While scientists are searching

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                                      Human Space Exploration
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        While scientists are searching the cause of the Columbia disaster, NASA is moving aheadwith plans to develop a new craft that would replace shuttles (航天飞机) on space station missions by 2012 and respond quickly to space station emergencies.
        The space agency released the first set of mission needs and requirements several days agofor orbital space plane ( 轨道航天飞机), which would be designed to transport a crew of fourto and from the International Space Station.
        Although it includes few specifics, the plan states the orbiter (轨道航天飞机) will besafer, cheaper and require less preparation time than the shuttle. It would be able to transportfour crew members by 2012 —— though it would be available for rescue missions by 2010. NASAsays the craft should be able to transport injured or ill space station crew members to "definitive(决定性的) medical care" within 24 hours.
       The release of the requirements showed NASA remains focused on the long-term prioritiesof space exploration, even as questions exist concerning the loss of Columbia and its sevenmember crew on February 1, 2003.
      Expels at Marshall Space Flight Center in Huntsville, Alabama, have been working foryears on a successor to the shuttle. The project, known as the Space Launch Initiative (倡议),was divided last year into two parts -- one focusing on a future launch vehicle, the other on aspace station orbiter. The orbiter is expected to be ready sooner.
     The program's managers say NASA officials have told them not to alter Space Launch Initiative in light of the Columbia disaster.
   U.S. President George W. Bush asked Congress for about US $1 billion for Space LaunchInitiative in 2004, funds that would be almost equally split between the Orbital Space Plane andNext Generation Launch Technology.