Officials also were studying a rip in astronaut Rick Mastracchio's glove that brought Wednesday's spacewalk to an early end, and they were loathe to authorize another one until they knew more. But they said those concerns wouldn't stand in the way of crucial repairs.
"If we decided we needed to go do this, I would feel very comfortable doing it. We've done a lot of spacewalks without any glove problems," said John Shannon, the mission management team's chairman.
Mission Control told Endeavour commander Scott Kelly early Thursday that a repair decision was expected by mid-day.
Endeavour's crew spent early Thursday learning what to do if NASA orders the repairs. Astronauts on the ground have been practicing the techniques so they can send the crew precise instructions and answer any questions.
Teacher-astronaut Barbara Morgan also spent time Thursday speaking with students at the Challenger Center for Space Science Education in Virginia, which was founded by the families of the doomed shuttle's crew. Morgan was Christa McAuliffe's backup for the 1986 flight.
The chat was moderated by the commander's widow, June Scobee-Rodgers.
Morgan squirted soapy water on her face to demonstrate how astronauts keep clean, and crewmate Al Drew brushed his teeth, spitting into a towel because sinks won't work without gravity. Morgan also encouraged the children not to give up on science, even if they think they are not good at it.
"Oftentimes when you're not good at something, it's just that you haven't learned how to yet," she said.
When asked if she had had any special teachers in her life, Morgan said the seven members of the fallen Challenger crew were mentors "that have meant more than anything to me."
"They were my teachers and I believe they are teaching us today still," said Morgan, who held an emblem from the fallen crew up to the camera as the event ended.
Later Thursday, Morgan planned to talk by ham radio with students at the Idaho school where she taught before moving to Houston in 1998 to become the first teacher to train as a full-fledged astronaut. That chat could not be broadcast because she was using the ham radio.
NASA has spent nearly a week agonizing over the 3½-inch-(9 centimeter)-long, 2-inch-(5-centimeter)-wide gouge that resulted from a debris strike at liftoff. Part of the gouge, a narrow 1-inch (2.5-centimeter)-strip, cuts all the way through the tiles, exposing the thin felt fabric that serves as the final thermal barrier to the ship's aluminum frame.
The exposed area and the gouge itself are so small that NASA is not worried about a Columbia-type catastrophe at flight's end. They are concerned, however, that the underlying aluminum structure might be damaged enough by the searing heat of re-entry to warrant lengthy post-flight repairs.
Officials have to balance those fears with the risk that astronauts wearing 300-pound (136-kilogram) spacesuits and carrying 150 pounds (68-kilograms) of tools could bang into the shuttle and cause more damage as they try to fix the gouge.
Putting the wrong amount of the caulk-like repair goo into the gash or failing to put it in exactly the right spot could make the problem worse, Shannon said.
The unprecedented patching job on Endeavour, if approved, would be performed on the next spacewalk, now set for Saturday, a day later than originally planned to give engineers more time to analyze the situation. That could keep Endeavour and its crew of seven at the space station at least an extra day.
Astronaut Clay Anderson, who joined Mastracchio on Wednesday's spacewalk, was taking digital pictures on Thursday of all the gloves on board so NASA could make sure they were in good shape. Mastracchio has a spare glove on board.
The repair job would not be anywhere near where Mastracchio was Wednesday, when he and Anderson moved two rail carts and an antenna base into new positions on the orbiting outpost. They also added new antenna parts to improve voice communications.
The next spacewalk will be trickier if shuttle repairs are ordered. Under the latest scenario, Mastracchio and astronaut Dave Williams would apply black paint to the white gouge and squirt in the caulk-like goo, while balancing themselves on the end of the shuttle's 100-foot (30.5-meter) robot arm and extension boom.
"We want to clear the fact that it might have been a generic problem with gloves before we go back out the door," Mission Control told Kelly.