Animation of Endeavour in orbit with its payload bay filled with the P6 truss structure bound for the international space station. Photo: NASA TV/Spaceflight Now

Unlike previous shuttle dockings with the space station, Endeavour's crew will approach from directly below, a rendezvous procedure similar to one used during shuttle linkups to the Mir space station.

For visualization purposes, the station's direction of motion is called the velocity vector, or V-bar for short. By convention, points in front of the station are said to be on the positive V-bar while points behind are considered negative.

The imaginary line connecting the station with the center of the Earth is called the radius vector, or R-bar. Points directly below the station along this line are considered positive while points above are considered negative.

Endeavour, then, will approach the station along the plus R-bar. The station will be oriented with its long axis aligned with the velocity vector, with Unity in "front" and Zvezda behind. Unity's nadir and zenith ports will be oriented as their names imply.

This orientation, or attitude, is known as "XVV," i.e., the station's X axis is aligned with the velocity vector.

Endeavour's approach to the space station from below as shown in NASA animation. Photo: NASA TV/Spaceflight Now

The terminal rendezvous sequence will begin with a rocket firing as Endeavour trails the station by about eight nautical miles. After catching up, Jett will guide the shuttle up from directly below. As he nears the station, he will yaw the shuttle 180 degrees so its tail is pointing in the direction of motion and its nose toward the minus V-bar.

In past shuttle-station or shuttle-Mir dockings, the two spacecraft were oriented so the offset between their centers of mass was minimal.

But this time around, the offset is large and Jett will have to carry out a near perfect approach to prevent any "tip-off" motions, when the two vehicles make contact, that could prevent the docking mechanism from working properly.

"The biggest difference for this docking and all our previous dockings is when we actually dock with PMA-3 there'll be a fairly large offset between the center of mass of the orbiter and the center of mass of the space station," Jett said.

"We're a little bit concerned about the post-contact dynamics. Our capture envelope and our capture probabilities are a little bit less than on previous missions, we'll actually be flying to a little bit tighter tolerances."

In fact, Jett will have to make sure the shuttle's docking ring is aligned to within three inches of its counterpart on the space station to ensure a uniform fit between the docking hooks and latches.

Shuttle Endeavour docks to the Pressurized Mating Adapter 3 on the Unity node as seen in animation. Photo: NASA TV/Spaceflight Now

Assuming a good linkup, the shuttle will begin controlling the station's orientation for the duration of Endeavour's mission, moving the lab back into an "X-POP" attitude with the station's X axis perpendicular to the orbital plane. In this orientation, the long axis of the station is broadside to the velocity vector.

The re-orientation to an X-POP attitude is necessary to ensure solar heating on the Z1 truss. The truss houses four massive gyroscopes that will be activated next year to keep the station stable and properly oriented.

Before it was launched, engineers discovered critical gyro speed sensors could fail when exposed to the cold temperatures they might normally experience in orbit between launch and activation.

The X-POP attitude helps alleviate that concern. After the P6 solar arrays are installed, enough power will be available to run internal heaters and the station's orientation will not be so critical.

Shortly after Endeavour docks, the astronauts will open a hatch leading into PMA-3 and position critical equipment and supplies - water, laptop computers, etc. - for later transfer to the station. The final hatch leading into Unity and the rest of the station will not be opened until all three spacewalks are complete.

Garneau, meanwhile, will power up Endeavour's Canadian-built 50-foot-long robot arm and lift the P6 arrays out of the shuttle's cargo bay.

Animation shows the P6 truss being lifted out of Endeavour's payload bay using the shuttle's robotic arm. Photo: NASA TV/Spaceflight Now

"For thermal reasons, we need to get P6 picked up and put into a thermal park position," Reeves said. "P6 has no power on it at this time, it's totally passive. The only thing we have going on it are some battery operated remote temperature sensors so we can tell what the temperature is on the module. We will hold it in that position overnight and get ready for installation the next day."

But Garneau will not be able to lift the array straight out of the cargo bay. As the shuttle is docked to the station, PMA-2 will be directly over the forward part of the orbiter's payloadbay.

"PMA-2 is directly over the bay, it's parallel to the bay, and of course the P6 is almost 45 feet long," Garneau said. "So it cannot be lifted straight out of the bay or it would hit PMA-2. At the same time, you can't move it back a great deal or you'll hit the back of the cargo bay.

"And so the removal of the P6 from the cargo bay, which has been worked out very carefully, is to essentially lift it a little bit and then sort of tilt the tail up so we can sort of lift it out with the aft part sort of going up first."

Assuming all of that goes well, the stage will be set for one of the most dramatic days in the history of space station assembly: Installation and deplolyment of the P6 solar arrays atop the Z1 truss.

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