"There is no problem with that at all. We've discussed that back and forth," German astronaut Gerhard Thiele told Helms and Voss from NASA's Mission Control Center in Houston, Texas.

A bit disappointed, the veteran astronauts took the news in stride.

Said Voss: "That was the back-up plan."

The switch to Plan B came amid a somewhat clumsy eight-hour, 56-minute excursion that nonetheless set a new spacewalking endurance record while paving the way for a shuttle-borne moving van to be mounted to the station late Sunday.

Now nestled in shuttle Discovery's payload bay, the Italian-built cargo carrier is filled with five tons of supplies and equipment that Helms, Voss and incoming station commander Yuri Usachev will need during a four-month tour of duty at the outpost.

Look for Discovery mission specialist Andy Thomas to grapple the Leonardo module with the shuttle's robot arm about 9:42 p.m. EST (02:42 GMT Monday).

About the size of a family mini-van, the 21-foot (6.4-meter) freighter then must be attached to a berthing port on the station's Unity module so that it can be unloaded.

"It's quite a demanding task," said Thomas. "It weighs something like 20,000 pounds (9,000 kilograms), so we have to do it fairly carefully. It does require a certain amount of precision in alignment and orientation to make sure it mates properly with the station."

The mounting of the moving van will come on the heels of a spacewalk that got off to a bit of a bumbling start early Sunday, one that put Helms and Voss behind schedule from the get-go.

First, Helms inadvertently bumped a bag of toxic rocket fuel detection equipment as she exited the shuttle's airlock, casting it adrift in Discovery's cargo bay.

"Uh-oh!" she exclaimed.

Not wanting to add to the increasing amount of space junk in low Earth orbit, Voss quickly grabbed the end of the shuttle's 50-foot (15-meter) robot arm so crane operator Jim "Vegas" Kelly could whisk him within reach of the floating sack.

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

"A good catch by Jim Voss," NASA flight commentator Rob Navias added.

Minutes later, however, Voss fumbled a small fixture used to attach tools to portable spacewalking work platforms. The device somehow came loose from a safety tether and then tumbled out of the shuttle's cargo bay and into the void.

Shaking off the slip-ups, the two spacewalkers moved on to their initial task: Prepping a shuttle docking tunnel for a move from one Unity berthing port to another, work needed to make room for the mounting of the Leonardo cargo carrier.

A veteran of two previous spacewalks, Voss then ran into trouble disconnecting electrical cables routed between the docking tunnel and the Unity module, which is a pressurized passageway that links all parts of the station.

"It's just not my day, apparently," Voss said.

The cable work ultimately was completed, but Voss then had to fetch a spare tool attachment fixture in order to carry out other chores outside the station.

That put the astronauts about an hour behind schedule as they began work aimed at preparing the outpost for the mid-April arrival of the station's Canadian-built robot arm.

~

Considered the centerpiece of the Canadian Space Agency's contribution to the station project, the 57-foot (17-meter) robot arm must be put in place and activated before outpost assembly can continue.

A technological marvel, the construction crane will be able to "inch-worm" 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 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 managed to attach a mounting platform for the robot arm to the top of the station's Destiny laboratory, a bus-sized research facility delivered to the outpost last month.

And the spacewalkers also 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 arm.

Helms and Voss, however, didn't have time to hook up the wiring harnesses - a job that must be done so that the Canadian crane can be operated from computer workstations to be set up inside the Destiny lab.

Instead, they retreated to Discovery's airlock as previously planned and sat for almost two hours while robot arm operator Thomas moved the docking tunnel from one port on Unity to another, clearing the way for the Italian moving van.

The idea was to keep the suited astronauts on stand-by in case anything went awry with the docking tunnel move, and in fact, Thomas had a bit of trouble mating the tunnel to its new port. The Australian-born astronaut had to carefully edge the tunnel up to Unity twice before the two parts locked in a metallic embrace.

Somewhat ironically, Helms and Voss set a new record for the longest spacewalk in U.S. history while they were waiting.

Cooling their heels in the airlock, the two eclipsed a dramatic 8-hour, 29-minute excursion during which three astronauts -- Pierre Thuot, Richard Hieb and Tom Akers -- snatched a wayward spacecraft that weighed 4.5 tons from orbit with their gloved hands.

Known throughout NASA as "The Grab," the satellite rescue came during the maiden voyage of shuttle Endeavour in May 1992.

Helms and Voss also set a new record for the longest spacewalk at the international outpost. The pair eclipsed a 7-hour, 55-minute foray carried out by astronauts Tammy Jernigan and Dan Barry during a mid-1999 flight to the then-fledgling station.

The unfinished wiring work now will fall to Thomas and Discovery mission specialist Paul Richards, the two of whom will set out on a follow-up spacewalk at 11:47 p.m. EST Monday (04:47 GMT) Tuesday.

Thomas and Richards also will be setting up a stowage platform and spare cooling system equipment outside the Destiny lab. In addition, the two will scale to the top of the 17-story station to fix a metal bar that failed to latch firmly when a U.S. electric power tower was delivered to the outpost late last year.

Despite the problems on their spacewalk, Helms and Voss both appeared to enjoy working outside the complex as the station and Discovery soared in tandem some 235 miles (376 kilometers) above the planet.

A rookie spacewalker, Helms took a moment to gawk at the panorama as the joined shuttle-station complex flew over the Sea of Arabia.

"Oh my goodness. Take a look at the view!" Helms urged her spacewalking partner.

"Yeah, it's really pretty," Voss replied.

Riding on the end of the shuttle's robot arm like a telephone repair worker in a cherry picker, she also took time to send special greetings to her mother - Dori Helms of Albuquerque, N.M.

"While I have a minute here, I just want to say happy birthday to my mother. I think she's 68 today, and Mom, I just couldn't think of a better way to spend your birthday. Sorry I'm not with you, though."

Her Mom was listening in, too.

"We received a phone call at (Mission Control) from your Mom, and she heard your message," Thiele told Helms.

"Marvelous!" the astronaut replied with a hearty laugh. "Tell her I'm having the time of my life."