Cassini Joins Rings and Moons of Saturn

NASA’s Cassini spacecraft successfully fired its rocket motor Wednesday evening, becoming the first spacecraft to orbit the ringed planet Saturn. Whoops of joy erupted and engineers hugged one another in the control room in Pasadena’s Jet Propulsion Laboratory as the craft beamed back word that it had successfully completed the perilous maneuver.

“We have burn complete,” mission communicator Todd Barber said to cheers and high fives. “Welcome to Saturn.”

For a tense 96 minutes, engineers had listened to a warble of a tiny radio signal from Cassini indicating that its rocket was firing and slowing the craft, which had taken seven years to complete its 2.2-billion-mile journey to Saturn.

The signal finally stabilized, precisely on schedule, at 9:12 p.m., a sign that the rocket had fired for its full allotted time, slowing the craft just enough for it to be captured by the gravity of the massive planet.

Cheers and handshakes erupted again minutes later, at 9:30 p.m., when Cassini pointed its large antenna toward Earth and sent back a “blast of data” indicating that all was well, an official said. The craft then turned its antenna away once more as it snapped a hurried series of photographs of Saturn’s rings during its closest approach to them.

“We got it,” a voice crackled over the control room’s loud speakers.

“There is a 32nd moon gracing the Saturnian skies tonight,” Barber added.

The arrival of the craft at Saturn represents the end of an era. Cassini’s $3.3-billion cost, massive size and payload of 18 scientific instruments set it apart from recent interplanetary missions launched under the “faster, better, cheaper” slogan.

With the new emphasis at NASA on a return to the moon and a manned mission to Mars, it seems unlikely that there will be another mission to Saturn in the next few decades, and almost certain that none will be so complex.

At least 1,500 scientists and engineers at JPL worked full or part time during the early 1990s to bring Cassini to Wednesday’s successful rendezvous.

Wednesday had been a day of anxiety and anticipation as the craft neared the largely gaseous planet. Engineers had sent their last commands to Cassini on Tuesday and could only “chew their nails” as the craft got closer to the “hair-graying” maneuvers of entry, program manager Robert Mitchell said.

The craft had to operate on its own because, with Saturn 900 million miles from Earth, radio signals take 83 minutes to reach it.

At 7:11 p.m. Wednesday, the craft hurtled through the gap between Saturn’s F and G rings at a speed of 49,000 mph. Minutes later, the craft began its 96-minute burn.

During the burn, the school-bus-sized Cassini consumed nearly 2,000 pounds of fuel, about a third of the total amount on board. The rest is reserved for changing the craft’s orbit during the four-year mission.

The rocket firing knocked about 1,400 mph off the craft’s speed, but because Cassini was falling into Saturn’s gravity well, the probe speeded up during the burn, to 68,000 mph, its maximum speed since the craft was launched Oct. 15, 1997.

Since then, the craft has traveled billions of miles, swinging by Venus, Earth and Jupiter to get gravitational slingshot boosts propelling it on its way to Saturn.

After the rocket burn, Cassini passed once more through the gap between the F and G rings -- toward the outer edge of Saturn’s ring system -- taking extensive pictures before and after the passage. It could not take pictures of Saturn during its close approach -- about 12,400 miles above the planet’s cloud tops -- because the orientation required to slow the craft pointed the cameras in the wrong direction.

The spacecraft made extensive measurements of the planet’s magnetic field, however, in an effort to understand its composition.

The JPL team expected to begin receiving the first pictures of the rings sometime between 5 and 6 a.m. today. Until then, they wouldn’t know what quality of pictures to expect because the cameras would not be able to resolve individual particles of rock and ice in the rings. The pictures “could be spectacular, and they could be blurry and hazy,” Mitchell said. “The latter is perhaps the more likely outcome.”

Cassini is scheduled to spend four years orbiting Saturn, making at least 76 orbits, visiting its largest moon, Titan, 42 times and flying by several of the planet’s other 30 known moons. The craft could continue orbiting the planet for as long as 15 years.

On Christmas Eve, Cassini will release the Huygens probe, which will plunge to the surface of Titan three weeks later. Scientists are particularly excited about the probe because Titan is the only moon in the solar system known to possess a dense atmosphere.

The project is co-funded by NASA, the European Space Agency and the Italian Space Agency.