If NASA managers had realized early on that Columbia had suffered a catastrophic breach in its left wing during launch - either by obtaining satellite imagery or, more likely, by having the astronauts stage an inspection spacewalk - they might have had time to mount a repair spacewalk or even an emergency rescue mission with the shuttle Atlantis, the chairman of the Columbia Accident Investigation Board said today.

But both scenarios, while emotionally dramatic, had virtually no chance of being executed in reality because of two assumptions the CAIB made that restricted the NASA study to technical feasibility alone. The first assumption was that NASA managers had conclusive proof of a catastrophic breach by the third or fourth day of Columbia's mission. They had no such evidence. The second assumption was that agency managers would commit another orbiter to flight without knowing whether it might fall victim to the same problem. Given NASA's past history, that's extremely unlikely.

"This whole question that we asked, and NASA's whole response, is based on a set of assumptions ... which set conditions which were not present in January of 2003," CAIB Chairman Harold Gehman told reporters today in a teleconference. "In other words, we set a scenario here that was not the scenario of the Columbia. Nobody told us on day 4 that we had a hole in the leading edge of the left wing. So these are two different sets of conditions."

But the bottom line, he said, "is that it's all feasible. There are no show stoppers. But it all turns out to be extraordinarily (difficult)."

Investigators believe Columbia re-entered the atmosphere Feb. 1 with a sizeable breach in the leading edge of its left wing, a breach presumably related to the impact of external tank foam insulation that broke away and hit the orbiter 82 seconds after liftoff Jan. 16.

Engineers first saw the foam strike the day after launch while reviewing long-range tracking camera footage. NASA managers ordered an engineering analysis to determine the severity of the problem and ultimately concluded the impact had not caused any catastrophic damage. Initial requests for spy satellite imagery to examine the impact site in detail were halted.

The day of the accident, shuttle program manager Ronald Dittemore said shuttle managers didn't ask for imagery because past experience indicated it would not have helped all that much and because there was nothing the crew could have done even if a major problem had been spotted.

"Recall a year or two ago, we lost the drag chute door (from a shuttle)," Dittemore said Feb. 1. "Right at liftoff, it fell off. And we actually tried to take some pictures of the back end of the vehicle to see what was really there so that we can understand our thermal heating in that case, and those pictures that we received were not very useful to us. So that was part of our background.

"Combine that - our feeling that we didn't believe the pictures would be very useful to us - with the fact that there was not much, there was zero that we could do about it, and in this case, we elected not even to take the pictures.

"We believed that our technical analysis was sufficient. We couldn't do anything about it anyway. We were in the best possible position, and so we elected not to take any pictures from any other sources, and that's the way it played out."

Gehman said today the CAIB commissioned the repair/rescue mission study in part because board members were disturbed by repeated comments from NASA and contractor engineers and managers that nothing could have been done to save Columbia's crew even if they had known about the breach.

"To us, the area that we thought becomes more problematic when you deal with a question like this is the implications of the decisions made regarding the photography and the foam strike analysis and whether or not you should get on-orbit photography changes from being kind of a bureaucratic, administrative, fumbling, bumbling, to a much more serious life and death kind of a decision process," he said.

"Because it turns out a lot of people were saying, well it doesn't make any difference if you take photography or not because there's nothing we can do, it doesn't make any difference if the foam strike analysis was good, bad or indifferent because there was nothing that you could have done anyway.

"Now, those kinds of benign administrative decisions which were taken now look more ominous because now it looks like maybe there was something you could do. That's the area we were concerned about."

Ever since the shuttle's loss, reporters, politicians and the public have wondered whether anything could have been done to save Columbia and its seven astronauts if NASA had realized the severity of the problem. An internal NASA study concluded the astronauts would not have been able to survive entry on board Columbia, even if they managed to dump 15 tons of equipment overboard to minimize entry heating.

But in a subsequent study commissioned by the CAIB and discussed today for the first time by Gehman, agency engineers concluded the crew might have been able to patch the breach during an emergency spacewalk. Whether the makeshift patch would have protected the wing long enough for Columbia to make it through the region of peak heating is unknown. A better option, although only marginally so, would have been to rush the shuttle Atlantis to the launch pad for an emergency rescue mission.

Columbia had enough food, water and power to remain in orbit more than a month. The limiting consumable was lithium hydroxide, a chemical used to scrub carbon dioxide form the shuttle's air supply. Normally, NASA does not allow the partial pressure of CO2 to rise above 2 percent. But by relaxing those restrictions and accepting levels of up to 3.5 percent or so, the shuttle's air supply would have supported the crew for 30 days, until the evening of Feb. 15.

