The shuttle approaches the space station from below. Photo: NASA TV/Spaceflight Now

Space shuttle Atlantis is nearing the completion of its two-day pursuit to catch the international space station with the orbital linkup scheduled for 1650 GMT (11:50 a.m. EST) Friday. Our status center will have live updates.

Shuttle skipper Ken Cockrell will begin final approach to the complex with a critical rocket firing known as the TI burn at a point about 9.2 miles directly behind the target.

To visualize the shuttle's approach, it helps to keep the station's current configuration and orientation in mind.

The outpost currently consists of three pressurized modules attached end to end: The Russian command module, Zvezda, on one end; the multi-hatch U.S. Unity node on the other; and the Russian-built, NASA-financed Zarya module sandwiched in between.

The Soyuz spacecraft that ferried Bill Shepherd's crew to the station is docked to Zvezda's aft port.

The Unity module features six hatches or ports: One on each end and four around its circumference. Attached to one of the end hatches is a pressurized mating adapter - PMA-1 - that connects to the Zarya module.

Another mating adapter, PMA-2, is attached to the other end hatch while a third, PMA-3, is attached to Unity's downward-facing, or nadir, port.

A large structural component known as the Z1 truss is bolted to Unity's upward-facing, or zenith, port. The Z1 truss houses the four control moment gyroscopes that will be used to maintain the station's orientation.

A huge set of solar arrays is bolted to the top of the Z1 truss. Its two wings, oriented at right angles to the long axis made up of the three pressurized modules, stretch 240 feet from tipi to tip.

Installed during the most recent shuttle flight last December, the P6 solar array truss also includes large radiator panels to dissipate the heat generated by the Destiny module's electronics systems.

As Atlantis approaches the station, the complex will be oriented with its long axis - the Unity/Zarya/Zvezda axis - in the direction of its orbital motion with Unity in front and Zvezda behind. The Z1 truss and the P6 array will be facing deep space while PMA-3 will be facing Earth.

The Destiny laboratory module, which features a hatch on each end, must be installed on the Unity end hatch currently occupied by PMA-2. PMA-2 will be removed and reattached to the lab's far hatch during Atlantis' mission.

But first, Cockrell must guide the shuttle to a docking with Unity's downward-facing PMA-3.

The terminal rendezvous sequence will begin about two-and-a-half hours before docking with the TI burn. Final approach is set up to bring Atlantis, in a nose-forward, wings-level, belly-to-Earth orientation, to a point about 600 feet directly below the station.

The imaginary line connecting the station to the center of the Earth is known as the "r-bar," or radius vector. By convention, points below the station are considered positive while points above are considered negative.

Cockrell will approach the station along the plus r-bar for a docking at the downward-facing PMA-3.

But given the station's orientation, he first will have to yaw the shuttle 180 degrees so it is flying tail first below the station. Otherwise, PMA-1 and the Zarya module would prevent the crew from removing Destiny from the shuttle's cargo bay after docking.

Atlantis performs the yaw maneuver during the rendezvous as seen in animation. Photo: NASA TV/Spaceflight Now

The yaw maneuver is fairly tricky, combining two different types of orbiter motion at the same time.

"It's called the tail forward maneuver," Cockrell said. "And when I was first presented with this, I thought, 'Okay, tail forward maneuver. Put in some control inputs and the orbiter swings around.'

"It turns out, it's a little more of an art than it is of a science and it's taken a lot of practice for us to begin the tail forward maneuver so that we come out of it at the appropriate place and don't require a lot of thruster firings that waste fuel.

"And so, if there's a point you want to key into the rendezvous and note our response to something we've done, that would be one of them," Cockrell said. "Because it can either go well or it can go bad. It won't ruin the rendezvous. But, if it goes badly, we may not have as much fuel as we'd like (left over)."

The tail forward maneuver should be complete by the time Atlantis is 360 feet or so below the station. That will prevent any chance of possible thruster exhaust plume damage to the space station.

"From that point on, we drive in to 170 feet," Cockrell said. "If there's any timing we need to do to make the docking occur at a specific point over the Earth, then we wait at 170 feet to make that timing come true.

"From there, we drive in to 30 feet. And at 30 feet, we zoom the camera that's looking at the station right up through our docking adapter, zoom it all the way in, look at the target ... and determine whether there's any error in pitch, yaw or roll of the station.

"And if there is, we just, by visually using a little scale and ruler, measure how much error there is, apply it to a formula, put it into the autopilot ... so that we can then manually fly it on up to the station. That happens at 30 feet.

"Once the pointing is done and then when the timer counts down to the right amount if we're trying to hit a specific spot over the Earth at docking time, we push on in."

Illustration of Atlantis nearing its linkup with the station. Photo: NASA

From that point, he said, "it's just trying to keep your 200,000-pound vehicle lined up with (the station) to an accuracy of about three inches."

"We need to be no less, no more, than three inches off in our position," Cockrell said.

While the job is no doubt challenging, mission specialist Bob Curbeam said in a NASA interview that Cockrell - known as "Taco" - has no problem pulling it off to perfection in NASA's shuttle simulators.

"Space rendezvous is an absolutely amazing thing to me," Curbeam said. "You have these two objects sitting in orbit, moving at over 17,000 miles an hour. But yet, you've got to control these two vehicles so you get closure between them to less than a tenth of a foot per second.

"So it's a pretty amazing feat in itself," he said. "And I bow down to Taco every time I see him do it when we're in the simulator and do it so well. If all goes well hooks and latches in the docking system will pull the two spacecraft together a few minutes before noon EST Friday.

A vestibule between the shuttle's hatch and a hatch on the end of PMA-3 then will be pressurized and after leak checks, the hatches will be opened and the two crews will finally greet each other.

A brief welcoming ceremony is planned that will be carried live on NASA television.

"I'm sure they'll be happy to see us because we'll have some fresh food and some gifts and maybe some chocolate, or whatever they'd like, we'll be bringing to them," Cockrell said. "And then, we'll get straight to work."