The international space station currently consists of eight pressurized modules stretching some 167 feet. Its current mass is 555,244 pounds.

At the back end of the outpost is the Russian Zvezda command module featuring two solar arrays and an aft docking port that can accommodate Progress supply ships, European Automated Transfer Vehicles or manned Russian Soyuz capsules. An airlock/docking module called Pirs is attached to a downward-facing port on Zvezda's front end. The command module's forward port is attached to the Russian Zarya module, a supply and propulsion unit equipped with its own pair of no-longer-needed solar arrays. Zarya's front end features a downward-facing docking port used by Progress and Soyuz spacecraft.

Zarya's front end is bolted to a short U.S. tunnel segment called a pressurized mating adapter that, in turn, is attached to NASA's Unity module, a multi-hatch node with six ports. Its starboard, or right-side port, connects to the U.S. Quest airlock module while its upper zenith port accommodates the Z1 truss that houses the station's four stabilizing gyroscopes.

Unity's downward facing port provides a temporary mounting point for a pressurized mating adapter that will be used later.

Unity's left hatch is not currently occupied. Its forward port is attached to the U.S. Destiny laboratory, occasional home to the station's Canadarm 2 robot arm, a marvel of engineering that is capable of moving, end over end like an inchworm, from work site to work site on the station's solar array truss. Destiny also is the mounting point for a sophisticated multi-joint Canadian robot that can be attached to Canadarm 2 for robotic installation of standardized components.

On the forward end of Destiny is another connecting node called Harmony. Harmony's right-side port leads to the European Space Agency's Columbus research module while it's left-side port leads to Kibo, a roomy Japanese laboratory equipped with its own robot arm and an upper logistics module used to store spare parts and equipment.

During Endeavour's mission, the pressurized cargo module carrying the water recycling gear and other equipment bound for the station will be temporarily attached to Harmony's downward-facing port. Endeavour itself will be docked to a pressurized mating adapter on the front end of the Harmony module.

On top of the Destiny module is the station's main solar array truss, which is mounted at right angles to the long axis formed by the pressurized modules. When complete, the power truss will stretch some 356 feet across.

The S0 truss segment sits in the middle atop the lab, flanked by the S1 and P1 truss elements (there are no S2 or P2 components). S1, S0 and P1 house a variety of electrical components and the station's main ammonia cooling system, including huge articulating radiator panels. The cooling system features two independent ammonia loops - loop A and B - that feature large ammonia reservoirs, pumps, cold plates and the plumbing required to route the coolant through the big rotating radiators to dissipate heat.

The left side of the solar array truss is now complete and includes two sets of solar arrays: P4 and P6, separated by a short spacer known as P5. A massive solar alpha rotary joint, or SARJ, in the P3 segment rotates the outboard solar arrays to track the sun as the station orbits the Earth.

The right side of the solar power truss is not yet finished. It currently is made up of one set of arrays - S4 - and the spacer segment, S5. The final set of solar arrays, S6, is scheduled for launch on the next shuttle assembly mission in February 2009.

Unlike the port SARJ, the starboard rotary joint has suffered extensive degradation since it was activated in orbit and auto-track is no longer enabled.

The problem was first noticed in the summer of 2007 when telemetry indicated high currents and vibration in the mechanism. When spacewalking astronauts took a look inside last Fall, they discovered extensive contamination in the form of metallic shavings and degradation to one face of the drive gear bearing race.

The 10-foot-wide toothed drive gear at the heart of the joint is held in place by 12 trundle bearing assemblies, or TBAs, which distribute the load via roller bearings locked onto the gear's three faces, or races. A powerful motor drives the main gear and outboard arrays. One TBA was taken off the starboard SARJ in a subsequent spacewalk and returned to Earth for analysis. It was replaced with a pristine unit.

During Endeavour's mission, "we intend to go on the starboard side and remove the remainder," Suffredini said. "There are 12 trundle bearings, we've replaced one. We intend to remove and replace 11 other trundle bearings in order to bring those all home to help us with (confirmation of the) root cause.

"We will take that opportunity to clean up that race and then we'll also lubricate that race. What we're doing here is we're trying to modify the system just enough so that when we do have to rotate it, we minimize both the vibrations associated with the damage to the race and the contamination that's currently on the race, that helps us with structural life. In addition to that we want to reduce the amount of current required to drive this joint to make sure we never reach the maximum current the motor can drive to. So that's what these two steps do."

