The opposition mission also reduces time by cutting the time of the stay on the surface of Mars to 0.1 years. The orbit that this mission takes also loops very close to the sun, raising problems of heat control and especially radiation dosage. To help reduce flight time, more energy could be added to the ERV for the return trip. A table follows that compares the two missions and the time they take. Flight Times and Stay Times of Mars Missions Min.
Radiation is also an important factor in the design of a mission.
Cosmic rays are a constant hazard while solar flares can be fatal in large doses. If a nuclear engine is used, that is also another source of concern. For more info, see Space Medicine.