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Cassini Nears Strange Saturn Moon

10:24 AM Jun. 11, 2004 PT

Scientists at the Jet Propulsion Laboratory may get some clues to a mystery Friday when NASA's Cassini space probe comes within 1,250 miles of Phoebe, one of Saturn's most mysterious moons.

Phoebe has boggled astronomers for over 100 years because of its dark surface and retrograde orbit, which sends it spinning around Saturn in the opposite direction from the planet's 30 other known moons. Many scientists now believe the 137-mile-wide moon may be a captured asteroid that has remained unchanged since it was first formed in the outer solar system. If Friday's flyby proves that theory to be true, data gathered from Cassini's instruments could shed new light on how planets are formed.

Today's the Day. "The key thing here is that Phoebe is such a unique object in the Saturn system (that) we wanted to make sure we got a chance to observe it," said Amanda Hendrix, a planetary scientist with the JPL in Pasadena, California. "We're basically getting to sample an object that came from farther out in the solar system, and we're getting to do that for free."

Of course, the Cassini mission itself is far from free -- the orbiter and its attached Huygens probe will cost NASA, the European Space Agency and the Italian Space Agency over $3.26 billion by the time the mission is over. But the timing and trajectory of Cassini's main mission to Saturn puts it on a path that swings by Phoebe, allowing researchers to study the quirky moon as a bonus.

Cassini will spend approximately 30 hours photographing Phoebe up close and taking measurements with other instruments. The climax of the event will come at 1:56 p.m. PDT on Friday, when the orbiter will pass within 1,250 miles of the moon.

In contrast, the Voyager 2 spacecraft that photographed Phoebe in 1981 only came within 1.4 million miles of Phoebe. Though the images taken during that mission helped scientists get a better understanding of Phoebe, the photos this time around will be 1,000 times more detailed and are expected to reveal features of the moon that have never before been seen.

The photos will help JPL scientists get "a really good idea of the size and exact volume of Phoebe," said Hendrix. "This can be coupled with tracking data to know the mass better than is known now, and this will tell us about density and internal structure -- whether it's made up of ice, or rock and ice."

All this information will be compared with what is known about asteroids in the Kuiper Belt, an area of icy debris 7.5 billion to 9.3 billion miles from the sun. If the data match, scientists will be able to say with a good deal of certainty that Phoebe is indeed a "Centaur" -- an asteroid that has migrated from the Kuiper Belt into the inner solar system.

The data from the Phoebe flyby will also be compared with data collected from a later flyby of Iapetus, one of Saturn's larger moons. Scientists hope the comparison will reveal whether the dark side of Iapetus is composed of dust from Phoebe.

Cassini is scheduled to rendezvous with Saturn on June 30, where it will become the first spacecraft ever to orbit the planet. The orbiter will spend the next four years studying Saturn and its moons.

NASA officials are crossing their fingers in hopes of some big surprises along the way: most notably, the discovery of more satellites, or small moons, orbiting Saturn. "Cassini will almost certainly discover new moons around Saturn," said Hendrix, noting that the prior Galileo space probe found more around Jupiter.

Another big moon event will take place on Dec. 24, when Cassini releases the European Space Agency's Huygens probe above Titan, Saturn's largest moon. The probe will drift through Titan's atmosphere, taking as many measurements as possible before crashing into Titan's surface. Titan is especially intriguing to astronomers and astrophysicists because it contains its own atmosphere, which essentially makes it a planet that has been caught by Saturn's gravitational pull.

NASA officials are still unclear how the Cassini mission will end once its scheduled observations are completed. If the spacecraft's instruments and power systems show little sign of wear in 2008, the space agency could decide to extend the mission, just as they did with the Mars Exploration Rovers. Or, if time and the stresses of space have taken their toll, Cassini could get sucked in by Saturn's gravitational pull and burn up in its atmosphere, much as the Galileo spacecraft did in Jupiter's atmosphere.

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Cassini Slips Into Saturn Orbit

PASADENA, California -- Like thread through a buttonhole, the Cassini space probe successfully slipped through a gap in the rings surrounding Saturn -- not once, but twice -- on Wednesday night, kicking off a four-year mission to study the second largest planet in our solar system.

Along the way, the bus-sized spacecraft fired one of its two engines for nearly 96 minutes, made more than a dozen pre-programmed flips, and managed to avoid hitting any pebble-sized particles that might lie between Saturn's rings. And it did it all at speeds reaching 69,000 miles per hour.

