A robotic handshake in space as the Canadarm2 holds the Spacelab pallet for Endeavour's arm to grapple. Photo: Spaceflight Now/NASA TV

With ground teams "on the brink" of restoring the space station's main computers to relatively good health, the Endeavour astronauts were tentatively cleared to undock from the outpost Sunday, clearing the way for space tourist Dennis Tito and his two Soyuz crewmates to float aboard early Monday.

"It's been a very, very productive day in space," said Randy Stone, director of mission operations at the Johnson Space Center in Houston. "We have accomplished a number of the very important items that had to be done today to allow us to plan for an undocking tomorrow.

"Things have gone extremely well," he said. "We're very pleased with where we are right now."

But the work is not yet complete. Before Endeavour can undock, computer engineers must succeed giving the station at least two operational command-and-control computers with at least one working hard drive between them.

A formal decision on whether to proceed with undocking must be made by 11 p.m. or so, before the computer revival is complete. That's when Russian flight controllers overseeing the Soyuz's approach to the station need to know whether to continue the rendezvous or to stretch it out for a day-late arrival.

Around 9:30 p.m., as the astronauts were going to bed, Robert Thirsk in mission control told the crew "the plan is still to undock tomorrow. However, if the (computer) troubleshooting takes a couple of steps backwards, then of course that may change. But the plan is to undock."

Tito and his two cosmonaut crewmates blasted off early today. Assuming Endeavour departs Sunday, the Soyuz will dock with the station at 3:52 a.m. EDT Monday. The crew will enter the outpost around 5:20 a.m.

The Destiny laboratory module is equipped with three C&C machines, any one of which can oversee operation of the station's critical control systems. A still-unknown software problem knocked out all three machines in domino fashion earlier in the week, disrupting an otherwise smooth space station assembly mission.

As of today, two computers had been restored to near normal operation. C&C-3, which had an outright hard drive failure, is up and running with critical control software uplinked directly into its DRAM memory.

C&C-2 is operating normally, although it experienced hard drive access problems of its own earlier today. Engineers said that was a known condition and worked this evening to restore the drive to operation.

C&C-1 was replaced last night by an identical payload control computer. Engineers in Houston are working overnight to load it with C&C software. If all goes well, all three computers - and two hard drives - will be operational by Sunday morning, clearing the way for Endeavour to depart.

"We're now on the brink, I believe, of total success bringing the computers back to total redundancy," Stone said at an evening news briefing.

Aboard the space station, Expedition Two commander Yury Usachev, Susan Helms and James Voss closed out the day by joining their seven shuttle colleagues for a quiet meal before turning in for the night.

"We're going to have our last crew meal together, it seems," Helms radioed ground controllers around 8:40 p.m. "It's kind of a sad event, but I guess that needs to be done. Thanks again for all your support. It's a pleasure to work with everyone on the ground there."

Endeavour is scheduled to undock at 1:34 p.m. Sunday as the two spacecraft sail 240 miles above the mid Pacific Ocean. If all goes well, commander Kent Rominger will guide the spaceplane to a landing back at the Kennedy Space Center around 9:04 a.m. Tuesday.

Aboard the station, meanwhile, Tito and his two crewmates - Talgat Musabayev and Yury Baturin - will be settling in for a planned six-day stay after delivering a fresh Soyuz lifeboat to the orbital outpost. They are scheduled to depart late next Saturday, returning to Earth in the station's original Soyuz.

Robert Cabana, a veteran shuttle commander and senior space station manager, said the Expedition Two crew will enjoy a light schedule next week while ground engineers continue work to test C&C software and to fully restore all the software needed for normal operations.

The primary goal of the 104th shuttle mission was to deliver the $600 million Canadarm 2 space crane to the station, along with several tons of supplies, equipment and experiment hardware.

The 57-foot-long arm was attached last Sunday and checked out during and after a spacewalk last Tuesday. But later that night, C&C-1 suddenly dropped off line and NASA's computer nightmare began.

Today, with C&Cs 2 and 3 in operation, Helms and Voss used the arm to move the 3,000-pound cargo pallet that carried it into orbit into position so the shuttle's robot arm could latch on and move it back into Endeavour's cargo bay for return to Earth.

Moving one joint at a time to avoid taxing the still-fragile computer system, Helms and Voss began the oft-delayed operation at 4:01 p.m. as the space station sailed 240 miles above the south Pacific Ocean.

Twenty seven minutes later, the maneuvers were complete and the Spacelab Pallet was in position for Canadian astronaut Chris Hadfield, operating Endeavour's 50-foot robot arm, to latch onto a grapple fixture on the other side of the pallet.

At that moment, the U-shaped cargo pallet was suspended between the two robot arms like a giant wishbone, clearing the way for Helms and Voss to unlatch the station arm's snare-like grapple.

Because neither of the lab's operational C&C computers could use its built-in hard drive, the astronauts and ground controllers had to take turns issuing commands to the arm and its systems, working step-by-step through a tedious checklist to make sure they didn't inadvertently cause additional computer problems.

One hour and five minutes after the procedure began, Voss moved a hand controller on a computer work station in the Destiny laboratory module to detach the Canadarm 2 from the Spacelab Pallet, completing the handoff at 5:06 p.m. as the shuttle sailed over the western Indian Ocean in orbital darkness.

"As we were grappling the pallet we were flying over Canada," said Hadfield. "So it is just an amazing tribute to all the people that have worked on these things. Congratulations to everybody on the ground who made this happen."

The astronauts originally planned to carry out a series of tests with the new space crane, including a dress rehearsal of maneuvers that will be required in June to install the station's main airlock. But mission manages decided Friday to defer the airlock dry run and to power down the arm after today's pallet handoff.