In March 2004, President Bush ordered NASA to complete space station assembly and retire the shuttle by the end of fiscal 2010, freeing up money to support development of a new manned spacecraft to replace the shuttle. The new Orion crew capsule, expected to debut around 2015, will ferry astronauts to and from the station and eventually back to the moon as part of a long-range push to establish a permanent lunar base in the early 2020s.

NASA now views the space station as a test bed for technology development and to collect the medical data needed for future long-duration stays on the moon or voyages to Mars. Completing the station is equally or even more important to the European and Japanese space agencies, which have spent billions developing flight hardware and facilities only to suffer through repeated delays, most recently because of the 2003 Columbia disaster.

Before European and Japanese research modules can be launched, however, the station's solar array truss must be built out to provide the necessary electrical power.

The international space station currently consists of six pressurized modules. At the back end of the outpost is the Russian Zvezda command module featuring two solar arrays and an aft docking port that can accommodate Progress supply ships, Soyuz crew ferry capsules and the European Space Agency's upcoming Automated Transfer Vehicle.

A conmbined airlock/docking module called Pirs is attached to a downward-facing port on Zvezda's front end. The module's forward port is attached to the Russian Zarya module, a supply and propulsion unit equipped with its own pair of solar arrays. Zarya's front end features a downward-facing docking port used by Progress and Soyuz spacecraft.

Zarya's front end is bolted to a pressurized mating adapter that, in turn, is attached to NASA's Unity module, a multi-hatch node with six ports. Its starboard, or right-side port, connects to the U.S. Quest airlock module while its upper zenith port accommodates the Z1 truss and the now partially stowed P6 solar arrays.

The P6 arrays provided interim power to the U.S. segment of the space station during the initial stages of assembly. An interim cooling system mounted inside P6 was used to dissipate the heat generated by the electrical systems in Destiny lab module. During the most recent shuttle flight last December, NASA activated the station's permanent electrical and cooling systems and the port wing of the P6 array - P6-4B - was retracted. If all goes well, the starboard wing - P6-2B - will be retracted during Atlantis' upcoming mission.

Unity's downward facing port is used by cargo modules brought up by the shuttle and its port hatch is home to another pressurized mating adapter, PMA-3.

Unity's forward port is attached to the Destiny laboratory module. On the forward end of Destiny is another pressurized mating adapter, PMA-2, used as a docking port by visiting space shuttles. The lab module also is home to the station's Canadarm 2 robot arm, a marvel of engineering that is capable of moving, end-over-end like an inchworm, from work site to work site on the solar array truss.

On top of the lab module is the station's main solar array truss, which is mounted at right angles to the long axis formed by the pressurized modules.

The S0 truss segment sits in the middle atop the lab, flanked by the S1 (starboard 1) and P1 (port 1) truss elements. S1, S0 and P1 house four critical electrical equipment and the station's main ammonia cooling system, including huge articulating radiator panels.

Electricity from the solar arrays, known as "primary power," is routed to components in the S0 truss called main bus switching units, or MBSUs. The four MBSUs take that 160-volt primary power and route it to transformers known as DC-to-DC Converter Units, or DDCUs, which lower the voltage to a precisely controlled 124 volts DC. This so-called "secondary power" is then directed to the station's myriad electrical systems using numerous electro-mechanical switches known as remote power controllers.

The eight solar array wings on the completed space station, four on each side, will feed power through separate lines to the MBSUs. For redundancy, power from four SAWs will flow to a pair of major circuits - 1 and 4 - while power from the other four SAWs will be directed to a second pair of circuits - 2 and 3.

The cooling system features two independent ammonia loops - loop A and B - that include large ammonia reservoirs, pumps, cold plates and the plumbing required to route the coolant through the big radiators to dissipate heat.

The loop A and B pumps were powered up during Discovery's visit last December. Expedition 14 commander Michael Lopez-Alegria and Williams completed the cooling system activation during spacewalks early this year, repositioning large fluid jumpers to route ammonia from the permanent system in loops A and B to heat exchangers in the laboratory module. The interim cooling system then was disabled.

S1 and P1 each feature three sets of ammonia radiators but only one set of cooling panels on each segment is currently extended. In August, the Zarya module's two solar panels will be folded up to provide the clearance necessary for the eventual extension of the other S1 and P1 radiators.

