WASHINGTON — NASA is inviting scientists to provide ideas for the agency’s retooled robotic Mars exploration program, which in the wake of NASA’s exit from a joint sample-collection campaign with Europe aims to send a $700 million mission to the red planet in 2018 or 2020.

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Dan Leone is the NASA reporter for SpaceNews, where he also covers other civilian-run U.S. government space programs and a growing number of entrepreneurial space companies. He joined SpaceNews in 2011.Dan earned a bachelor's degree in public communications...