Animation of the Z1 truss being lifted out of Discovery's payload bay by the robot arm. Photo: NASA TV/Spaceflight Now

The day after docking, Wakata plans to pluck the massive truss from the shuttle's cargo bay and carefully mount it atop Unity. Melroy, working beside Wakata on the shuttle's aft flight deck, will control the operation of the interlocking common berthing mechanisms that will latch the Z1 truss into place.

Later that day, the astronauts will connect ground straps between Unity and the Z1 truss, working in a cramped vestibule where the truss is attached to the node. Other crew members will enter the Russian Zarya module to begin stowing fresh supplies.

The Z1 truss houses four large gyroscopes that will be used to keep the station stable and to change its orientation from time to time without having to burn limited on-board supplies of rocket fuel.

The gyros will not actually be used until after the U.S. laboratory module arrives in late January.

But during ground testing, a critical Hall effect sensor that monitors how fast a gyro is spinning failed after prolonged exposure to low temperatures like those the real Z1 gyros might see over the next five months.

Animation shows the Z1 truss after it is turned 180 degrees and mounted to the Unity node. Photo: NASA TV/Spaceflight Now

As a result, engineers removed the heater units on the flight hardware, reset their thermostats to a higher temperature and re-installed them in the Z1 truss. In addition, laptop computer routines were written to give the station's on-board crew the ability to directly control the heaters if necessary.

"They're still doing some tests to make sure all that works out correctly," Gerstenmaier said Sept. 14. "So there's still some work they have to do to verify those procedures to get that in shape. But the hardware's ready to go."

There are other temperature concerns as well.

The Z1 truss is not equipped with any active heating or cooling systems and it is very sensitive to temperature. Once Discovery is docked to the station, the Z1 truss must be installed within 20 to 25 hours to prevent ammonia lines from over heating.

But Z1 temperature constraints also pose problems for the shuttle.

"We installed a canopy, or a series of blankets, over the airlock to shade our water lines that run between the orbiter and the airlock," Gerstenmaier said. "That was done because the sun angles that the station wants are incompatible with the water lines that are running to the airlock."