A paper presented at this year's European Planetary Science Congress has shown that a round trip to Mars—six months in one direction, would expose a human being to about 600 mSv of radiation. To put that into perspective, that is about 600 chest x-rays.
Just one trip to Mars and back would have an astronaut experience about 60 percent of their radiation dose limit, for their entire career. That limit, is explained in a report by Futurism as the amount of exposure that a person can face without "substantially increasing" their risk of getting cancer, sustaining central nervous system damage, and like health disorders that can prove to be catastrophic.
These readings were reportedly made by the ExoMars Trace Gas Orbiter (TGO) in 2016. The probe was launched under partnership between the European Space Agency (ESA) and Roscosmos (Russian space agency). The TGO's main mission was to calculate radiation levels on a journey to and from the red planet as well as monitor Martian atmosphere. As of now the orbiter is studying the Martian atmosphere from an altitude of about 400 km from the surface.
TGO data, notes the report, only accounts for the journeys, to and from Earth to Mars and does not account for the time spent orbiting the planet, or the time that will eventually be spent on its surface. Mars' atmosphere is about a one percent the density of Earth's, so all the radiation that is deflected or absorbed by Earth through its active magnetosphere and atmosphere will not happen on Mars, it is likely that a lot of it will reach the surface.
The TGO will continue to orbit Mars till the ExoMars Rover by ESA lands on the red planet on 2021, says the ESA after which, it will assist with ground missions there. Among its many science missions till then will include its on board instruments mapping the Martian subsurface in search of hydrogen to a depth of up to a metre, it has improved spatial resolution when compared with previous measurements.
