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

After a two-day orbital chase, the shuttle Endeavour's crew closed in on the international space station today for a tricky, off-kilter docking that will set the stage for a spacewalk Sunday to attach a $600 million set of solar arrays.

Commander Brent Jett, pilot Michael Bloomfield, flight engineer Marc Garneau and spacewalkers Joseph Tanner and Carlos Noriega were awakened at 8:06 a.m. over the southern Atlantic Ocean, trailing the station by just 850 miles and closing.

Today's docking will differ from past shuttle-station linkups. Instead of looping up in front of the target and descending from directly above, Endeavour will approach from directly below using procedures first developed for dockings with the Russian Mir space station.

"It's different than what has been done on the previous ISS missions," Jett said in a NASA interview. "We're actually digging out some of the old, old procedures and going back and dusting those off and tweaking them, updating them a little bit."

Jett and Bloomfield are scheduled to fire Endeavour's maneuvering jets at 12:33 p.m. -- just after the start of the Army-Navy football game in Baltimore -- to begin final approach from a distance of about eight nautical miles.

During a video downlink Friday, Navy Cmdr. Jett, Marine Corps Lt. Col. Noriega and Navy veteran Tanner sent down an enthusiastic "Go Navy! Beat Army!" cheer for use during the CBS Sports broadcast of the game.

But football will not be on the astronauts' minds today. Assuming the final stages of the rendezvous go smoothly, Jett plans to guide Endeavour to an offset docking with the space station at 2:59 p.m.

Looking on from inside will be station commander William Shepherd, cosmonaut Yuri Gidzenko and flight engineer Sergei Krikalev, working through their 31st day aboard the orbital outpost.

While they will be able to chat with Endeavour's crew by radio, they will not be able to welcome them aboard until late next week, after three spacewalks are completed to attach the $600 million P6 solar array truss to the space station.

The terminal phase initiation burn at 12:33 p.m., along with any mid course corrections that might be required, will bring Endeavour to a point 600 feet directly below the station around 2:03 p.m.

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

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.

At 2:03 p.m., then, Endeavour will reach the plus R-bar directly below the station. The complex will be oriented with its long axis aligned with the velocity vector, with the U.S. Unity module in "front" and the Russian Zvezda command module 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.

Two minutes after reaching the plus R-bar, at a range of about 500 feet, Jett will begin a seven minute maneuver to yaw the shuttle 180 degrees so its tail is pointing in the direction of motion and its nose toward the minus V-bar.

"We will essentially swap ends of the orbiter," Jett said. "Instead of flying nose forward, we will do a maneuver that puts the tail of the orbiter forward and that puts us in the proper orientation for docking.

"The way we will be handling things in the cockpit is pretty standard for most of the crews. I will take over manual control of the vehicle at about two-thousand feet from the station, and from that point I'll be flying the vehicle based on what I see out the window and what information I'm getting from my other crewmates.

"I'll be getting range and range rate information through a laser, a trajectory control system laser, that gives us very precise information. If that's not working, Carlos will be operating a handheld laser in one of the overhead windows and he'll be able to give me range and range rate information as we close in.

"You can see, you know, range with your eye, you can see closure with your eye, but our profile is very demanding," Jett said. "We are looking for very specific ranges with specific closure rates all the way up to docking. And that's where my job comes is to make sure that happens and then make sure we get docked successfully."

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

By 2:18 p.m. or so the station will be in its final docking orientation. Ten minutes later, with the shuttle just 150 feet below, its solar arrays will be "feathered," or turned edge on to the orbiter to minimize the effects of any thruster plume impingements during final approach.

Orbital sunset will occur at 2:36 p.m. and the remainder of the approach and docking procedure will be carried out in darkness.

Jett should have Endeavour within about 30 feet of the station at 2;48 p.m. when he will briefly halt his approach until the two spacecraft move into contact with a Russian ground station at 2:52 p.m. One minute later, Jett will begin the final "push to dock."

Endeavour's docking mechanism, located in the cargo bay just aft of the crew module, will latch onto a downward-facing port on the U.S. Unity module called pressurized mating adapter No. 3, or PMA-3, that was installed by a shuttle crew in October.

In past shuttle-station or shuttle-Mir dockings, the shuttle and its target 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 have Endeavour aligned within inches to prevent any "tip-off" motions when the two vehicles make contact that could prevent the docking mechanism from working properly.

"One of the first things that got my attention on this mission ... is the fact that as we close in and actually make contact to get capture with the station the CG (center of gravity) of the shuttle is not going through anywhere close to the CG of the station," Jett said.

"In all the other dockings we've had as the shuttle's approached and actually made contact with the station, the CG of the shuttle was pretty close to being in a line through the CG of the station. Not exactly, but there was a significant amount of mass along that line.

"In our configuration, the CG of the station is way out over the nose of the shuttle and our CG is back in the payload bay," Jett said. "So, you can imagine, if you took two objects and had them close together and then they just made contact at the tip, the post-contact dynamics could be substantially different.

"So that has gotten the most attention in my mind in terms of what does that do to our probability of capture. We don't want to be the first shuttle ever to bounce off the station and not capture. ... So that sometimes keeps me up at night."

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.

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.

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

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.

Canadian astronaut 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.

"There is no interface between P6 and the shuttle," lead flight director William Reeves explained Friday. "There's no electrical power to any heaters on P6 and no conditioning of its equipment. So from the time of launch until we install it and get the systems operational we have kind of a thermal clock running. ... We need to pick up P6 after docking and put it in this thermal hover position to keep it at the right temperature."

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 payload bay.

"PMA-2 (connecting Unity to Zarya) 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 deployment of the P6 solar arrays atop the Z1 truss.