The flight of the U.S. space shuttle Columbia has gotten a reprieve. The U.S. space agency NASA says a cooling problem aboard the orbiter is not serious enough to abort its mission to upgrade the Hubble Space Telescope. Shuttle is ready to retrieve Hubble for the maintenance work.

Mission managers have monitored the reduced flow of freon coolant in one of two required radiators since Columbia lifted off on Friday. They say the flow, although reduced slightly, is stable and does not warrant termination of the mission.

The radiators dissipate heat from shuttle equipment during takeoff and reentry into the atmosphere. The flow problem in one of the freon circuits is believed to be the result of blockage by debris.

But shuttle program manager Ron Dittemore says the debris is apparently not moving around, allowing the freon to flow well enough to cool the shuttle alone without support from the second loop. The mission, he says, will continue.

"We're going to press on as planned and the guys will continue to monitor the loops very carefully," said Mr. Dittemore. "I suspect that the engineering teams will go back and scrutinize all their calculations one more time. But other than that, I think this is behind us."

This means Columbia's astronauts will retrieve the Hubble observatory from orbit with the robot arm Sunday as scheduled and begin five days of maintenance spacewalks on Monday.

Columbia is NASA's oldest shuttle, and this is its first flight after undergoing a two-and-a-half year overhaul. Mr. Dittemore says NASA engineers believe that the offending debris in the coolant flow fell into the line during that maintenance. As a result, the problem apparently does not affect the other shuttles. "Because we believe that is the primary culprit, we are not indicting the entire fleet and don't believe we're going to add any more inspections to the other vehicles," said Mr. Dittemore. "We will not call for any additional inspections."

The main goal of Columbia's mission is to install a new camera that increases the telescope's visual power ten times. Astronomers hope it will allow them to see the universe's first stars and galaxies as their faint light arrives from the distant past, when the cosmos was only a few hundred thousand years old. They also hope for the first direct observation of planets around other stars in our galaxy.

Other Hubble hardware to be installed during the five spacewalks includes new solar wings and a generator to distribute their power, a new gyroscope to point the observatory at its target, and a small cooling system to restore an infrared camera that has been dormant since 1998.