 These modules, called "Heavy Cosmos modules", helped engineers develop technology necessary to build "Mir".Salyut 7 was deorbited on 7 February 1991.After the second generation, plans for the next generation of Salyut stations called for the cores DOS-7 and DOS-8 to allow, for the first time in spaceflight, the addition of several modules to a station core and to create a modular space station.