A group of students at National Cheng Kung University in Taiwan are developing a CubeSat mission to study radiation hazards on an Earth-Mars trajectory for future human spaceflight. Possible secondary missions include demonstrating an Earth-Mars free-return trajectory, contributing to the search for Near Earth Objects, and imaging the host mission with which the CubeSat will be hitching a ride. A paper on the project is available here.
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So, where do you hitch a ride to Mars?
Would not the effort be better spent developing a launch means?
Then payloads to wherever will be a simple matter.