An artist's concept of the new PMA 3 docking port being removed from its platform in Discovery cargo bay. Photo: NASA TV/Spaceflight Now

With one complex spacewalk successfully completed, the Discovery astronauts are gearing up to carry out a second excursion Monday to attach a new $20 million shuttle docking port to the international space station's Unity module.

Astronauts Michael Lopez-Alegria and Peter "Jeff" Wisoff plan to begin the 6.5-hour spacewalk around 10:30 a.m. when they switch their spacesuits to internal battery power before floating out of Discovery's airlock.

On Saturday, the shuttle crew attached a $273 million truss to the Unity module. The Z1 truss houses four stabilizing gyroscopes and the station's main communications antennas. It also will serve as a mounting point for a set of solar arrays scheduled for attachment in December.

On Sunday, astronauts Leroy Chiao and William McArthur carried out a near-perfect six-hour 28-minute spacewalk to electrically hook up the truss and to deploy its S-band and Ku-band antennas. See our full story on the spacewalk.

Unity has four radial hatches and one on each end. Discovery is docked to the forward hatch while the Russian Zarya module is attached to the aft port. The Z1 truss is now locked to Unity's upward-facing zenith hatch - hence the name "Z1" - while PMA-3 will be attached to the module's Earth-facing, or nadir, hatch.

The shuttle Endeavour will use the PMA-3 docking port for the first time in early December when the station's first set of U.S. solar arrays will be attached to the top of Z1 truss. It also is required early next year when the U.S. laboratory module Destiny is attached to PMA-2, Discovery's current docking port.

PMA-3 was carried into space bolted to a Spacelab pallet in Discovery's cargo bay. Lopez-Alegria and Wisoff will manually unbolt the mating adapter so robot arm operator Koichi Wakata can position it for attachment to the station.

And it will not be easy.

As the station is docked to the shuttle, the Z1 truss faces Discovery's tail while Unity's nadir port faces directly ahead toward the orbiter's nose.

Wakata will have to attach PMA-3 in the blind as he maneuvers it above the shuttle's crew cabin, relying on verbal cues from the spacewalkers and data from the Canadian-built Space Vision System.

The Space Vision System uses a dedicated computer, targets painted on station components and cargo bay television cameras to geometrically determine a component's exact orientation and position with respect to the space station or the shuttle.

"Koichi Wakata takes the arm and grabs this thing and lifts it up over the nose of the orbiter and sticks it on the nadir docking port of the Node," Wisoff explained in a NASA interview.

Illustration shows the new docking port being transported from shuttle bay toward installation on Unity. Photo: NASA TV/Spaceflight Now

"And while he's doing that, Mike and I will be watching that operation, giving Koichi voice indications of how well the alignment is, if necessary. Again, he will be using a (computerized television) system to help him do that.

"But if there are any problems with it, since we're already out EVA, we'll be there to be able to give him voice indications of how to help him dock it," Wisoff said.

The task will be slightly harder than usual because one of two cameras mounted on the end of the robot arm specifically to help Wakata position PMA-3 for attachment is not working because of a short circuit Saturday.

But given Wakata's flawless performance so far, it is doubtful he will have any problems.

"They've assured me there's no problem whatsoever," said lead flight director Chuck Shaw. "There's a high degree of confidence."

But first, Wisoff and Lopez-Alegria have to unbolt PMA-3 from its pallet. Here is how Lopez-Alegria described the work in a NASA interview.

"There are 16 bolts that attach PMA-3 to the structure," he said. "First of all, Jeff will go up and cycle some latches that are not holding it in place - the bolts are - but will be used later as we release it to sort of constrain its motion. We'll cycle that and make sure it works OK.

"Then each of us will start releasing the torque on these 16 bolts, kind of opposite each other as we go, kind of like on a clock face. Once the torques have been released, we'll go back through and, using a Pistol Grip Tool - which is basically a very complicated, glorified, cordless screwdriver - we're going to continue to unbolt each of those bolts until the PMA is actually free.

"Of course, before any of that happens, Koichi will have grappled the arm to the PMA. And once it's been released all the way, we will open the latches again that Jeff had cycled previously. It'll back away. And, while it's being maneuvered into its installation position, we're going to go off and do some other tasks.

"I'm going to be relocating two sets of Circuit Interrupt Devices, or CIDs, on the Z1 for use on future flights. And Jeff will be removing some launch restraint caps that are on four different attach points that the P6 Truss will be set on top of the Z1 Truss.

"In the meantime, PMA-3 will have been maneuvered into position. And then Jeff and I are actually going to position ourselves close to the interface and give Koichi any cues that we think he needs as to how it's lined up.

"So, we're going to use sort of man-in-the-loop visual feedback, you know, like the guy in the garage that tells you to bring your car back. Kind of a guidance.

The spacewalkers will assist in aligning the PMA 3 to Unity as seen in this artist's concept. Photo: NASA TV/Spaceflight Now

"Once that's done and we have four latches indicating ready to latch and once they're latched, Jeff and I will again go off and do some other tasks. He will continue releasing the other two of those launch restraints. And I will go and relocate some APFRs.

"APFR stands for Articulating Portable Foot Restraint, which are things that we use to stand in to give us some stability when we're trying to do a certain task.

"In this case, they will be positioned on the Z1 under the future location of some toolboxes, which we'll be bringing up on a later EVA. Once the PMA-3 has been bolted on to the Node, then Jeff will start setting up so the arm can ungrapple the PMA.

"It'll be maneuvered over onto another location where Jeff will start installing the equipment necessary for him to ride the arm on top of one, or in one of those APFRs.

"In the meantime, I'll be releasing two sets of cables that are attached to PMA-3, which will end up being eventually plugged into the Node. And those are for power and data. And, Jeff will actually at that point have gotten on the arm, ridden over there, and the two of us together, I'll basically be his helper. I'll look at the cables, make sure the pins are okay, hand him the connector, and he'll actually make the connections.

"At that point, we'll, I'll try to do some tidying up, making sure the cables are lying, you know, nicely against the structure while Jeff tears down the arm. And then we'll both ingress the airlock, and that'll be EVA 2."