I thought the Probe to/past Saturn had an ion drive, the one launched like 8-10 years ago. (no idea on date random guess) I recall hearing about this before, and the plus to the ion drive is its over all speed is far higher even though it starts out ridiculously slow, but the total out put over 5+ years is what makes it so appealing. Plus instead of firing it off and hoping for the best or it just being lost after the mission you can redirect it later on.