Animation shows the PMA being removed from its temporary storage site on the Z1 truss. Photo: NASA TV/Spaceflight Now

The primary goal of the mission's second spacewalk is to move pressurized mating adapter No. 2 from its temporary mounting point on the Z1 truss and to attach it to the far end of the Destiny module to permit subsequent shuttle dockings.

Jones and Curbeam also plan to install a so-called power and data grapple fixture to Destiny's hull that later will serve as the mounting point for the space station's Canadian-built robot arm.

Other tasks include attachment of a slidewire for use by future spacewalkers and installation of thermal covers over the massive trunnion pins used to mount the lab in the shuttle's cargo bay.

While the spacewalk is going on, flight controllers will begin testing the control moment gyroscopes, making for another busy day in space for the astronauts and ground crews alike.

"The first spacewalk is basically to prepare Destiny for activation," Curbeam said. "The second spacewalk is basically to get ready for the rest of the assembly sequence."

A half-hour or so before Jones and Curbeam exit Atlantis' airlock, Ivins will grapple PMA-2 with the robot arm. But the common berthing mechanism on the temporary attachment fitting is a manually operated system.

"We have to get Tom up there first before we can do anything," Cockrell said. "He has to come up and be ready to unlatch PMA-2 from the Z1. Marsha grapples PMA 2. Tom unlatches it. And then she takes it down to the end of the lab and installs it on the leading edge of the front-end cone of the lab.

"PMA 2, on the front of the lab, will be where the next several flights dock to. So, it's important that it gets installed there; otherwise, there's a break in the assembly process."

But the crew will take its time tightening the 16 bolts in the lab's active common berthing mechanism.

PMA No. 2 is attached to the end of Destiny. Photo: NASA TV/Spaceflight Now

"Because it's been in a different thermal condition up on Z1 than the front of the lab has been, we will only partially bolt it down," Cockrell said. "We'll bolt it down tight enough to where she can let go of it with the arm, but then it needs to sit there and wait 12 hours while the temperatures equalize on the two interfaces.

"We'll come back to it later and do the final torquing of those bolts," he added. "It depends on how smoothly that goes what else we get done on EVA two."

Once PMA-2 is attached to the lab, Ivins will be free to help Jones and Curbeam attach the power and data grapple fixture to Destiny's hull.

Jones, anchored to the end of the arm, will retrieve the grapple fixture from its mounting point in the cargo bay while Curbeam peels back micrometeoroid shielding protecting Destiny's outer skin where the PDGF will be installed.

"It's just a grapple fixture that you've seen on all sorts of payloads on shuttle and space station elements," Jones said. "But this one's a little bit more capable in that it provides electrical connections into the laboratory from the outside.

"And not only electrical connections but a video feed so that the cameras on the arm can be monitored from inside the laboratory and later from other workstations inside the station cupola, for example."

The new grapple fixture -- highlighted in light blue -- is installed on Destiny. Photo: NASA TV/Spaceflight Now

It will take the spacewalkers two hours or so to complete the installation, attaching power lines, installing a video signal converter for a fiber optic link to the interior of the station and then re-installing the debris shielding.

"There are a number of other minor outfitting chores on the EVA and installing a slide wire for translation safety up and down the lab, installing foot restraints for other work that we're going to be doing and other crews will be doing on the spacewalk.

"But the major tasks are getting that docking port relocated and then providing the foundation for the robotic operations outside."

The spacewalk is scheduled to last about six hours. Overnight, while the crews sleep, flight controllers in Houston will send commands to complete the tightening of the bolts in the lab's forward common berthing mechanism to firmly lock PMA-2 in place.

Hatches between Atlantis and the space station will remain closed throughout the next day. The astronauts will enjoy a bit of off-duty time and carry out another in the series of reboost operations to increase the station's altitude.

The third and final spacewalk of the mission - the 100th in U.S. space history - will begin the following day.

"We'll have left the lab in good shape," Jones said. "It'll have all of its power and cooling capability. We'll have relocated the docking port so future crews can come to the right docking port. ... Those are the key things.

"Then on the third EVA, we're going to set our sights on just enhancing the situation of the space station with regard to spares."

As seen highlighted in light blue, the S-band antenna assembly is mounted to the station. Photo: NASA TV/Spaceflight Now

First, Jones and Curbeam will mount a spare S-band antenna array on the Z1 truss to provide a backup in case the station's main S-band antenna package malfunctions at some point.

The spacewalkers then will install a frying pan-like shutter on the lab's Earth-facing window and connect power and data lines between the lab and PMA-2.

"Those power lines will provide both electricity flow to the heaters on the PMA and then also some data feedback into the station so that, when an orbiter docks, you can transfer information and commanding from the orbiter through to the rest of the space station via these lines," Jones said.

With their primary tasks complete, Jones and Curbeam will make their way all the way up the Z1 and P6 trusses - a point more than nine stories above the cargo bay - to inspect the linkages in a system used earlier to extend the two deployable solar arrays.

During deployment of the array booms in early November, one of the four bars in the linkage system did not rotate away from the central truss far enough to completely lock in place. Jones and Curbeam will inspect and photograph the suspect linkage to determine what, if anything, might need to be done down the road to correct the problem.

Jones and Curbeam will close out their third excursion by staging a demonstration of techniques that might be needed down the road to rescue an incapacitated spacewalker.

The spacewalkers practice the incapacitated test in a giant water tank in Houston. Photo: NASA TV/Spaceflight Now

Taking turns, one spacewalker will pretend to be unconscious while the other tows him the length of Atlantis' payload bay and maneuvers him into the shuttle's airlock.

Two techniques will be tested. In one, called the daisy chain method, the active spacewalker will hook his waist tether to the passive spacewalker's suit.

In the other, called the strap method, both ends of the passive spacewalker's waist tether will be hooked to the his suit making a loop. The active spacewalker will put his arm through the loop to pull the incapacitated crew member along with both hands free.

In all of the testing, the incapacitated crewman is "going to be pretty incapacitated, that's the whole idea," said shuttle flight director Robert Castle. "Being able to talk, I think, isn't really the point. The point is, if he can't move his limbs, do we have the right way to move him back to the airlock? We think we do, we'd just like to demonstrate it."

For his part, Jones said he anticipates no problems.

"We've done it in the water tank and we don't think managing somebody even as big as Bob is going to be a big problem in zero G," he said at a pre-flight news conference. "As long as you can keep him from developing a rate (of motion) in one direction or another, you can keep him under control.

"I think that's the hardest part, actually maneuvering the mass of three or four hundred pounds of spacesuit and person into the airlock single handed. We're going to see if that's feasible.

"Our experience in the water has been that as long as you move slowly and carefully and you've got a presence of mind where your partner is, you can keep yourself from getting in trouble with the masses involved."

The spacewalk is expected to last about five hours. Hatches between Atlantis and the station will be reopened about an hour after the spacewalk concludes for a final day of joint work to outfit the lab and to complete equipment transfers before the shuttle crew departs.

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