"We have looked at everything," Collins told The Associated Press in an interview. "I wouldn't fly this flight if I didn't think it was a safe thing to do."
After Wednesday's successful spacewalk during which two pieces of gap-filling cloth were removed from the shuttle's exterior, CBS News Correspondent Mark Strassmann reports that Deputy shuttle program manager Wayne Hale says just one concern lingers: a hole torn in a thermal blanket near the cockpit window.
"The only thing that we've to go work on is if the blanket came off," Hale said. "Where would it go and what would it do?"
A fourth unplanned spacewalk may be necessary to take care of the problem.
The soonest the spacewalk could occur would be Saturday, which would delay Discovery's scheduled undocking from the international space station and result in the shuttle's return to Earth being pushed back a day until at least Tuesday. The shuttle is scheduled to return Monday.
Mission managers planned to meet Thursday morning to decide whether another spacewalk to repair the ship is needed.
Meanwhile, as Discovery orbited the Earth on Thursday, the shuttle's crew sent down images of the planet below and each crewmember took a few minutes to discuss space exploration, its costs and remembered those who didn't make it home.
"We choose to do these things not because they are easy, but because they are hard," astronaut Wendy Lawrence said. "And certainly, space exploration is not easy and there has been a human price that has been paid."
Astronaut Charles Camarda said Columbia's loss in 2003 reaffirmed the need for those who explore to be vigilant about the risks involved.
Columbia was doomed by a piece of insulating foam shed from its external fuel tank during launch. The foam pierced a wing and caused the shuttle to disintegrate over Texas, killing all seven astronauts.
"In that accident, we not only lost seven colleagues, we lost seven friends," Camarda said. "They believed in space exploration. They knew the risks, but they believed in what they were doing. They showed us that the fire of the human spirit is insatiable."
Based on the Columbia tragedy, Discovery's astronauts have spent a majority of their test flight mission inspecting their ship, making repairs and testing out new repair techniques.
Collins said her crew had thoroughly studied the procedures before Wednesday's unprecedented spacewalk when astronaut Stephen Robinson removed two worrisome pieces of filler material from the shuttle's belly. It was the first time an astronaut has ventured below the ship.
NASA engineers thought the tile filler removed Wednesday might cause the shuttle to overheat during its descent through the atmosphere and lead to another Columbia-type disaster.
Robinson told the AP that he was careful to make sure his body and none of his tools contacted the shuttle's fragile belly during the task.
"I had my hand out and I had that purposely out there sort of as potential bumper," he said. "If I was going to hit anything on the orbiter, I wanted it to be my fingers in a springy position."
Hale said tile fillers, like the ones removed Wednesday, have probably protruded on previous flights, but this time the agency spotted them and was able to repair them because of the new inspection techniques put in place after Columbia.
Robinson was barely back inside the shuttle and out of his spacesuit Wednesday when Mission Control informed the crew about the chance of another spacewalk to deal with the torn thermal blanket.
The concern is that a roughly 1-foot section of the blanket could rip away during re-entry, whip backward and slam into the shuttle, perhaps causing grave damage. Engineers expected to know by late Thursday whether the danger is real and whether any blanket trimming is required.
"I think in the old days we would not have worried about this nearly so much," Hale said. "I am very hopeful that we will be able to put this issue at rest."
Also Thursday, astronauts finished up their testing of new repair techniques.
They demonstrated a proposed repair method for small holes in the carbon panels that line the shuttle's wings. That repair technique, tested in Discovery's mid-deck on samples brought to space, was the last of three techniques set to be tested on orbit by Discovery's crew.
Astronaut James Kelly told Mission Control the astronauts got 40 minutes of video from the experiment, which he called a success.
Perhaps Wednesday's successful spacewalk also encourages the astronauts. Although this was the first time an astronaut has ventured beneath the ship, CBS News Space Consultant Bill Harwood called the emergency repair job "a piece of cake."
"All the talk over the past couple days about this risky this how it's never been done before … well, it's never been done before, but he sure made it look easy," Harwood told The Early Show.