Results from radiation environment measurements aboard ExoMars Trace Gas Orbiter in Mars science orbit in May 2018–December 2019

Abstract ExoMars is a joint ESA - Rosscosmos program for investigating Mars. Two missions are foreseen within this program: one consisting of the Trace Gas Orbiter (TGO), that carries scientific instruments for the detection of trace gases in the Martian atmosphere and for the location of their source regions, launched on March 14, 2016; and the other, featuring a rover and a surface platform, with a launch date of 2022. In March 2018 TGO was inserted into circular Mars science orbit with a 400 km altitude. The dosimetric telescope Liulin-MO for measuring the radiation environment onboard the ExoMars TGO is a module of the Fine Resolution Epithermal Neutron Detector (FREND). Here we present recent results from measurements of the charged particle fluxes, dose rates and estimation of dose equivalent rates at ExoMars TGO science orbit, provided by Liulin-MO dosimeter. The obtained data from May 2018 to December 2019 show that: 1) Increase of the dose rate, dose equivalent rate and flux is observed during this period, which corresponds to the increase of galactic cosmic rays (GCR) intensity during the declining of the solar activity. Measurements in two perpendicular directions along axes X and Z of TGO show that the average fluxes for the period are 3.08 cm−2 s−1 and 3.18 cm−2 s−1, the average dose rates are 352 ± 35 μGy day−1 and 366 ± 36 μGy day−1, the average dose equivalent rates are 1.6 ± 0.33 mSv day−1 and 1.65 ± 0.34 mSv day−1; 2) There is slight dependence of the flux distribution on the Martian latitude and longitude; 3) Data from two independent instruments is also evaluated and compared to: good agreement between the GCR count rates time profiles from Liulin-MO, neutron detectors of FREND and High Energy Neutron Detector (HEND) onboard Mars Odyssey orbiter is observed. The comparison between simulations of dose rate carried out with OLTARIS tool using the GCR model of Matthia et al., 2013 and Liulin-MO measurements during the transit to Mars and on the high elliptic orbit show that the measured dose rate behind the shielding of the detectors of Liulin-MO is about 25% higher than the simulated values. The data obtained confirm the worsening of the radiation conditions in the interplanetary space. Our evaluations show that with respect to the values measured during TGO transit to Mars (April–September 2016) in December 2019 the particle flux in the free space has increased at least by 16% and the dose rate – by 23%. The GCR dose rate in free space during the minimum of 24th solar cycle, based on Liulin-MO data is significantly higher than the dose rate measured by CRaTER instrument onboard Lunar Reconnaissance Orbiter during the minimum of 23rd cycle.