During the first three spacewalks, Stefanyshyn-Piper and Bowen will begin the process of removing and replacing the starboard trundle bearings, using grease-impregnated towels to wipe up and capture metal filings and other debris, a scraper to remove debris stuck to the race and a grease gun to apply fresh lubrication. The astronauts will work on opposite sides of the big drive gear, cleaning and lubricating relatively small sections at a time. Ground controllers will rotate the gear as required to bring fresh segments into reach. It will take most of three spacewalks to complete the starboard SARJ cleaning and lubrication.

"Some of the wipes are going to have grease, some of them are just going to be plain, regular wipes," said Stefanyshyn-Piper. "We're going to have a scraper tool, because some of the particles are fairly hard and they think we'll need to lay some grease to contain everything and it may require some scraping. And so we're going to have all these tools out there. We've thought long and hard about what are the best ways to go out and do this job. We came up with our plan, it's worked in the pool, but there's a lot of things you just can't do in the pool. ... The best we can do is, we know how things behave in space and we know what's the best way to contain your tools and we're just going to do the best that we can."

If all goes well, the starboard SARJ will be commanded to rotate in an auto-track test during crew sleep after the third spacewalk. Sensors will measure vibration levels and electrical loads to determine how successful the astronauts were in reducing friction. As a precaution, the astronauts also plan to lubricate the port-side SARJ during the fourth spacewalk.

The port-side SARJ has not experienced the same sort of friction, apparently because of a pre-launch vacuum test that caused a lubricant inside the trundle bearing rollers to leak out. As the port SARJ operated in orbit, the rollers smoothed the extruded lubricant across the bearing races, reducing friction and, apparently, helping the big gear resist the sort of stress that damaged the starboard SARJ. While both mechanisms have been operating in a vacuum in space, engineers with Boeing believe the way the pre-launch vacuum test was conducted created a different set of conditions more conducive to coaxing out the internal lubricant.

During Endeavour's mission Kimbrough will spend most of the fourth spacewalk adding additional lubrication to the port SARJ as preventive maintenance.

But the damage to the starboard SARJ is extensive and engineers do not believe they will be able to return to full-time auto-track operations. Instead, the gear will be repositioned as required throughout the year to improve solar energy generation. If the servicing goes as expected, the gear should remain operational for years to come.

Even so, continued operations on the damaged drive gear represents an interim fix at best. NASA managers would like to implement a long-term solution that would permit a resumption of auto-track to meet the electrical demands of the completed station while preserving redundancy.

Up to this point, the only long-range option appeared to be switching to a redundant outboard race ring. The SARJ mechanisms include two drive gears for just this eventuality and all that would be needed to switch from one to the other would be to reposition the trundle bearing assemblies and the redundant drive motors. But no one expected SARJ problems this early in the station's operational life and engineers do not want to switch to the backup drive gear - and accept a critical loss of redundancy for the rest of the station's life - unless absolutely necessary.

Instead, engineers have come up with an alternative approach, one that amounts to major surgery. During the final currently planned shuttle flight in 2010, spacewalking astronauts will begin the process of attaching a new drive gear to the damaged one.

"We intend to bring up another race and we will attach it to the race that's damaged and then roll on that race and save the outboard race for later in the life of the international space station," Suffredini said.

Installing a new race ring, or drive gear, will permit the station to continue operating with its current software and still preserve a backup drive gear in case of additional problems down the road.

"The downside to that is we have to basically separate the truss at a joint that wasn't made to separate on orbit," Suffredini said. "This is not the joint we put together on orbit, this is a joint that was assembled prior to flight and flew as an integrated truss. We referred to it at the time as S3/S4.

"We have a technique for doing that," he said. "We have to build the hardware, basically build some jack screws and we'll attach them to where we had some launch locks. And we'll basically separate this joint about 10 inches and we'll slip this new race ring in, install it and then pull it back together. To do all that, it won't happen tomorrow. It'll probably take us to late in 2010 before we have all this hardware ready to go and can get this race ring on orbit. But that is the current plan."

A detailed electrical analysis shows the station will have enough power for normal, or near-normal, operations until the new race can be installed.

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