Mars One has quietly stopped work on a pair of robotic precursor missions it once called "the first step in Mars One’s overall plan of establishing a permanent human settlement on Mars."

Mars One, the project to send humans to Mars on one-way missions that has attracted public interest and professional skepticism, said Dec. 7 is it delaying its timetable for those missions by several years.

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Jeff Foust has more than a decade of experience writing about space policy, entrepreneurial ventures and regulatory affairs. In 2001, he established spacetoday.net to aggregate and summarize the day's space-related news stories. In 2003, he started The...