Station crew gears up for risky repairs

Two U.S. astronauts will set out on a risky repair job at the International Space Station Wednesday -- one that will expose them to electrical current and raise the possibility of losing a massive American solar wing.

Station commander Peggy Whitson and flight engineer Dan Tani are scheduled to head outside the U.S. Quest airlock at 5:20 a.m. EST, but if past performance is any indication, the excursion could start up to an hour or so earlier.

You can watch the action unfold live here in The Flame Trench. We'll have live NASA TV coverage of the outing beginning at 4 a.m.

Whitson and Tani will attempt to remove and replace a broken motor drive that is designed to pivot solar arrays that stretch 240 feet from tip to tip.

The Bearing Motor Roll Ring Module also serves as a conduit for high-voltage electricity, transmitting it from the solar arrays through the station's central truss and ultimately to outpost systems. And it also serves as a mechanical connector that links the arrays with the central truss.

Here's a look at where the broken motor -- which is about the size of a garbage can and weighs more than 200 pounds -- is located, and the hand-over-hand path the spacewalkers will take to get to it:















Here's a closer look at a BMRRM ("Broom"):

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Here's a look at the connectors that the spacewalkers will be working with:

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And a look at the fasteners that physically connect the motor to the station's central truss:

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NASA astronauts Tom Marshburn and Sunita "Suni" Williams have been working in a giant swimming pool near Johnson Space Center in Houston to develop the procedures that Whitson and Tani will follow to remove and replace the motor.














You can check out a video of the prep work in the Sonny Carter Neutral Buoyancy Laboratory here: Spacewalk Procedures Prep.

And you can see the detailed plan and timeline for the seven-hour foray here: Spacewalk Plan.

Tani said the spacewalkers would have to be especially careful working with the electrical connectors.

"We're dealing with a solar array that produces kilowatts of power, so we have to be very conscious of when we are going to be opening connectors that will expose us to that power," he said. "So the bulk of the activities, or the critical activities, will have to be performed at night when the solar array is not producing any power, or much power at all."

The astronauts also will have to take great care not to inadvertently let loose the solar arrays, which have a wingspan greater than that of a 747 jumbo jet.

"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 on to the canister in order not to lose the solar array," Whitson said. "That would lose us a whole lot of style points."

"This is kind of a garbage-can-sized device that not only transmits all of the power from the solar array to the truss structure where it is accumulated and given to the station but also provides the mechanical connection," Tani added.

"So to replace this item we need to use latches that are already in place and make sure that those are tied down so that the solar array doesn't go away. That is probably the biggest danger of this EVA."

The astronauts also intend to perform further inspections of a 10-foot-diameter joint that enables the wings to turn like a Ferris wheel as the station circles Earth. The Solar Alpha Rotary Joint, or SARJ, is designed to keep the arrays optimally pointed at the sun to maximize solar energy collection and electrical output.

Engineers shut it down late last year after detecting higher-than-normal voltage levels and vibrations, and metal shards were found within the joint during subsequent spacewalking inspections. The filings indicate that internal bearings might be grinding, but the root cause of the problem still has not been determined. The broken motor and the rotary joint both must be fixed to maximize electrical output at the station, a key to completing the assembly of the outpost.

"We'll provide just additional data to help them figure out what's going on and come up with the best plan to get that (joint) back into action," Tani said.

Added Whitson: "It's an important step for us to get that power generation back up to where it ought to be."