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."http://www.wired.com/news/space/0,2697,64055,00.html?tw=wn_tophead_2