Endeavour's successful delivery of Tranquility, the final US habitable module, leaves NASA with four more shuttle missions - to complete delivery of spare parts and science experiments - to finish the station's outfitting. The US then intends to retire its three space shuttles before the end of the year, after which the space station will be supplied by craft from Russia, Europe and Japan. Astronauts will hitch rides exclusively on Russian Soyuz capsules, while cargo will arrive on unmanned carriers. __