Damage to one part of the shuttle Endeavour's heat tiles penetrates right through the thermal shielding and has NASA making some high-pressure decisions about whether spacewalk repairs are needed.
"If it's required, we'll do it. If it's not, we won't," John Shannon, chair of the NASA mission management team, told reporters Sunday in Houston.
"As to which way we'll go, we'll see through the analysis over the next 24 to 48 hours."
Throughout the day, Canadarms 1 and 2 were used to manoeuvre around Endeavour to reach the shuttle's underbelly with a laser camera built in Ottawa. The laser allows a three-dimensional image to be built of the damage.
"They got right on the five different targets that we asked them to go look at," Shannon said.
Most of the nicks don't appear to be problems, he said.
A major gouge did occur at the 58-second mark of liftoff on Saturday. The gouge occurred when a chunk of insulating foam broke off and ricocheted off the fuel tank, striking the shuttle.
The gouge is nine centimetres long and five centimetres wide. The gouge appears to into the felt pad below and possibly exposes some of the shuttle's aluminum skin.
"Not only did we get really good imagery, we got a good laser scan of it as well," Shannon said of the gouge.
Scientists will make a "point cloud" map of the gouge and do a thermal analysis on it to see how the damaged area might hold up upon re-entry, he said.
In addition, the exact same damage can be mimicked on test tiles at the Johnson Space Centre in Houston, he said.
A decision will likely be made by Monday -- or Tuesday at the latest -- on whether repairs are necessary.
Once a decision is made, Canadian astronaut Dave Williams and U.S. astronaut Rick Mastracchio will likely be the two assigned to perform any necessary patchwork, NASA has said.
In 2003, the shuttle Columbia burned up on re-entry as a result of damage to its heat-resistant tiles sustained during launch.
The Canadian connection
Canadian technology played a key role in inspecting a worrisome gouge on the belly of the space shuttle Endeavour.
Astronaut Chris Hadfield says the Canadian robotic technology is an integral part of international efforts in space.
"We couldn't fly the shuttle without the ability to inspect it, we couldn't build the station without the Canadarms," Hadfield said.
As a small Canadian tribute, Mission Control woke the astronauts on Sunday morning by playing Shania Twain's "Up" for them.
"A special good morning to you today, Dave," the controller said. Williams said he really enjoyed the wake-up music.
Williams became the third Canadian to walk in space on Saturday during his inaugural spacewalk after floating out of the International Space Station.
He and Mastracchio accomplished their major task -- installing a new two-tonne beam on the station -- just under an hour later.
That spacewalk lasted just over six hours.
Their next spacewalk is scheduled to start Monday at 11:31 a.m. ET to replace a faulty control movement gyro on the space station, one of four such gyros.
NASA has also made a decision to extend the mission's length to 14 days following successful operation of the new station-to-shuttle power transfer system.
There will be a fourth spacewalk on Friday. The shuttle will undock from the space station on Aug. 20 and land on Aug. 22.
With a report from CTV's Sarah Galashan and files from the Associated Press
