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There are a number of historical planetary missions that have produced 2 flying instances. Examples include Voyager, Viking, MERS, some of the Mariners missions. Other missions have only one instance: Mars Observer, MRO, Magellan, Cassini and so on.

What is the cost difference between producing a mission with one instance vs. a mission with two identical instances. I'm looking for specific citations to clarify and answer this point.

This question presumes that the production of the second instance occurs at the same time as the first instance. That is to say in parallel, not produced at a later time.

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    $\begingroup$ In some cases it's totally free: two are made just in case. If the primary spacecraft was successful, then the backup could be modified and repurposed for another mission(e.g. Chang'e 1 and 2 were born as identical twins then 2 evolved to its own mission after 1 was successful). $\endgroup$
    – user3528438
    Dec 19, 2021 at 5:46
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    $\begingroup$ Some components, like the RTG power source are inherently expensive and would incur great cost to duplicate. Most of the cost lies with R&D and testing though, and would not be at all duplicated between copies. $\endgroup$
    – CuteKItty_pleaseStopBArking
    Dec 19, 2021 at 12:15
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    $\begingroup$ Obligatory Contact quote: "First Rule in government spending: why build one, when you can have two at twice the price?" $\endgroup$
    – Jörg W Mittag
    Dec 20, 2021 at 11:07
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    $\begingroup$ Its an interesting problem but unfortunately there is no easy answer to this because the basic cost is going to relate to the amount of reuse or upgrading of equipment from previous missions for either the "flight model plus spare" or the "flight model only" approach. If you tried to do a survey you'd very quickly run into the problem of not being able to compare like with like. I think it would l be hard to answer unless you had the manufacturer's private costings to hand. $\endgroup$
    – Puffin
    Dec 20, 2021 at 21:13
  • $\begingroup$ @Puffin. I agree that Voyager won't directly compare with MER and so on. It would be interesting to know the range of costs $\endgroup$
    – Freddo411
    Dec 21, 2021 at 15:34

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