At 0236 GMT Thursday, the craft will begin orbit insertion by firing its rocket engine for about 96 minutes to slow down enough to be caught in the gravitational force of Saturn.

The maneuver is critical to the 3.3-billion-dollar mission.

"Everything has to go just right," said Robert Mitchell, program manager for the Cassini-Huygens mission at NASA's Jet Propulsion Laboratory in Pasadena, California.

"The burn must occur for all 96 minutes," he said, no matter what.

"The computers must keep the sequence going, even in the event something unexpected should happen. The spacecraft has been programmed to continue even in the event of an emergency," he said. Because it takes one hour and 24 minutes to sent a message to the craft -- and another hour and 24 minutes to get a response -- there is no time to question orders.

"We don't want Cassini to call home if a problem arises. We want it to keep going. That is precisely what we've told the spacecraft: 'Don't stop, keep going until you've put in all 96 minutes of burn'," he said.

During its orbit insertion, the probe will fly closer to Saturn than it will at any other moment of its four-year mission to come, giving it the chance to study the planet from about 20,000 kilometers (12,427 miles) away.

"In a sense, Cassini and the Huygens probe are like time machines that will take us back to examine a world we've never seen before, a world that may resemble what our own world was like 4.5 billion years ago," said Jean-Pierre Lebreton, the European Space Agency mission manager and project scientist for the Huygens probe.

On December 25, Cassini will free Huygens on a path to Saturn's largest moon. The craft will become the first to land on a natural satellite of a planet other than the Moon of Earth.

SPACE.WIRE