The maneuver, known to mission controllers at the Jet Propulsion Laboratory as Saturn orbit insertion, or SOI, slowed Cassini down just enough to be snared by Saturn's gravitational pull. Without the move, Cassini would have soared past Saturn into the depths of the outer solar system, losing its chance to enter into orbit around the planet and effectively canceling the $3.3 billion mission, which is being jointly managed by NASA, the European Space Agency and the Italian Space Agency.

Robert Mitchell, NASA's Cassini program manager, said he was more impatient than worried in the moments before the spacecraft signaled that it had begun the maneuver as planned. "I was looking at my watch a lot, wondering, 'How much longer?'" he said. "The most significant moment was when we first got the signal back from Cassini that said everything was all right. That was really where there was a moment of concern."

The faint radio signal, which JPL mission controllers would use over the next few hours to monitor Cassini's status, began to trickle back to Earth from Cassini at 7:36 p.m. PDT, after an expected delay of approximately one-and-a-half hours -- the time it takes radio signals to travel from Saturn to Earth.

Officials in the mission control room applauded hesitantly after first detecting the signal and listened for further news. "We dodged the first bullet," announced Cassini development manager Chris Jones.

But the crowd in the control room abandoned its reservation just minutes later, letting out cheers and trading high-fives as a translation of the radio signal indicated that Cassini's engine had begun its planned 96-minute long burn. The brief celebration was followed by at least a half dozen bursts of cheering and applause over the next hour and a half as the incoming signal revealed that Cassini was meeting each of its goals for the SOI as planned.

Still, the most exciting moment came at 9:12 p.m. when mission controllers announced that Cassini had completed its engine burn, marking the first time a spacecraft had ever entered into orbit around Saturn.

Project planners and most everyone else in the control room jumped out of their chairs, many of them hugging and shaking hands. "It really confirmed today that Cassini is a great instrument," said Jean-Pierre Lebreton, project scientist for the European Space Agency's Huygens probe that is piggybacking on Cassini.

Though the SOI marked the end of a six-and-a-half year journey through 2.2 billion miles of space for Cassini , it was just the beginning of its main mission to study Saturn, its rings and its moons.

Only minutes after completing the engine burn, the spacecraft reoriented itself to begin taking measurements of Saturn's magnetic field and photos of its rings. It was the ideal time to gather this data, since the maneuver placed Cassini within 12,400 miles of Saturn's cloud tops -- the closest it would ever come to the planet or its rings in the entire mission.

By midnight, however, the spacecraft had sped back through Saturn's rings and had signaled back to the control room on Earth that it was ready for its next task.

The probe will now use its 12 instruments to learn as much as possible about the Saturnian system over the next four years. During that time, it will complete 74 orbits around Saturn, 44 fly-bys of the mysterious moon Titan and 13 fly-bys of other moons.

Saturn has 31 known moons -- most of them vastly different from one another -- and scientists say they hope to discover even more by the time the mission is over. They also hope to learn what many of those moons are made of and whether they contain elements that are considered to be the building blocks of our solar system and of life itself.

To that effect, Cassini has already proved that it is living up to its expectations. On June 11, the spacecraft came within 1,250 miles of Phoebe, a dark, icy moon that orbits Saturn in the opposite direction of all its other known moons. That fly-by revealed that Phoebe harbors pockets of frozen carbon dioxide, a feature that makes it highly likely that Phoebe originated in the Kuiper belt, a region of debris left over from the birth of the solar system.

On Friday, Cassini will make its first pass by Titan, the only moon in our solar system to have its own atmosphere. The dense smog circling the moon is thought to hide vast ethane lakes, raging lightning storms and a large object thought to be a tall continent or a deep crater. The conditions mirror those on Earth billions of years ago, according to planetary scientists.

"It's really a planet in its own right," said Cassini project scientist Dennis Matson, suggesting that Titan may once have orbited the sun before being captured by Saturn's gravitational pull.

Cassini will study Titan again and again in the coming years. However, the climax of these fly-bys will come on Christmas Eve and in the weeks immediately after. That's when Cassini releases the Huygens probe, allowing it to drift down into Titan's atmosphere, where it will take as many measurements as possible before crashing into the moon's surface or splashing into an ocean of ethane.

"I just hope -- maybe dream -- that we'll see oceans on Titan," said Breton. "It would be very exciting to land in an ocean." But, he conceded, the actual composition of Titan's surface is anybody's guess at this point.

What Cassini measures and photographs from above and what Huygens gathers on its way down is sure to be considered a discovery.

"I don't know what we're going to see," said Breton. "We'll have to wait six months to tell you."

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