Emily LakdawallaMay 17, 2007

A billion dollars won't get you back to Enceladus or Titan

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 ElementsMission Selection Rationale
Moon orbiters with or without in-situ element1. Titan orbiter + landerMultiple complex architectural elements -- too expensive
2. Enceladus orbiter + landerMultiple complex architectural elements; large propulsion delta-V -- too expensive
3. Titan orbiter + aerobotMultiple complex architectural elements -- too expensive
4. Titan orbiterLong dwell at Titan with new instrumentation enables complete and improved mapping of surface and upper atmosphere. Cost: $1.586 billion
5. Enceladus orbiterDelta-v too costly even with Titan aerocapture into Saturn orbit -- too expensive
Saturn orbiter with or without in-situ or sample return element6. Saturn orbiter + Titan landerMultiple complex architectural elements -- too expensive
7. Saturn orbiter + Titan aerobotMultiple complex architectural elements -- too expensive
8. Saturn orbiter + Titan atmosphere sample returnSample 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 returnMultiple complex architectural elements -- too expensive
10. Saturn orbiter; Titan and Enceladus cyclerFewer 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 flybysMore 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 flybysNew instrumentation could provide moderate science return beyond Cassini. Cost: $1.553 billion
Single flyby with or without in-situ or sample return element13. Flyby spacecraft with Titan landerMultiple complex architectural elements -- too expensive
14. Flyby spacecraft with Enceladus instrumented impactorOnly a few seconds of unique science prior to impact, not compelling -- low science return
15. Flyby spacecraft with Enceladus hard landerMultiple complex architectural elements -- too expensive
16. Flyby spacecraft with Titan atmospheric probeInsufficient science increment beyond Huygens, low science return. Cost: $1.087 billion
17. Flyby spacecraft with Titan aerobotMultiple complex architectural elements -- too expensive
18. Titan atmospheric sample returnLimited atmospheric sample and return to Earth -- low science return
19. Enceladus plume sample returnPotential 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 flybyProvides no advance over Cassini -- low science return
21. Single Enceladus flybyScience return not compelling even with new instruments compared to Cassini -- low science return. Cost: $0.826 billion
In situ only22. Titan landerNew 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 landerPotentially valuable science, large propulsion delta-V to reach surface -- too expensive
24. Titan aerobotNew 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.

Artist's concept of a Titan balloon
Artist's concept of a Titan balloon Tibor Balint

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