CU astronomers jubilant as Cassini returns images of Saturn's rings

This is a narrow-angle camera image of the outer part of Saturn's A ring taken after the successful completion of the Cassini orbit insertion burn. At this time, the spacecraft viewed the rings from the north side. The sun illuminates the rings from the south side. NASA/JPL/Space Science Institute

By David Brand

PASADENA, Calif. -- Just hours after Cassini-Hugyens rocketed into orbit around Saturn at 7:36 PDT (10:36 EDT) on June 30, the spacecraft sent back 61 images of the giant planet's rings that Cornell researchers acclaimed as spectacular and surprising. "The images are mind-boggling, just mind-boggling. I'm surprised at how surprised I am at the beauty and clarity of the images," said Carolyn Porco, the imaging team's principal investigator, speaking at a press briefing at the Jet Propulsion Laboratory (JPL) July 1.

Said Porco, who is with the Space Science Institute in Boulder, Colo., describing one picture of the Encke Gap, a small gap in Saturn's A ring, "I'm still not convinced it's real. It's so sharp."

Cornell astronomers Joseph Burns and Peter Thomas, two of her team members who are helping with the analysis of the images, which show the closest views ever obtained of Saturn's rings, said they were looking at "spectacular" structures never seen before.

"Some images, as Carolyn said, are textbook examples. But textbooks are just simulations, and we didn't actually know they were true," said Burns. "But after looking at 61 pictures, the structures around the rings are very surprising. We certainly didn't expect to see anything like that."

As one example, Burns cited periodic bands threading through the rings and "kinks" visible in the main ring. He also noted the clear interactions between some of Saturn's 31 known moons and the rings.

"Considering the lighting conditions and that the spacecraft camera was snapping these off with the shortest possible exposure, they came out very well," said Thomas.

Thomas said straw, or mottled "wisps," seen in one ring image were such a surprise "that they literally ground us to a halt." The mottling, he said, "was something we didn't even have a prediction for."

Three Cornell researchers who helped return the Cassini spacecraft's spectacular images of Saturn's rings to Earth celebrate their achievement July 1 at the Jet Propulsion Laboratory in Pasadena, Calif. Holding a model of the spacecraft, which went into orbit around the giant planet June 30, are, from left, Joseph Burns, the Irving Porter Church Professor of Engineering, Theoretical and Applied Mechanics and professor of astronomy, astronomy senior research associate Peter Thomas, and astronomy postdoctoral researcher Matthew Tiscareno. David Brand/Cornell News Service

After sending back images of Saturn's moons Mimas, Tethys, Rhea and Iapetus, Cassini disappeared from view as it began orbiting Saturn on the opposite side of the sun from Earth. It remained out of communication until July 12, when it reappeared from behind the sun.

It had taken the spacecraft seven years to travel the 2.2 billion miles from Earth to Saturn, and so distant is the planet, the second largest in the solar system, that it took 84 minutes for signals to reach Earth confirming that the spacecraft had reached its target. The spacecraft will spend four years making an extensive tour of the giant planet's rings and moons. It is in an orbit that will take it 116.3 days to make one journey around the planet.

Other Cornell researchers on the imaging team, besides Burns, the Irving Porter Church Professor of Engineering, Theoretical and Applied Mechanics as well as professor of astronomy and vice provost for physical sciences and engineering, and Thomas, a senior researcher in astronomy, are Joseph Veverka, professor and chair of the Department of Astronomy, and astronomy professor Steven Squyres. The team will analyze about 300,000 images of Saturn and its environs over four years. Also on the mission are Cornell astronomy professors Peter Gierasch and Philip Nicholson.

The narrow-angle camera on Cassini took the images soon after the main engine burn that put the spacecraft into orbit. The spacecraft was hurtling at 15 kilometers per second (about 34,000 miles per hour), so only pieces of the rings were targeted. The images were taken by Cassini as it flew about 10 times closer to the rings than it will at any point during the mission, passing about 20,000 kilometers (12,427 miles) above Saturn's cloud tops.

Before going into orbit, Cassini flew through debris gaps in two of Saturn's outer rings, a decision that was made on May 20 after multiple observations by Burns and his colleague at NASA-Ames, Jeff Cuzzi, who was an undergraduate at Cornell.

Well before Cassini began its approach to Saturn, Burns and Cuzzi were studying images of the area taken by the Voyager spacecraft (Voyagers 1 and 2 flew past Saturn nine months apart in 1980 and 1981), as well as ground-based observations and images taken by the Cassini camera every other day.

"We were trying to see whether or not anything might have changed in the region to make sure it is still the safest place to go," said Burns.

Burns' work on estimating the hazards of the rings did not stop on May 20. "Much of the work I do on the mission is a question of hazards, and you want that information early in the mission," he said. "How big are the particles in the rings, what is their size distribution and orbits?"

Burns and Cuzzi have been allocated time in the fall to get their new observations and to develop a model of where the debris in the rings is located.

One early result from the spacecraft that is intriguing scientists concerns Saturn's Cassini Division, the large gap between the A and B rings. While Saturn's rings are almost exclusively composed of water ice, new findings show the Cassini Division contains relatively more "dirt" than ice. Further, the particles between the rings seem remarkably similar to the dark material that scientists saw on Saturn's moon Phoebe. These dark particles refuel the theory that the rings might be the remnants of a moon. The F ring also was found to contain more dirt.

Another instrument on Cassini has detected large quantities of oxygen at the edge of the rings. Scientists are still trying to understand these results, but they think the oxygen may be left over from a collision that occurred as recently as January of this year.

"In just two days, our ideas about the rings have been expanded tremendously," said Linda Spilker of JPL, deputy project scientist for the mission.

Cassini, said Burns, will transform our knowledge of Saturn, with the almost certain discovery of new rings and new moons beyond the 31 already known. Voyager, he said, "essentially took a couple of beautiful photos of a glorious dance as it went by. We are going to have the motion picture."