All of the seven windows have covers that are closed when the windows aren't used, to protect the station from space debris impact. The "Cupola" was delivered together with the "Tranquility" node by STS-130 in February 2010. The Pressurized Mating Adapters (PMA) serve as docking ports on the USOS portion of the ISS. It converts the standard Common Berthing Mechanism to APAS-95, the docking system that was used by the Space Shuttle and the Russian Orbital Segment. Currently PMA-1 is used to connect the "Unity" node with the "Zarya" module on the ISS. Pressurized Mating Adapter-2 is located on the forward end of "Harmony", while PMA-3 is located in the zenith port of the same node. PMA-2 was the main Shuttle docking port, with PMA-3 being its backup, used only a few times. With the new Crew Commercial Program and the retirement of the Shuttle fleet, NASA built the International Docking Adapter, to convert PMA-2 and PMA-3 to the NASA Docking System. IDA-1 was supposed to dock with PMA-2, but was lost in the SpaceX CRS-7 launch failure. Thus IDA-2, which was brought by SpaceX CRS-9 and was supposed to dock to PMA-3, was shifted to PMA-2. IDA-3, the replacement for the lost IDA-1, was launched in July 2019 on SpaceX CRS-18 and was berthed to PMA-3. PMA-1 and PMA-2 were delivered with the "Unity" node on STS-88 in December 1998. The third PMA was delivered by STS-92 on October 11, 2000. The Integrated Truss Structure (ITS) houses vital equipment on the exterior of the ISS. Each segment of truss is given a designation of P or S, indicating whether the segment is on the port or starboard side, and a number which indicates its position on its respective side. The truss system itself consists of 12 total segments—four on each side, and one central segment—which are connected to the ISS by attachment points on the "Destiny" module. The thirteenth piece, known as the Zenith-1 (Z1) truss segment, is attached to the "Unity" module, and was originally used to hold the P6 solar arrays to provide power to the USOS.