stylistic:style_transfer, structural:reformatting
Third start

The launch of Soyuz TM-8 on 5 September 1989 marked the beginning of the longest human presence in space, until 23 October 2010, when this record was surpassed by the ISS. It also marked the beginning of Mir's second expansion. The Kvant-2 and Kristall modules were now ready for launch. Alexander Viktorenko and Aleksandr Serebrov docked with Mir and brought the station out of its five-month hibernation. On 29 September the cosmonauts installed equipment in the docking system in preparation for the arrival of Kvant -2, the first of the 20 tonne add-on modules based on the TKS spacecraft from the Almaz programme.

After a 40-day delay caused by faulty computer chips, Kvant -2 was launched on 26 November 1989. After problems deploying the craft's solar array and with the automated docking systems on both Kvant -2 and Mir, the new module was docked manually on 6 December. Kvant -2 added a second set of control moment gyroscopes (CMGs, or "gyrodynes") to Mir, and brought the new life support systems for recycling water and generating oxygen, reducing dependence on ground resupply. The module featured a large airlock with a one-metre hatch. A special backpack unit (known as Ikar), an equivalent of the US Manned Maneuvering Unit, was located inside Kvant -2's airlock.

Soyuz TM-9 launched EO-6 crew members Anatoly Solovyev and Aleksandr Balandin on 11 February 1990. While docking, the EO-5 crew noted that three thermal blankets on the ferry were loose, potentially creating problems on reentry, but it was decided that they would be manageable. Their stay on board Mir saw the addition of the Kristall module, launched 31 May 1990. The first docking attempt on 6 June was aborted due to an attitude control thruster failure. Kristall arrived at the front port on 10 June and was relocated to the lateral port opposite Kvant -2 the next day, restoring the equilibrium of the complex. Due to the delay in the docking of Kristall, EO-6 was extended by 10 days to permit the activation of the module's systems and to accommodate an EVA to repair the loose thermal blankets on Soyuz TM-9.

Kristall contained furnaces for use in producing crystals under microgravity conditions (hence the choice of name for the module). The module was also equipped with biotechnology research equipment, including a small greenhouse for plant cultivation experiments which was equipped with a source of light and a feeding system, in addition to equipment for astronomical observations. The most obvious features of the module were the two Androgynous Peripheral Attach System (APAS-89) docking ports designed to be compatible with the Buran spacecraft. Although they were never used in a Buran docking, they were useful later during the Shuttle- Mir programme, providing a berthing location for US Space Shuttles.

The EO-7 relief crew arrived aboard Soyuz TM-10 on 3 August 1990. The new crew arrived at Mir with quail for Kvant -2's cages, one of which laid an egg en route to the station. It was returned to Earth, along with 130 kg of experiment results and industrial products, in Soyuz TM-9. Two more expeditions, EO-8 and EO-9, continued the work of their predecessors whilst tensions grew back on Earth.