A CubeSat can provide a relatively cheap and easy way to improve the radiations environment knowledge for a Mars manned mission.
Such a CubeSat could be launched and jettisoned as a piggyback of another satellite going to Mars,
This video shows the presentation that Boris Segret gave to the 16th Annual International Mars Society Convention, held at the University of Colorado, Boulder, August 15-18, 2013. In the presentation he describes a CubeSat mission to Mars.
Watch CubeSat – Boris Segret
CubeSat on an Earth-Mars Free-Return Trajectory to study radiation hazards in the future manned mission http://meetingorganizer.copernicus.org/EPSC2013/EPSC2013-1088.pdf
The Mars Society http://www.marssociety.org/conventions/2013