"It's important to remember that we have two Freon loops, and the other loop is still operating at 100 percent capacity," said NASA mission manager Phil Engelauf.Only one of the two loops, he added, is needed to adequately cool spaceship electronics, preventing them from overheating and in a worst case, triggering systems failures that would make landing difficult if not dangerous.
What's more, an extensive NASA analysis of the situation showed the clogged loop could do the job by itself if necessary, Engelauf said. But if both coolant lines stopped working for some reason, the shuttle crew would be in trouble.
"If you lose both, you've got to get on the ground as soon as possible," Columbia commander Scott Altman told reporters in a space-to-ground news conference early Monday.
Altman and his six crewmates are scheduled to make a predawn landing at NASA's coastal Florida spaceport at 4:32 a.m. EST (0932 GMT) Tuesday.
The weather forecast for landing is favorable, with only a slight concern that isolated showers could pop up off the coast of Cape Canaveral. NASA flight rules call for a landing attempt to be called off if rain creeps with 34.5 miles (55.2 kilometers) of a shuttle runway.
The Columbia crew also will have a second landing opportunity at 6:13 a.m. EST (1113 GMT).
NASA officials decided not to call up a back-up landing site at Edwards Air Force Base in California because the weather at the agency's shuttle homeport is expected to be good enough to give the crew a go-ahead to land here.
The situation with the shuttle's cooling system cropped up shortly after Columbia reached orbit on March 1. Ground engineers and the shuttle crew noted that a Freon loop critical to cooling spaceship electronics was operating outside NASA safety limits.
Specifically, the flow of coolant through the line dropped by more than a third, from an expected 300 pounds per hour to 195 pounds per hour, and then stabilized at about 208 pounds per hour.
NASA's safety limit, including margin for the possibility of sensor inaccuracies, is 236 pounds per hour. Longstanding flight rules, meanwhile, call for a mission to be cut short if one of the loops fails, or if coolant flow within either of them drops below safety limits.
Mission managers, consequently, considered ordering the shuttle crew to abandon their Hubble servicing mission and return to Earth early.
"There were a few nervous moments," Altman noted. "As I look back on this mission, we had kind of a rough start."
The prime concern was not how well the clogged cooling line would operate in orbit. It was whether it would perform properly during atmospheric reentry and landing.
Once a shuttle's cargo bay doors are opened in orbit, two large radiators located on their inner walls dispel heat from spaceship electronics.
But the doors swing shut before atmospheric reentry, leaving internal water spray and ammonia boilers to cool shuttle electronics. And those electronics generate more heat during launch and landing than they do in orbit.
An extensive, two-day analysis, however, showed that the clogged line would provide adequate cooling during landing even if the second loop for some reason failed. And the Columbia crew ultimately was given a go-ahead to rendezvous with Hubble on March 3.
"That was the big concern - would the one degraded cooling loop have enough capability to get us to the ground," Altman said. "And (engineers on) the ground decided... that it would. We were capable of (surviving) a next worse case failure, and that's what allowed us to stay up here the rest of the time."
During the next five days, spacewalkers equipped the telescope with a fresh set of solar wings, a new power switching station and a replacement for a faulty pointing control device.
The astronauts also outfitted Hubble with the most powerful planetary camera ever launched into orbit, and they rigged up a high-tech cooling system in a bid to resuscitate the telescope's dormant infrared instrument.
"It's unbelievable we got everything we set out to do accomplished," Altman said. "I think we're all just elated that we got to stay up here and complete the mission."
Columbia's return to Earth will mark only the 19th night landing in 108 shuttle flights.
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