U. of Colorado instruments approach Saturn aboard Cassini spacecraft

06/22/04

NASA's Cassini-Huygens spacecraft carrying a $12.5 million University of Colorado at Boulder instrument package is expected to enter Saturn's orbit June 30, beginning a four-year mission to probe the planet, its fabulous ring system and bizarre moons.

Launched Oct. 15, 1997 from Cape Canaveral, Fla., the NASA spacecraft has traveled more than 2 billion miles during a roundabout, 6.7-year journey to the ringed planet. The most ambitious planetary mission ever, the $3 billion international project is managed by the Jet Propulsion Laboratory in Pasadena, Calif.

The Cassini orbiter is equipped with 12 scientific instruments and the deployable Huygens probe, which is carrying six instruments that will parachute into the thick atmosphere of Titan, Saturn's largest and most intriguing moon.

"I'm looking forward to this encounter, and I am more eager than anxious," said Professor Larry Esposito of the Laboratory for Atmospheric and Space Physics, team leader for the Ultraviolet Imaging Spectrograph, or UVIS. "I don't have the time or energy to worry about things that are out of my control."

Although the spacecraft has been taking data on Saturn since late in 2003, the first major order of business is to successfully enter Saturn's orbit June 30, he said. Cassini is expected to slow enough to be captured into Saturn's orbit, passing through a gap between the F-Ring -- which Esposito discovered in 1979 while analyzing data from Pioneer 11 as it passed by Saturn -- and the G-Ring.

The UVIS instrument package has a set of telescopes to measure UV light reflected by or emitted from Saturn's atmosphere, its rings and its moon atmospheres and surfaces, he said. The data collected can determine their compositions, distribution, aerosol content and temperatures.

"Saturn is surrounded by a huge cloud of hydrogen gas that is leaking out of the top of its atmosphere," he said. "Our team hopes to learn more about the energetic processes that create the cloud."

UVIS also includes a high-speed photometer to determine the radial structure and dynamics of the ring system by measuring fluctuations of distant starlight passing through the rings and ring gaps, known as stellar occultation. "We will conduct 60 star occultations in October at up to 10 times the highest resolution and 50 times the sensitivity we measured during the Voyager 2 flyby," Esposito said.

The rings are "incredibly active, exhibiting waves, wakes, ripples, bends and kinks" that seem to wax and wane over time, he said. The rings' waves, which appear to be tied to gravitational tugs from Saturn's inner satellites, can spiral somewhat like the grooves in a phonograph record or ripple like waves on a pond.

The F-Ring discovered by Esposito is a braided and kinked object located just outside the main rings visible from Earth. The exotic ring appears to be held in place by two "shepherd" satellites, Pandora and Prometheus, Esposito said.

"Saturn's rings have a violent history and appear to have been created by the break-up of small moons in the recent past," he said. "We are fairly certain the ring particles consist of ice and rock ranging in size from sugar granules to houses."

The researchers hope to resolve how gravity, magnetic energy and other forces hold the enigmatic rings together. The main ring system -- which Esposito said is so large it would barely fit in the space between Earth and its moon -- should provide "a local lab of sorts to help us understand similar phenomenon in our larger astrophysical system like accretion discs around black holes."

The UVIS science and engineering team also includes CU-Boulder co-investigators George Lawrence, Bill McClintock, Charles Barth, Ian Stewart and Josh Colwell. It also includes Justin Maki, a 1996 CU-Boulder graduate now at JPL.

The spacecraft will spend four years orbiting Saturn, including 69 close encounters of seven of the planet's 31 known moons. The primary focus of the moon encounters will be Titan, which will be the recipient of 45 flybys and where the spacecraft will release the Huygens probe to its surface on Christmas Eve.

The probe will freefall for 20 days, then parachute into Titan's atmosphere and commence 2.5 hours of intensive data collection, transmitting information back to the Cassini orbiter. The data then will be relayed to Earth ground stations.

In addition to taking "chemical fingerprints" of Saturn's gases and measuring their temperatures and compositions, LASP's UVIS team is particularly interested in the atmosphere of Titan. Titan is the only solar system body other than Earth that is rich in nitrogen, he said.

"When Voyager flew by Titan in 1980, some thought there might be a chance to detect evidence of life there," said Esposito, also a professor in CU's astrophysical and planetary sciences department. Even though Titan's temperature was later measured at a frigid minus 290 F -- dashing most hopes for life -- many scientists believe it may resemble a primordial Earth.

Titan's surface may contain lakes of liquid methane and ethane, and organic molecules may constantly be raining down from the moon's thick clouds onto its surface.

"We plan to measure the composition of Titan's clouds in order to understand their chemistry and how they cool or warm the moon," said Esposito.

"Titan is almost like a little world," he said. "We should be able to look back in time and see what types of pre-biological chemistry are occurring."

LASP has been building ultraviolet spectrometers for NASA since the Mars missions in the mid-1960s, said Esposito. "The main thing we have learned from our planetary exploration experience is that there will always be surprises and serendipitous discoveries."

CU's UVIS package also contains a hydrogen-deuterium absorption cell designed and built with the participation of the Max Plank Institute of Lindau, Germany. Since all the universe's deuterium -- a heavy form of hydrogen -- is thought to have formed during the big bang, the ratio of the two elements should shed light on the solar system's earliest history.

Cassini was launched on a Titan IV/Centaur rocket built by Lockheed Martin engineers in Colorado, who also built the spacecraft's main engine, 16 thrusters, and assembled a camera that will ride on the Huygens probe.

The Cassini effort is a very intensive, four-year mission," said Esposito. "We can investigate, ask new questions and reprogram our spacecraft orbits to answer them. The magnitude of discoveries on this mission will be tremendous."

Source: Eurekalert & others

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