I learned in my life in a high tech company, though, that "simple" additions often turn out not to be simple and could drive up cost rapidly. For those of us outside of the planetary mission design world, it's hard for us to understand which additions might truly simple additions and which would be unacceptably complex and costly.
The recently published Decadal Survey mission concept studies offer a peak into some of these tradeoffs. Several reports explicitly explore several versions of missions with a variety of goals and instrument costs. In this blog entry, I'll look at examples of the cost tradeoffs for possible missions to Ganymede and Enceladus. The concept studies don't cover all options I would have liked to see discussed. For example, how much would it drive up the costs of an Enceladus orbiter to have an instrument or two to study Titan during several flybys? Or, what would be the design and cost impacts of adding several Europa flybys for a Ganymede orbiter? Still, these reports offer insights that often aren't available to the public. They also were all carried out under the same ground rules and using the same FY15 dollar costs, making comparisons between them reasonable.
The following table list a number of Ganymede and Enceladus mission options. The Ganymede missions differ both in the number of instruments and in the length of time in Ganymede orbit. The Enceladus missions would all orbit that moon for 12 months (except for a multiple flyby mission), but differ in the number of instruments. Several of the Enceladus options were also ranked for relative science value.
Option numbers are from the reports listed at the end of this blog entry. Click on image for a larger version.
The Ganymede mission options range from $1.3B to $1.7B. Looking at the charts in the table, most of the difference in costs appears to be driven by costs associated with building and operating additional instruments rather than the longer time in orbit. This is dramatically shown by adding up the costs to build and test the instruments suites for the Ganymede orbiter. They are $62M, $96M, and $190M for the three options (reserves do not appear to be included in these numbers, so the real costs probably could be higher by ~50%). Additional instruments also require additional operational costs and additional teams of scientists to plan instrument usage and analyze the results. (Instrument costs given do not have reserves included; total mission costs do.)
The Enceladus orbiter missions range from $1.6B to $2.7B. The costs of individual items wasn't detailed in charts, but by examing the graph showing relative cost elements, it appears that this difference again is largely driven by the costs of building and operating different instrument suites.
Costs of individual instruments vary considerably. A magnetometer is only a few million dollars. A mass spectrometer, radio and plasma wave package, or a subsurface radar isseveral tens of millions of dollars.
Even for a specific instrument type, costs can vary considerably. The simpler Ganymede mass spectrometer apparently would cost ~$25M to build while the more sophisticated Enceladus mass spectrometer would cost ~$57M. (Note: For some items, I'm making educated guesses to determine which instrument costs in the reports go with which instruments.)
Neither the Ganymede nor Enceladus mission reports spell out the cost of a narrow angle camera (NAC), which would be useful for exploring their target worlds and really useful for observing the rest of the Jupiter or Saturn systems. However, it appears that instrument #10 in the Ganymede report at ~$13M is the NAC and the Io observer NAC is listed at ~$17M. These are simple cameras compared to those proposed for the Titan and Europa flagship missions that proposed NACs costing ~$54M and ~$43M respectively. A lot of capability is given up to keep the costs of these Decadal Survey concept missions below the Flagship mission costs.
Editorial Thoughts: My favorite instrument to add to an Enceladus mission would be a 2-micron imager that would use a spectral window in the atmosphere for high resolution imaging of Titan. The specific costs of that instrument wasn't spelled out in the reports. However, assuming that its cost might be similar to that of a NAC (similar optical and mechanical design but a different sensor, I think), then the final cost to the mission might be $35-40M with design, fabrication, testing, operation, analysis.
I would hate to see a mission go to Enceladus and not carry this instrument. However, it might be that adding this instrument would bust the budget and jeopardize approval of the mission. In my experience, engineers are very creative at exploring all the options. Then they become hard nosed about what has to be left out to fit within the financial, manpower, expertise, and weight restrictions. If this instrument can be flown within those restrictions, I expect that it will be. In the meantime, I can imagine what such a mission might do.
Reports can be downloaded from: http://sites.nationalacademies.org/SSB/SSB_059331
Ganymede Orbiter Concept Study
Enceladus Flyby & Sample Return Concept Studies*
Enceladus Orbiter Concept Study
*Despite its title, this study examined a range of missions from multiple flybies to several flavors of orbiters to samples return and landers.