A NASA decision last month to replace an instrument on the Europa Clipper mission with a less expensive, but less capable, alternative is leaving scientists concerned about the ability of the mission to meet some of its objectives.

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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...