"It was a very easy decision to add the task to EVA number three to go and remove those two gap fillers," Wayne Hale, deputy manager of the space shuttle programme, told a press briefing on Monday in Houston, Texas.
Nasa is uncertain about how the material could affect the shuttle when it re-enters the Earth's atmosphere. "We investigated that at length. The team has been working for three days," Hale said.
"At the end of the day the bottom line is, there is large uncertainty because nobody has a very good handle on aerodynamics at those altitudes, at those speeds."
"This is the new Nasa. If we cannot prove that it's safe, then we don't want to go there," he said.
"This is the new Nasa. If we cannot prove that it's safe, then we don't want to go there"
Wayne Hale, space shuttle programme deputy manager
During the mission's third spacewalk on Wednesday, an astronaut will attempt to pull out the dangling pieces of gap filler or, if that proves too difficult, cut them off.
Hale said the shuttle could safely re-enter the atmosphere with the errant gap filler removed, even though for one of the two pieces, "there's a possibility that heating could get into that gap".
"For one flight we are well within our safety margins," he added.