As expected, NASA's mission management team today formally cleared the shuttle Columbia's crew to press ahead with a full-duration mission to repair and upgrade the Hubble Space Telescope. The decision was made after engineers were able to show that reduced flow in one of Columbia's two coolant loops was still sufficient to support a normal end-of-mission landing even if the second, healthy coolant loop failed.

"The decision of the management team is we have the confidence that Freon coolant loop 2 is good and stable and that the (degraded) flow rate that we see in Freon coolant 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," said shuttle program manager Ronald Dittemore. "And so the team decided that we would press on with the nominal mission and not make any changes."

Dittemore said engineers believe the blockage in coolant loop 1 is caused by debris that shook loose during the "shake, rattle and roll" of ascent and lodged in a downstream filter. If there was any similar debris in loop 2, it would have shaken loose during the same period. The fact that loop 2 continues to function normally gives engineers confidence no such problems will affect it later.

"If there's anything in loop 2 it would have broken loose by now," Dittemore said. "Since it passed that screen, we feel highly confident it will remain that way."

As mentioned below, the problem does not affect Columbia's systems when it is configured for normal orbital operations with its payload bay door radiators deployed. The issue is the performance of loop 1 when the doors are closed. With the radiators stowed, and with additional electronic equipment turned on, the heat load in the shuttle's aft avionics bays is higher and the reduced flow rate in loop 1 is more of a factor.

Even though the flow rate is slightly lower than allowable under NASA's conservative flight rules, a re-analysis of the actual heat loads aboard Columbia indicate loop 1 still has enough cooling capability to support a landing even if the currently healthy loop 2 should fail completely.

Mission manager Phil Engelauf, in a teleconference with reporters, said most of NASA's flight rules were written before engineers had real world experience flying the space shuttle. As such, most are very conservative. He said after 107 shuttle missions, NASA now has an enormous database of actual in-flight experience and in this case, that experience gave engineers confidence the coolant loop redline could be safely relaxed somewhat.

"The philosophy of the rule is, when one Freon loop can no longer support the heat loads for entry, you call it failed and you decide to come home next PLS (primary landing site) because you're down to one loop and if you lose that good loop you can't support the heat loads for entry," he said. "That's the philosophy of the rule.

"Now, that's pretty hard to implement in real time because you need a number of what it takes to support the entry loads. Most of the rules we fly today in that six-inch-thick flight rule book have been on the books since before we started flying."

At that time, he said, engineers considered the certification limits for the hardware, the results of testing and the predictions of mathematical modeling to determine how much Freon would need to move through the system to carry away the heat engineers calculated would be generated. The result was written into the flight rule governing the Freon coolant loops.

"Well, we've got 107 flights of experience under our belts now," Engelauf said. "We've continued to evolve the math model, we've got performance history of the flight hardware that we didn't have before."

The result is more accurate knowledge of how each shuttle actually performs in space, how much heat is actually generated and how much Freon flow is absolutely required.

When engineers studied telemetry from Columbia, they saw numbers that were slightly below the printed redline. But after around-the-clock analysis, they concluded the flow rate was sufficient to cool Columbia during entry even if loop 2 failed outright.

"And I would submit to you guys if we arbitrarily declared next PLS and aborted the mission for a very small violation of the rule, without going and looking at the real data to see if we couldn't do better than that, you guys would be all over us for not doing our jobs," Engelauf said.

Columbia's crew went to bed before the mission management team met. They will be awakened at 9:22 p.m. After breakfast, they will get to work for the final stages of the shuttle's rendezvous with the Hubble Space Telescope. If all goes well, astronaut Nancy Currie, operating Columbia's robot arm, will grapple Hubble around 4:15 a.m. Sunday.