What is the main idea of the news item? [originaltext] These four astronauts

These four astronauts are about to make an historic journey. Mission Specialist Rex Walheim, Commander Chris Ferguson, Pilot Doug Hurley and Mission Specialist Sandra Magnus make up the last crew to fly aboard a NASA space shuttle.
    All members of this Atlantis crew have flown on space shuttles before. But this time it is different, because NASA is retiring the shuttle fleet at the end of this mission.
    The astronauts readied for this mission with ascent training in a simulator, water survival training, running through emergency scenarios—such as a fake fire on a mock-up of the space station, preparing in virtual reality labs and more simulators, reviewing the tools they might need in space and practicing post-landing exits from a mock-up of the shuttle.
    The crew says it is ready for this final mission.
    This last shuttle flight will help to restock the International Space Station. Although NASA is retiring its shuttle fleet, the United States has committed to funding the space station through 2020.
    The orbiting lab is as capable as it is today because of the shuttle program. The space outpost where people have lived continuously for more than a decade remains a visible legacy of the 30-year-old shuttle program.