Slowly but surely, the former Russian Air Force pilot then will extend the 45-foot (13.6-meter) boom, using it to snare a second, identical crane now stored on the outside of the U.S. half of the station.

With an assist from Walz, Onufrienko aims to snake the second boom by a fragile station solar array so that it too can be attached to the Pirs airlock.

Maximum clearance: About a foot (30 centimeters).

"It's going to be very interesting to watch," said Daryl Schuck, an engineer in NASA's spacewalk projects office at Johnson Space Center in Houston. "Moving a large mass around can be complex and needs to be conducted very carefully."

Launched to the station last September, the Pirs airlock serves as an orbital portal for spacewalks being staged out of the Russian segment of the 17-story international outpost.

A first Strela boom was mounted to one side of the airlock during a spacewalk last November. The second Strela boom is to be attached to the opposite side of Pirs.

Doing so will enable both booms to be operated either separately or simultaneously to move cargoes and spacewalkers around the exterior of the station's Russian segment.

The cranes are capable of hoisting cargoes that weigh as much as three tons. They are expected to play a key role in the assembly of a Russian electrical power tower and its associated solar arrays after their arrival at the outpost around 2005.

The two booms also will be used to move spacewalkers and cargoes around the outer hull of the Russian half of the station during routine maintenance of the outpost.

The second crane actually has been at the station longer than the first.

Ferried up to the outpost in parts on May 1999 and May 2000 shuttle missions, the boom was assembled during the later flight and then stowed outside a pressurized mating adapter that links the U.S. and Russian sides of the station.

Pairing the two cranes on Pirs will involve extending the first boom past a solar wing jutting from the side of the station's Russian Zarya space tug, which was launched in November 1998 and served as the initial building block of the outpost.

Once the second crane is snatched, Onufrienko then will have to carefully retract the first boom so that neither of the lifting devices ram into the Zarya solar array.

Walz, consequently, will be called upon to mount a specially designed clearance bracket on the outside of the Zarya module. The first boom then will be routed through the bracket to make sure that neither of the cranes hit the solar wing.

Onufrienko, meanwhile, has done similar work in the past. The veteran outpost commander paired up two Strela booms outside the Mir space station's core module during a March 1996 spacewalk.

"The Russians have had experience with this," Schuck noted. "They seem fairly comfortable and confident with it, and I anticipate this will go very smoothly."

The crane work represents the primary goal of the first of two spacewalks that are planned at the station this month. Onufrienko and Walz also aim to mount a ham radio antenna on the outer hull of the station's Russian-built crew quarters during the planned six-hour excursion.

Three more ham radio antennas are to be installed on the outside of the "Zvezda," or "Star," crew module during a Jan. 25 spacewalk to be carried out by Onufrienko and U.S. flight engineer Daniel Bursch.

Metal deflectors also are to be set up near Zvezda steering thrusters during that outing to prevent toxic rocket exhaust from damaging the outer hull of the bus-sized crew compartment.

Contamination monitors also will be installed in that same area. In addition, Onufrienko and Bursch will retrieve an external materials science experiment during the second sortie.

Launched Dec. 5 aboard shuttle Endeavour, Onufrienko and his Expedition Four crew are in the midst of a planned five-a-half-month aboard the international outpost. The trio remains scheduled to return to Earth in mid-May.