"If we can see our way clear with good engineering rationale to continue and fly that mission, that's what we are supposed to do and that's what you guys pay us for."

Columbia's astronauts were sleeping when mission managers gave ground controllers a green light to proceed with plans for a high-flying Hubble rendezvous at 4:14 a.m. EST (0913 GMT) Sunday.

The six-man, one-woman crew, however, already had been anticipating the call.

"To be honest, we've gone on the assumption that we're here to stay, that we're going to do our job, and we've just kind of kept that thought foremost in our minds," shuttle skipper Scott Altman said in a space-to-ground interview early Saturday.

The shuttle-Hubble hook-up will get underway early Sunday as Columbia robot arm operator Nancy Currie attempts to snatch the 13-ton telescope out of open space and place it atop a Lazy Susan-like work platform in the ship's cargo bay.

Four stories tall, the telescope will tower over the shuttle as ground controllers command its golden solar arrays to roll up like bamboo shades at 8:12 a.m. EST (1312 GMT).

A day later, the astronauts will set out on five consecutive days of spacewalking Hubble maintenance and repair work.

Their aim is to equip the telescope with a fresh set of solar wings, install a new planetary camera, resuscitate a dead science instrument, replace a faulty pointing control device and overhaul the observatory's electrical system.

Fresh off a $100 million overhaul of its own, Columbia ran into trouble almost immediately after the spaceship reached orbit Friday.

Ground engineers and the crew noted that a Freon loop critical to cooling spaceship electronics was operating outside longstanding NASA safety limits.

Specifically, the flow of coolant through the line dropped by more than a third, from an expected 300 pounds per hour to 195 pounds per hour.

NASA's safety limit, including margin for the possibility of sensor inaccuracies, is 236 pounds per hour. The flow through the line since has stabilized at about 208 pounds per hour and is expected to remain at that rate for the remainder of the mission.

Similar to coolant in a household air conditioning system, the Freon 21 that flows through the loop is used to keep sensitive spacecraft electronics from overheating.

The big issue, meanwhile, is not the performance of the suspect line in orbit. Rather, engineers are more concerned about how it will operate during landing.

The reason: Once a shuttle's cargo bay doors are opened in orbit, two large radiators located on their inner walls dispel heat from spaceship electronics.

But the doors swing shut before atmospheric reentry, leaving internal water spray and ammonia boilers to cool shuttle electronics. And those electronics generate more heat during launch and landing than they do in orbit.

NASA flight rules, meanwhile, call for a mission to be cut short if one of the shuttle's two associated Freon coolant loops fails, or if coolant flow within either of them drops below safety limits.

Faced with the prospect of abandoning the Hubble mission, NASA engineers spent the better part of the past two days contemplating the situation.

Ground teams reviewed the analysis that was done to establish current safety limits. They combed over data to refine their estimates of actual coolant flow rates within the suspect line. And they pinpointed the most probably cause of the problem.

As it turns out, extensive work was done on both of Columbia's Freon lines during a recent overhaul, and engineers think a small piece of "weld slag" -- or solder -- probably broke loose as a result of launch vibrations and then clogged up the suspect pipe.

At the same time, the review team concluded that if any additional debris were lurking within either coolant loop, it already would have surfaced in the wake of Columbia's thundering climb into orbit.

"The shake, rattle and roll of ascent really is a very dynamic test. Not only do you get a lot of vibration but you get a lot of acoustic vibration," said NASA shuttle program manager Ron Dittemore.

"We believe that screen, if you want to call it a screen, is a severe screen, and that if the Freon coolant loops were going to have any foreign debris in them, we would see that debris shake loose and move during that most dynamic phase of flight."

Also part of the evaluation: A survey of electronic equipment that could be turned off, if necessary, to ensure that the partially blocked coolant loop will provide adequate cooling during atmospheric reentry.

The bottom line: Engineers now think the clogged line would keep spacecraft electronics from overheating even if the fully functional second loop for some reason failed.

"There are just a lot of different ways that we can make the situation better, and try to recover a position where we can fly the full mission, rather than throw away a perfectly doable mission here for what amounts to a technical violation (of safety limits)," Engelauf said.

"So we still think we're meeting the spirit of the flight rule."

All bets, however, are off if the coolant flow within the clogged line gets worse, or if the second Freon loop fails at some point during flight.

Said Engelauf: "We would probably have to terminate the mission."

Mission managers, however, think the chances of that scenario are remote.

"The team unanimously felt very confident that we can press forward on this mission with no risk, and that we'll be able to complete the mission as planned," Dittemore said.

The planned landing time: 4:35 a.m. EST (0935 GMT) March 12.