A U.S.-European spacecraft ends a seven-year journey to Saturn late Wednesday to become the first man-made body to orbit the ringed planet. Closer to Earth, U.S. and Russian space station crewmen will try again to undertake a risky maintenance spacewalk after last week's attempt failed.
After two successful Mars rover landings in January, the U.S. space agency NASA hopes for another precision entry to a different planet, Saturn. Mission officials say the Cassini spacecraft is on target to begin a four-year orbit of the giant gas planet and eight of its 31 moons, including the one of most interest, the Earth-like Titan.
"We're about to start on a delicious smorgasbord of scientific opportunities that lie before us," says Cassini project scientist Dennis Matson. "We have the planet Saturn, the fantastic ring system, Titan, icy satellites and the gigantic magnetic bubble that surrounds most of that, the magnetosphere."
NASA says that Saturn, with its own miniature planet system, is a model that can offer clues to how the larger solar system formed out of gas and dust 4.5 billion years ago.
Cassini, on automatic pilot, will guide itself up between the rings with its broad dish antenna deflecting dangerous particles, then will fire retrorockets to slow down enough to allow the planet's gravity to capture it into orbit.
The maneuvers are tricky, but Cassini team chief Julie Webster says she does not expect problems. "This whole mission has been an incredibly smooth one to fly," she says. "We loaded the last command we were going to send to it late Saturday night, early Sunday morning, and we've just been clocking it out ever since. We expect this to go very, very smoothly."
In December, the European Space Agency's Huygens probe detaches from Cassini to sample the atmosphere of the moon Titan, which scientists believe was like that of Earth millions of years ago before life appeared.
Cassini's arrival coincides with an unrelated space event, a spacewalk by the U.S.-Russian crew of the International Space Station. Astronaut Mike Fincke and cosmonaut Gennady Padalka will dress in Russian space suits to restore electricity to one of three gyroscopes, which point and stabilize the station. Two are sufficient to do the job, but NASA and the Russian space agency want a working spare.
Their first attempt failed Thursday when Mr. Fincke's oxygen tank lost pressure. Russian technicians say a switch failed to close completely.
Cooling problems with the U.S. spacesuits are forcing use of the Russian outfits, even though the work site is close to the American side of the station. This makes the spacewalk riskier than usual, because the two men must exit from the Russian hatch and travel a longer than normal distance to do their job. Unlike the U.S. gear, their suits do not have jet backpacks for use if their safety tethers break and the gloves are bulkier than the U.S. versions. The men will also be out of view of the Russian module's antennas, which could disrupt radio transmission.
But NASA space station operations manager Mike Suffredini says restoring the gyroscope is important enough to let the spacewalk proceed.
"This is one that we can go do and should go do, and so we have been through all that and believe this is the right posture to be in," he said.
The spacewalk is scheduled to last six hours, 90 minutes of which is for movement between the work site and the Russian hatch.