IIE is a 200 AU mission that is still really difficult. Hence, many of us believe that (1) the next step past Voyager needs to be taken and that scientific case can be made, (2) speed is important, and (3) one has to be realistic about what can – and cannot – be accomplished with that next step. A large launch vehicle, upper stage, Jupiter gravity assist, and REP continues – at least to me – to look like the current best bet, but I am always open to practical suggestions. To get to the “interesting” region of the sky as seen by Cassini MIMI and IBEX instruments in the last couple of years, the next window for a Jupiter gravity assist opens in ~2024 – and that could be done.
Problems closing the engineering loop for high energy density power systems
It is perhaps also worth noting that nuclear electric propulsion has been looked at – and in some detail under NASA’s Project Prometheus. The problem is that the power system needs to have a specific mass no greater than ~30 kg/kW (something noted by Ernst Stuhlinger back in the 1960′s — Stuhlinger literally wrote the book on ion propulsion) to have an advantage in speed delivered by nuclear electric propulsion (NEP). But that has to include the mass of the system for dumping the waste heat of the reactor (from the second law of thermodynamics) as well as its mechanical supports. The Prometheus architecture came in at over twice that, and that is the problem. To date all NEP designs come in underpowered when engineering closure on the system as a whole is examined. Think of Hiram Maxim’s steam-powered airplane versus the gasoline-powered airplane of the Wright Brothers. This is ultimately the problem with VASIMIR as well – a more mass-efficient means of providing the wall-plug electricity is needed, if it is to ever become a real system.