"OK. I got it. Nice work Vegas," Voss said.

"A good catch for Jim Voss," NASA flight commentator Rob Navias said.

Working in concert with two robot arm operators aboard Discovery, Helms and Voss aim to help move a shuttle docking port at the station, clearing the way for an Italian cargo carrier to be temporarily mounted to the outpost for orbital unloading.

But more importantly, the spacewalkers face a flurry of work that is key to preparing for the scheduled mid-April arrival of the station's Canadian-built robot arm. And if any major problems crop up, the $60 billion outpost construction project could come to a halt.

The late night sortie represents "what I've always referred to as 'The Big Day' on this flight," said NASA lead flight director John Shannon.

He called the spacewalking work "assembly-critical."

"It's a lot of important stuff," Shannon added. "It seems like it's this waterfall of activities that you have to get right in order to keep the assembly sequence going."

With the 17-story station looming above them, Helms and Voss began making their way to a conical shuttle docking port on the Earth-facing side of the U.S. Unity module, a pressurized passageway that links all parts of the growing station.

The spacewalkers plan to disconnect various cables and remove a communications antenna so that robot-arm operator Andy Thomas can move the so-called Pressurized Mating Adapter to another one of Unity's six berthing ports.

That, in turn, will pave the way for the Italian moving van to be attached to the nadir port of Unity. Five tons of supplies and internal equipment for the station's U.S. Destiny laboratory subsequently will be unpacked from it.

Nicknamed Leonardo, the cylindrical module ultimately will be stuffed full of a ton of discarded station equipment and trash and then stowed back in Discovery's cargo bay for a return trip to Earth.

Helms and Voss then will begin prepping the outside of the Destiny lab for the delivery of a 57-foot (17-meter) Canadian robot arm.

Considered the centerpiece of the Canadian Space Agency's contribution to the station project, the robot arm must be put in place and activated before outpost assembly can continue.

A technological marvel, the construction crane will be capable of "inch-worming" from work site to work site outside the outpost, crawling to places the shuttle's fixed robot arm cannot reach.

That capability will be required on all future station assembly missions as NASA and its 15 international partners add new labs, solar power wings, connecting nodes and support trusses to the outpost, which eventually will span an area nearly as large as two football fields.

To that end, Helms and Voss plan to attach a mounting platform for the so-called Space Station Remote Manipulator System to the top of the Destiny lab, a bus-sized research facility delivered to the station by a visiting shuttle crew last month.

What's more, the spacewalkers aim to put in place a metal pallet laced with wiring harnesses that will route electrical power from the lab to a grapple fixture that will serve as an anchor for the robot arm.

The wiring harness then will be hooked up so that the robot arm can be operated from sophisticated computer workstations inside the lab. The workstations will be unloaded from the Italian moving van and then set up by the station's second resident crew, which includes Helms, Voss and incoming Russian commander Yuri Usachev.

The so-called Expedition Two crew taxied to the station aboard Discovery and will remain on the outpost for at least the next four months.

The trio will be replacing outgoing station skipper Bill Shepherd and two Russian cosmonauts, Yuri Gidzenko and Sergei Krikalev, the three of whom are nearing the end of a four-and-a-half-month "shakedown cruise" aboard the outpost.

Shepherd and his cosmonaut colleagues are scheduled to depart the station next Saturday. They'll return to terra firma aboard Discovery, which remains scheduled for a March 20 landing here at NASA's Kennedy Space Center.