the internal volume of the airlock and further into the MLM pressurized adapter; for conducting scientific experiments in the internal volume of the airlock; for conducting scientific experiments outside the lock chamber on an extended table and in a special organized place. Development In the 1990s, Roscosmos' original plan for the Russian Orbital Segment included two Russian Research Modules and a Universal Docking Module based on the FGB design to be located at Zvezdas nadir port. A backup flight article for FGB-based Zarya, known in production as FGB-2, was originally planned to serve as the Universal Docking Module, though its construction had been halted at 70% completion in the late 1990s. By August 2004, Roscosmos decided to scrap the two research modules in favour of flying FGB-2 as a single research module known as Nauka. A joint Astrotech-Energia concept for a Commercial Enterprise Module-based Nauka, proposed to Roscosmos around this time, was rejected. In 2005, Roscosmos brokered a deal with the ESA to launch their European Robotic Arm with Nauka, with its spare elbow joint to launch with Rassvet. While Roscosmos had originally hoped Nauka would be ready for launch by 2007, multiple delays pushed the prospective launch date back progressively to 2014. Acceptance tests of Nauka in late 2013 found leaks in the propulsion system's fueling valve and contamination, so Energia returned the module to Khrunichev for twelve to eighteen months of repairs. Naukas prospective 2015 launch was delayed again after more fuel valve leaks damaged the module's exterior plumbing, necessitating its replacement. Metallic dust contamination in the module's fuel tanks in 2017 caused three more years of delays, as initial repairs were unsuccessful, and the installation of a new single-use fuel system based on the Fregat upper stage was considered as a solution. However, in late 2019, the original tanks were successfully repaired, rendering the planned Fregat-based tanks redundant and paving the way for a 2019 or 2020 launch. Targeting an early-to-mid-2020 launch, Nauka's Proton-M launch vehicle was assembled and shipped to the Baikonur Cosmodrome in February. However, Nauka needed another fuel tank valve replacement, along with further tests necessitated by an expiring warranty following years of delays. The testing – which could only take place at Energia in Russia instead of Kazakhstan, where Baikonur is located – was completed in May 2020, around the same time the new fuel tank valves were shipped to Baikonur. Efforts to