Meet the STS-121 Crew: The Spacewalkers

This NASA illustration depicts the STS-121 spacewalkers as they test the stability of Discovery’s orbital boom during their first spacewalk. CREDIT: NASA/JSC.

Two astronauts are preparing to take at least two strolls in space outside NASA's Discovery shuttle after it launches into orbit Saturday.

STS-121 spacewalkers Piers Sellers and Michael Fossum are charged with some vital tasks during their 12-day spaceflight, not the least of which is making pivotal repairs that will make or break the future of the International Space Station (ISS).

A mix of experience and enthusiasm, Discovery's spacewalking team will test heat shield repair methods and fix a vital cable reel on the space station's Mobile Base Transporter - a railcar that is vital for ISS construction.

"It's been about two years of pretty hard work," Sellers said of his mission training in a prelaunch interview.

Discovery's STS-121 mission is slated to launch at just past 3:48 p.m. EDT (1948 GMT) on July 1 on NASA's second shuttle flight since the 2003 Columbia tragedy. STS-121 pilot Mark Kelly will assist Sellers and Fossum from inside Discovery, NASA officials said. The pair could make an additional spacewalk if orbiter resources allow.

Funnyman takes the lead

A veteran of one shuttle flight and three spacewalks, Sellers is by far considered the funnyman of the STS-121 mission by his crewmates and will lead the spaceflight's multiple spacewalks.

"We all make fun of each other," said Sellers, 51, adding that humor can break tension during training. "You have to think of training as a marathon, it goes on for many months, and then you get a slip and sometimes if can feel like it's getting frustrating. And then you feel like laughing."

A native of Crowborough, Sussex in the United Kingdom, Sellers joined NASA's astronaut corps in 1996 and flew aboard the Atlantis shuttle during its ISS-bound STS-112 mission in 2002.

"It's great just to sit there and think about where you are," Sellers said of shuttle flights. "And think, this is a remarkable place to be with the world spinning through space below you, and you in this little ship floating around it."

Sellers, who is married and has two grown children, will ride at the end of Discovery's 50-foot (15-meter) orbital boom during the first STS-121 spacewalk to check its stability as a work platform for heat shield repair. He and Fossum will then repair the Mobile Base Transporter's cable reel system.

"As long as [it] can move up and down, we can build the space station. Right now it's broken and we're going to try to fix it," Sellers said of the complicated task. "It's not a sure thing by any means."

A third spacewalk, to check a series of heat shield repair techniques, will likely be added later in the mission, the experienced spacewalker said.

"I would be very surprised if we didn't do it," Sellers said, adding that Discovery's cryogenic fuels should be ample enough to support the third spacewalk unless the orbiter is stuck on the launch pad for several days for some reason. "That would require some bad luck."

First-time spacewalker

Aiding Sellers in the STS-121 spacewalk is Michael Fossum, 48, who will make his spaceflight debut with Discovery's planned July 1 launch.

"I love the space business," said Fossum, who grew up in McAllen, Texas, in a preflight interview. "I wanted to be part of it one way or another."

A former U.S. Air Force test pilot and colonel in the USAF Reserves, Fossum joined NASA in 1993 - five years before he became an astronaut - to test the potential of Russian Soyuz spacecraft for use on the ISS. He helped in an ISS redesign and later served as a flight test engineer on NASA's X-38 space station crew escape vehicle.

Fossum, who is raising four children with his wife Melanie, considers his spaceflight another notch in his family history, which has been packed with explorers since his first ancestors came to the U.S. from Norway, Ireland and Germany more than 130 years ago.

"It's really a large part of what makes me who I am," Fossum said. "The romantic in me likes to think that I'm living just a little fraction of that life."

Outside NASA, Fossum holds scouting high on his list of activities and is himself an Eagle Scout from his days with the Boy Scouts of America.

"I know that it has changed my life in many significant ways and...I like the opportunity now to help work with the kids," Fossum said in a NASA interview, adding that his troop has given him some gruff about missing this summer's major camping trip. "The guys give me a hard time, telling me I'm going to be missing next summer's High Adventure activity because I'm working on another one for NASA."

Despite the busy schedule facing the STS-121 astronaut, Fossum hopes for a free moment to absorb the feat he and his crewmates are accomplishing.

"I hope to have a few minutes just to appreciate the opportunity that I've got," Fossum said. "To have it sink in without having to rush to the next task."

Robert Z. Pearlman contributed to this story.

• Meet the STS-121 Crew: The Flyers

• STS-121 Crew to Fly on July 1
• Return to Flight: NASA's Road to STS-121
• The Great Space Quiz: Space Shuttle Countdown