For my first choice of compelling missions from the Planetary Science Decadal Survey Mission List, I picked the Lunar Polar Volatiles Explorer mission to send a rover to the Moon to assess the volatiles there. This mission has great science and "astronaut scouting" potential. However, in my first post in the series, I said that
Planetary Science should not have to be warped beyond recognition into a substitute Robotic Precursor program just because Congress isn't wise enough to adequately fund Robotic Precursors.
While robotic "astronaut scouting" has a great deal of value, we should let Planetary Science be Planetary Science, and the Jupiter and Saturn systems (if I may group them together) are certainly top-tier Planetary Science subjects. As a result, it's just a matter of choosing one of the Jupiter and Saturn missions from the Decadal Survey list for the second-place spot.
The front-runner for a Jupiter or Saturn mission is probably the Jupiter Europa Orbiter (JEO). There are a number of reasons to pick this one as my second most compelling mission:
The Vision for Space Exploration that this blog takes its name from specifically identifies robotic missions to Jupiter's moons (in that case the Jupiter Icy Moons Explorer (JIMO):
Conduct robotic exploration across the solar system for scientific purposes and to support human exploration. In particular, explore Jupiter’s moons, asteroids and other bodies to search for evidence of life, to understand the history of the solar system, and to search for resources;
However, the VSE also mentions missions to Saturn to follow Cassini, such as a Titan balloon. I'd suggest that the VSE would have also considered Enceladus had it been written long enough after Cassini's work at Saturn started.
A Europa orbiter has been studied in detail for many years. Such a mission has been a high priority for the Planetary Science community for years, too. JEO also has the potential for mutual observations with ESA's Jupiter Ganymede Orbiter if that mission is selected. JEO would not only study Europa from orbit with instruments like a laser altimeter, ice-penetrating radar, and many others to find out about Europa's likely subsurface ocean, deep interior, ice shell, and surface, but it would also conduct numerous flybys of Ganymede, Callisto, and Io, and would be able to study the entire Jupiter system during its long tour towards Europa orbit. It would also be able to conduct its mission years before an Enceladus orbiter. In addition, researchers have had many years to consider the Galileo results, whereas Cassini is still in operation and could still change our perspective on just how an Enceladus mission should be conducted.
Now that I've made the case for JEO, I'm going to select an Enceladus orbiter as my second most compelling mission instead. Enceladus wins me over for 2 reasons: the JEO price tag is a bit scary (even the Enceladus orbiter is not cheap), and Enceladus has those plumes! The Titan Saturn System Mission would also study the plumes of Enceladus, to say nothing of also conducting a staggeringly ambitious investigation of Titan and the entire Saturn system with a spacecraft bristling with instruments and carrying a Titan lake lander and a balloon, but again the estimated cost is too high ($3.248B for the floor mission, not counting partner costs) for my faster-better-cheaper instincts.
The Decadal Survey looked into a number of Enceladus missions, including flybys, landers, and orbiters (see "Enceladus Flyby and Sample Return Concept Studies" on the Planetary Science Decadal Survey Mission & Technology Studies page). The conclusion was that Enceladus landers would be better done after an orbiter mission, and orbiter missions would return more science than sample return missions of similar cost. The "Enceladus Orbiter Concept Study" available on the same page gives more details about orbiter mission options.
Like JEO, an Enceladus Orbiter would conduct a tour of its destination planetary system before intense study of its destination moon. The tour presented includes 40 flybys of Titan, Rhea, Dione, and Tethys, and 20 flybys of Enceladus itself, before the Enceladus orbit phase. The main goals of the orbiter would be to study the source of the plumes, the composition, rate, and dynamics of the plumes themselves, the geology of Enceladus, the internal makeup of Enceladus including the subsurface ocean, scouting for future landers, and studies of other moons it would fly by. Instruments would include a camera designed for the tiger stripe region of Enceladus, a thermal imaging radiometer, a mass spectrometer to measure the makeup of the plumes while the orbiter goes through them, a dust analyzer, and a magnetometer to study the moon's magnetic field for clues about the subsurface ocean as Galileo did for Jupiter's icy moons.
Little technology development would be needed for the mission. The baseline mission cost in FY15 dollars is projected to be $1.613B, including a significant amount for post-launch work like conducting the journey to Saturn and the science phase over many years.