The Z1 truss is lifted out of Discovery's payload bay by the shuttle's robotic arm. Photo: NASA TV/Spaceflight Now

After recovering from a potentially crippling short circuit, the Discovery astronauts today successfully attached a $273 million structural truss to the international space station to accomplish one of the primary goals of the 100th shuttle mission.

Japanese astronaut Koichi Wakata, operating Discovery's robot arm, precisely positioned the nine-ton Z1 truss within a few inches of the Unity module's zenith port shortly after 2 p.m.

Shuttle pilot Pamela Melroy then sent commands through a laptop computer to retract four latches in Unity's common berthing mechanism, or CBM, that pulled the truss snugly into place.

Sixteen motor-driven bolts then engaged, initially with a force of some 10,250 pounds each and ultimately with up to 19,300 pounds each to ensure a rock-solid, airtight fit between the truss and the Unity module.

"And Houston, Discovery. We've completed the final torquing and the bolts look good," Melroy radioed at 3:10 p.m. "The torque looked very much on the low end of what we saw in testing in Huntsville. So I think everything looks great, we'll proceed on with capture latch closure."

"We see the same down here. Great work, thanks," replied astronaut Ellen Ochoa in mission control.

The Discovery astronauts planned to spend the rest of the day installing electrical grounding straps between Unity and the truss and moving equipment into the space station.

Before moving on to other chores, Melroy radioed her thanks to engineers and flight controllers on the ground "for helping us have this go so smoothly."

"We copy, and thanks to you for executing it," Ochoa replied. "And we have a lot of happy CBM folks down here."

An artist's illustration of the Z1 truss undergoing a calibration test for the Space Vision System. Photo: NASA TV/Spaceflight Now

The work to install the Z1 truss took longer than expected because of an as-yet-unexplained short circuit that knocked three critical systems out of action.

The 14-amp power spike tripped a five-amp circuit breaker and cut power to an upward-pointing payload bay keel camera; a device that allows shuttle systems to electrically communicate with the space station; and a computerized television system needed to accurately position the truss for attachment.

The short circuit occurred at 8:57 a.m. as Wakata was unlimbering Discovery's 50-foot-long robot arm and gearing up to grapple the Z1 truss mounted in the back of Discovery's cargo bay.

The truss houses four stabilizing gyroscopes and the lab's main communications antennas. It also will serve as a temporary mounting point for a huge set of solar arrays scheduled for delivery in early December.

The truss was designed to be mounted on the upward-facing, or zenith, port - hence the name Z1 truss - of the Unity module. As the station is docked to the shuttle, the port actually faces Discovery's tail.

Because of the station's orientation with respect to Discovery's flight deck - Unity's zenith port is invisible on the far side of the complex - Wakata was not able to see the common berthing mechanisms needed to actually attach the truss to the station.

Instead, he planned to use the Canadian-built space vision system, which uses a laptop computer, cargo bay TV cameras and small targets on the station modules and the truss to determine the Z1's exact orientation and position.

The short circuit knocked the space vision system out of action. But the astronauts were able to connect a spare computer to an alternate power source to restore Wakata's electronic eyes.

They also were able to switch to a backup orbiter interface unit, or OIU, to restore electronic communications between the shuttle and the station.

The keel camera, one of several that Wakata originally planned to use to position and align the truss for attachment, appears out of action for the duration of the flight.

An artist's illustration of the Z1 truss about to be mounted to the Unity node. Photo: NASA TV/Spaceflight Now

To add additional margin, astronaut Michael Lopez-Alegria was positioned by a viewport in the Unity module to give Wakata verbal cues as to the truss's exact alignment.

"If you're comfortable with L-A being back in the node doing that, I think that's the way we'll end up pressing on, we'll just up the gain on that cue," lead flight director Chuck Shaw told commander Brian Duffy.

"OK, Chuck, I think that sounds like a great plan," Duffy replied. "I know how hard you guys are working it and we'll be ready to go and follow your directions."

"Copy that," said Shaw. "This is a good sim."

"Unbelievable is a little more like it," Duffy responded.

The crew finally began remotely unlatching the truss from its carrier pallet around 11:30 a.m., running some two hours and 20 minutes behind schedule because of the work to recover from the short circuit.

Even without the space vision system, the astronauts were confident they could have successfully installed the Z1 truss.

"We've been assessing the assets we have available up here and we know the payload bay cameras are working and the elbow camera is working," Duffy said. "Koichi has some terrific pictures of pre-install positions from cameras B and C that he got out of the VR lab, so we have that.

"And then, of course, when he gets in closer we'll have Mike being able to look at it. So we're pretty optimistic, but we just thought we'd let you know where we think we stand."

"Thanks, Brian. We concur with all those comments," astronaut Ellen Ochoa replied from Houston. "I think we're pretty optimistic down here, too. But we're kind of proceeding slowly. There's no need to press on real quickly. We do want to understand what piece of equipment caused the bus to go down before we try to do any kind of workaround for that."

"OK, yeah, we understand that, thanks a lot," Duffy said. "Just keep us informed, we appreciate it."