  Context: Environmental soft protons have affected the performance of the
X-ray detectors on board Chandra and XMM-Newton, and they pose a threat for
future high energy astrophysics missions with larger aperture, such as ATHENA.
  Aims: We aim at estimating soft proton fluxes at the ATHENA detectors
independently of any modelisation of the external fluxes in the space
environment.We analysed the background data measured by eROSITA on board SRG,
and with the help of simulations we defined a range of values for the potential
count-rate of quiet-time soft protons focused through the mirror shells. We
used it to derive an estimate of soft proton fluxes at the ATHENA detectors,
assuming ATHENA in the same L2-orbit as SRG.
  Results: The scaling, based on the computed proton transmission yields of the
optics and optical/thermal filters of eROSITA and ATHENA, indicates that the
soft proton induced WFI and X-IFU backgrounds could be expected close to the
requirement.
  Conclusions: No soft proton fluxes detrimental to the observations have been
suffered by eROSITA during its all-sky survey in orbit around L2. Regardless of
where ATHENA will be placed (L1 or L2), our analysis suggests that increasing
somewhat the thickness of the WFI optical blocking filter, e.g. by 30%, would
help to reduce the soft proton flux onto the detector, in case the planned
magnetic diverters perform worse than expected due to soft proton
neutralisation at the mirror level.
