The successful end of Discovery’s mission — the second and final post-Columbia shuttle test flight — marks the beginning of a four-year sprint for NASA to finish the half-built international space station (ISS) without compromising astronaut safety.

Commanded by veteran NASA astronaut Steven Lindsey, Discovery’s success opens the gate for 15 future orbiter missions to build out the ISS by Sept. 30, 2010.

Beginning as soon as late August, NASA plans to conduct a series of shuttle flights to the ISS that first will be used to build up the station’s power and support systems, and then involve the installation of new modules and laboratories that have been waiting for NASA’s shuttle fleet to resume full flight status.

Up next is the STS-115 launch, a flight by the shuttle Atlantis that is slated to occur between Aug. 28 and Sept. 7. The goal of that mission is to deliver a 17.5-ton solar array and a truss segment to the ISS.

NASA officials noted that Atlantis is scheduled to conduct one of the most complicated ISS construction missions ever, but also said they are confident the agency is ready. “Yes, I think the conclusion is that the shuttle is back,” said Wayne Hale, NASA’s shuttle program chief. “We have the team that is now practiced and battle-hardened ready to go …” on to the next mission.

STS-116, the third shuttle mission planned for this year and the second for Discovery, is scheduled to launch Dec. 14 and will involve the installation of the new solar array to be delivered by Atlantis in August and the delivery of another truss segment.

“That’s a very challenging [work] flow for us,” Stephanie Stilson, NASA’s flow manager for Discovery, said in an interview before the most recent Discovery mission launched July 4. “We are going to do the best we can to get everything done in the time allotted … it’s very important that we launch in December if we can, and we’re going to do everything we can to get there.”

Discovery also must be ready to serve if needed as a rescue ship for the Atlantis mission that is scheduled to begin around Aug. 28. “Every mission depends on the successful completion of the previous one,” said NASA astronaut Mark Polansky, commander of the STS-116 mission.

Shuttle officials expect to use at least 110 days, with an extra five days if needed, to be ready to fly Discovery at the opening of its next launch window Dec. 14.

“Right now, everything is per plan and I see no reason we couldn’t get to late August to launch Atlantis,” NASA launch director Michael Leinbach said July 17. Initially, NASA’s window for the STS-115 launch extended through mid-September, but a schedule clash with the planned Russian Soyuz launch of the station’s Expedition 14 crew prompted a shortening of that window by a week to about Sept. 7.

The Spacecraft Shuffle

Russia’s Federal Space Agency hopes to launch Expedition 14 commander Michael Lopez-Alegria, flight engineer Mikhail Tyurin and Japanese space tourist Daisuke Enomoto toward the ISS aboard a Soyuz TMA-9 spacecraft Sept. 14, but can only do so if NASA launches Atlantis by Sept. 3, NASA officials said.

“We don’t want to have dual operations at the station with both the Soyuz crew and the shuttle crew at the same time,” said William Gerstenmaier, NASA’s associate administrator of space operations, adding that simultaneous ISS construction and crew change missions places too many astronauts aboard the station. “We just can’t have that many folks on the space station doing the diverse tasks that are there.”

Michael Sufferdini, NASA’s ISS program manager, also has said that onboard shuttle systems can interfere with the autonomous Kurs docking systems used aboard Russian spacecraft. A Russian-built Progress cargo ship also will have to be jettisoned to free up a docking port for the incoming Soyuz in that timeline as well, he added.

To work around the scheduling headache, Russian space officials agreed to delay their Soyuz launch to Sept. 18 if necessary to allow a two-day buffer between Atlantis’ departure and the Soyuz arrival.

Russia’s Federal Space Agency cannot move the Soyuz launch past Sept. 20 because it would result in a night landing for the spacecraft, which will be unfavorable for the helicopter recovery crews charged with seeking out the spacecraft on the unlit steppes of Central Asia’s Kazakhstan.

“We’re going to work with our Russian partners to have them go ahead and move the Soyuz such that we don’t have this overlap between the shuttle and the Soyuz at the same time,” Gerstenmaier said. “We’ve balanced our needs and their needs together to get the right answer for the space station.”