This blog entry continues the series to pick the 5 missions that I personally find most compelling for the next decade. I'm under no illusion that I will persuade anyone (especially anyone who influences government spending). However, I find a well argued (and I hope these will be) argument to help me form my own opinions. Please provide your opinions, too, in the comments.
My first choice went to a series of missions to lead to sample return from Mars (Mars Trace Gas Orbiter, the 2018 NASA and ESA rovers, and technology development for eventual sample return) and the second choice to a Venus mission to study the other major terrestrial planet. My third and fourth missions would go to explore the ice-ocean moons of Jupiter and Saturn.
The last several posts have looked at options for exploring those ice-ocean moons. The options range from the Flagship Jupiter Europa orbiter, to a Ganymede orbiter from NASA or ESA, to an Enceladus orbiter (with Titan and other moon flybys), to a Titan lake lander, airplane, or balloon. Writing about these options has been an education for me (and I hope interesting to my readers). In this final entry in this series on ice-ocean moons, I'll look at one way to prioritize these missions.
My ranking of missions is influenced by a senior scientist who has reminded me in emails that you get what you pay for. A mission done too cheaply is one that eventually you'll refly to get the information that was really needed to answer the key questions. With this in mind, here is how I would prioritize the missions:
1) Jupiter Europa Orbiter - A Flagship mission to study Europa and the Jupiter system would return more information, I believe, for the dollar than any other mission choice. However, this choice comes with risks and collateral costs: Assuming that Mars will be the Decadal Survey's highest priority, will there be sufficient funds to fly this mission? Will there be sufficient plutonium to power the spacecraft and would any plutonium remain for other missions? Is the technology mature enough that the risk of major cost overruns is low? I don't have answers to these questions and lack the information to make informed guesses. That's why they pay the Decadal Survey the [metaphorical] big bucks.
2) Either an Enceladus orbiter or a Titan lake lander/submersible: Either mission seeks to explore a constrained set of questions: the cryovolcanic activity and interior of a small moon or the chemistry and properties of an exotic sea.
3) If the Jupiter Europa Orbiter mission doesn't fly, then my third choice would be a Ganymede orbiter that explores that ice-ocean moon in depth and performs several flybys of Callisto. I also hope -- although the mission concept study did not address this possibility -- that this mission would also make several flybys of Europa to further our knowledge of its interior and surface.
4) Either an airplane or balloon to explore the atmosphere and remotely observe the surface of Titan in detail. Placing these options last was a hard call. From the personal arm chair explorer perspective, I would pick either the Titan airplane or balloon as my first choice. I want to see the surface up close and personal as if I was flying above it. However, both missions would suffer from severe limitations on the information they could return to Earth without an orbital relay spacecraft. If either flies, I suspect that we'll eventually refly the mission when a capable orbiter is also sent to Saturn or Titan to enable high bandwidth data return and in-depth coverage of many locations rather than a few. (However, the Galileo mission with its crippled antenna showed how much science can be done with limited bandwidth -- these would be good missions without the data relay.)
These choices assume that all missions have an equal chance of being selected. In reality, each faces significant challenges on the path to selection. Each must fit within an available budget and beat out tough competition to be selected. If any of these missions surmounts these obstacles, then any would be an excellent choice.
In a perfect world, several of these missions could fly, funded from different NASA and ESA budgets. NASA might fund a Flagship Jupiter Europa Orbiter and a small Enceladus flyby or orbiter mission that also acts as a data relay for an ESA Titan balloon mission while a Titan lake lander is funded from NASA's Discovery program. An ESA Ganymede orbiter would also explore that world. In our less than perfect real world, I would be happy to see at least one and, if all gods smile, two of these missions fly. Which one or two will be the result of hard looks at budgets and winning out over tough competition from other excellent missions.