International space station towers above the Endeavour. Photo: Spaceflight Now/NASA TV

Engineers have been unable to reinitialize a second command-and-control computer aboard the international space station. Tests of the lab's new Canadian robot arm are on hold pending a decision by mission managers to press ahead with just one operational C&C computer, something officials said Friday they did not want to do.

Meanwhile, a Russian Soyuz spacecraft carrying two cosmonauts and millionaire space tourist Dennis Tito blasted off from the Baikonur Cosmodrome at 3:37:20 a.m. EDT. The shuttle Endeavour currently is scheduled to undock from the station around 12:01 p.m. EDT Sunday, clearing the way for the Soyuz to dock at 4:05 a.m. EDT Monday.

But unless the station's ongoing computer problems are cleared up soon - and that does not appear likely - NASA may be forced to ask the Russians to delay the arrival of the Soyuz by one day so Endeavour can remain docked to the outpost through Monday.

The Russians agreed to consider such a request in a compromise launch agreement reached Friday. But based on the partners' recent track record, convincing the Russians to actually delay the Soyuz arrival may prove difficult.

"As you know, (the Soyuz) launched not too long ago," astronaut Lisa Nowak told the station crew this morning in a daily planning conference. "And there's a good possibility the shuttle will still be there on Monday.

"We are discussing having the possibility of the Soyuz dock on Tuesday," she went on. "This has, of course, not been finalized, we just wanted to let you all be aware of the potential of both vehicles being in the vicinity at the same time."

"Yeah, we understand that," station astronaut James Voss replied. "And we also know how close the tail of the orbiter is to where they're going to be docking that thing."

"It is our position that the Soyuz cannot dock while the shuttle is actually still attached to the station," Nowak said.

"Sounds like a good position to me."

Russia's Soyuz rocket carrying Dennis Tito lifts off from Baikonur carrying space tourist Dennis Tito and crew.

The Soyuz must dock at the Zarya module's Earth-facing, or nadir, port. On final approach, the spacecraft would pass within about 25 feet of Endeavour's tail, if the shuttle was still in place. NASA managers have said repeatedly that is not an option because of safety concerns.

Endeavour's crew originally planned to undock today, but cascading computer failures crippled the ship late Tuesday and after four days, computer experts have been unable to restore all three of the lab's critical C&C computers to operation.

NASA sources say engineers are not optimistic about getting the computer system back to full health in time for a Sunday undocking. In fact, it's not at all clear as of this writing if the problem can be resolved before a Monday departure.

If NASA managers and their Russian counterparts ultimately agree to extend Endeavour's stay to Monday, Dennis Tito and his Soyuz crewmates would be forced to cool their heels in a nearby parking orbit, delaying their arrival from Monday to Tuesday.

Endeavour's crew, in theory, could remain docked to the station through Tuesday morning by giving up one of two landing weather delay days or by shortening the time between undocking and re-entry. That would result in the Soyuz docking while the shuttle was still attached and while NASA has said it would not consider such a scenario, the ongoing computer trouble could force reconsideration.

But engineers hope it will not come to that and they're hopeful about getting a second C&C computer back on line shortly.

C&C-2 remains up and running but engineers have not yet been unable to load fresh software into the other two computers in the command-and-control set. C&C-3 has a faulty hard drive and engineers are attempting to load software directly into its DRAM memory. Initial attempts were unsuccessful.

The C&C-1 computer was replaced last night by the station crew, which substituted an unneeded payload control computer in its place. Engineers have attempted to load that computer's hard drive with the necessary software but again, they have been unsuccessful.

"For the big picture, we still do not have C&C-1 and C&C-3," she said. "We've been trying to load C&C-3, several times, unsuccessfully. A recent development, we think we found an error in some of the files and maybe that will solve the problem.

"On the side, we're also discussing the possibility of beginning robotic operations without getting another C&C. We're looking at the possible loads associated with shuttle undocking and the Soyuz docking, the various configurations if there was a failure. But we are going to need to wait for an MMT (mission management team) decision a little later here to get a final decision on that if we don't get a second C&C before then."

"So we can expect to not start our robotic operations until after the MMT makes the decision, Lisa?" Voss asked.

"Unless we get a C&C before then, that is correct."

The computer work has taken its toll on the space station crew. Alarms have been ringing in the middle of the night as ground engineers change computer configurations, disrupting the crew's sleep. Given already long work days, Voss sounded tired today when he said "we'd appreciate a reasonable night's sleep."