In the current tally of your votes for missions you would like to see the Decadal Survey prioritize (if you haven't voted, please do so!), the Io Observer currently has the second highest number of votes for New Frontiers missions. (The current leader is a Titan lake probe.) Today, I'll summarize the mission concept prepared for the Decadal Survey for the Io mission.
The concept has many simularities to the Io Volcano Observer Discovery proposal that I wrote about almost two years ago. This is not surprising since both take their starting point from the same JPL concept study conducted in 2008. The mission would place an spacecraft into a highly inclined (>45 degree) orbit around Jupiter that would encounter Io at each perijove. The inclined orbit keeps the spacecraft out of the highest radiation fields except during the actual flyby. This strategy allows the mission to avoid the high-cost radiation mitigation strategies of the proposed Jupiter Europa mission and instead use the more modest design approaches employed by the upcoming Juno Jupiter orbiter. (The Io Observer would have less than half the radiation exposure of Juno by the end of their nominal missions.)
At each Io flyby, the spacecraft would map the illuminated hemisphere in eight colors at resolutions of less than 1 km/pixel. Selected areas would be mapped at 10-100 m/pixel in four colors and small areas at less than 10 m in panchromatic. A thermal mapper would provide high resolution thermal imaging of the moon during the flybys and whole disk heat flow maps at ~200 km/pixel when Io slips into Jupiter's shadow and the spacecraft is between flybys. Observations of Jupiter's atmosphere could also be made between flybys.
The six to ten encounters in the nominal mission would employ a variety of encounter strategies at Io. There would be at least three passes each on the day and night sides of Io. Two flybys would study magnetic induction by conducting encounters at high but opposite magnetic latitudes. Two or more flybys would encounter Io at the same latitude to allow searches for changes on the surface. At least one flyby would occur below 100 km above the surface, and one encounter might take the spacecraft through a volcanic plume.
The instrument list proposed would be modest: a narrow angle camera with filters for multispectral imaging, a thermal mapper operating between 2-20 μm, an ion and neutral mass spectrometer to measure the composition of material ejected from Io, and a magnetometer. One option studied would add a fast-imaging plasma spectrometer to map the distribution of ions in the space around Io. Charts tying science goals to instruments note that an ultraviolet spectrometer and a near-IR spectrometer also would make valuable contributions (with the latter also noted as useful for studies of the Jovian atmosphere).
The Io Observer Decadal Survey concept and the Io Volcanic Observer Discovery concept have two key differences. The analysis for the Survey concept concluded that solar power was more practical than plutonium ASRGs. The Discovery concept was based on ASRGs. At the time the Discovery concept was publicly discussed, it would have been powered by ASRGs. That concept's study, however, was funded to determine the types of missions that would be enabled by ASRG units. I understand that the PI submitted a proposal to the current Discovery mission selection; I don't know what power source was proposed. An advantage for proposing an ASRG system for a Discovery proposal would be that NASA would pick up the costs for the ASRGs but not for solar panels. In addition, ASRG's would allow all instruments to be mounted to the spacecraft body, while solar panels would require the remote sensing instruments to be mounted on a more expensive scan platform.
The second key difference between the two concepts is in the cost estimates. The Survey concept would fit within a New Frontiers budget while the Discovery concept would cost several hundred million dollars less. Yet the two concepts appear quite similar. It could be that the team estimating the Survey concept was too conservative -- under the ground rules established by the Survey, conservative estimates were encouraged and each concept had to carry hefty cost reserves. The PI for the Discovery concept may be too optimistic, although he is very experienced and has well established credibility. A similar difference exists between the Survey's Titan lake probe cost estimates in the small Flagship range and a similar Discovery proposal, with the Discovery proposal again led by a very experienced, credible team. Several of the Survey concept reports speculated that it might be possible to significantly reduce costs below their estimates with a focused design-to-cost approach.
Appendix: Science goals from the report
"Understand the eruption mechanisms for Io’s lavas and plumes and their implications for volcanic processes on Earth, especially early in Earth’s history when its heat flow was similar to Io’s, and elsewhere in the solar system.
"Determine Io’s interior structure, e.g., whether it has a magma ocean, and implications for the coupled orbital-thermal evolution of Io and Europa.
"Determine the magnitude, spatial distribution, temporal variability, and dissipation mechanisms of Io’s tidal heating.
"Investigate the processes that form Io’s mountains and the implications for tectonics under high heat-flow conditions that may have existed early in the history of other planets.
"Understand the composition, structure, and thermal structure of Io’s atmosphere and ionosphere, the dominant mechanisms of mass loss, and the connection to Io’s volcanism.
"Determine whether Io has a magnetic field.
"Understand Io’s surface chemistry, including volatiles and silicates, and derive magma compositions (and ranges thereof), crustal and mantle compositions and implications for the extent of differentiation, and contributions to the atmosphere, magnetosphere, and torus."
"Improve our understanding of Jupiter system science, including meteorology, aerosol structure, tropospheric composition, and auroral phenomena on Jupiter, composition and temporal variability of Europa’s exosphere, Jovian magnetospheric processes, and small inner moons and rings of Jupiter."
The Decadal Survey mission concept studies can be found here.