One of the main reasons why the Solar System hasd been isolated is that more money has been spent on pork technology like VASIMR than actual missions; & that the cost benefit analysis of this work after the fact has generally proven to be extremely poor. Look at SEP-related technology: true, it makes medium delta v missions like Dawn more feasible; but it isn't even necessary for a Mercury orbiter (qv Messenger). Apart from some sample return architectures, it has little value for Mars & Venus missions, is almost useless for the Moon (pace SMART-1); & even with medium term extensions of our solar power technology, will struggle to operate at Jovian distances much less out in deep space.

(Didn't stop a bunch of labs fighting over the deep space propulsion initiative; especially when it was decided that a good use for the first year's budget was putting up a bunch of buildings instead of actually doing any research. With the money already allocated by Congress, the cash which has already been wasted on the propulsion initiative would pay a fair slab of the short term New Horizons cost)

VASIMR is the same: a vaguely plausible SEP derivative with few obvious markets in real world space programming; but plenty of press releases to make it look almost useful. Like all SEP systems (only more so), it's too inflexibly bulky for most robotic missions; unnecessary for manned flights (notwithstanding Dr Chang-Diaz's sometimes fruity claims), it's a solution in desperate search of a problem that can justify the huge expenditures required to even demonstrate that the system might be work; & is therefore only being given a starvation funding by space agencies. That's all its worth at this stage.

Bear in mind, however: there are plenty of good deep space missions which could be flown using off-the-shelf technology if labs were more interested in flying missions than building buildings & creating labs.

Neptune & Uranus fast flybys deploying small atmsopheric probes are perfectly feasible using buses based on New Horizons & its rival, POSSE; & would generate massive science returns on a budget.

Existing SEP-powered systems (ie, none of this advanced propulsion vaporware) can easily reach Jovian Trojans & could be used as a disposable part propulsion for a Centaur mission; & (closer to home) could supply the full delta v for a comet orbiter & probably sample return as well (asteroid sample returns are also quite easy with SEP-based systems).

Of course: if someone would sepent a few millions (not billions) to properly spacerate a small sail, then our options would really expand.

Robert