Free Astronomy Magazine September-October 2019

47 SEPTEMBER-OCTOBER 2019 ASTRONAUTICS T he human ex- ploration of Mars (illustration below) is partly braked by the im- possibility of ade- quately defend- ing astronauts from space radia- tion. Recent data from the ExoMars Trace Gas Orbiter (side illustration) showed that dur- ing a six-month journey to the red planet an as- tronaut could be exposed to at least 60% of the total radiation dose limit recom- mended for their entire career. [NASA, ESA/D. Ducros] The intravehicular dose is totally unpre- dictable because the interaction between heavy ions and the spacecraft hull causes the fragmentation of the former into a cas- cade of lighter secondary particles, whose nature and energy depend on those of the original ion and the targeted mate- rial. This process, called “spallation,” actually in- creases the destructive potential of the incoming flow and, depending on the dynamics of the phe- nomenon and the materi- als involved, it may turn out that a thicker shield proves more deleterious than a lighter one, lead- ing to the creation of a larger subset of particles. Not knowing the GCR makeup that interacts with a given spacecraft, most effective shielding materials can only partially reduce the so-called “intravehicu- lar dose,” the flow of particles that invades the cockpit and its inhabitants.