The first major objective of the mission will be accomplished within a few hours of Endeavour's docking with the international space station when the Expedition 6 crew officially replaces Expedition 5. That will occur after custom-fitted Soyuz lifeboat seat liners and pressure suits have been moved into the station and tested.

The next day, the combined shuttle-station crews will begin implementing the second major objective of the mission, installation, activation and checkout of a massive $390 million truss segment known as "port 1," or P1.

The international space station currently consists of four large pressurized modules - the U.S. Destiny laboratory, the multi-hatch Unity node, the NASA-financed Russian-built Zarya module and the Russian command module, Destiny. These modules form the central core of the station and they fly through space like coupled train cars, with Destiny leading the way.

Attached to right, or starboard, side of the Unity node is the Quest airlock module. Another pressurized module, a Russian airlock/Soyuz docking compartment called Pirs, is attached to a downward-facing port on the Zvezda module.

A fresh Soyuz lifeboat, delivered by a so-called "taxi" crew Nov. 1, is currently docked to Pirs while an unmanned Progress supply ship is docked to Zvezda's aft port.

Extending straight up from Unity like the mast of a ship, is a U.S. truss segment known as Z1 that houses the station's stabilizing gyroscopes and serves as a temporary mounting point for the P6 solar array truss. At the top of the P6 truss, two huge solar array panels extend to either side like great wings.

Astronauts currently are building the station's main solar array truss, a beam anchored to the top of the Destiny module that eventually will extend some 180 feet to either side.

Huge solar arrays will be mounted on each end of the completed truss while ammonia lines, pumps and massive radiators mounted inboard of the arrays will dissipate the heat generated by the station's electronics.

Rails running the length of the truss on its forward face will allow the station's robot arm to move to various work sites atop a mobile transporter. The rails also will allow spacewalkers to move equipment using small railcars called CETA carts.

The central segment of the truss, S0, was bolted to the Destiny module in April. The first starboard-side segment, S1, was attached in October. The P1 truss segment aboard Endeavour is the first port-side section of the beam.

S1 and P2 include the lab's ammonia cooling system and radiators, three folding sets of panels on each segment that will extend 75 feet toward the rear of the station when fully deployed. The P6 truss segment atop Z1 currently provides power and cooling for the station. It eventually will be moved to the port side of the main truss currently under construction.

S1 and P1 are virtually identical. But installation of the new truss segment will be very different from the techniques used to install S1 in October.

During that flight, the station's Canadarm2 space crane pulled S1 from the shuttle Atlantis' cargo bay and attached it straight-away to the right side of S0. Two spacewalkers then made a variety of electrical connections, attached ammonia jumpers between S0 and S1 and installed nearly three dozen "spool positioning devices," or SPDs, to prevent quick-disconnect fittings from locking together too tightly.

They also installed an S-band antenna system and mounted two remotely controlled television cameras, one on the lab module and one on the end of S1.

For Endeavour's mission, installation will follow a very different course, one designed to set the stage for build-out of the port-side of the truss next year.

"I'd say probably 60 to 70 percent of the tasks are identical, sort of a mirror image, although we are executing a lot of them differently," said Lopez-Alegria.

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