The costs of integrating ion propulsion into the already mature MESSENGER design*may* have been non-trivial. Also, one must consider the MESSENGER teaming arrangement (which includes JHU/APL as the prime spacecraft builder). For various reasons (e.g., mass restrictions, science payload, in-house expertise, lack of confidence in ion propulsion, etc.), they may have preferred conventional propulsion instead. I do know many of the principals on the MESSENGER team, though I admit I do not have any firsthand knowledge that any of the above is true. That said, I would argue that it is a plausible explanation, especially given that the vast majority of Discovery proposals (all of which, by defintion, are "inner planetary" missions) do not utilize ion propulsion. I suspect, though, with the success of DS1's demonstration that many future missions will seriously consider the option.

Alex R. Blackwell
University of Hawaii