An upcoming NASA selection between very different robotic spacecraft for a $700-million mission scheduled to fly by 2010 is emblematic of the strains that are likely to beset the U.S. agency-and its international partners-for years to come as they struggle to map the new pay-as-you-go U.S. exploration program. In choosing between a vehicle to return lunar samples from the Aitkin Basin at the Moon's south pole, and an orbiter that would circle Jupiter from pole to pole to see if the gas giant has a solid core, NASA managers must strike a balance between human space spectaculars close to home and the equally spectacular science possible deeper in space.
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