Twenty years after the most recent spacecraft encounter with Saturn, another spacecraft is speeding toward the beautifully ringed planet. And this one will stay.
When NASA's Voyager 2 spacecraft sped past Saturn in August 1981, scientists sought answers to questions raised by two other spacecraft that had visited Saturn in the previous two years.
That was 20 years ago. No spacecraft has reached Saturn since then, but one is now on its way there.
The Cassini-Huygens mission of NASA and the European Space Agency will reach Saturn on July 1, 2004. Cassini will become the first craft to orbit Saturn, and a half year after arrival, its piggybacked Huygens probe will descend onto Saturn's moon Titan. Scientists will use them to answer questions remaining from the earlier explorations.
Dr. Ellis Miner, an astronomer at the Jet Propulsion Laboratory since 1965, has worked on missions to Saturn and six other planets. "Whenever we investigate a planet in detail," he says, "we discover answers to a lot of questions, but we inevitably raise even more new questions."
Saturn's First Close-Ups
Miner was assistant project scientist for Project Voyager as Voyager 2 approached Saturn 20 years ago. When that spacecraft was launched in 1977, no spacecraft had visited Saturn. However, while Voyager 2 was on its way toward Saturn, Pioneer 11 got the first close look at the spectacularly ringed planet, then Voyager 1 studied it in more detail. So, scientists learned much about the Saturn system by the time Voyager 2 got there. They learned Saturn had more moons, more rings, more heat and faster winds than anyone had known. They discovered that its biggest moon, Titan, hides beneath a thick, nitrogen-rich atmosphere resembling the early Earth's.
"We made several adjustments to the planned observations by Voyager 2 based on what we had seen with Voyager 1," Miner recalls. "For example, we discovered some moons with the Voyager 1 imagery, and we wanted to make sure we got some new observations of those moons to refine their orbits and see if we could learn something about their sizes and surface characteristics as we flew by."
Flight engineers programmed Voyager 2 to get a better look at one of Saturn's middle-sized inner moons, Enceladus, after Voyager 1 images showed that Enceladus had an interestingly bright surface. "We got those closer-up pictures of Enceladus showing a surface that had obviously melted in the geologically recent past," says Miner. Tidal forces on Enceladus from Saturn's gravity may be warming that moon's interior the way the tidal tug from Jupiter heats the Jovian moons Io and Europa.
The assortment of follow-up observations attempted with Voyager 2 probably contributed to a temporary jam in movement of the platform on which the spacecraft's camera is mounted. "We found so many things to do with Voyager 2 after the Voyager 1 flyby, that may have led to the scan-platform problem, because we had the thing swinging back and forth in the sky so fast and so furiously that we think that's what drove the lubricant out of the gears and caused the gear train to seize," Miner says.
The problem prevented picture-taking temporarily after the spacecraft's closest approach to Saturn. A few days later, engineers sent up commands to try a low-speed turn for a view back toward Saturn that had not been possible with Voyager 1 because of its trajectory. Scientists anxiously watched monitors in the operations center at JPL to see if the maneuver would work.
"It was a few seconds later than we anticipated when something finally showed up on the screen, but at first none of us could make out what it was," Miner says. "Then it dawned on me that looking back in that direction, Saturn was upside down in the monitor because of the direction things were pointed, so I ran up to the screen and said, 'Look! Here's the ring, and here's Saturn, and here's a shadow.'"
The look-back pictures showed the rings as seen from the unlit side, with sunlight coming through them, revealing additional information about the particles making up the rings. The scan platform worked for taking pictures of the outer moon Phoebe as Voyager 2 headed out of the Saturn system, and for the spacecraft's later flybys of Uranus and Neptune.
During a press conference at the end of Voyager 2's Saturn flyby, a reporter asked JPL's Dr. Ed Stone, project scientist for the mission, what percentage of the expected science had been obtained during the flyby. "Ed's answer was a classic," Miner recalls, "He said, 'Two hundred percent.' And it's true. We got a lot more than we anticipated despite that seizure of the scan platform. I still look back on it, though, and wish we had been able to get those close-up pictures of Tethys [the next moon out from Enceladus] and the other things we had planned in that time period when the platform was frozen."
The Next Explorer: Cassini
Miner began working on the Cassini-Huygens mission to Saturn in 1990, shortly after Voyager 2 completed its Neptune flyby. This mission has three main differences from the earlier Saturn explorers that will enable it to answer many questions they left open, he explains: "One, we have a probe so we can study Titan in place. Two, it's an orbital mission so we can watch things happen over a long period of time. And three, the complement of instruments on Cassini is both more extensive and far more sophisticated than the one we had on Voyager."
A few of the questions for Cassini and Huygens are: Does Titan have lakes of liquid hydrocarbons? What happens in its atmosphere? Do Saturn's winds and rings change much in the span of a few years? How do Saturn's moons and magnetic field affect the rings? Does Enceladus have active ice volcanoes?
Cassini was launched four years ago. It passed Jupiter last December. Twenty years after Saturn's most recent visit from Earth, the next emissary is on the last leg of its journey and has orders to stick around once it arrives.