The shuttle Atlantis is poised for blastoff Sunday on a long-awaited flight to restart assembly of the international space station three years after the Columbia disaster derailed construction. Atlantis and its six-person crew will deliver a $372 million set of solar arrays to the outpost, kicking off the most ambitious series of manned space flights since the Apollo moon program.

"In my opinion, every one of these flights we're flying in the next 12 to 18 months ranks right up there as the most complicated flights we've every flown, including Hubble Space Telescope repair missions," said Paul Hill, mission operations manager at the Johnson Space Center.

"The fact that we're going to go conduct a series of them for 18 months, each one of which is absolutely necessary for the next one to happen, without a doubt makes this the most complicated, most complex 18 months of manned spaceflight we have ever experienced.

Said shuttle commander Brent Jett: "Every crew likes to say boy, this is one of the most complex missions we've every flown. They're all that way. And they'll all be that way until we stop flying in 2010."

With Jett and rookie pilot Chris Ferguson at the controls, the shuttle Atlantis is scheduled to rocket away from pad 39B at the Kennedy Space Center at 4:30 p.m. Sunday, Aug. 27, just six weeks after shuttle Discovery landed to close out the second post-Columbia mission.

The quick turnaround was possible because Discovery came through its flight in excellent condition, with no major systems problems and no major heat shield damage caused by foam insulation falling away from its external fuel tank.

Atlantis will fly with a virtually identical tank, one with foam insulation around nearly three dozen external fittings that is classified as "probable/catastrophic" in NASA's risk matrix. That means there's a 50-50 chance of catastrophic damage over the 100-flight design life of a shuttle.

NASA is developing a redesign for the so-called ice-frost ramps, but it will not be ready for flight until next year. NASA Administrator Mike Griffin approved Discovery's launch last month over the objections of the agency's top safety manager and its chief engineer. Facing a 2010 deadline for completing the space station and retiring the shuttle, Griffin decided the impact of another long delay was worse than flying with what he considered to be an acceptable risk.

This time around, even though the ice-frost ramps remain officially classified as probable/catastrophic, there was no dissent and Atlantis was cleared for launch on the 116th shuttle mission without objection.

Joining Jett and Ferguson aboard Atlantis will be flight engineer Dan Burbank, a former Coast Guard helicopter pilot who flew a real rescue mission in "the perfect storm," Canadian astronaut Steve MacLean, veteran Hubble spacewalker Joe Tanner and Heidemarie Stefanyshyn-Piper, a former Navy salvage diver.

Mounted in the shuttle's cargo bay is one of the heaviest payloads ever launched to the space station, a $371.8 million extension to the lab's main solar array truss that weighs some 34,977 pounds.

Forty-five feet long and 16 wide, the Boeing-designed payload features two new solar arrays and a massive rotary joint that will permit the giant panels to slowly turn like a giant waterwheel to track the sun as the station circles the globe at five miles per second.

The solar alpha rotary joint, or SARJ, is the main component of what NASA calls the P3, or port 3, truss element. The solar arrays, their electronics and cooling radiators make up the P4 truss element. Both are bolted together and NASA refers to the combination as the P3/P4 truss.

"It's been six years since our payload has been at Kennedy," MacLean said when the crew flew to Florida to prepare for blastoff. "It's been four years since Atlantis has been in preparation (for launch) and for us as a crew, it's been four and a half years as well. And finally, on Sunday, we're going to get to walk out to the pad for launch.

"For me, walking out to the pad on Sunday will be much like walking into an Olympic stadium for your athletic event. Many countries will be participating in a spirit of international cooperation and our families and our friends who believe in what we do will be in the front seats of the stadium. So I invite you all to watch what we do over the next week. It will be exciting. It's complex what we do, it's not easy. But with a team like this that I've been working with for the last four years and especially with the focus and dedication of the teams on the ground, I promise you we'll bring home a gold medal."

Getting P3/P4 attached to the station's port truss will require one of the most ambitious flight plans ever put together for a shuttle mission.

Assuming an on-time launching, Atlantis will dock with the space station Tuesday around 12:40 p.m. Just two hours later, Burbank, operating the shuttle's robot arm, will lift the P3/P4 truss out of the cargo bay and hand it off to MacLean, who will be operating the station's more massive Canadarm 2. Canadarm 2 will be mounted atop a mobile transporter on the far end of the port truss.

"Normally, once we finish the rendezvous, we open the hatches, everybody says hello and we get down to doing some (equipment and supply) transfers and a few things like that," said lead shuttle flight director Paul Dye. "We have so much on this mission that on this day, we're going to do the rendezvous and then we're going to ... take the payload, the P3/P4 truss segment, out of the bay and hand it off to the station before the crew goes to bed.

"So while we are equalizing air pressure between the spacecraft and opening the hatches and getting things connected and shaking hands and greeting, we're going to have crew on the aft flight deck getting ready to pull the P3 out."

Because the 50-foot-long orbiter boom sensor system, or OBSS, will be in place along the right side of Atlantis' payload bay, Burbank will have about one inch of clearance as he unberths P3/P4. To get past the OBSS, he will have to move the arm in a complex sequence, being careful not to bang into anything along the way.

"We'll grapple the payload, we'll move it up, back a little bit, up a little bit more to where we're clear, then out to the side and then we have a large automated maneuver which will put it into what we call the handoff position so the station can go and grab that with the big arm," Dye said.

"This is a pretty significant activity by itself and putting it on rendezvous day makes this a pretty long and very interesting day. Once we have it held in the handoff position, the station arm will move in and grapple. Once they've got it, we'll release it with our arm and at that point, it becomes (the station's) piece of equipment."

