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Close Enoughto Touch Cassini Spacecraft Approaches Saturn

Posted on: Thursday, 17 June 2004, 06:00 CDT

The destination is within clear view, and a beckoning sight it is. Saturn and its creamy pastel bands of thick atmosphere shimmer in pale sunlight, and the majestic rings of dust and rock set it apart from the sun's other worlds.

Dancing about in rhythmic orbits are 31 known satellites, of which the most mysterious and inviting is the planet-size Titan. After a nearly seven-year voyage from Earth, the Cassini spacecraft is fast approaching the moment that scientists have dreamed of and planned for over the better part of their careers. The spacecraft is scheduled to swing into orbit around Saturn on the evening of June 30.

Expectations are high for the $3.3 billion American-European mission, which is planned to last at least four years, and could keep going for as much as a decade.

"The Saturn system represents an unsurpassed laboratory, where we can look for answers to many fundamental questions about the physics, chemistry and evolution of the planets and the conditions that give rise to life," Edward Weiler, associate administrator for science at NASA, said in a statement.

Scientists dare not predict the discoveries waiting to be made as the spacecraft focuses its cameras and instruments repeatedly on Saturn and its signature rings and takes the measure of the icy moons during at least 76 orbits. "Prepare to be amazed," Carolyn Porco, chief of the mission's imaging team, said in an interview last week.

Anxiety also is rising, though a compensating air of optimism seems to prevail. The spacecraft's performance over the 2.2 billion- mile flight so far, has been virtually trouble free. "There's not a single thing to point to and say, 'I'm worried about that,'" said Robert Mitchell, project manager at the Jet Propulsion Laboratory in Pasadena, California, where the mission is being directed. Still, when the time comes for the Cassini to thread the gap between two dust rings, and the main engine starts to slow the craft for capture by Saturn's strong gravity, Mitchell confessed, "I'll be in mission control with my fingers crossed."

The spacecraft has already had its first Saturn encounter, passing by Phoebe, the planet's small dark outermost moon on Friday. Scientists theorize that in Phoebe they may be getting their first close examination of an object from the outer reaches of the solar system. Everything about its appearance and motions suggests that the 137-mile-diameter Phoebe originated far beyond the outer planets and that it was flung toward Saturn, which captured it into its orbit.

Saturn itself, second to Jupiter in size, now looms so large with respect to Cassini that the planet's full girth no longer fits inside the frame of the craft's narrow-angle camera. The Cassini's two cameras are expected to take as many as 500,000 pictures in the next four years.

If all continues to go well, the spacecraft the 4,700-pound (about 2,100-kilogram) Cassini orbiter and the attached 700-pound Huygens, to be released in December to investigate the atmosphere and surface of Titan will arrive at Saturn below the plane of the spreading rings. It is to pass through the gap between the F and G rings. Recent photography, including observations by the Hubble Space Telescope, shows no sign of hazardous debris in Cassini's path, Mitchell said. Three previous spacecraft have flown through the region without harm.

Once through the passage, the 96-minute firing of the engine is to begin braking the Cassini's velocity. Should the main engine fail, there is a backup. As soon as the Cassini settles into orbit, it is to turn its cameras down for a long close look at the rings, searching for evidence of how particles of dust cluster there, then dissipate and gather again in ever transient structures. The spacecraft will also be taking some of its closest pictures of Saturn at this point, as it begins the first of its orbits among the planet's family of moons.

Porco, a planetary scientist at the Space Science Institute in Boulder, Colorado, said the initial four-year orbital tour had been plotted in detail "to give us great flexibility in observing all the important targets in the Saturn system."

In 45 years of exploration, spacecraft have visited all the planets except Pluto. They have orbited Venus, Mars and Jupiter, and landed on Venus and Mars. Three spacecraft have flown by Saturn, but this will be the first attempt to orbit the giant planet.

The mission's object of greatest curiosity is Titan. Forty-five of the 76 Cassini orbits will include Titan flybys, coming as close as 600 miles from its surface on some encounters. Its substantial atmosphere, like Earth's, is thick with nitrogen, though it has no free oxygen. "Titan is like a time machine taking us to the past to see what Earth might have been like," said Dennis Matson, the project's chief scientist. "The hazy moon may hold clues to how the primitive Earth evolved into a life-bearing planet."

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