From Earth to Mars: Radiation in Interplanetary Space

Coupled finite-difference solutions to an adjoint form of the Parker heliospheric transport equation have been solved using a random-walk method. The heliosphere has been represented as a 140 AU sphere with the sun at 100 AU from the boundary in the bow-shock direction and 40 AU from the center of the heliosphere. The dependence of the solar wind on latitude is included, varying from 400 to 800 km/s. The tensor diffusion coefficient was collapsed to a simple scalar. Solar activity levels were represented by a recent solar minimum, a recent solar maximum, and a worst-case Gleissberg minimum of zero solar modulation. Doses from two solar-particle events were calculated, one based on the 29-30 September 1989 event Ground Level Event no. 42 (GLE 42), and the other on the Carrington event which was assumed to have the same time and energy structure as GLE-42. The cosmic-ray dose rate in the Earth-Mars interplanetary space is 55 centisievert/year (cSv/year) through 20 g/cm² of polyethylene. The dose delivered from a “worst case” solar particle event could disable the crew without a storm shelter. Calculations of the radiation intensity in interplanetary space and on the surface of Mars using cosmic-ray spectra and solar-particle spectra transported to those locations are presented.