SPACE CENTER, Houston – After much soul-searching and analysis, NASA cleared Shuttle Discovery (search) to return to Earth next week, concluding Thursday that there was no need to send the astronauts out on another spacewalk to repair a torn thermal blanket near a cockpit window.
Mission managers could not guarantee that a piece of the blanket won't rip off during re-entry and slam into the spacecraft, but they said the chance of that happening was remote and that it would be riskier to try to fix the problem.
"The lowest risk, the best choice and the unanimous decision of the engineers in the management team is that we should re-enter as is," deputy shuttle program manager Wayne Hale (search) said in a news conference.
Discovery's astronauts awoke late Thursday prepared to pack up. They planned to transfer a cargo container from the station back into the shuttle's cargo bay in preparation for the ship's undocking from the station, set for Saturday.
NASA (search) had been considering sending the astronauts out to snip away part of the blanket for fear a 13-inch section weighing just under an ounce could tear away during the latter stages of descent and strike the shuttle, perhaps causing grave danger.
Wind tunnel tests hurriedly conducted in California on real thermal blanket samples showed it is possible tiny pieces of the fabric might shred off, Hale said. In the worst situation, he noted, there is a 1.5 percent chance that the entire 13-inch section would come off and hit the shuttle.
It's possible, under that remote circumstance, that the cloth could strike the rudder speed brake and create a hole and 6-foot-long crack, but even that would not be enough to endanger Discovery and its crew of seven, Hale said. He noted, however, that there are a lot of assumptions and variations in that chain of analysis, "so is that the absolute worst thing that could happen? Well, no, it's not."
"I am not here to tell you that we are 100 percent confident that there is no risk during re-entry. That would be untrue and foolish to even try to make that case," he said. "But I am here to tell you is that we've assessed this risk to the very best of our engineering knowledge and we believe that it is remote, small, whatever adjective you want to put with that."
Discovery is scheduled to land back at Cape Canaveral, Fla., before dawn on Monday.
Had the astronauts been asked to repair the blanket, it would have been the fourth spacewalk of the 13-day mission. It also would have been the second time during the flight that the astronauts had to step outside to repair the shuttle's thermal protection and reduce the risk of another Columbia-type tragedy during the trip home, when the spacecraft passes through the blowtorch heat of re-entry.
Mission Control radioed the "good news" up to the astronauts before they went to sleep.
Hale pointed out that there is a "profound" difference in the way NASA is handling such problems now, compared with the way it did before and during Columbia's doomed flight in 2003. Many of the safeguards put in place for this flight — like photographing the shuttle from every angle and using lasers to hunt for any cracks in its thermal shielding — were the direct result of the tragedy.
"We put the orbiter under a microscope," Hale said. "I mean, we're out there with every little spot, scuff, ding, you name it. It's not ignored. It's not brushed off. It's evaluated by a large team of engineers who are very knowledgeable about the orbiter."
Noted astronaut Stephen Robinson from orbit: "We know more about our shuttle before re-entry than any crew has ever known before."
Taking part in Thursday's mission management team meeting were a few of the engineers who had serious concerns about the foam strike to Columbia but did not express them to their bosses while the shuttle was still in orbit.
Hale, the team's chairman, said everyone agreed the best thing to do for Discovery was to leave the thermal blanket alone. But he said one of the engineers who did not speak out during the Columbia flight wanted managers to understand the uncertainties that still remain from "this admittedly very rapid set of tests and analyses that have been run."
Hale said mission managers really had only three options: try to fix the blanket under the commander's side window in a complex spacewalk with no good repair strategy; order the astronauts into the space station, discard Discovery into the ocean and launch a rescue shuttle, hopefully before the oxygen runs out in one to two months; or bring Discovery home as is.
After a lengthy meeting, managers unanimously chose the latter.
Earlier in the day, Discovery's commander, Eileen Collins, said in an interview with The Associated Press that she's been sending messages to her two children, ages 9 and 4, "telling that I love them and that I miss them and not to worry about us."
"I feel very confident coming back because we have looked at everything," she said.