta Scorpii in the constellation Scorpius on the other side of the rings and almost 990 light years away.
Measuring the star's light shining through the rings will provide the best measurements yet of the number of rings, their width and the size of the gaps between them. It should also tell scientists if there are any small moons embedded inside the rings that serve like shepherds to keep the rings apart.
"Each drop and each gain in starlight as we make our passage of the rings will tell us something new about the rings," Project scientist Ed Stone said today. "If there are moonlets inside the rings we should see them blocking the star's light in a much more defined way than the ring material does."
But photographs taken today of the gaps between the three largest rings of Saturn failed to turn up a single embedded moonlet, which sent many Voyager scientists scurrying for new explanations of what divides the rings and keeps them apart.
"We've had one camera looking over one of the rings and it sees nothing, we've had another camera looking at the two other rings and it sees nothing either," Dr. Bradford A. Smith of the University of Arizona said. "We now find ourselves at a point where we had hoped not to be."
One thing that the telescopic cameras on Voyager II did observe today was the 1,100-mile-an-hour jetstream that circles the planet on the north and south sides of its equator, whirling around like an interplanetary speedway. One scientist jokingly called it the "Saturn 500."
Four times faster than the jetstream observed around the equator of Jupiter, Saturn's highest winds are blowing the tops of its green and yellow clouds from the west toward the east at a speed that almost defies the laws of physics. Sweeping along huge brown globs that appear to be Saturnean clouds, the jetstream is behaving for all the world like the jetstreams do on Earth or on any other planet with a turbulent and busy atmosphere.
"We see the classic low pressure zones on either side of the jetstream that we see on Earth," Hunt said. "It's the fastest jetstream we've seen yet in the solar system but it's still nothing more than a classic jetstream."