This model depicts a space station concept design from 1986 known as "dual-keel." This design placed the station's modules on a central truss to provide a stable microgravity environment. As ongoing evaluations of various proposed space station designs raised concerns over costs and practicality, this space station design, and others, such as Space Station Freedom (see items A20060465000 and A20060466000), evolved into the International Space Station (ISS). The ISS is a cooperative effort of many nations with the goal of establishing a permanent human presence in space. Construction of the ISS began in November 1998 and was completed in 2010.