Micrometeoroid Impacts on to the SRG/eROSITA X-Ray Telescope

The X-Ray Observatory eROSITA on board the Spectrum Roentgen Gamma (SRG) satellite has been operational in a halo orbit around the Sun-Earth Lagrange point L2 since summer 2019. Between December 2019 and January 2022 the eROSITA instruments registered at least seven non-nominal events which are likely related to micrometeoroid impacts, leading to permanent local defects in the X-ray CCD sensor. Each of the micrometeoroid events showed different features and effects in the CCD. In order to test the hypothesis that the defects in the CCDs are indeed related to micrometeoroid impacts, we performed numerical simulations with two interplanetary dust models, taking into account the exact times of the events, orbital position including velocity vector, and the telescope pointing and eROSITA instrument field-of-view. 

In order to identify time intervals when the SRG space telescope traversed cometary meteoroid trails, we used the IMEX dust streams in space model (Soja et al. 2015, Astron Astrophys, 583, A18). The model generates trails for 420 comets available in the JPL Small Body Database (SBDB) as of 1 August 2013, and simulates the dynamics of individual dust particles released from these comets in the size range of 100 micrometers to 1 centimeter, taking into account all relevant forces (solar and planetary gravity, solar radiation pressure, and Poynting-Robertson drag). 

Our simulations reveal at least two candidate cometary trail crossings of SRG in the L2 point with predicted micrometeoroid fluxes sufficiently large that individual particle impacts have to be expected. However, a comparison of the particle impact directions predicted by the model with the telescope pointing during the impact events are in disagreement for all events, implying that none of the registered events is likely connected with dust impacts from a dedicated cometary micrometeoroid trail. However, based on these simulations, it became clear that caution is needed during time periods of cometary trail crossings when high particle fluxes are expected. Here, telescope pointings should avoid looking into the radiant direction of a meteoroid trail.

In a second analysis step, we tested the hypothesis that the events registered by eROSITA may be related to sporadic impacts of particles from the zodiacal dust cloud. To this end, we used the Interplanetary Meteoroid Environment Model 2 (IMEM2; Soja et al. 2019, Astron Astrophys, 628, A109). The model integrates the orbits of cometary and asteroidal particle distributions in the size range 1 micrometer to 1.25 millimeters over 1 Myr, including solar gravity and radiation forces, as well as particle disruptions due to mutual particle collisions. It provides the distribution of dust particles from different cometary and asteroidal sources in the inner solar system. Our results show that many of the registered events occurred when eROSITA was pointing towards directions of increased sporadic interplanetary dust flux as predicted by the model. Several of the events occurred when the telescope was pointing in the direction close to toroidal zones of the sporadic interplanetary dust flux. The modelled dust fluxes predict that approximately two to five micrometeoroid impacts of approximately 1 micrometer sized particles are expected within eROSITA’s operational timeframe of 2.5 years, in rough agreement with the number of events registered by eROSITA. 

Acknowledgements: The IMEM2 model and the IMEX Dust Streams in Space model were developed under ESA funding (contracts 4000114513/15/NL/HK and 4000106316/12/NL/AF ‐ IMEX).