Space station commander Peggy Whitson and flight engineer Dan Tani are preparing for a critical, riskier-than-usual spacewalk Wednesday to replace a faulty solar array motor assembly on the right side of the lab's power truss. The motor, which malfunctioned Dec. 8, is needed to pivot a solar blanket from side to side to improve power generation. A different problem in a massive rotary joint used to turn the right-side solar panels like a giant paddle wheel will take longer to resolve. But a successful motor swap-out Wednesday should permit the station to generate the electricity needed to permit attachment of new European and Japanese research modules in February, March and April.

The spacewalk is a bit riskier than most for two reasons: A mistake managing the latches that hold the motor and its housing in place could result in the solar panel's inadvertent release; and because of the shock hazard associated with unplugging and replugging power cables that route 160-volt electricity from the array into the station. To eliminate any chance of a potentially fatal shock, the work will take place when the station is in Earth's shadow and the arrays are not generating any significant power.

"The choreography for the EVA will be very complex, both on orbit and with the ground," Tani said. "Because we're dealing with a solar array that produces kilowatts of power, we have to be very conscientious of when we're going to be opening connections that will expose us to that power. So the bulk of the activities will have to be performed at night when the solar array is not producing any power, or much power, at all."

The bearing motor roll ring module, or BMRRM (pronounced "broom"), is roughly the size of a garbage can and weighs more than 200 pounds. Replacing it is complicated, Whitson said, "because it's really the guts of what's holding the solar array in place. And so Dan and I will have to coordinate when we release and grapple onto the (motor housing) canister in order not to lose the solar array. That would lose us a whole lot of style points!"

The planned six-and-a-half-hour spacewalk, the 101st devoted to station assembly and maintenance since construction began in 1998 and the first so far this year, is scheduled to begin around 5:20 a.m. Whitson and Tani will exit the Quest airlock module and make their way to the far end of the right-side, or starboard, solar array truss segment. They will await the start of an eclipse period before beginning the repair work.

Here is a timeline of events, including when live television from the station is possible (in EST):

02:00 AM...Crew wakeup 
03:55 AM...Oxygen pre-breathe procedure 
04:00 AM...NASA TV coverage begins
04:09 AM...ISS TV downlink window opens
04:45 AM...ISS TV downlink window closes
05:12 AM...ISS TV downlink window opens
05:20 AM...Spacewalk begins
05:49 AM...ISS TV downlink window closes
06:10 AM...ISS TV downlink window opens
06:22 AM...ISS TV downlink window closes
06:47 AM...ISS TV downlink window opens
07:00 AM...Failed BMRRM removal begins in eclipse
07:35 AM...ISS TV downlink window closes
08:24 AM...ISS TV downlink window opens
08:30 AM...New BMRRM installation begins in eclipse
09:10 AM...ISS TV downlink window closes
10:00 AM...ISS TV downlink window opens
10:05 AM...Solar alpha rotary joint inspection begins
11:20 AM...ISS TV downlink window closes
11:35 AM...ISS TV downlink window opens
11:50 AM...Spacewalk ends 
12:59 PM...ISS TV downlink window closes

"We think the actual remove and replace will take around two-and-a-half hours," said Kieth Johnson, NASA's lead spacewalk planner. "The way we have this planned out, we need to do the critical tasks in an eclipse. So the crew members are going to come out of the airlock and they'll have about 90 minutes to run through all the set up procedures to get them into position. As part of that, they're going to review all the steps that they need to do during the eclipse and they'll go through and make sure all the tethering and everything is routed. We hope to have about 10 minutes of wait time until the eclipse comes up before they jump into the actual tasks of disconnecting the connectors and driving the bolts to remove the failed broom.

"We have about a 35-minute eclipse period," he said. "And we've been told from the engineering community that about the first minute and the last two minutes of that eclipse are unusable because we're waiting for the power to ramp down at the beginning and then to ramp back up again at the end. So roughly 32-and-a-half, 33 minutes of that time is useful to get the steps complete.