During a shuttle flight last September, the P3 truss segment and P4 solar arrays were bolted to P1 (there is no P2 or S2). P3 features a massive dual-motor solar alpha rotary joint, or SARJ, which is designed to rotate the outboard solar arrays like a giant paddle wheel to track the sun. P4, like P6, S4 and S6, features solar blankets that stretch 240 feet from tip to tip. In December, a short spacer truss, known as P5, was bolted to the outboard side of P4.

With the attachment of the S3/S4 segments during Atlantis' flight, NASA will resume building out the right side of the main power truss to provide the power necessary for the eventual attachment of the European and Japanese research modules late this year and early next.

During shuttle mission STS-120 in October, the flight that will carry the Harmony node into orbit, the P6 solar array now mounted atop the Z1 truss will be moved to the left end of the main solar power truss, bolted to P5 and redeployed.

"The robotics part we know how to do, I'm not especially concerned about that," said Hill. "But both of those solar arrays have to redeploy. And if we see the same type of behavior on 2B, with frayed guide wires and things like that, I am concerned about how is that going to affect the redeployment of those two solar arrays?"

Moving and redeploying P6, he said, is "probably the sportiest robotic ops we're going to do for the whole assembly."

On that same flight, Harmony will be bolted to Unity's left-side hatch. After the shuttle departs, station astronauts will use the lab's robot arm to remove PMA-2, the shuttle docking port, from Destiny's front end, attach it to Harmony and then move the two components back to the front of the lab module.

This is an especially critical point in the station's construction because space shuttles will be unable to dock at the outpost until PMA-2 and Harmony are in the proper location.

"After shuttle undocks, pulling off PMA-2 from the front of the lab, moving it over to the node and then re-installing the node on the front of the lab, that's huge," Hill said. "Because first, there's no shuttle there so there's only three crew members to get all that done with very limited views outside. And, from the time we pull the PMA off until the whole thing is complete, there's not a shuttle docking port. So that'll be sporty. But all the robotics necessary to do it are well within our experience base."

If all that goes well, including complex spacewalks to route power and cooling to Harmony, Atlantis will return to orbit on mission STS-122 in December to attach the Columbus research module to Harmony's right-side hatch and make the required electrical and cooling connections.

With Columbus in place, powered and cooled by the station's main solar array truss, NASA will turn its attention to launching two modules for the Japanese Space Agency. First up is a pressurized experiment module next February that will be temporarily mounted to the upper hatch on Harmony. The huge Kibo research module then will be launched next April and bolted to Harmony's left-side hatch. The experiment module then will be moved to an upward-facing port on Kibo.

Nine additional shuttle flights are required after that to carry up a final set of solar arrays - S6 - a cupola, a third and final node, supplies and spare parts. Station crew size will expand from three to six in 2009. Here is the current manifest:

DATE........STS/ISS........Orbiter......MISSION

06/08/07...STS-117/13A.....Atlantis.....S3/S4 solar array truss
08/09/07...STS-118/13A.1...Endeavour....S5 spacer segment; supplies
10/20/07...STS-120/10A.....Discovery....Harmony connecting node; 
........................................P6 relocation
12/06/07...STS-122/1E......Atlantis.....Columbus research module
02/14/08...STS-123/1JA.....Endeavour....Japanese experiment module;
........................................Canadian dextrous manipulator
04/24/08...STS-124/1J......Discovery....Japanese Kibo research module
08/08/08...STS-125.........Atlantis.....Hubble Space Telescope mission
10/02/08...STS-119/15A.....Endeavour....S6 solar array truss segment
11/20/08...STS-126/ULF-2...Discovery....Supplies
TBD........STS-127/2JA.....TBD..........Japanese exposed experiment
........................................facility
TBD........STS-128/17A.....TBD..........Crew equipment (6-person
........................................capability)
TBD........STS-129/ULF-3...TBD..........Supplies
TBD........STS-130/19A.....TBD..........Supplies
TBD........STS-131/ULF-4...TBD..........Contingency re-supply flight
TBD........STS-132/20A.....TBD..........Node 3, cupola
TBD........STS-133/ULF-5...TBD..........Contingency re-supply flight

"The international space station originally was conceived as a world class research facility," Suffredini said last year. "It will continue to provide that capability for research that you can do in space that can open our eyes to problems on the ground. Today, we have a number of plans to up the research capability, the research suite of hardware, during the assembly process such that we will be able to utilize it.

"However, what we look to ISS for as well is the exploration," he said. "We will use it to test systems that we plan to utilize on the moon and Mars, it's going to be really critical that we build highly reliable systems. The best place to test systems, particularly that have to operate in a zero-gravity environment, would be on the space station."

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