CAPE CANAVERAL, Fla. - Astronauts aboard shuttle Discovery embarked Tuesday on a third spacewalk in as many days, heading into the home stretch of a weeklong flurry of construction work at the International Space Station.

During NASA's most ambitious station assembly mission to date, the astronauts already have outfitted the outpost with the first piece of its girder-like truss and a new shuttle docking port.

Astronaut Bill McArthur works in Discovery's cargo bay before heading up to the International Space Station.

The job now at hand: Rigging up a pair of power converters that eventually will route electricity to station from two giant solar wings to be delivered later this year.

"The first two [spacewalks] were definitely the most difficult, but the next two are just as important," NASA spacewalk engineer Daryl Schuck told reporters Monday.

"Now it's kind of like when you're on a race track," added flight director Chuck Shaw. "You're on the back curve, and now we're coming around to finish up."

Running right on schedule, astronauts Leroy Chiao and Bill McArthur donned bulky spacesuits and then floated out into open space just after 10:30 a.m. Eastern Daylight Time (14:30 GMT).

Hooking up to braided-steel safety tethers, McArthur inadvertently brushed up against an airlock valve cap, which drifted up and bounced off both the station's new truss and the shuttle's robot arm before tumbling off into space.

"It was a nice billiard shot," McArthur told flight directors in NASA's Mission Control Center in Houston, Texas. "It caromed off the corner of the truss and then off the arm, and now it's become the latest addition to the tiny bodies orbiting the Earth."

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Electrical engineers

Equipped with pistol-grip electric screwdrivers, the spacewalkers then set out to unbolt two 129-pound (58 kilogram) power converters from the sidewalls of the shuttle's cargo bay.

Astronaut Bill McArthur floats near Discovery's airlock moments after Tuesday's spacewalk begins.

The power converters are to be lugged up to a new 9-ton truss that was mounted to the top of the 13-story outpost last Saturday. The high-tech power tools then will be used to bolt the converters in place on the side of the truss.

Shaped like a huge cube, the $273 million truss will serve as a temporary mounting platform for a pair of power-producing solar arrays to be launched aboard shuttle Endeavour on November 30.

The massive arrays will have a wingspan of 240 feet (73 meters) once unfurled in orbit.

The power converters will play a key role in routing electricity to the station from the arrays. Like large transformers on Earth, the boxy units will reduce and regulate the amount of voltage being fed into the outpost.

"You know, it's a bit analogous to the fact that power from the electrical company comes on these high-voltage power lines, and so you have to reduce the voltage. And that's what we're doing," McArthur said in a preflight interview.

"We're also trying to keep [voltage] regulated to a very steady level so that we don't see power fluctuations in our equipment inside the station."

Mission Discovery

Look here for the latest news from NASA's STS-92.

McArthur and Chiao also plan to finish up some wiring work during their planned 6.5-hour excursion, routing a few additional electrical cables to the truss and the new shuttle docking port, which was mounted to the outpost Monday.

A bit of mop-up work is on tap, too.

The astronauts will stow a toolbox outside the station and remove a keel pin that had been used to hold the new truss in the shuttle's cargo bay. The lengthy pin will be repositioned inside the truss to make way for the soon-to-be-delivered solar arrays.

The fourth and final spacewalking excursion remains scheduled for Wednesday.

During that foray, astronauts Jeff Wisoff and Michael Lopez-Alegria will prepare the top of the truss for the upcoming solar-array installation. They'll also test jet backpacks that spacewalkers would use to fly back to the station if inadvertently cast adrift.

Discovery's six-man, one-woman crew also will spend a day inside the station, delivering supplies for the outpost's first full-time tenants, who are due to take up residence at the station in early November.

The astronauts are to depart the station Friday and then land back at Kennedy Space Center at 2:10 p.m. EDT (18:10 GMT) Sunday.