Cassini enters Saturn orbit
By Michael Coren and Stefano Coledan
CNN
 |  This image of Saturn was captured by Cassini as it approached the planet. |
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|  VIDEO |
 The Cassini mission to enter Saturn's orbit is a success.
 The Cassini spacecraft's rendezvous with Saturn has been decades in the making.
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| CASSINI-HUYGENS MISSION |  Saturn and its moon Titan
SATURN: Planet second in size to Jupiter with a diameter of 74,898 miles (120,511 km). Seven rings of ice and rock particles with 31 known moons. Visited by Pioneer 11 (1979), Voyager 1 (1980), Voyager 2 (1981).
TITAN: Largest Saturnian moon. May harbor organic compounds similar to those predating life on Earth. Temperature is minus 290 degrees Fahrenheit.
CASSINI ORBITER: Launched Oct. 15, 1997. Spacecraft is 22 feet long and weighs 12,593 pounds (5,667 kg). Runs on nuclear power. Will orbit Saturn 76 times over four years.
HUYGENS PROBE: Spacecraft is 8.9 feet in diameter and 705 pounds (317 kg). Will be released from Cassini on Dec. 24 and enter Titan's atmosphere on Jan. 14.
MISSION COST: $3.3 billion, shared by NASA, ESA, Italian Space Agency. |
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(CNN) -- After a seven-year journey, NASA's Cassini probe has become the first spacecraft ever to orbit the giant ringed planet Saturn.
Cassini entered orbit shortly after midnight Wednesday ET (0400 GMT Thursday) after threading a gap between two of Saturn's rings.
The maneuver brought Cassini within 12,500 miles (20,112 kilometers) of Saturn's cloud tops.
"It was kind of a nail biter throughout," said Cassini program manager Robert Mitchell. "What you saw here was the result of a lot of work on the part of a lot of people, and it all paid off just perfect."
But the first images of Saturn's rings may not be something to write home about, Mitchell warned.
"We're going to have pictures of the rings. What will they look like? Well, we really don't know," he said. "For somebody wanting to look at the 'gee-whiz' kind of picture, we might not have that."
Earlier, there were cheers at Mission Control when the craft began a 95-minute engine burn to slow it down and place it into orbit.
The craft risked shooting past Saturn if the burn failed.
Cassini has spent the last seven years making a 2.2 billion-mile journey to study Saturn's atmosphere, magnetic field and weather patterns -- including its lightning and stormy ammonia clouds.
"It's going to be the star of the show for the next four years," Mitchell said.
Saturn, the second-largest planet in our solar system, has intrigued scientists for centuries. Its unique atmosphere and the wild chemistry on its many moons could shed light on how the planets -- and possibly life -- formed in our solar system.
The $3.3 billion craft will train its 12 scientific instruments on the planet and moons over the next four years. In December, it will deploy a probe, called Huygens, to land on Saturn's largest moon, Titan. That moon may hold chemical clues to how life began on Earth.
The craft also will send back the closest images ever taken of the rings -- consisting of ice, rock and dust -- orbiting the gas-giant.
But first, Cassini needed to navigate a hazardous passage through those rings and insert itself into orbit, at times passing just 12,000 miles or so above the clouds of Saturn.
Through the rings
To ward off dust-size particles as it passed through the rings, Cassini's main antenna acted as a shield during the 98,500-mile (158,000-km) transit.
Afterward, Cassini fired its main engine for 95 minutes -- a minute less than planned -- to slow down to slide into orbit around the planet.
During the planned four-year study of Saturn and its colorful entourage of 31 moons, Cassini will never pass again as close to the planet or its rings as it did early Thursday.
On December 24, it will deploy Huygens, a saucer-shaped probe now attached to the spacecraft. That 705-pound probe will plunge through the atmosphere of Titan, one of Saturn's moons, and reach the surface before shutting down.
Huygens will peer beneath layers of smog that shroud the moon with an atmosphere surprisingly similar to Earth's -- 4 billion years ago. Titan's nitrogen-rich atmosphere, possible seas of liquid ethane and tar-like permafrost are thought to resemble conditions on early Earth.
Scientists hope Titan will have the same organic compounds that led to life on primordial Earth and will shed new light on how the planets and life evolved.
"We may find some highly complex molecules that may have been on Earth long before there was life here," said Jean-Pierre Lebreton, project scientist for the European Space Agency's Huygens probe. "Going to Titan now is like going to Earth and going back in time."
Reaching the planets
The Cassini mission is one of the most advanced and ambitious ever attempted. The interplanetary spacecraft was designed to be the most "sophisticated and reliable ... spacecraft ever built for exploration of the planets," NASA said.
At 22 feet long and 13 feet wide, weighing nearly 12,600 pounds loaded with fuel and the Huygens probe, it is also one of the largest. Its propellant load alone has more mass than both Voyager I and II combined, two craft that preceded Cassini's visit to Saturn about 20 years ago.
The electronic brain of Cassini, a sophisticated computer directing its movements, is now a standard feature on spacecraft. It uses a relatively new family of integrated circuits -- the first civilian application of this technology -- that is 10 times more efficient than previous spacecraft at just a fraction of the size.
These computer chips, along with the radio transponders from Cassini, were pulled from the spacecraft's production line for use on NASA's Mars Pathfinder and the Near Earth Asteroid Rendezvous, or NEAR, missions
First steps
Cassini's journey to Saturn began at a meeting between European and American scientists in 1982. The Saturn orbiter and Titan probe were conceived as a mission that would benefit the scientific and, technological sectors of the scientists' respective countries.
 An artist's deptiction of the surface of Titan as the Huygens probe makes its descent. | |
Seven years later, the U.S. Congress approved funding for the Cassini mission and Cassini-Huygens was launched in 1997.
However, the original design process -- aimed at producing multiple spacecraft with standardized components -- was dramatically scaled back. Engineers eliminated most of Cassini's moving parts in favor of simplicity.
The designers replaced tape recorders with solid-state recorders, installed gyroscopes without spinning components and dropped an articulated antenna arm for one bolted to the spacecraft.
This reliance on nonmoving components meant Cassini must rotate its entire body in order to communicate with Earth or collect data by aiming its instruments. These acrobatics are accomplished with two main engines (one as a backup) and 16 smaller thrusters controlling the craft's orientation. But it also lessens the risk of problems on the spacecraft.
NASA also designed Cassini to be independent because the time lag of one hour and 24 minutes -- the time for light to travel 930 million miles from Saturn to Earth -- means controllers on Earth cannot immediately react to unexpected problems.
Instead, Cassini operates on its own.
"The spacecraft has been programmed to continue even in the event of an emergency," said Mitchell in a statement. "We don't want Cassini to call home if a problem arises, we want it to keep going."
|  Scientists hold a news conference -- with a model of the Cassini probe in the foreground -- after Saturn orbit was achieved early Thursday. |
That forced engineers to give Cassini an "inner ear," or computer software, that recognizes the spacecraft's orientation at all times. It gauges its position in space, as well as its location relative to the Earth, sun, Saturn and stars.
To stay warm, Cassini also sports nuclear-powered heaters for cold interplanetary nights. More than two dozen layers of insulation wrap the spacecraft's sensitive innards from frigid space and meteorites.
The spacecraft also carries a DVD record of 616,400 handwritten signatures from 81 countries around the globe, including the mission's namesakes, Jean-Dominique Cassini and Christiaan Huygens, lifted from 17th-century letters.
On Cassini's four-year mission, it will make 52 close encounters with a handful of Saturn's 31 moons and orbit Saturn 76 times.
