Observations of neutron radiation environment during Odyssey cruise to Mars

In April 2001 Mars Odyssey spacecraft with High Energy Neutron Detector (HEND) onboard was launched to Mars. HEND/Odyssey was switched on measurement mode most of transit to Mars to monitor variations of spacecraft background and solar activity. Although HEND/Odyssey was originally designed to measure martian neutron albedo and to search for martian subsurface water/water ice, its measurements during cruise phase to Mars are applicable to evaluate spacecraft ambient radiation background. The biological impact of neutron component of this radiation background should be understood to take it into account in planning future manned missions to Mars.  We modeled spacecraft neutron spectral density and compare it with HEND measurements to estimate equivalent neutron dose rates during Odyssey cruise phase, which corresponds to the solar maxim period (23^th solar cycle). It was found that Odyssey ambient neutron environment during May – September 2001 produces 6.3±1.0 mSv per day in energy range 0-15 MeV or about 23 mSv per day if extrapolated to 0-1000 MeV energy range for sun quiet intervals without Solar Particle Events (SPEs). The occurrence of SPEs may additionally increase the total neutron radiation dose accumulated for 6 months of Odyssey cruise phase up to 10%. We have also extrapolated HEND/Odyssey measurements to the different periods of solar cycle and found that during solar minimum (maximum of GCR flux) neutron equivalent rate during cruise to Mars could be as high as 40 mSv per day.  These values are in good agreement with results reported for a similar measurement made with an instrument aboard the Mars Science Laboratory during its cruise to Mars in 2011-2012.