Astronaut Dave Williams, the third Canadian to walk in space, is working with a fellow astronaut to install a space station beam onto the orbiting lab's frame.
Williams and Mission Specialist Rick Mastracchio added a newly delivered $11-million truss segment, part of a power-generating solar array that will be delivered on future missions.
The spacewalk began at 12:28 ET on Saturday and is expected to last for more than six hours. It is the first of two scheduled spacewalks for Williams.
He may be asked to perform a third if NASA extends shuttle Endeavour's stay in orbit.
On the ground, NASA engineers continued to review camera and radar images on a troubling gouge located on the underside of the shuttle. NASA believes the damage, about 7.6 square centimetres, was caused by ice that broke off the fuel tank a minute after liftoff.
The gash will be inspected on Sunday to determine if the damage will need to be repaired, which could mean a fourth spacewalk for the astrounauts.
The shuttle comes equipped with a repair kit complete with three types of patch materials -- an addition to every mission since the 2003 Columbia disaster.
ISS construction underway
Dr. Steve MacLean, the Chief Astronaut for the Canadian Space Agency, says the truss assembly for the International Space Station is Saturday's main objective.
MacLean did a spacewalk last September, on an Atlantis mission, and knows the drill well.
The truss is no small addition either. Roughly the size of a car and weighing in at 2,200 kilograms, the truss segment will be moved into position using the Canadarm2 before the astronauts secure it to the station.
Once it is in position, Williams and Mastracchio "will be outside when they do the final assembly there, and they actually will use their own eyes to do the final check-ups on guiding it into its location," MacLean says.
In a departure from earlier missions, the work will be done with fewer automated manoeuvres and more control being given to the astronauts. They can use their judgment while floating in space.
The spacewalk comes with its own unique challenges. The truss must be attached with bolts and electrical connections must be made, and all the while the astronauts will have to resist the spectacular views of Earth.
"I don't know if it's a challenge, but it's a tremendous amount of fun, they're out on the very end of the truss, and the view from there," MacLean says. "That's just a wonderful thing to be out there, and you do need to keep your focus, because actually a mistake out there is worse than a mistake, for example, inside the cargo bay of the shuttle."
Apart from the distracting view, there is also a small chance that micrometeorites, could force the astronauts to cut the spacewalk short.
The International Space Station has about a one in 100,000 chance of being hit by the small bits of rock and metal. The space shuttle has a smaller chance, and the astronauts even smaller, but training is done to prepare for such an eventuality.
"When you're out on a spacewalk, if you get hit by a micrometeorite, let's say you get a hole the size of a nickel in your leg after being hit, you have to get back to the station. With a hole that size you probably have 30 minutes to get back to the Station, and where Dave is, it's probably about 10 or 15 minutes to get back."
MacLean is quick to point out that space is a very big place, and the probability is very small, but it is something they will be watching for.
Improved view on second walk
These same challenges may be heightened on the second spacewalk, scheduled for Monday, when Williams will have an even more spectacular view, with his feet firmly attached to the end of the Canadarm2.
Williams and Mastracchio will be working to replace a faulty Control Movement Gyro (CMG), which helps the Station maintain its position, and Williams will carry the 600-kilogram CMG while standing on the end of the Canadarm2.
"We have three of them, recently one broke down, so we have only two that are working, we need two to be working, so we need to get the third one in there in case another one fails so we always have attitude control," MacLean says. "These momentum wheels help us maintain the attitude of the station without using fuel, so it's a lot cheaper."
Whether there is a third spacewalk will likely not be determined until the shuttle's fifth day in space. If the assembly of a new system that allows power to be transferred from the Station to the shuttle is successful, then the mission could be extended from 11 to 14 days, and allow for the additional spacewalk.
MacLean believes it is likely the mission will be extended, but says it is not at all critical that the final spacewalk is completed.
"The third spacewalk that Dave might do, is basically a spacewalk where they do some clean-up, do some preparations for future flights, they have a communications antenna to replace, and so there's a series of tasks that need to get done," he said.
"The difference is though, if they don't quite finish the third spacewalk that Dave is on, that's not that critical, because we have a long list of items that we need to get done and some of them we can almost do any time."
