From my examination of the mission concept studies from the Decadal Survey, it appears that there were two classes of missions examined. The first were flagship class missions costing over ~$2B. These were the three missions composing a Mars sample return, the Europa Jupiter System Mission, and the Titan Saturn System Mission. (There were also some Flagship-scale options in other reports.) In addition, the Survey commissioned a number of concept studies for missions that might fit within the New Frontiers class of missions (~$650M for principle investigator costs; ~$1.2B for NASA's fully burdened costs). All costs in the studies were for FY15 costs, when the burdened costs of a New Frontiers mission (assuming 3% inflation per year) would be ~$750M for the PI costs and ~$1.4B for the fully burdened costs*. (The Survey reportedly will recommend specific Flagship and New Frontiers-class missions; it will not recommend specific missions for the lower cost Discovery missions.)
Several New Frontiers class missions were studied for the icy moons of Jupiter and Saturn:
- Several incremental flavors of a Ganymede orbiter that would also conduct several flybys of Callisto
- A number of variations of Enceladus missions that included flybys, orbiters, landers, and flyby sample returns along with flybys of other moons of Saturn
- Four variations of probes to float on or descend into one of the polar lakes of Titan
Option numbers are taken from the reports listed below. Missions in the Enceladus Flyby & Sample Return Concept Studies report were ranked by the relative value of the science they would be expected to return. Click on table for a larger image.
In this post, I'll begin looking at the orbiters of Ganymede and Enceladus. Unfortunately, the concepts studies for Enceladus landers and flyby sample returns determined that these missions are premature. For landers, we don't understand the nature of the surface (fluffy snow or rock hard ice? gentle plains or steep slopes?) and for sample returns there are uncertainties associated with the design of the sampling mechanism (a derivative of the Stardust aerogel collector) requiring "a significant technology development" as would issues of ensuring sterilization of the return probe for planetary protection. The studies concluded that for Enceladus, an orbiter represents the most attractive target for the next mission (after Cassini) to this moon.
In several ways, a mission to Ganymede and Enceladus have similar requirements. Both must travel to and operate in the outer solar system. Both would study icy ocean worlds, and hence their list of desired instruments are similar. However, there are also important differences. A Ganymede orbiter is close enough to the sun that solar panels could be used. An Enceladus orbiter is far enough from the sun that the safe bet is on plutonium-powered spacecraft (ASRG's). An Enceladus orbiter would study a tiny moon so close to its dominant planet that a polar orbit would be unstable. Instead, the southern polar geysers and terrain would have to be studied during a series of flybys prior to orbit insertion. The final orbit could not exceed 60 degrees latitude to ensure a stable orbit.
While missions with costs in the $1.3-1.6B range are possible for both moons, the capabilities of the instruments suites would differ considerably. For $1.6B, the Ganymede orbiter would carry the full suite of desired instruments. For the same cost, the Enceladus orbiter would have to forgo desirable instruments such as a narrow angle camera, an imaging spectrometer to study the composition of surface materials, and an ice penetrating radar to directly detect the presence of a subsurface ocean. These instruments could be added to an Enceladus orbiter, but only by increasing costs to almost twice that of a New Frontiers-class mission.
Either mission, however, would substantially expand on the knowledge of their target moons. A Ganymede orbiter has been a priority mission for NASA for several years and is currently in contention as an ESA mission. The discovery of active geysers at Enceladus have made it a priority since it provides our only near term option to directly sample the composition of an icy moon's ocean in the next two decades. In lieu of a Flagship mission to study the moons of either Jupiter or Saturn, these seem to be worthy missions.
In my next post, I'll look at the impact of instrument costs on options for exploring these two moons.
*Note: The budgets for New Frontiers missions are something of a mystery to me. The PI budget is stated in the Announcements of Opportunity, and the fully burdened cost can be derived from NASA's New Frontiers budget line and include some obvious big ticket items like launchers. The Decadal Survey studies seem to be giving cost estimates somewhere in between these two numbers. As near as I can determine, the budget for a New Frontiers mission using the items included for FY15 would be between $1.0B and $1.1B.
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.