Solar electric propulsion (SEP) provides a unique capability for interplanetary missions, although spacecraft designs based on solar power, developed by large flexible arrays, lead to formidable vehicle interactions with attitude control. Control techniques must be developed for both thrusting and non-thrusting modes of operation. The challenges to be faced are the development of new control concepts and the design analysis for vehicle/control interaction. Control design tradeoffs are discussed to show the rationale for present designs and to indicate future concerns. It is concluded that the state-of-the-art of control system design is mature enough to support an SEP spacecraft for such heliocentric missions as rendezvous or slow flyby of the comet Encke or out-of-the ecliptic missions in the 1980s.