STS-120: Delivering Harmony and Repairing a Torn Solar Array

STS-120 was a NASA assembly mission to the International Space Station. Its main payload was the Harmony node, while another major task was relocating the P6 truss and its solar arrays. The two projects added laboratory interfaces and improved the power configuration, preparing the station for the European Columbus and Japanese Kibo laboratories.

Thales Alenia Space in Turin, Italy, designed and built Harmony under an agreement between NASA and the European Space Agency. Boeing supplied several subsystems and the Active Common Berthing Mechanisms. The module measured about 7.2 meters long and 4.4 meters in diameter; preflight material listed its weight as 31,500 pounds, or about 14.3 metric tons. It provided a passageway for crew and distributed air, electricity and water. The berthing mechanisms aligned and locked adjacent pressurized modules into airtight connections.

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Discovery launched from Pad 39A at Kennedy Space Center, Florida, on 23 October 2007 at 15:38:19 UTC. Commander Pamela Melroy and pilot George Zamka led a seven-person crew that included ESA astronaut Paolo Nespoli. Scott Parazynski and Douglas Wheelock performed the main spacewalk tasks, while Stephanie Wilson participated in robotic-arm operations and other work.

Discovery made contact with the station on 25 October at 12:39:57 UTC and subsequently completed the rigid connection. Daniel Tani replaced Clayton Anderson aboard the station; transferring the seat liners marked the change in crew assignments. Anderson returned to Earth on Discovery, while Tani remained with Expedition 16.

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During the first spacewalk on 26 October, Parazynski and Wheelock prepared Harmony for unloading and installation, removed a failed antenna and disconnected some truss lines. A robotic arm installed Harmony temporarily on the Unity node. During this flight it had yet to occupy its final location at the forward end of the Destiny laboratory. The station crew would later relocate a docking adapter and move the node.

On 28 October, Parazynski and Tani separated P6 from the Z1 truss. The station and shuttle robotic arms transferred it together, and the third spacewalk installed it at the outboard end of the port-side P5 truss on 30 October. P6 had previously supplied power from a temporary position; relocation allowed its arrays to redeploy at the end of the station’s truss.

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During redeployment, the 2B array opened successfully, but the 4B array tore after extending to 25 folded bays when a guide wire snagged. The fourth spacewalk, originally intended to continue inspecting the starboard Solar Alpha Rotary Joint, was reassigned to repairing the array. Repairing the damage and completing deployment became an urgent task for the station’s future power supply.

On 3 November, Parazynski reached the damaged area using the shuttle’s Orbiter Boom Sensor System attached to the station’s robotic arm. With Wheelock assisting, he repaired the array so that it could extend fully. The fourth spacewalk lasted 7 hours 19 minutes; all four spacewalks totaled 27 hours 14 minutes. The emergency work used time allocated to inspections and experiments, and a demonstration of the Tile Ablator Dispenser was postponed.

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Alongside the torn array, the starboard rotary joint showed abnormal behavior. The crew collected joint debris and returned it to the Materials Failure Analysis Laboratory at Kennedy Space Center. A 2008 technical report documents three sampling tapes examined through scanning electron microscopy, energy-dispersive spectroscopy and metallography. Most debris was identified as the nitrided layer of 15-5 PH stainless steel. Fractures in the larger fragments were confined to that hardened layer, while the smaller particles showed substantial mechanical damage. These physical samples turned orbital observations into testable materials evidence.

Discovery undocked on 5 November at 10:32:03 UTC and landed at Kennedy Space Center on 7 November at 18:01 UTC, ending a flight of approximately 15 days. Harmony’s delivery, the relocation of P6 and the solar-array repair were complete. The station crew and later missions continued Harmony’s final relocation and subsequent rotary-joint repairs.

References: [2] [3]