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During the old space race days, launching double or multiple scientific probe copies seems to have been the standard. It should have the advantage of sharing the one time fixed development costs while less than doubling the manufacturing and operational costs given diminishing marginal costs of reiteration.

Based on Pioneer, Viking, Voyager and other duplicate missions (even to some degree Galileo/Cassini and MSL/Mars2020 and the Soviet Venus missions), how should one estimate the marginal cost today of duplicating a Cassini class orbiter to Uranus with a copy to Neptune at the same time (albeit on separate launchers a year apart)? Would major design differences of the spacecrafts and their instruments be worthwhile to turn it into two unrelated missions?

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  • $\begingroup$ related, but not identical: space.stackexchange.com/questions/9535/… $\endgroup$ – Hobbes Jun 22 '17 at 15:32
  • $\begingroup$ In books I have read by NASA engineers, it's repeatedly been described as "pennies on the dollar", since the vast majority of the cost is design and testing. I imagine the Uranus and Neptune orbiters would be extremely similar, though not a "twin" mission, so you'd save most of the money on instruments, etc but need to have different accommodations for radiation, communication, trajectory, aerobraking, etc. $\endgroup$ – Deimophobia Jun 22 '17 at 18:56
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    $\begingroup$ But testing has to be done not only with the first probe, but also with the second. $\endgroup$ – Uwe Jun 25 '17 at 12:02
  • $\begingroup$ The total cost for any mission going to the outer planets is going to be high for numerous reasons, many of which are unavoidable (e.g., buying and testing RTGs is not cheap, radiation hardening is expensive, electromagnetic cleanliness is very expensive, etc.). The launch vehicle alone is typically somewhere between \$250M and \$500M, depending on the size/weight of the spacecraft, which is included in mission costs. However, mission budgets are generally spread over at least 10 years, so any mission is fractions of a penny for every tax dollar. $\endgroup$ – honeste_vivere Jun 30 '17 at 18:17
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Partly shared costs:

  • Research & Development (assuming missions and probe equipment are aligned)
  • Ongoing monitoring and support crews

One Time Costs:

Much of the marginal cost difference comes the following, which vary depending on how closely the additional missions align with the original.

  • Additional launch vehicle(s) (rockets)*

  • Manufacturing costs for a second probe

  • Software revisions to adapt to needs of the additional mission(s)

  • Planning for additional mission(s)

Limiting Factor:

While the costs listed above are significant, the limiting factor may be fuel. The duplicate missions mentioned in your question are powered by what NASA calls a Radioisotope Power System (RPS) also known as a Radioisotope Thermoelectric Generator (RTG) which rely on availability of Plutonium-238 as a long lasting thermal energy source.

Availability of Plutonium-238 for space missions is so limited, that a duplicate mission may be declined on basis of fuel availability, not cost. As of February 11, 2018, NASA states they have 17 kg of Plutonium-238 remaining that is compatible with current RPS designs. Each mission takes about 4 kg.

Therefore a significant cost to produce a duplicate mission is acquisition of Pu-238 or research towards an alternate long lasting power source.

Other considerations

The specific price for the categories listed above have changed over the years because of economic and political factors. I believe these categories cover the most significant costs, but others may exist.

*reusable rockets which promise cheaper cost per flight were unavailable for the earlier missions mentioned.

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For a new development, which pretty much all interplanetary spacecraft are, a rule of thumb is that a second flight article, fully qualified and ready to go, will add 30% to 40% to the development cost. The development cost is the pre-launch Phase A-D cost of just the spacecraft, which does not include the launch vehicle(s) or operations.

This assumes that the second spacecraft is built at the same time as the first, using the same subcontracts, same personnel, same facilities, and same design. If there are any changes to the design, e.g. for different science or different targets, then the savings over a separate development can diminish very quickly.

The different targets thing was briefly attempted for the CRAF-Cassini mission, which planned to use a common spacecraft bus for two very different missions, one a Saturn orbiter delivering a Titan probe, and the other an asteroid flyby and comet rendezvous mission, delivering a comet probe. However the CRAF part was cancelled very early in development, so we'll never know how that would have turned out in terms of cost.

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