"We are not in a condition where we have to come home early at this time," he added. "We believe that we are safe to continue on orbit for the next 24 hours while we continue to look at further information and refine our analysis."

That was good news to the astronauts, who were informed of the latest plan in their morning mail. The six-man, one-woman crew were awakened for their first full day in space at 9:22 p.m. EST (0122 Saturday GMT).

"Thanks for your words on the big picture," Columbia commander Scott Altman radioed down to colleagues in NASA's Mission Control Center in Houston. "And it's great to be in space again."

Just 90 minutes after Columbia set sail for the Hubble telescope, a potential showstopper cropped up aboard NASA's oldest orbiter, which is making its first flight since being sidelined for a $100 million overhaul.

A warning message seen on computers both onboard Columbia and at Mission Control indicated that fluid line coolant flow within one of two crucial shuttle radiators had dropped to a level that approached safety limits.

Coolant loops within the radiators -- which are located on the inner side of Columbia's two cargo bay doors -- play a key role in dissipating heat generated by spaceship electronics throughout a flight.

Strict NASA flight rules call for a shuttle to return to Earth as soon as possible if either of the radiators or their internal coolant loops fail. The safety rule aims to guard against the chance that both radiators might break down during a flight, completely shutting down critical cooling systems.

Dittemore said engineers at first thought that the situation was grim -- that mission managers might have to order the crew to abandon the telescope repair mission and return to Earth.

The slow coolant flow within the suspect cooling line, however, returned to normal once the shuttle's cargo bay doors were opened and has remained steady ever since.

"Our initial look at the system shows it to be stable, even with the degradation," Dittemore said, adding that the problem would not put the shuttle crew in immediate jeopardy.

"I don't think there should be any cause for us to be in any concern (about) the state of the spacecraft," he said. "I think it's in a good, stable configuration. It's certainly safe."

Engineers nevertheless will continue their analysis of the situation overnight, briefing mission managers at a meeting now scheduled for 1 p.m. EST (1800 GMT) Saturday.

"All we want to do is have some time to make sure that we understand the configuration that we do have, that we understand the degradation that did happen, and that we do some analysis to prove to ourselves that we can complete the full mission without any undue risk," Dittemore said.

A similar situation cropped up during Columbia's last flight, a July 1999 mission to deploy NASA's $1.5 billion Chandra Observatory, a Hubble-class x-ray telescope.

In that case, restricted coolant flow was seen in the same portside radiator, but the five-day mission went on as planned. Columbia's flight is slated to last 11 days.

As it stands, Columbia and its crew remain scheduled to rendezvous with the 13-ton telescope at 4:14 a.m. EST (9:14 GMT) Sunday. Five consecutive days of spacewalking telescope maintenance and repair work remain scheduled to begin Monday.

During the excursions, the astronauts plan to equip Hubble 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.

A full mission, meanwhile, would lead to a landing here at Kennedy Space Center March 12.