STS-119 (Discovery / ISS S6 arrays)

Background

By the late 2000s, the International Space Station had grown into one of the most complex engineering structures ever assembled in orbit, yet it remained operationally incomplete. The station's integrated truss structure — a 109-metre backbone running perpendicular to the habitation modules — was designed to carry four pairs of photovoltaic solar array wings, each capable of generating roughly 31 kilowatts of electrical power. Three of those four pairs had already been delivered and deployed across earlier shuttle missions, but the fourth and final pair, designated S6, still waited on the ground. Without it, the station could not reach the full power capacity required to support a six-person permanent crew, a milestone NASA and its international partners were pressing toward in 2009.

STS-119 was assigned to close that gap. Discovery, one of the shuttle fleet's most-flown orbiters, was selected as the vehicle. The integrated truss segment S6 — a structure weighing roughly 14,000 kilograms — was loaded into the payload bay along with the Solar Array Wings that would unfurl from it. The flight also served a crew-rotation function: mission specialist Koichi Wakata of JAXA would remain aboard the station as a member of Expedition 18, replacing Sandy Magnus, who had been living on the station since October 2008.

Crew and Preparation

Commander Lee Archambault led a seven-person crew with broad collective experience. Pilot Tony Antonelli was making his first spaceflight, as were mission specialists Joseph Acaba and Richard Arnold, both former teachers who had been selected as part of NASA's educator-astronaut programme. Steven Swanson, a veteran of STS-117, served as a mission specialist alongside John Phillips, who had previously lived aboard the station during Expedition 11. Koichi Wakata, a veteran of multiple shuttle flights, rounded out the crew and brought continuity to the station's long-duration research programme.

The mission had experienced a short delay from its original February target, caused in part by a hydrogen flow-control valve issue that required inspection and resolution before the vehicle could be cleared to fly. Discovery lifted off from Launch Complex 39A at Kennedy Space Center on 15 March 2009.

Flight and Assembly Operations

Discovery reached orbit approximately eight and a half minutes after launch, and the crew began the standard programme of post-insertion checks, payload bay inspections, and thermal protection system surveys that had become mandatory following the Columbia accident of 2003. Rendezvous with the station proceeded over the following two days, with Archambault executing the now-standard Rendezvous Pitch Manoeuvre — a slow backflip that allowed station crew members to photograph Discovery's heat shield tiles for ground analysis.

After docking, the primary work of the mission began. The S6 truss segment was lifted out of Discovery's payload bay using the station's robotic arm and handed off through a carefully choreographed sequence of robotics operations before being positioned at the starboard end of the integrated truss. At approximately 55 hours and 33 minutes into the mission, the installation was completed: S6 was bolted into its permanent position, and the station's truss structure was, for the first time, physically whole.

Three spacewalks — conducted by Swanson, Arnold, and Acaba in various pairings — supported the installation and subsequent deployment of the solar array wings. The two new wings, each stretching approximately 34 metres when fully unfurled, were deployed and verified to be functioning normally. With their successful checkout, the station achieved its full power-generating capacity, closing an assembly chapter that had been underway since the STS-97 mission in late 2000 first brought solar arrays to the P6 truss location.

Wakata formally transferred to the station crew during the docked period, and Magnus returned to Discovery for the journey home. The handover marked the beginning of a new phase in the station's operations: for the first time, a Japanese astronaut would serve as a long-duration resident, a milestone in the international character of the programme.

Discovery and the station parted ways at approximately 250 hours into the mission. The shuttle performed its deorbit burn at around 306 hours and 50 minutes after liftoff, committing the vehicle to reentry over the Pacific. Discovery touched down on Runway 15 at Kennedy Space Center approximately 40 minutes later, at roughly 307 hours and 30 minutes mission elapsed time, completing a flight of just under thirteen days.

Legacy

STS-119 occupies a precise and significant place in the history of the station. With the S6 arrays generating power, the station could at last support the six-person Expedition crews that the partnership had long planned, enabling a near-doubling of the science programme conducted on board. The delivery also fulfilled a commitment made to the international partners that the backbone infrastructure of the station — power, cooling, and structure — would be in place before the permanent crew size was increased.

The completion of the truss also represented a kind of quiet milestone in NASA's understanding of large-scale on-orbit construction. What had seemed an almost impossibly ambitious assembly programme in the 1990s had been executed, flight by flight, into an operating reality. STS-119 was not the last shuttle mission to the station, but it closed the structural story of the truss, a chapter that had required more than a decade of planning and nearly thirty assembly flights to write.

For the crew members who flew the mission, it represented the culmination of years of training oriented specifically toward this task. For the station programme as a whole, it was the moment when the hardware finally matched the ambition.