NASA green-lights Hubble mission

'The shake, rattle and roll of ascent is a very dynamic test'

Columbia's liftoff early Friday was delayed from Thursday because of cold weather at Cape Canaveral in Florida.

JOHNSON SPACE CENTER, Texas (CNN) -- NASA engineers on Saturday afternoon gave the crew of the space shuttle Columbia a green light to continue the Hubble Space Telescope repair mission, despite the underperformance of one of the shuttle's two freon cooling systems.

"We have the confidence that freon cooling loop 2 is good and stable," said Ron Dittemore, shuttle program manager, after a 2 p.m. EST management meeting on the matter. "The flow-rate we see on freon cooling loop 1 is large enough that it would be able to support a full nominal entry if called upon to do it all on its own."

In this decision, shuttle mission managers are bending a rule of flight that dictates that if one of the craft's two redundant cooling systems isn't fully operative, a shuttle must return to Earth. At least one of the two systems is required for adequate cooling as the shuttle re-enters the planet's atmosphere, when temperatures reach more than 2,000 degrees Fahrenheit.

The port side freon cooling loop in Columbia's payload bay -- cooling loop 1 -- was sluggish shortly after launch. Ground controllers say they suspect it's blocked with debris. For hours, uncertainty threatened the 11-day mission to repair and refurbish the 12-year-old Hubble telescope.

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But in reviewing the situation, Dittemore said, engineers looked at Friday's launch of the shuttle as the decisive test. "The shake, rattle and roll of ascent is a very dynamic test," he said, so severe that "if the freon cooling loops were going to have any foreign debris in them -- which we think is the primary, most likely root cause (of the problem) -- that we would see that debris shake loose and move during that most dynamic phase of flight.

"If there was anything in loop 2, it would have broken loose by now. ... That's why we believe that we saw the debris in freon cooling loop 1. That (the launch) is the time it was going to break loose. We've seen it, it's not going to move anywhere, based on our discussions. We believe it's going to remain stable and support the remainder of the flight."

Ironically, it may have been during Columbia's recent $164 million overhaul that debris entered the cooling system, Dittemore said. Both cooling systems were broken apart and reassembled as part of that work.

The shuttle is expected to rendezvous with Hubble early on Sunday.

No change in return

Dittemore clarified that Saturday's thumbs-up from engineers is expected to mean Columbia is on-schedule for a March 12 return to Earth. One of its sister ships, Atlantis, is slated for an April 4 launch on an international space station run -- that mission, Dittemore said, shouldn't be compromised by the cooling-system issue on Columbia, since Atlantis has had no overhaul.

This isn't the first time a problem has threatened to terminate a shuttle mission. Several years ago, a fuel cell aboard Columbia malfunctioned shortly after the craft went into orbit: The shuttle returned to Earth three days later.

First launched in 1981, Columbia is NASA's oldest operational shuttle. The craft flew 26 missions before undergoing its overhaul following its last flight in November 1999.

Columbia is carrying $172 million in new equipment for Hubble, including new solar-panel wings, a power-control unit, a steering mechanism, a more advanced camera and a system to restart a disabled infrared camera.

The mission calls for the astronauts to install the new equipment in five spacewalks during the mission, including repairs never meant to take place in orbit: replacement of a power-control unit and the work on Hubble's infrared camera.

NASA scientists say they're particularly nervous about the power-control unit repair because the power to the space telescope must be shut off and turned back on -- a sequence never before performed on the telescope.

High-flying track record

Hubble already has given scientists an impressive view of the universe. The orbiting observatory has watched a comet break up near the sun, spied the ruins of a stellar explosion 10 billion light-years away and investigated the rate at which the cosmos expands.

Improvements include installation of the Advanced Camera for Surveys -- a multiple-camera instrument that promises to increase 10 times the imaging capability of the bus-size observatory. Other new instruments, including a filter that blocks out bright light, should allow the observatory's technicians to improve Hubble's structural studies of massive black holes in the far reaches of the universe.

The Hubble Space Telescope, a joint venture of NASA and the European Space Agency, was launched in 1990. Because of a structural defect, it produced seriously blurred images until visiting shuttle astronauts made repairs in 1993.