"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.