Two Discovery astronauts on Friday successfully rewired the power system for half of the orbiting International Space Station [Images] during a spacewalk, while mission specialists Indian-American Sunita Williams [Images] and Joan Higginbotham operated the ISS' robotic arm.

Bob Curbeam and Christer Fuglesang began their spacewalk, the second of their 12-day space shuttle mission, at 1.41 pm CST, and finished a full hour earlier than scheduled.

The second half of the station's power system will be reconfigured during the mission's third spacewalk on Saturday.

The successful spacewalk, along with an extravehicular activity on Saturday, will set the stage for the station's expansion to support two new international laboratories and a crew of six in upcoming years.

"NASA [Images] is very happy about that," said Sunita Williams, after flight controllers reported all systems were operating as designed. "Excellent," Curbeam added.

Before the astronauts could begin their spacewalk, station flight controllers had to undertake the risky work of shutting down about half of the station's systems, including some lights, communications gear, ventilation fans and back-up computers.

The spacewalkers headed out of the hatch of the Quest airlock about 30 minutes ahead of schedule, and finished their tasks at 6.41 pm CST, an hour earlier than planned and exactly five hours after they started.

Before heading back into the station, Curbeam and Fuglesang also relocated two small handcarts that run along rails on the station's main truss, put a thermal cover on the station's robotic arm and installed bags of tools for future spacewalkers.

The Discovery crew will begin Saturday with a wake-up call at 8.47 am CST, and spend the day transferring supplies and equipment from the shuttle to the ISS, later participating in a news conference and also enjoy a bit of off-duty time.

NASA astronaut William Oefelein, aboard the ISS, says the work was "excellent" as astronauts Robert Curbeam and Christer Fuglesang brought power back online to sections of the station.

The spacewalkers will suit up again on Saturday to rewire the other half of the ISS.

While the wiring is being worked on, the station must cut power to those sections.

As a result, it becomes a race against time to power up the critical functions of the ISS before computers overheat or worse.

Friday's spacewalk marked the 75th in support of ISS assembly and maintenance and the 47th staged from the space station itself.

It also marked the fifth career EVA for Curbeam, who now ranks 22nd on the list of all time spacewalkers.

Fuglesang now has two EVAs under his belt, a total of 11 hours and 36 minutes, both of which were performed during the current mission.

NASA is studying the possibility of a fourth spacewalk that could call for Curbeam and Fuglesang to help fully retract a P6 solar array left half stowed on Wednesday after repeated attempts to fold it away completely from inside the ISS failed.