Going to Mars will be a huge challenge for humanity. There are several risks to leaving the protection of our planet, and one of them is being exposed to cosmic radiation without the magnetic cocoon generated by the Earth. In a new study, which will be presented at the European Planetary Science Congress in Berlin this week, researchers have estimated just how much radiation astronauts are likely to expect.

According to the work, the people undertaking the first interplanetary journey will be exposed to at least 60 percent of the total dose limit currently recommended for an entire career as an astronaut. The measurement is an estimation for what a six-month there and a six-month back trip would entail and doesn’t consider the radiation experienced on the ground.

The measurement is based on the data collected by ExoMars, the European-Russian orbiter that is currently studying the composition of the Martian atmosphere. The spacecraft is equipped with the Liulin-MO dosimeter, which has been recording radiation levels since its launch in March 2016. With that data, researchers have the likely dose that an astronaut would get on a round trip to Mars.

“One of the basic factors in planning and designing a long-duration crewed mission to Mars is consideration of the radiation risk,” Jordanka Semkova of the Bulgarian Academy of Sciences and lead scientist of the Liulin-MO instrument, said in a statement.

“Radiation doses accumulated by astronauts in interplanetary space would be several hundred times larger than the doses accumulated by humans over the same time period on Earth, and several times larger than the doses of astronauts and cosmonauts working on the International Space Station. Our results show that the journey itself would provide very significant exposure for the astronauts to radiation.”

It is also important to consider other factors. The probe crossed interplanetary space during a period of declining activity for the Sun. The Sun can both help and make things worse. Increased activity tends to reduce the number of cosmic rays that penetrate the inner Solar System, but solar flares and eruptions can throw more energetic particles at the astronauts.

The ExoMars orbiter is only half of the ExoMars project. A rover called ExoMars 2020 will arrive on the Red Planet in 2021 and will have a radiation sensor that will also provide data from the surface. A serious proposal for Mars will require foolproof plans to protect astronauts from the danger of radiation.