The upcoming Discovery mission set to launch early next month will have two of its shuttle astronauts performing a couple spacewalks and possibly a third one. The lucky future STS-121 spacewalking cowboys, Piers Sellers and Michael Fossum, are getting ready for at least two excursions outside Discovery for some space station repairs.

Their first tasks require them to safe a faulty guillotine-like device and then replace a cable reel on the space station’s Mobile Transporter (MT). In December last year, the MT’s interface umbilical assembly (IUA) accidentally fired its cable cutter severing primary power, data and video to the transporter.

They will also test whether NASA’s 50-foot extension of the spacecraft’s robotic arm (orbital boom) can be used as a work platform. The orbital boom is almost twice the length of Discovery’s robotic arm and the test would confrim if it can be used by astronauts to repair heat shield damage in occuring in otherwise inaccessible areas. The test will have the two spacewalkers bouncing on the boom over Discovery’s payload bay and then re-positioned near the station’s solid truss.

The orbital boom was developed after the Columbia accident to allow the space crew to scan their heat shield and determine their vehicle’s condition. Its orbiter boom sensor system (OBSS) cameras and laser-ranging scanners can search for blemishes or breaches on heat-resistant tiles and panels.

During their second venture outside, Sellers and Fossum will replace the transporter’s Trailing Umbilical System (TUS) which provides power and data to the MT. They will also install a spare pump for the station’s thermal control system located outside Quest airlock.

The third spacewalk, if time and shuttle power resources permit, will have Fossum and Sellers testing shuttle heat shield repair techniques. That space excursion would involve the two spacewalkers to apply a black, heat-resistant material called non-oxide adhesive experimental (NOAX) on cracks in the same type of reinforced carbon carbon panels lining Discovery’s wing leading edges.