The Outer Planets Assessment Group or OPAG met two weeks ago, and the presentations from the meeting were recently posted online. One of the notable items from the meeting was the discussion of a feasibility study that was done to determine whether it's possible to follow up Cassini-Huygens (which cost more than three billion dollars) with a less expensive mission to Enceladus and/or Titan, one that would cost an easier-to-swallow under-a-billion. The answer, unfortunately, is no.
Kim Reh, Ralph Lorenz, and John Spencer explored two dozen different missions. They looked at orbiting Titan or Enceladus, and dropping a lander (or not) or, for Titan, an "aerobot," which is to say, a balloon of some type. They looked at orbiting Saturn and doing Titan and/or Enceladus flybys, like Cassini does now, and sending lander or aerobot to Titan, or flying a sample return sub-craft through Titan's atmosphere or Enceladus' plume. They looked at flyby spacecraft that might or might not include a lander, Enceladus impactor, or Titan probe, aerobot, or sample return (the coolest one of these would essentially have done the Stardust mission through Enceladus' plume). And they looked at lander- or aerobot-only missions. Twenty-four possible missions in total, and in the end, none of them can be done for less than a billion dollars, except for one (a single Enceladus flyby without a lander) for which the science return would be so low as to make the effort not worth the expense.
Below is one table from their study, which I think is interesting because it lists all 24 possible missions that they considered for a future mission to Titan and/or Enceladus. Of these, most (17) were immediately ruled out because expected to be either so expensive or so science-poor that they were not worth following up on in the study. Only seven (shown in bold text below) were carefully costed. The only other mission besides the Enceladus flyby that came in at close to a billion was a Titan atmospheric probe -- which is to say, another Huygens. Reh, Lorenz, and Spencer found that improvements in science instruments since the development of Cassini were not great enough to make a second Huygens worth doing.
|24 Possible Missions to Titan and/or Enceladus|
|Architectural Elements||Mission Selection Rationale|
|Moon orbiters with or without in-situ element||1. Titan orbiter + lander||Multiple complex architectural elements -- too expensive|
|2. Enceladus orbiter + lander||Multiple complex architectural elements; large propulsion delta-V -- too expensive|
|3. Titan orbiter + aerobot||Multiple complex architectural elements -- too expensive|
|4. Titan orbiter||Long dwell at Titan with new instrumentation enables complete and improved mapping of surface and upper atmosphere. Cost: $1.586 billion|
|5. Enceladus orbiter||Delta-v too costly even with Titan aerocapture into Saturn orbit -- too expensive|
|Saturn orbiter with or without in-situ or sample return element||6. Saturn orbiter + Titan lander||Multiple complex architectural elements -- too expensive|
|7. Saturn orbiter + Titan aerobot||Multiple complex architectural elements -- too expensive|
|8. Saturn orbiter + Titan atmosphere sample return||Sample integrity not assured (loss of volatiles, polymerization during sampling process); does not sample diverse locations, multiple complex architectural elements -- too expensive|
|9. Saturn orbiter + Enceladus plume sample return||Multiple complex architectural elements -- too expensive|
|10. Saturn orbiter; Titan and Enceladus cycler||Fewer flybys at Enceladus than at Titan. Insufficient increase in understanding beyond Cassini even with improved instrumentation -- low science return|
|11. Saturn orbiter with multiple Titan flybys||More flybys at Titan than at Enceladus. Insufficient increase in understanding beyond Cassini even with improved instrumentation -- low science return|
|12. Saturn orbiter with multiple Enceladus flybys||New instrumentation could provide moderate science return beyond Cassini. Cost: $1.553 billion|
|Single flyby with or without in-situ or sample return element||13. Flyby spacecraft with Titan lander||Multiple complex architectural elements -- too expensive|
|14. Flyby spacecraft with Enceladus instrumented impactor||Only a few seconds of unique science prior to impact, not compelling -- low science return|
|15. Flyby spacecraft with Enceladus hard lander||Multiple complex architectural elements -- too expensive|
|16. Flyby spacecraft with Titan atmospheric probe||Insufficient science increment beyond Huygens, low science return. Cost: $1.087 billion|
|17. Flyby spacecraft with Titan aerobot||Multiple complex architectural elements -- too expensive|
|18. Titan atmospheric sample return||Limited atmospheric sample and return to Earth -- low science return|
|19. Enceladus plume sample return||Potential value of science return is very high, mission is high-risk: >10 km/s sample capture speeds and long duration >18 years. Cost: $1.378 billion|
|20. Single Titan flyby||Provides no advance over Cassini -- low science return|
|21. Single Enceladus flyby||Science return not compelling even with new instruments compared to Cassini -- low science return. Cost: $0.826 billion|
|In situ only||22. Titan lander||New instrumentation enables surface chemistry; radioisotope power system enables long-term meteorological and seismic monitoring (new science) -- multiple-battery landers not considered due to probable too expensive. Single battery lander provides insufficient science. Cost: $1.397 billion|
|23. Enceladus lander||Potentially valuable science, large propulsion delta-V to reach surface -- too expensive|
|24. Titan aerobot||New instrumentation for chemistry, structure and long term meteorological & seismological monitoring (new science) -- no surface sampling. Cost: $1.384 billion|
So if we're going back to Enceladus or Titan after Cassini is over, it looks like we're going to have to be willing to spend 1.5 to 2 billion dollars. If you'd like to see the whole feasibility study, it's available (all 105 pages of it) in PDF format from the OPAG website.