The Martian rover Spirit held the call signs MER-A and MER-2. Opportunity was called MER-B and MER-1. The assignment A=2 seems backwards; logic would suggest A=1. Why is this so?

Based on comments in a previous question, answered by @MarkAdler. Here it is as a proper question.

  • 3
    $\begingroup$ This is a nice way to convert that comment to a proper FAQ, thanks for doing it. $\endgroup$
    – Polygnome
    May 20, 2019 at 6:58

1 Answer 1


On MER we numbered the builds, but used letters for the launches. So MER-1 was built before MER-2. MER-A would both launch and arrive first, and MER-B would launch and arrive second. When we started, we weren't sure which hardware would launch first. Due to how the design testing split across the two flight units was arranged, it turned out that MER-2 became MER-A (Spirit), and MER-1 became MER-B (Opportunity).


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