NASA astronauts Piers Sellers and Michael Fossum completed three spacewalks during the STS-121 mission, vital steps towards NASA’s goal of finishing assembly of the international space station (ISS) and verifying the integrity of the Space Shuttle Discovery’s heat shield for a safe return to Cape Canaveral.

During the first spacewalk July 8, Sellers and Fossum spent most of their time determining the stability of Discovery ‘s 30-meter robotic appendage to determine its stability for the delicate work that might one day be necessary to fix an orbiter’s damaged heat shield.

“My first impression is that it damped out a lot faster than I thought it would,” Sellers said of the extended arm’s movements after the tests were completed.

Sellers and Fossum each spent time bouncing on the end of the 15-meter inspection boom. They mimicked typical spacewalk activities — such as grabbing a camera or reaching for tools — then moved close to the main truss of the ISS for some mock shuttle tile and wing panel repairs.

Developed in direct response to the Columbia tragedy, the boom has been used to scan the shuttle’s heat shield for damage.

Tony Ceccacci, lead shuttle flight director for Discovery’s STS-121 spaceflight, said the July 8 tests would help engineers decide whether repairs can be staged from the boom as is, or whether a specialized workstation would have to be developed to attach to its tip.

Sellers and Fossum also installed a blade blocker into a cable cutter system on the top side of the Mobile Transporter to prevent it from inadvertently slicing a power, video or data cable as the Mobile Transporter moves along the station’s main truss.

The Mobile Transporter is a vital piece of ISS hardware because it serves as a moving base for the station’s robotic arm and a carrier for large pieces of hardware, such as a new solar array tower slated to be delivered to the orbital laboratory in late August.

The repair allowed the Mobile Transporter to move into position for the July 10 spacewalk, when the two astronauts replaced the railcar’s baby grand piano-sized Trailing Umbilical System , and delivered a 631-kilogram spare pump module to the ISS, which transports liquid ammonia through the space station’s cooling system.

While there were no major problems, Sellers and Fossum had their share of frustrations during the second spacewalk .

The astronauts had to work through stuck bolts , a stubborn spring that prevented initial attempts to install the new cable reel in place, and a lot of bumping that ultimately knocked Sellers’ emergency thruster system loose.

Sellers was connected to Discovery or the ISS with two tethers at all times and not in danger of drifting free, but spacewalk planners were concerned the Simplified Aid for EVA Rescue (SAFER) thruster system could separate from his spacesuit and pose a debris hazard for the orbiter or station. Fossum later re-engaged the latches to lock Sellers’ SAFER system into its fittings.

Tomas Gonzalez-Torres, lead spacewalk officer for the STS-121 mission, said his spacewalking astronauts and ground crew performed above the bar during the orbital repair, and were on schedule — and ahead at some times — through the six-hour-and-47-minute activity.

The third and final spacewalk of the STS-121 mission was added to the spaceflight — along with a 13th flight day — once flight controllers found the shuttle’s power supply could support it, NASA said.

The two astronauts tested basic shuttle heat shield repair techniques during their final spacewalk July 12.

Sellers and Fossum evaluated methods of applying an experimental heat-resistant material onto samples of the same type of panels that line the orbiter’s wing edges and nose cap.

Sellers said their primary task — testing a putty-like heat shield repair material dubbed NOAX — was more akin to a “meticulous lab experiment” rather than the heavy lifting of their earlier spacewalk to repair the space station’s Mobile Transporter.

NOAX, which is short for non-oxide adhesive experimental, is a sticky black substance that carries the initial consistency of peanut butter before it is worked into place in orbit, NASA officials said. The space agency described the material as a pre-ceramic polymer that is impregnated with carbon silicon carbide powder.

Engineers designed NOAX as a coating, crack and gouge filler for the black panels that protect Discovery’s wing leading edges and nose cap. Made of a carbon composite called reinforced carbon carbon (RCC), the panels endure the hottest temperatures when the shuttle re-enters the Earth’s atmosphere.

Discovery’s STS-121 pilot Mark Kelly orchestrated the spacewalk from inside the orbiter, while crewmates Stephanie Wilson and Lisa Nowak worked the space station’s robotic arm.

Sellers and Fossum also tested a new infrared video camera . The spacewalkers used the camera to record two, 20-second videos of Discovery’s wing leading edges. They then used the camera to study two of the intentionally damaged RCC samples in the shuttle’s payload bay, as well as two others that had undergone a test repair.

Ceccacci said the camera could be used by hand or be mounted to the tip of Discovery’s orbital boom if its images prove useful for heat shield inspections.

Sellers also used the camera to observe parts of the ISS, including its radiators and one spacewalker — Fossum — as he crawled across the station’s hull.

The July 12 spacewalk marked the third extravehicular activity of Fossum’s spaceflight career and the sixth for Sellers. It was the 21st spacewalk staged from the station’s Quest airlock.