CAPE CANAVERAL - The shuttle Atlantis astronauts closed their spaceship's cargo bay doors on Thursday, aiming to conclude the final US space shuttle mission with a predawn landing at the Kennedy Space Center in Florida.Touchdown at the shuttle's home base was targeted for 5:57 a.m. local time. RELATED: Last US space shuttle leaves Int'l Space Station US space shuttle Atlantis leaves Earth on final flight (video)"Severe clear," was how NASA mission commentator Rob Navias described the weather outlook for landing, a far cry from the cloudy skies that threatened Atlantis' launch 13 days ago on a cargo run to the International Space Station.Atlantis' return from the 135th shuttle mission caps a 30-year program that made spaceflight appear routine, despite two fatal accidents that killed 14 astronauts and destroyed two of NASA's five spaceships.The last accident investigation board recommended the shuttles be retired after construction was finished on the space station, a $100 billion project of 16 nations. That milestone was reached this year. Details of a follow-on program are still pending, but the overall objective is to build new spaceships that can travel beyond the station's 250-mile (400-km) orbit and send astronauts to the moon, asteroids and other destinations in deep space.The shuttles' retirement opens the door for a new commercial space transportation industry, with NASA relying on US firms to delivery cargo to the station starting next year and to fly its astronauts there by about 2015.Until space taxis are available, Russia will take on the job of flying crews to the station, at a cost of more than $50 million per person.The primary goal of Atlantis' flight was to deliver a year's worth of supplies to the station in case NASA's newly hired cargo suppliers, Space Exploration Technologies and Orbital Sciences Corp, encounter delays preparing their new vehicles for flight.The final shuttle crew included just four astronauts -- commander Chris Ferguson, pilot Doug Hurley, flight engineer Rex Walheim and Sandy Magnus -- rather than the typical six or seven astronauts, a precaution in case Atlantis was too damaged to safely attempt the return to Earth. With no more shuttles available for a rescue, NASA's backup plan was to rely on the smaller Russian Soyuz capsules.NASA added a rescue plan after the 2003 Columbia accident.