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.

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.