NASA managers are increasingly optimistic problems with one of the shuttle Columbia's two coolant loops will not force an early end to a dramatic mission to overhaul the Hubble Space Telescope.

"The analysis and the discussions thus far are such that people are feeling more and more optimistic about the prospects of remaining on orbit and continuing the flight nominally," astronaut Mario Runco told the astronauts early today. "The discussion is still on going and as you know and would have expected, people are sharpening their pencils and taking a real close look at this.

"The mission management team is going to meet at noon (CST) today and that is when a final decision will be made, one way or the other. But again, I'd like to couch this in a more optimistic light. That is the nature of the discussion at this point, it's just that a final decision has not been reached at the present time."

"I appreciate you putting those words together for us," replied commander Scott Altman. "We will stand by for the final decision."

Shortly after launch Friday , one of the shuttle's two main Freon coolant loops suffered a reduced flow rate, dropping to just above NASA's flight rule redline of 211 pounds of Freon-21 per hour. Shortly thereafter, the shuttle's cargo bay doors were opened and the Freon in loop 1 was routed through radiators in the port-side payload bay doors. The flow rate dropped again, due to the increased volume, but this isn't an issue for on-orbit operations, when many of the avionics systems used for re-entry are powered down.

It's a different story for landing, however, when the radiators are stowed and the heat generated by the shuttle's electronics must be rejected by boiling water or ammonia. The redline in question reflects the ability of coolant loop 1 to cool electronic gear in an aft avionics bay housing equipment that must be powered up for entry.

To protect against the possibility of problems with the second coolant loop, NASA flight rules require a quick landing if the flow rate in either loop falls below about 211 pounds of Freon-21 per hour when the radiators are not deployed. Before Columbia's cargo bay doors were opened Friday, the flow rate in this "rad bypass mode" hit 195 pounds per hour, in technical violation of the flight rule. The flight rule does not apply to flow rates when the radiators are deployed.

Lead flight director Bryan Austin said today engineers believe the flow rate in loop 1 is stable, i.e., it won't get much worse in rad bypass, and that loop 2 appears healthy. Engineers are justified in re-assesing a pre-defined flight rule, he said, to reflect the real-world conditions aboard Columbia.

As for the wisdom of changing flight rules on the fly during the heat of a very important mission, Austin said "sometimes in the real world it doesn't always line up exactly how you wrote it down - if this happens, then we'll do this - and what we're taking is a real-world scenario and then bringing a flight rule up to date."

"The assumptions bethind that flight rule will not change at all," he said. "What we're doing is bringing them up to date in terms of how we're operating the vehicle ... so the actual end item number will change, but the assumption, which is truly the foundation of the rule, will not change at all."

Regardless of the redline limit vs the actual flow rate, engineers say loop 1 provides sufficient cooling to permit a normal entry if it doesn't get worse. And engineers do not think it will. They believe the reduced flow is the result of a small bit of debris lodged in an internal Freon-line filter. They also believe that if any similar debris was present in loop 2 it would have shown up by now. So far, loop 2 is operating normally.

Before a round of media interviews this morning, Altman asked Runco for an update on the cooling issue.

"The flow rate on loop 1 is sufficient to support an entry," Runco replied from Houston. "The main focus of attention right now is the integrity of Freon loop 2. The concern is it having the same problem as Freon loop 1 because of all the work that has been done."

Columbia, NASA's oldest space shuttle, is making its first flight in two-and-a-half years following a $100 million overhaul and upgrade of its own. The problem with coolant loop No. is believed to be debris lodged in one of the Freon lines.

Fielding questions from reporters, Altman and payload commander John Grunsfeld said they were too busy preparing for the upcoming work on the Hubble Space Telescope to spend much time worrying about the impact of the coolant loop issue.

"We've been extremely busy up here (and) we haven't had a lot of time to ponder that," Grunsfeld said. "Of course, we do know that it's a serious problem back there and one that shouldn't be ignored and so we're letting the smart folks on the ground really worry for us. If we had to come back, we know that Hubble's important enough that Hubble would be well taken care of."

The astronauts spent their first full day in orbit readying Columbia and its robot arm for the rendezvous and capture of Hubble early Sunday. They also have readied the shuttle's airlock for the upcoming spacewalks and are in the process of checking out the four spacesuits that will be used in five back-to-back EVAs beginning around 1:30 a.m. Monday. So far, no problems of any significance have been encountered.