International Space Station PMA-3 docking module relocated
Ground-based robotics teams remotely commanded the International Space Station’s robotic Canadarm2 to move the Space Shuttle-era Pressurized Mating Adapter (PMA) 3 to the forward end of the outpost. The March 26, 2017, relocation was part of a multi-year effort to prepare the ISS for future commercial crew spacecraft.
PMA-3 was located on the Tranquility module’s port-side Common Berthing Mechanism (CBM), a place it had been for seven years. The relocation effort moved the docking module to the Harmony module’s space-facing CBM, right next to PMA-2, which is located on Harmony’s forward CBM.
The purpose of Boeing-built PMAs was to convert the 50-inch (127-centimeter) U.S. module CBM hatches to the 31-inch (80-centimeter) Russian-designed APAS-95 docking hatch used by the Space Shuttles and Zarya module. It is 6.2 feet (1.9 meters) long and 6.2 feet (1.9 meters) at its widest diameter.
The small, 2,600-pound (1,200-kilogram) docking module was brought to the station along with the Z1 truss segment during the STS-92 mission in October 2000. It was originally attached to the Earth-facing port of the Unity module. Back then, the station consisted of only three large components: Zvezda, Zarya, and Unity. PMA-1 and PMA-2 were attached to either end of Unity during its 1998 launch.
PMA-3 was placed on the Earth-facing port of Unity, allowing the next two Space Shuttle missions in 2000 and 2001 to dock below the ISS instead of at the forward end. This enabled Endeavour and Atlantis to attach the P6 truss and Destiny laboratory during STS-97 and STS-98, respectively. It was the only time this docking module was used.
Since then, it has been moved to various ports around the outpost to make room for other modules. Additionally, it has been used as a makeshift “storage closet”.
In March 2001, it was moved to the port side of Unity to make room for Multipurpose Logistics Modules that the Space Shuttles would periodically launch to resupply the outpost.
Then, August 2007, PMA-3 was moved back to the Earth-facing port of Unity to allow for Harmony, which arrived during mission STS-120, to be temporarily attached. After the Space Shuttle left, Harmony was relocated to the forward end of Destiny.
PMA-3 was moved back to the port-side CBM of Unity in 2009, but, a year later, it was moved to the space-facing port of Harmony temporarily to allow for the Tranquility module to be attached to Unity. Once it was in place, the docking module was moved to the port-side CBM of Tranquility where it stayed until March 26, 2017.
Just two days before the move, a spacewalk was conducted. Extravehicular Activity (EVA) 40 was performed by Expedition 50 Commander Shane Kimbrough of NASA and Flight Engineer Thomas Pesquet of the European Space Agency. The primary purpose of the excursion was to detach various power and data cables connecting PMA-3 to Tranquility.
Once complete, Earth-based robotics teams controlled Canadarm2 to grapple onto the small module. Then the CBM bolts physically holding PMA-3 to the station were released. Over the next couple hours, the arm moved the docking module over to its new location.
After PMA-3 was confirmed in the correct pre-latch position, Harmony’s CBM bolts were commanded to turn, reconnecting the module into its final location.
Next, on March 30, Flight Engineer Peggy Whitson and Kimbrough will venture out on EVA-41. The primary task for that spacewalk will be to connect new power and data cables to PMA-3, readying it for the arrival of International Docking Adapter (IDA) 3 sometime in 2018.
The reason for the adapter modification to the docking module is that the Commercial Crew Program spacecraft from SpaceX and Boeing will be using a more modern NASA Docking System which follows the International Docking System Standard.
The new adapter uses low-impact technology and can be used for both docking and berthing, as well as both autonomous and piloted dockings. It will also sport a 31-inch (80-centimeter) hatch, just like APAS-95.
Since APAS-95 is not compatible with the NASA Docking System, two International Docking Adapters were developed. The first one, IDA-2 (IDA-1 was lost during the CRS-7 launch mishap) was attached to PMA-2.
IDA-2 was launched in 2016 inside a SpaceX Dragon cargo ship’s trunk. Once the spacecraft was attached to the station, the Special Purpose Dexterous Manipulator, Dextre, removed the adapter from Dragon’s trunk and placed it just in front of PMA-2. Then spacewalking astronauts went outside to complete electrical connections and permanently mount it to the Shuttle-era adapter.
With work on IDA-2 complete, it is now ready to support either a SpaceX Crew Dragon or a Boeing CST-100 Starliner. The first flights are expected in 2018.
While a second IDA is not required, it allows for redundancy and direct handovers of U.S. crews. IDA-3, which replaces the lost IDA-1, will launch aboard SpaceX’s CRS-16 Dragon.
Video courtesy of NASA Johnson
Derek Richardson has a degree in mass media, with an emphasis in contemporary journalism, from Washburn University in Topeka, Kansas. While at Washburn, he was the managing editor of the student run newspaper, the Washburn Review. He also has a blog about the International Space Station, called Orbital Velocity.