Secrets Of The Rings


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For a spacecraft that had spent nearly seven years in flight and journeyed more than 2.2 billion miles to get where it was going, the Cassini-Huygens Saturn probe did a funny thing when it closed in on the ringed planet last week: it hid.

Traveling at a breakneck 54,000 m.p.h.--four times its cruising speed--the ship was no longer flying toward the planet but falling toward it, on a high-speed trajectory that could send it skimming past Saturn and back out into space. If the ship was going to enter a stable orbit, it would have to fire its little braking rocket for 96 min., until it reached the right speed and position to dart upward through a gap in Saturn's rings and begin circling the giant world. But when it comes to the dense rivers of ice and rubble that form the planet's rings, the word gap is an imprecise term. Even a seemingly clear opening can be swarming with dust and particles. A collision with a bit of cosmic buckshot no bigger than a marble could destroy the ship. Making matters worse, the 930 million miles separating Saturn and Earth mean that even moving at the speed of light, radio instructions take 84 min. to travel out and another 84 to come back. Thus the ship would be operating entirely alone during its high-wire maneuver, all of its commands preloaded into its computer. As Cassini-Huygens approached the gap, it carried out, as preinstructed, one final step to protect itself: it turned around and pointed its large, dish-shaped antenna forward--a makeshift shield to protect the fragile hardware behind.

"Engine turn-on," announced the propulsion engineer at NASA's Jet Propulsion Laboratory in Pasadena, Calif., at 7:36 p.m. (P.T.), when the signal came down that the ship's fire had been lit. For the next hour and a half, the room was largely quiet. It was not until 9:12 that the same engineer spoke the words that meant the engine firing was over and the spacecraft had survived.

"We have burn complete," he said.

At that, the mission controllers, who in recent years have whipsawed between the devastation of the shuttle disaster and the celebration of the Mars landings, once again had reason to cheer, whoop and slap one another on the back.

For a nation struggling with war and terrorism, as well as alienation from some longtime allies, the scientific triumph was a moment to remind the world what the U.S. still does better than anyone else ever has, marshalling its imagination and technological prowess to send robotic emissaries into outer space. It didn't hurt the effort to mend international fences that the mission is being conducted in collaboration with the European and Italian space agencies. And it didn't hurt the domestic sense of pride that the arrival at Saturn occurred in the run-up to the Fourth of July.

"This is a story of human accomplishment," exulted Carolyn Porco, leader of the Cassini imaging team. "How can anyone not be excited about that?"

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