In a somewhat riskier than usual procedure, two spacewalking astronauts installed a replacement solar array positioning motor today aboard the international space station. Subsequent tests confirmed its ability to move the panel as required to maximize power production, clearing the way for attachment of European and Japanese research modules.

"Good news from the electrical systems officer here in mission control," NASA commentator Rob Navias reported. "A good rotation of 3 degrees of the beta gimbal assembly for the S4-1A array driven by the new motor, the new bearing motor roll ring module, that was installed a few hours ago by Peggy Whitson and Dan Tani."

The new motor was installed during a 35-minute pass through Earth's shadow that began just before 7 a.m. The old motor was removed, the new motor installed and four of five power and data cables reconnected before the station moved back into sunlight.

Waiting for the next night pass to avoid any risk of electric shock from the huge array, the astronauts reconnected the final, primary power cable, allowing electricity to flow through the new bearing motor roll ring module, or BMRRM (pronounced "broom").

"Word now received in mission control that the J1 connector is routing good power through the new BMRRM," Navias said at 9:06 a.m.

"Yee ha! Excellent," Whitson exclaimed.

"How long 'til we know if the BMRRM works?" Tani asked. "Or if the problem was cleared, I guess."

"And Dan, once (you are) clear of the worksite we'll be able o move it and know if the problem is solved," said astronaut Tom Marshburn in mission control.

"Great."

Just after 10:15 a.m., flight controllers sent commands to the motor to rotate its arrays 3 degrees. The motor worked as expected, indicating today's repair work was successful.

"The BGA, or beta gimbal assembly, that rotates to pivot the solar arrays to face the sun rotated 3 degrees," Navias reported. "They're in the process of conducting a latching test right now. That will be followed by the unlatching of the beta gimbal assembly once again and then a full 360-degree rotation. But so far so good, the BMRRM operating as advertised."

Just before 11 a.m., commands were sent to start the full rotation. Downlink television from the space station showed the huge solar array slowly turning as Whitson and Tani floated in the foreground.

"Peggy and Dan, just so you know, from our data down here that motion is good," Marshburn radioed. "The currents and rates are good. Also, the three (circuit breaker) switches that popped open a couple of months ago are now closed and staying closed, primary, backup and controls. So we're looking good so far."

"Excellent," the spacewalkers said in unison.

"Awesome work, you guys," Marshburn said.

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 long axis 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.

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 made the BGA problem serious in the near term was 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.

With the BMRRM replacement behind them, Whitson and Tani moved to the starboard SARJ and removed thermal covers to continue the ongoing troubleshooting. They reported seeing the same sort of damage noted earlier and used adhesive tape to collect additional samples of the metallic debris coating one surface of the race ring.

As the spacewalk moved through the six-hour mark, the astronauts agreed to extend the excursion to remove another cover from the SARJ before heading back to the space station's airlock.

Additional coverage for subscribers:
VIDEO: REMOVED BMRRM PLACED IN STOWAGE CONTAINER PLAY
VIDEO: THE NEW REPLACEMENT BMRRM MOTOR IS INSTALLED PLAY
VIDEO: FAILED BMRRM MOTOR REMOVED FROM STATION PLAY
VIDEO: SPACEWALKERS PREP OLD BMRRM FOR REMOVAL PLAY
VIDEO: PREVIEW OF THE BMRRM REPLACEMENT SPACEWALK PLAY
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