CU's Burns looks at gaps in Saturn's rings for Cassini

Burns

Thomas

By David Brand

PASADENA, Calif. -- As the Cassini-Hugyens spacecraft was to arrive at Saturn at 10:36 last night (June 30), among the most anxious participants was Joseph Burns, Cornell's vice provost for physical sciences and engineering. As a member of the Cassini imaging team, he has been jointly responsible for observing the gaps in two of the planet's outer rings that the spacecraft had to slip through to become the first mission to orbit Saturn.

The decision to go through these gaps in the ring debris was made on May 20 after multiple observations by Burns and his colleague at NASA-Ames, Jeff Cuzzi, who was an undergraduate at Cornell. "I am looking at the place where the spacecraft is going to make sure there is nothing there," said Burns, who is the Irving Porter Church Professor of Engineering, Theoretical and Applied Mechanics as well as professor of astronomy.

The fear is that something might have changed since May 20 in one of the two entry rings, known as the "G" ring. "We ask ourselves the question: Are we basing our orbit insertion strategy on old data?" he said.

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 at 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.

The $3 billion-plus mission is the result of an international cooperation between NASA and 17 European nations. After a nearly seven-year journey, the spacecraft will make 76 orbits of the giant planet over four years, touring Saturn's rings, icy moons, magnetosphere and sending a probe onto Titan, the planet's largest moon.

This graphic illustrates that despite Saturn moon Phoebe's bumpy, irregular topography, it has a fairly round shape. A digitally rendered shape model of Phoebe was constructed using Cassini imaging data obtained before and after the spacecraft's close flyby of the Saturnian moon June 11, 2004. The average diameter of Phoebe is about 133 miles. The four views of the model are each separated by a 90 degree rotation; the upper left is centered at 0 degrees west longitude. The others show regions of the moon centered at 90, 180 and 270 degrees west longitude, as labeled. NASA/JPL/Space Science Institute

The mission, 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."

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. 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.

Other Cornellians on the Cassini imaging team are Joseph Veverka, professor and chair of the Department of Astronomy; Steven Squyres, professor of astronomy; and Peter Thomas, senior researcher in astronomy. Two other Cornell professors of astronomy on the mission are Peter Gierasch, on the composite infrared spectrometer team, and Philip Nicholson, on the visual infrared mapping spectrometer team.

Thomas appeared at a Jet Propulsion Laboratory press briefing on June 23 to describe the great geological variety found on Saturn's outer moon, Phoebe, which Cassini flew by on June 11. The 137-mile-wide Phoebe, mission scientists conclude, is likely a primordial mixture of ice, rock and carbon-containing compounds similar to material seen on Pluto and Neptune's moon Triton.

Thomas, a specialist in small satellites and small-satellite encounters, described images of "materials streaming down" in large landslides on the surface of Phoebe, revealing a number of surface materials, including individual boulders. "The largest boulder here, casting a big shadow, is about 300 meters across -- that would squash the Rose Bowl and most of the surrounding parking blocks pretty well," he commented.

One thing Phoebe is not is a captured asteroid, a long-held theory. "Phoebe is definitely not an asteroid," said JPL scientist Bonnie Buratti, a 1983 Cornell astronomy graduate. Instead, said JPL mission scientist Amanda Hendrix, "Phoebe probably is a captured object. Water ice is a telltale sign that Phoebe originated somewhere in the solar system beyond the asteroid belt [between Mars and Jupiter] and was formed somewhere in the outer solar system."