MacLean will leave P3/P4 parked overnight on the left side of the shuttle just below the station's unfinished truss. Before the crew goes to bed, they will change the orientation of the shuttle-station complex, rolling 45 degrees to port. That will aim the left side of the truss down toward Earth and help keep the stowed solar array blankets warm.

The next day, MacLean will move P3/P4 up to the station's P1 truss (there is no P2 element). Once properly aligned, motorized bolts in P1 will be powered up to lock the new truss in place.

As soon as three of the four attachment bolts are engaged, Tanner and Piper will exit the station's Quest airlock and begin a complex spacewalk to make critical electrical connections required to power heaters and other systems necessary to keep the new arrays alive.

Electrical power to the U.S. section of the space station currently is provided by solar arrays mounted atop a short truss, known as Z1, that extends up in the zenith direction from the station's multi-hatch Unity module.

Those arrays, known as P6, will be moved down to the main solar array truss next year and attached to a short spacer, known as P5, that will be bolted to P4 during the next shuttle assembly mission in December.

To wire in the new P4 arrays, flight controllers in Houston will carefully power down the station's two P6 electrical channels, one at a time, so Tanner can hook up 13 umbilical cables between P1 and P3/P4.

"There are about a hundred pages of ground commanding that have to go on in order to get things powered down in the right order and get things switched over and powered back up again afterwards," station flight director John McCullough said in an interview. "It's one of the more tightly choreographed EVAs that we've done as far as ground and crew interaction."

The P4 solar arrays are packed up like venetian blinds in four large "blanket boxes." Tanner and Piper will prepare those boxes for deployment and start the process of readying the SARJ for operation.

The next day, MacLean and Burbank will carry out a second spacewalk to complete preparations for SARJ operation and solar array deploy. That night, flight controllers will send commands to extend the arrays one mast bay in a confidence test before the astronauts take over the next morning. First, they will extend each array to 49 percent and then, after letting the sun heat them up a bit, the rest of the way.

The stepwise approach is being taken because of problems encountered in December 2000 when the P6 array was deployed. When the first P6 wing unfurled, several solar cell panels stuck together, resulting in a jerky motion that caused a tension cable to unwind and jump from its spool.

The second P6 array was deployed in high-tension mode, which prevented additional problems. Engineers now believe the "stiction" was caused by subtle effects of atomic oxygen coating the arrays and low temperatures. For the P3/P4 deploy, the arrays will be extended in high-tension mode with enough solar heating to preclude similar problems.

Fully extended, the 38-foot-wide arrays will span 240 feet from tip to tip. Some 66,000 solar array cells will generate nearly 66 kilowatts of usable power. A dozen massive batteries will provide power when the station moves into Earth's shadow and ammonia lines inside accordion-like radiator panels extending 44 feet will shed the heat generated by the electrical circuits.

But the new arrays will not provide power to the station until the next shuttle mission in December when the left side of the P6 array is retracted. In its current position, the port wing of the P6 array is at right angles to the P4 wings, extending into the area where P4 eventually will rotate.

With the new arrays deployed, Tanner and Piper will stage a third spacewalk the following day to complete SARJ activation, to carry out critical repairs on the station's S-band antenna system and to install a thermal blanket around components of the lab's high-speed KU-band antenna.

For Jett, the number one concern for mission STS-115 is the timeline.

"I try to worry about the things we have control over," he told CBS News in an interview. "All the hardware issues, whether the SARJ is going to work right, I'll let somebody else worry about that. But the timeline is something we have control over and when you look at the first five days of this mission, we've never put a mission together with that many major activities in the first five days, Hubble or no Hubble. It's unprecedented and it's going to be really tough.

"The second thing I spend a little time (thinking about) is the unberth of P3/P4. The clearance is extremely tight. ... Dan is going to do a little maneuver while the payload is still coming out of the bay, he's going to slide it to port about two inches to give us a little bit of extra clearance on the (inspection) boom.

"When you have a big payload like that, you'd like to try to move it in one axis at a time when you're working with real tight clearances. So Dan's actually going to be moving it up and to the port side of the shuttle at the same time. How the payload actually responds to that, when he stops that sideways maneuver, will you get a little oscillation? Our clearance on the left side will be about an inch. So that's gotten our attention."

If all goes well, Atlantis will undock from the space station Sept. 4. The next day, the crew will carry out heat shield inspections to make sure Atlantis hasn't been hit by any orbital debris or micrometeoroids since they carried out a post-launch inspection on the second day of the mission.

Landing back at the Kennedy Space Center is targeted for a few minutes past noon on Sept. 7. Assuming the shuttle's external tank performed well and no major impact damage occurs, NASA managers will relax a post-Columbia requirement to launch in daylight, opening up more space station launch windows and clearing the way for a night launch of Discovery Dec. 14 on mission STS-116.

It all starts Sunday when Atlantis takes off on its first flight in four years to begin an assembly sequence that's been on hold since Columbia went down on Feb. 1, 2003.

"The restart of assembly, obviously, fulfills the first objective of the president's vision, that is, return the shuttle to flight and complete the space station," Jett said. "We have to do that as an agency and this mission marks the restart of the assembly sequence."

But Atlantis' flight, he said, "is just one of the assembly missions we have to get done. Every crew and every mission control team has to be on their game when we fly these assembly missions. We've got to do them right. We can handle some setbacks, we can deal with some problems but we have to perform these missions well and we have only a limited amount of time to do it. They're all important.

"If we do our jobs right, we'll get to 2010 and we'll retire the shuttle safely, we'll see it come to a wheels stopped not too far from here and we'll have a complete station. That's what we're really all looking forward to."

PREVIEW REPORT PART 2 --->