Bruce Moomaw sent out the following e-mail with more information on the proposed Argo (Jupiter/Trojan)-Saturn-Neptune/Triton-Kuiper belt mission.
This is a follow-up by me to Emily's excellent piece on the "Argo" New Frontiers mission concept today ( http://www.planetary.org/blog/article/00001729/ ). It was inspired largely by the fact that -- in her original description of the mission last March ( http://www.lpi.usra.edu/opag/march_08_meeting/presentations/hammel.pdf ) -- Dr. Hammel emphasized the strong possibility that it might also fly by a Jovian Trojan on its way out to Saturn, rather than flying by Jupiter itself. (In fact, she described two possible missions of this sort on pg. 3 of her presentation.) These objects are looking more and more important scientifically, especially given both their rather puzzling origin and the fact that it now looks as though they total almost as much mass as the entire regular Asteroid Belt.
The question -- as with Alan Stern's similar "New Horizons 2", which would have flown by Uranus rather than Neptune on its way to a big KBO -- is whether such a mission would be worth the cost. On the other hand, "Argo" -- even without a Trojan flyby -- does include a very close flyby of Triton, which is a major item that "NH-2" didn't have.
Meanwhile, this summer's class of JPL Planetary Science Summer School devoted their yearly mission design effort this time to a possible Trojan-and-Centaur flyby:
(The Centaur 39P/Oterma [ http://en.wikipedia.org/wiki/39P/Oterma ] turns out to actually have an orbit between those of Jupiter and Saturn, suggesting that a mission to it could be solar-powered -- except for the danger that the solar panels would get machine-gunned by the particles in Oterma's coma.)
[Editorial note from Van: Bruce included Heidi Hammel’s complete reply in his e-mail. Since she sent the message to Bruce, I don’t want to reproduce it without permission. However she makes two interesting points: (1) the first Argo flyby can be either Jupiter or a Trojan, but Triton and a Kuiper belt object remain the priority and will dictate the trajectory flown; (2) even if another nation supplies an atmospheric probe for Neptune (or, by extension, Saturn) it would bust the Argo budget out of the New Frontiers cap.]