Has USA hit final frontier in human space exploration?

Just six years ago, President Bush laid out a vision of space exploration that harked back to NASA's halcyon days built on astronauts as explorers. Bush wanted to sling them from low Earth orbit to a base on the moon and then, perhaps, on to a first manned landing on another planet, Mars. But that was before huge federal deficits arrived, public support failed to show, and unmanned explorers scored successes -- namely the Hubble telescope and Mars rovers Spirit and Opportunity, which are still sending back signals years after they were expected to expire.

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