A couple of miscellaneous responses:

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This is in response to Larry J's post at
http://www.habitablezone.com/space/messages/17631.html

Larry J wrote:
I'm saying that due to the need for a lot of electricity, ion propulsion to the outer planets is tough unless you're talking about nuclear power. It's a different proposition altogether if you're talking about the inner planets. That's why I was a little surprised to see that MESSENGER doesn't use it.

Alex writes:
A couple of points should be noted. MESSENGER's baseline design was largely frozen during the two Discovery cycles in which it competed, the final one resulting in selection. Ion propulsion had not been technically validated at the time and was considered "unproven." In fact, Dawn, one of the two most recently selected Discovery missions, uses ion propulsion but was not downselected in previous cycles for the same reason. Even Dawn's PI, C.T. Russell of UCLA, admits that Dawn, which always had a very compelling science rationale, only became convincing to the reviewers from a technical viewpoint *following* DS1's successful technology demonstration. That said, I doubt whether MESSENGER could accomodate ion propulsion within its cost caps in any event. One will note, however, that BepiColombo, ESA's proposed Cornerstone mission to Mercury, *will* utilize solar electric propulsion, if it ends up flying.

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This is in response to Marcia's post at
http://www.habitablezone.com/space/messages/17634.html

Marcia wrote:
Can they use a combination of Ion propulsion AND RTGs for electrical power in the same craft?

Alex writes:
Actually, conventional RTGs have a comparable specific power performance to the current state-of-the-art Stirling Radioisotope Power Sources (SRPS). However, the advanced SRPS being contemplated might, when combined with a putative sub-kilowatt electric propulsion system, enable this method to fly on outer planetary spacecraft without radical mass reduction. One will note that the SPRSs carry about a factor of four less fuel (by mass) than typical RTGs in the first place, so SPRS-powered electric propulsion for, say, Pluto exploration is a definite possibility.

Alex R. Blackwell
University of Hawaii