CAPE CANAVERAL, Fla. - When NASA's shuttle Atlantis launches toward the International Space Station (ISS) Sunday, it will kick off an intricate series of missions that will--with each flight--leave the orbital laboratory closer to completion.

Atlantis' six-astronaut crew are set to launch spaceward at 4:30 p.m. EDT (2030 GMT) on Aug. 27 on NASA's STS-115 mission to deliver two new solar wings and a massive pair of trusses to build up the station's port side. The spaceflight is NASA's first major ISS construction mission since the 2003 Columbia accident.

"It's a good feeling to be back in the space station assembly business," Paul Hill, NASA's shuttle mission operations manager, told SPACE.com.

With a frenetic schedule, new equipment, two back-to-back spacewalks that must go as planned to deploy the new solar panels, and one final extravehicular activity (EVA), the STS-115 mission has been touted as one of NASA's most challenging to date. But in fact, it's just an appetizer.

"So it's kind of a mixed feeling," Hill said. "It's really exciting to be getting back into the construction business, but we're going back into that construction business on the hardest things we've ever done."

Hill said NASA's planned shuttle flights, particularly those over the next 18 months, will be a marathon of the most complex maneuvers ever conducted in space. While the agency and its partners have performed many of the necessary tasks individually, future ISS construction strings those actions together and requires a whole lot more of them, he added.

"The assembly of the station on these flights has no parallel in space history," Michael Sufferdini, NASA's ISS program manager. "It's like building a ship in the middle of the ocean from the keel up."

Physically, the space station is only 50 percent complete and contains only 40 percent of the pressurized volume--which includes crew living and work space--it will have once NASA's planned construction plan is fulfilled by 2010.

NASA plans at least 15 more shuttle flights to complete the $100 billion space station before retiring its three remaining orbiters. Once finished, the ISS will span more than 300 feet (91 meters) with enough living space to rival a five-bedroom home and support crews of up to seven astronauts, Hill said.

"We'll go from a space station that's been a marvel in orbit, but only had a reasonably small capability to do science, to a much larger space station that takes a much lower effort to keep...going," Hill added.

Meticulously choreographed spacewalks are planned not only for shuttle astronauts during ISS construction flights, but also for station crews as well to ready the outpost for each subsequent alteration.

Among the drastic changes in appearance is next summer's planned move of the station's Port 6 (P6) truss, which currently sprouts from the outpost's center with two solar arrays at the end, to the tip of the segments to launch aboard Atlantis.

Cooling systems must also be activated, and new trusses, solar arrays and two new laboratories--Europe's Columbus module and Japan's Kibo segment, which carries the largest pressurized volume of any ISS component--still wait here to be launched from NASA's Kennedy Space Center.

Launching the international components and fulfilling NASA's obligations to its partner nations in the ISS project, are critical not just to complete the ISS, but also to reach beyond low-Earth orbit, NASA said.

"In my mind, this world will not explore unless we go together," Sufferdini said.

• VIDEO: First Tasks of NASA's STS-115 Mission
• Gallery: Prepping Atlantis
• The International Space Station So Far: Five Years of Service, But Incomplete
• Complete Space Shuttle Mission Coverage
• NASA's STS-115: Shuttle Atlantis to Jump Start ISS Construction
• The Great Space Quiz: Space Shuttle Countdown