"We've discussed some back-out plans if we get to the end of the eclipse and we haven't gotten the new broom installed or are having problems, we can remove that broom, set it off to the side, we'll pull a cover over the beta gimbal assembly housing and the crew will have to wait until the next eclipse to start back in and finish the tasks," Johnson said. "What we're hoping to do is get the new broom installed, the center bolt latched and the vice clamp in place. If we have enough time, we're going to go through the rest of the connectors and get it all hooked up."

Astronauts Tom Marshburn and Sunita Williams practiced the repair procedure last week in a huge water tank near the Johnson Space Center.

"They'll want to be very meticulous, making sure they're making the right connections at the right time," Marshburn told reporters. "A little bit of a timeline crunch when they're performing some of the activities during the eclipse. On our run when we actually went through the timeline step by step last Friday at the Neutral Buoyancy Laboratory underwater in the suits, we found it's very doable to get all this done within the time constraints."

The starboard side of the station's power truss is made up of a solar alpha rotary joint, or SARJ, and two solar panels making up the starboard 4 - S4 - power module. One wing of the array is known as S4-1A and the other, extending in the opposite direction, is known as S4-3A. The two panels stretch some 240 feet from tip to tip. A second set of arrays, known as S6, will be attached to the starboard power truss next fall.

To maximize power generation, the arrays must be constantly repositioned to keep them face on to the sun as the station orbits the Earth. The starboard SARJ, which features a 10-foot-wide motor-driven gear supported by 12 so-called trundle bearings, is designed to turn the outboard arrays like a giant paddle wheel, completing one 360-degree rotation per 90-minute orbit.

But the sun's position relative to the space station's orbital path changes from day to day and simply rotating the arrays end over end is not enough to maximize power production. To permit the panels to be aimed to either side of the station's orbital path while the SARJ rotates them as required, each array wing is equipped with a beta gimbal joint. The beta gimbal assembly pivots the blankets from side to side about their long axis in a motion similar to changing the pitch of an airplane propeller.

The port-side of the station's power truss is finished and now features four solar array wings. The BGAs on those four wings are working normally, as is the port-side SARJ.

But only one set of arrays is in place on the right side of the truss and one of them - panel S4-1A - suffered a BGA failure Dec. 8. Engineers initially believed a cable or some other component might have been hit by space debris or a micrometeoroid. But during a spacewalk inspection by Whitson and Tani on Dec. 18, no such damage was found. Subsequent tests showed the problem involved a fault inside the BGA motor assembly itself, the BMRRM or broom.

"This kind of a garbage-can sized device not only transmits all of the power from the solar array to the truss structure where it's accumulated and given to the station, but also provides the mechanical connection," Tani said. "So to simply replace this item, we need to use latches that are already in place and make sure those are tied down so that the solar array doesn't go floating away. That's probably the biggest danger to this EVA."

The loss of a single BGA would not normally be a critical issue. But NASA is on the verge of launching European and Japanese research modules and the station needs all of the power it can generate.

What makes the BGA problem serious in the near term is that earlier this fall, engineers noticed high vibration levels in the starboard SARJ. Impromptu spacewalk inspections revealed unexpected damage to the surface of one bearing race surface and large amounts of metallic shavings, presumably the result of some sort of friction or grinding in the mechanism that eroded the outer layer of the bearing race in question.

Engineers still do not understand the root cause of the race ring damage. The current plan is to possibly lubricate the damaged race in the near term and then, during a shuttle visit later this fall, move the 12 bearing assemblies and two drive motors to a redundant inboard gear. But engineers do not want to consider such a drastic step until they figure out what is causing the problem with the active gear and race ring.

If time is available after Wednesday's BMRRM changeout, Whitson and Tani will move back to the starboard SARJ for additional inspections.

The goal is to collect additional data "to help the ground troubleshoot the problems the SARJ is experiencing," Tani said. "The ground just completed a good visual inspection of that SARJ using the camera on the station's robotic arm. Any data we will get looking under some covers we were not able to access during the last EVA (in December) will provide just additional data to help them figure out what's going on and come up with the best plan to get that SARJ back in action."