Space station commander Kenneth Bowersox and science officer Donald Pettit are gearing up for a planned six-and-a-half-hour spacewalk Wednesday to help deploy a huge radiator panel and to carry out other tasks necessary for ongoing station assembly work.

NASA television coverage will begin at 6 a.m. EST and the two astronauts are expected to float out of the Quest airlock module between 7 a.m. and 7:30 a.m. EST.

Expedition 6 commander Bowersox and flight engineer Nikolai Budarin, a Mir veteran, originally planned to carry out the spacewalk, or EVA, on Dec. 12, one week after the departure of the shuttle Endeavour that carried them into orbit. But a few days earlier, NASA flight surgeons "grounded" Budarin because of an undisclosed medical issue and assigned Pettit to take his place.

Citing medical privacy, NASA will not discuss astronaut health issues. While Budarin is a Russian cosmonaut, the spacewalk will be staged from the U.S. Quest airlock using NASA spacesuits and under existing international agreements, that gives U.S. flight surgeons the final say.

"Since this was a U.S.-based EVA, the U.S. surgeon made the call on whether Nikolai would be going outside or not," said station flight director Norm Knight. "Nikolai was disappointed about this. He has trained very hard for this EVA and he was ready to go from the training aspect of this."

In any case, Budarin's replacement by Pettit forced NASA to delay the spacewalk until this month to provide additional training time and to replan some of the tasks that originally were to have utilized the station's Canadarm2 space crane. Budarin is not qualified to operate the arm and instead, Bowersox and Pettit will move themselves around the station hand-over-hand and use foot restraints to anchor themselves to its hull.

"The primary change is we will not be using the SSRMS, or the station arm, to transport the crew from one worksite to another," Knight said. "Instead, we'll leave the arm in a static configuration such that we can use its cameras to provide excellent views of the tasks we'll be performing."

NASA currently is building the station's central solar array truss, a huge beam that eventually will stretch some 365 feet from tip to tip. Solar arrays will be mounted on each end of the truss while two inboard segments will house the ammonia radiators and plumbing needed to cool the station's electronics.

The truss currently consists of three segments: A central, S0, section mounted to the top of the Destiny laboratory module; the S1 truss on the right side of S0; and the P1 truss segment on the left side.

A motorized transporter on the forward face of the truss will be used later to move the station's Canadarm2 space crane to various work sites as required to continue the assembly of the truss and to perform needed maintenance. In a test, the transporter was moved from work site 4 on S0 to work site 7 at the end of P1 during the most recent shuttle visit last November when the P1 segment was attached to the station.

But the transporter got hung up on the way to work site 7 when it ran into a stowed UHF antenna boom. The boom was successfully deployed during a subsequent spacewalk and the transporter eventually reached work site 7 as planned. But by the time the transporter was in place, there was not enough time left to move it back to work site 4 on S0.

So before the spacewalk begins Wednesday, flight controllers plan to send commands to move the station's robot arm carrier back to work site 4 on the central S0 truss element. The hour-long move is scheduled to begin around 2:30 a.m.

"We like to move the MT in conjunction with an EVA just in case there's a problem with the move," Knight said. "Then we can then have the EVA crew go out and essentially rescue the MT and provide an alternate source of power to keep the heaters alive while we work any problems that may have been encountered."

No such problems are expected.

Bowersox and Pettit have two primary objectives. The first item on the agenda is to release launch locks holding one of three folded ammonia radiator panels in place on the newly installed P1 segment.

S1 and P1 each feature three ammonia radiator panels that will extend 75 feet when fully deployed. The central radiator beam on S1 was deployed last fall. The central set of panels on P1 will be deployed as soon as Bowersox and Pettit release the launch locks holding it in place.

While the ammonia cooling system in the truss will not be activated until later this year, engineers want to make sure the radiator deployment mechanism works as designed.

Once the radiators are deployed, Bowersox and Pettit will move down "under" the station to clean debris off an Earth-facing port on the U.S. Unity module. The currently vacant port is used to dock supply modules and the grit could cause problems achieving an air-tight seal. Bowersox, anchored in a foot restraint, plans to hold Pettit's legs so the science officer can dab up the debris using adhesive tape.

Next, the spacewalkers will float over to the S1 truss on the right side of the station and install a floodlight for use during future spacewalks. Bowersox, who will be free floating, will install a light fixture on a stanchion that, in turn, will be mounted and plugged in by Pettit, working from a foot restraint mounted on S1.

From there, Bowersox will float up to the Z1 truss atop the Unity module to retrieve tools from an externally mounted tool box. The tools in question will be brought inside for use by future spacewalkers. Pettit, meanwhile, will float up higher still, to the P6 solar array segment, to check on the health of an ammonia system that will be used later to charge various coolant lines.

The spacewalk is expected to end around 2 p.m.

"Hopefully at the end of this EVA, we're going to leave the station better than before and more complete," said Daryl Schuck, the Expedition 6 spacewalk director.

Pettit, an Expedition 6 backup crew member, never expected to fly aboard the station, much less make a spacewalk. But he was assigned to the Expedition 6 crew last summer when the original science officer, Donald Thomas, was grounded by an undisclosed medical problem.

Pettit was trained as a backup EVA crew member and Schuck said he completed six runs in NASA's water tank spacewalk training lab at the Johnson Space Center, along with multiple runs in its Russian counterpart.

"Don was originally a backup crew member, so he wasn't even supposed to fly," Schuck said. "So I think he probably just continues to shake his head and wonders what's next? He's thrilled to do it, he's extremely capable and very excited about it."

Schuck said Bowersox completed 12 to 15 water tank runs prior to launch and Budarin completed nine or 10. While Pettit's time in the tank is half that of his colleagues, his six runs more than met the minimum qualifications required.

"He was the backup EVA crew member, so he's been through six NBL runs and those included tasks very specific to what we're doing today so he's extremely prepared for that," Schuck said. "He has had sufficient training to do this.

"We find him to be an especially good student, he's a scientist by trade, he asks a lot of questions ... he's very receptive to our inputs and his basic skills and aptitude to do EVA are second to none."

This will be the 50th spacewalk devoted to station assembly and maintenance and the first so far this year. Going into Wednesday's excursion, 35 NASA astronauts, seven Russians, one Canadian and one Frenchman have logged 305 hours and 20 minutes in station spacewalk time. It will be the 16th spacewalk staged from the Quest airlock and the 25th station-based EVA.

"Look at the station we have today," said Melissa Gard, Expedition 6 program manager. Last year, she said, "we built the core, the backbone of the station, installed new truss segments, completed three missions, and I have to say that I feel like we're there again.

"For everything that we did last year, this year you're going to see some incredible things in space. We're going to complete three more expeditions, we're going to deliver three more research facilities, we're going to install four new truss segments and some great big solar arrays that will almost triple the capacity of the station's power generating capability. The station's about to get pretty big.

"We'll also break the record we set last year of the number of spacewalks conducted in a single year and that record-breaking year starts with the spacewalk that we'll conduct (Wednesday)."

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