A silver-and-black Pioneer spacecraft climaxed 6 1/2 years in deep space today when it made an historic two-hour passage of the rings of the planet Saturn.
The first spacecraft to reach Saturn from Earth, the 570-pound Pioneer II flew by and under the planet's three dazzling rings at 10:35 a.m. EDT and emerged from behind the rings two hours later. Still beaming a strong radio signal back to earth 960 million miles away, Pioneer clearly had made a safe passage of the rings and of the radiation belts that surround Saturn.
"We're getting the same telemetry signal we had before we passed under the rings," Flight Operations Chief Robert P. Hogan said just after 4 p.m. at the National Aeronautics Space Administration's Ames Research Center where Pioneer is directed."I think we can all assume we made it through."
Pioneer's flight by Saturn means more than a safe passage of the planet's rings and radiation belts. It means the spacecraft can spend Sunday and Monday photographing Saturn's giant moon Titan and can continue to take measurements of the rings from the dark side of the planet.
It also means that the two Voyager spacecraft following Pioneer to Saturn most likely can come close to the planet without fear of damage and that Voyager 2 can go on to a 1986 encounter with Uranus and possibly even Neptune at a later date.
"The excitement we felt today was the excitement that we know we can go to Uranus," said Voyager scientist Bradford Smith of the University of Arizona. "If Voyager is going to get to Uranus, it must follow the exact flight path that Pioneer took today."
The two-hour passage of the giant rings was not without its anxious moments. A device aboard Pioneer that measures meteoroid impacts recorded four hits as the spacecraft flew under the rings. Two of the impacts came within seconds of each other and the other two minutes apart. Scientists assumed the four impacts were from debris that had wandered out of the rings.
Pioneer also photographed a new ring farther out than the others and heretofore hidden from earthbound telescopes.Thin and dark -- no wider than 300 miles -- it was dubbed the F ring, and the space between it and the outermost previously known ring was christened the Pioneer Gap.
"We see this F ring as a very narrow fringe just outside the A ring, about 2,200 miles from the A ring," Dr. Larry Esposito of the University of Colorado said. "Even though we had never seen it before, this new ring is a very distinct feature."
Scientists have long known that there were four rings around Saturn, three of them bright enough to be seen from Earth with most telescopes. They have given them letter names, A through D. And they have suspected there is a fifth, or E, ring, in close to the A ring, though Pioneer saw no sign of it.
Pioneer also tracked what scientists believe to be a new moon of Saturn, lying in an orbit about 70,000 miles from the planet. While there was some suspicion it could have been the moon Janus, the object recorded by Pioneer was in a strange enough position to be an entirely new satellite. If it is a new moon, it would mean that Saturn has 11 moons and bring it within one of the 12 Jupiter has.
The spacecraft's photographs of the planet itself showed striking dark blue and green stripes across the face of Saturn just below the North Pole. The colors, which do not show up nearly as vividly from Earth, are believed to be due to a wide scattering of sunlight by the complex chemistry of the clouds in the upper regions of Saturn's atmosphere.
Photographs also show what are believed to be jet streams whirling around the planet at speeds of almost 300 miles an hour, the same phenomenon that is seen more distinctly in Jupiter's upper atmosphere. "We've seen the whorls that we see on Jupiter," Dr. Thomas Gehrels of the University of Arizona said. "The jet streaming on Saturn is not as evident as it is on Jupiter, but it's there."
Pioneer's pictures of the rings show distinct features in the outer A ring, suggesting that it is made of larger particles than many scientists believed. The second ring shows up on the photographs as the darkest of the three big rings and the third shows up as the brightest. The gap between the two outermost rings known as the Cassini Division (named for the 18th century Italian astronomer who identified it) reveals itself in the photographs to be even brighter than the brightest of the rings, leading scientists to believe that there is just enough debris inside the gap to be scattering the sunlight from below back toward the spacecraft to suggest that it itself is a ring and not a gap.
The path that Pioneer flew took it under the rings, meaning that the sun was illuminating the rings from behind. This is opposite from the way the rings are seen from Earth and gave the photographs an illusory quality that made the rings look dark instead of bright.
"It's like looking at a photographic negative," the University of Colorado's Esposito said. "But it gives us the chance to compare the spacecraft's photographs with our earthbound pictures to get a better idea of the size and dimensions of the rings."
Before flying under the rings, Pioneer encountered a sharp rise in radiation from the electrons and protons trapped near the planet by Saturn's magnetic field. As abruptly as the radiation had risen, it fell, as the spacecraft flew less than 1,000 miles from the outermost ring. By the time Pioneer was under the rings the radiation it had recorded earlier disappeared completely.
"What we saw here were the rings scouring out the radiation around the planet," Dr. John Simpson of the University of Chicago explained. The rings are big enough to sweep out just about all the particles the magnetic field traps in close to the planet.
Not only has Pioneer flown farther into the solar system than any spacecraft before, it has transmitted messages longer and from greater distances than any spacecraft launched toward the planets. When the Pioneer project began in 1967, it was not built even to fly to Saturn. The mission to Saturn was put in only after the spacecraft successfully flew by Jupiter 4 1/2 years ago, and the first plan for visiting Saturn was to fly right through the rings on a kamikaze mission.
"We've welcomed a new world into our book of knowledge," Dr. Thomas Young, deputy director of the Ames Research Center, said today.
Added the University of Arizona's Gehrels: "Today is a milestone. To understand the origins of the solar system, we must understand each body in the solar system. We have begun to add one more piece to the puzzle."