HOUSTON - Two shuttle astronauts are prepared to perform a busy pair of spacewalks--and likely one extra--on tap for NASA's upcoming Discovery mission slated to launch early next month.

STS-121 mission spacewalkers Piers Sellers and Michael Fossum are gearing up for at least two excursions outside Discovery to make space station repairs and test whether NASA's shuttle orbital boom--a camera-tipped, 50-foot (15-meter) extension of the spacecraft's robotic arm--can be used as a work platform.

Sellers and Fossum will join their STS-121 crewmates--commanded by shuttle veteran Steven Lindsey--in a planned July 1 launch toward the International Space Station (ISS). The mission will mark NASA's second orbiter flight since the 2003 Columbia accident and the last test mission before NASA resumes ISS assembly flights.

Balancing Act

The STS-121 spacewalkers first excursion outside will find them perched at the tip of Discovery's sensor-laden orbital boom.

Developed after the Columbia accident to allow shuttle crews to scan their heat shield and determine their vehicle's health, the orbiter boom sensor system (OBSS) carries cameras and laser-ranging scanners to search for blemishes or breaches on heat-resistant tiles and panels.

At 50 feet (15 meters) in length, the boom nearly doubles the length of Discovery's robotic arm, and that extended reach has led to the question of whether it could be used by astronauts to repair heat shield damage in previously inaccessible areas.

"The [STS-] 114 crew proved its stability as a sensor platform to go out and do close-in inspections of the leading edge and underbelly of the shuttle. The next part of the question is will it hold crewmembers stable enough outside to actually effect a repair if one were to be needed," said Fossum.

The two spacewalkers will conduct the boom stability tests, which Fossum has referred to as 'bouncing on the boom,' first over Discovery's payload bay and then re-positioned near the station's solid truss.

"I'll be pushing on [the truss], as if I am doing simulated repair activities just to verify if it holds us steady enough," explained Fossum.

The tests will begin with just Sellers attached to the boom's end while Fossum looks on, and then will advance to having both astronauts suspended, as would likely be needed during a real repair.

"It will be an eye opener being on the waving end of about 100 feet of robotic arm and boom, way out away from the shuttle, out of the payload bay," Fossum said.

Vital repairs

Sellers and Fossum will dedicate part of their first spacewalk and most of their second to work that is vital for the future of the ISS.

The primary--and complicated--tasks require the two astronauts to safe a faulty guillotine-like device and replace a cable reel on the space station's Mobile Transporter (MT).

"[The MT] is a mobile platform that can move along the forward structure of the station, providing a work-site platform for the station robotic arm," explained Tomas Gonzalez-Torres, Lead STS-121 extravehicular activities officer.

In December 2005, the MT's interface umbilical assembly (IUA) inadvertently fired its cable cutter, severing primary power, data and video to the transporter.

Earlier this year, ISS astronauts were unable to install a safing bolt that would prevent the cutter system from severing a backup cable to the MT. They instead removed the cable entirely, ensuring it would not be cut but rendering the transporter motionless until it could be repaired.

The railcar is critical for future ISS assembly missions to proceed.

"The first task of the first EVA will be the installation of an item called the blade blocker," said Gonzalez-Torres, "which will physically prevent the blade from possibly severing [the back-up cable]".

During their second venture outside, Sellers and Fossum will replace the transporter's Trailing Umbilical System (TUS).

"The TUS is basically an extension cord that provides power and data to the MT," said Gonzalez-Torres.

The spacewalkers will also install a spare pump for the station's thermal control system on the outside of the Quest airlock.

Third spacewalk possible

Sellers said he is confident that he and Fossum will be able to venture outside Discovery a third time during their mission to test shuttle heat shield repair techniques.

The planned spacewalk was officially pulled off the STS-121 schedule to allow more time for orbiter inspections, but could be added--along with an extra day--if shuttle power resources permit.

"I'd be very surprised if we didn't do it," Sellers told SPACE.com. "We're training as if we are going to do it and the whole mission training profile that we've been working to is based on the premise that we will do a third EVA."

"The reason we wouldn't do one is if we were to launch after a long time of sitting on the pad and our cryogenics supplies that supply electricity would have reduced--boiled off a bit--so we wouldn't have time to stay on orbit for the extra day," Sellers explained. "But that would sort of require some bad luck with weather or technical problems, day after day, for that to happen. So I am betting if we get off in the first few days of July, we'll do the third EVA."

The third EVA calls for the two spacewalkers to apply a black, heat-resistant material called NOAX (short for non-oxide adhesive experimental) on cracks in the same type of reinforced carbon carbon panels that line Discovery's wing leading edges but carried in a special experimentation box stowed in the payload bay.

STS-114 astronaut Stephen Robinson conducted a similar test during NASA's first return to flight mission, but only under the extreme cold of nighttime conditions, which in turn led to the formation of bubbles that--in an actual heat shield repair--could compromise the fix.

Sellers and Fossum are scheduled to test the NOAX material during daylight conditions, to check the repair technique's effectiveness.

SPACE.com staff writer Tariq Malik contributed to this story from Singapore.