The Spacewalk: Five Steps to Fix SARJ

Endeavour astronaut Heidemarie Stefanyshyn-Piper has replaced the first of four sets of bearings she hopes to replace today on a damaged International Space Station rotary joint.

She and Steve Bowen are said to be 30 minutes ahead of schedule. You can watch live by clicking the NASA TV picture at left.

Their rather unglamorous repair job 225 miles above Earth boils down to the following five steps, which take roughly two hours to complete:

First, remove thermal covers protecting a the Solar Alpha Rotary Joint, or SARJ.

The joint should allow the starboard solar wings to constantly track the sun, but a breakdown of lubrication last year made it risky to turn the joint.

Second, use a cordless drill to remove one of 12 trundle bearing assemblies, as shown at left.

By the end of today, the astronauts hope to have replaced 11 sets of bearings, which will be taken back to the ground for a failure analysis. NASA believes gold-plated lubrication failed to work properly, causing the bearings to grind against the outer surface of the rotary joint's gear ring, or race ring.

The picture above shows the joint's two exposed steel gear rings with their covers removed. Only one half, the outboard side, is used to turn the solar wings. NASA plans to ship up a new ring to replace the damaged one in 2010, to preserve the joint's backup capacity.

Third, clean the gear ring. Use a "wet wipe" like the one shown at left to spread grease over the gear ring. The wipe is a terrycloth mitt impregnated with a space grease called Braycote, which is dry and has the consistency of toothpaste.

Then use a scraper to pry up "pancakes" of metal debris ground into the gear ring, and a dry wipe to collect any loose metal shavings.

Fourth, lubricate the gear ring. Using a wet wipe or a grease gun like the one shown at left, spread a bead of grease over the three surfaces of the gear ring over which the bearings roll.

A grease gun with a J-hook nozzle is required to reach an inside surface of the ring.

Fifth, install a new bearing assembly and replace the joint's protective thermal covers, then move on to the next set.

Repeat.