In addition to the routine Soyuz and Progress flights, it was anticipated that "Mir" would also be the destination for flights by the Soviet "Buran" space shuttle, which was intended to deliver extra modules (based on the same "37K" bus as "Kvant"-1) and provide a much improved cargo return service to the station. "Kristall" carried two Androgynous Peripheral Attach System (APAS-89) docking ports designed to be compatible with the shuttle. One port was to be used for "Buran"; the other for the planned "Pulsar" X-2 telescope, also to be delivered by "Buran". The cancellation of the "Buran" programme meant these capabilities were not realised until the 1990s when the ports were used instead by US Space Shuttles as part of the Shuttle-"Mir" programme (after testing by the specially modified Soyuz TM-16 in 1993). Initially, visiting Space Shuttle orbiters docked directly to "Kristall", but this required the relocation of the module to ensure sufficient distance between the shuttle and "Mir"s solar arrays. To eliminate the need to move the module and retract solar arrays for clearance issues, a Mir Docking Module was later added to the end of "Kristall". The shuttles provided crew rotation of the American astronauts on station and carried cargo to and from the station, performing some of the largest transfers of cargo of the time. With a space shuttle docked to "Mir", the temporary enlargements of living and working areas amounted to a complex that was the largest spacecraft in history at that time, with a combined mass of .