Douglas Wheelock and Scott Parazynski will step out of the ISS at 10:28 a.m. GMT. The second spacewalk to be carried out as part of Discovery's trip to the station will involve Parazynski and Daniel Tani.
The main goal of the first three spacewalks will be to link the Italian-made Harmony Node 2 module to the ISS. The compartment will expand the interior of the station to around 500 cubic meters (18,000 cubic feet), and allow European and Japanese research laboratories to be connected to the ISS.
Other tasks include removing a broken antenna from the orbital station, and moving the station's P6 truss structure, a beam holding a U.S.-made, power-generating solar wing.
Typically, astronauts on the ISS make three spacewalks during a shuttle's visit. However, changes to the station's energy supply, with increased reliance on solar power, have allowed astronauts to remain in orbit for longer periods.