The station's robotic arm moves the S6 unit into position
Astronauts have installed the final truss, or backbone segment, to the International Space Station (ISS).
The Starboard 6 truss holds the fourth pair of solar arrays needed to fully power the orbiting outpost.
Friday will see the command given to unfurl the arrays, a procedure that has in the past run into difficulties with panels snagging as they roll out.
The Discovery shuttle brought the new power unit to the ISS, docking with the platform late on Tuesday (GMT).
Its astronauts Steve Swanson and Richard Arnold undertook the six-hour, seven-minute spacewalk required to install the truss.
Once the station's robotic arm had positioned the hardware in the right place, the pair connected bolts to permanently attach the S6 segment to its neighbour, S5.
The spacewalkers then plugged in power and data connectors to the truss and prepared a radiator to cool it.
They also opened boxes containing the new solar arrays and deployed the Beta Gimbal Assemblies containing masts that support the solar arrays.
The US space agency's Mission Control deployed the radiator.
Unfurling of the solar wings is expected at 1458 GMT, Friday.
Altogether, the station's arrays will generate as much as 120 kilowatts of usable electricity, says Nasa.
The addition of the final set of solar arrays will nearly double the amount of power available for scientific experiments aboard the station - from 15kW to 30kW.
Discovery's mission is the 125th to be made by a shuttle; the 28th to the ISS; and the 36th flight for Discovery itself.