To make it that long, the crew would have had to power down the orbiter and do everything possible to minimize physical activity and their consumption of oxygen. Staging more than one spacewalk likely would have eliminated the Atlantis rescue option because of air lost overboard when the airlock was cycled.

But the spacewalk repair option is interesting if for no other reason than NASA managers initially ruled out any chance for an on-orbit fix.

"They inventoried everything that was on board the Columbia," Gehman said. "There are two EVA suits. They devised a successful way to get out to the area of the damage without further damage to the TPS (thermal protection system). They devised a way that they thought they could work out there and they ... came up with a patch that they would jam stuff in the hole."

After plugging the breach, the spacewalkers would position a plastic water bag over the opening.

"Then, after a day or two of maneuvering the orbiter so that section of the wing was in the shade all the time, the water would freeze solid and then that holds the stuff in place," Gehman said. "They would then put something over the top of the hole, some Teflon tape or something like that, and then they would attempt to re-enter.

"No studies have been done on the thermal characteristics of this patch," he said. "Whether it would hold for 30 seconds, 60 seconds, 90 seconds or three minutes we have no clue. Those studies will be done, but we've got no clue, we can't put a number, a probability, that this patch would have worked. It kind of comes under the category of 'at least we would have done something.'"

The Atlantis rescue scenario is a bit more believable, but only just. At the time of Columbia's launching, Atlantis was being prepared for a flight to the international space station in March. It's tank and boosters were mated in early January and Atlantis was scheduled to be moved from its hangar to the Vehicle Assembly Building Jan. 29 for attachment and subsequent roll out to the pad.

Assuming an inspection spacewalk by flight day 5 - Jan. 20 - and a decision that day to begin around-the-clock processing of Atlantis, the NASA study concluded the shuttle could, in theory, be ready for a launch by Feb. 10 or 11. That corresponds to flight day 26/27 for Columbia's crew. The launch schedule assumes the elimination of non-essential countdown "holds," the elimination of non-essential pre-launch tests and simulations, no mechanical problems or processing snags and good weather.

"The study indicates if you have five or six 'ifs' that line up and you get an affirmative answer to each of the five or six 'ifs,' the launch of the second shuttle and the rescue was conceivable," Gehman said. "It isn't easy, it's not even highly likely. But it is conceivable.

"By the six ifs, I'm saying if you could have reduced the processing time satisfactorily and if you could go through the launch countdown and prelaunch preparations without a mechanical problem and if the weather was suitable and if the rendezvous was successful and the two orbiters could maintain station on each other and if the multitude of EVAs worked all right, you could have done this. It is possible."

Atlantis would carry four veteran astronauts and would launch at night, rendezvousing with Columbia within 24 hours. The shuttle's flight computers would use a software "load" developed for the March shuttle mission, patched to take into account altitude changes and other factors relating to an immediate rendezvous.

Columbia would be oriented with its payload bay facing Earth and its tail in the direction of travel. Atlantis would approach from below in a manner similar to past linkups with the Russian Mir space station. Picturing Columbia moving upside down and tail first to the left, Atlantis would approach from directly below with its left wing facing the direction of travel and its open payload bay facing Columbia. The long axis of Atlantis would thus be turned 90 degrees with respect to the long axis of Columbia.

Once on station at a distance of 50 to 60 feet, spacewalkers from Atlantis would carry two spacesuits and additional lithium hydroxide to Columbia, staying in contact with safety tethers at all times. The final two astronauts to leave Columbia would configure cockpit switches and on-board software to permit flight controllers to remotely deorbit Columbia.

"Columbia would be lost," Gehman said. "There was no way to recover Columbia. The last two astronauts who left the Columbia would flip certain switches in certain positions so Columbia could be deorbited on command from Houston. And then the Columbia would be lost, ditched in the ocean."

All 11 astronauts - four of them sitting on the floor of Atlantis's lower deck - then would return to Earth.

"Based on the verbal report the board got, it indicated the launch of the Atlantis was the less risky and the more probable (of the two options studied)," Gehman said. "The on-orbit repair was very, very unknown as to whether or not it would have worked or not. But obviously, it's inconceivable that we would have done nothing if we had known."

He said the decision to launch a rescue mission ultimately would have depended "solely on how good the knowledge of the damage to the orbiter was."

"In other words, if somebody came and said we've got a little tiny hole in the leading edge and we think the orbiter can re enter safely but it might not, then probably the decision to launch another orbiter would be very hard to come by," he said. "If on the other hand an inspection said 'holy mackerel, we've got a great big hole and the orbiter is doomed, the crew has no chance whatsoever,' then the probability they would have launched a rescue mission probably would have gone up."

He summed up the rescue scenario by saying "it's technically possible, very, very risky and a whole bunch of ifs have to line up. ... But I have no idea if it would have been successful or not."