The Prometheus project had as goal to use a nuclear fission reactor to power the ion propulsion since it would go so far away from the sun that it couldn't power that with solar cells.
A deep space manned made mission with that kind of propulsion is possible IMO but then you will have to start thinking about a huge vessel, 5% of its mass is a self-sustainable living environment, 5% engines, 90% propulsion mass, bit speculative of course.
Formerly known as iconized
