Spacecraft Impact Risk Flux Bound for Millimetre Grains at One Astronomical Unit

This paper examines Expected number of millimetre-class impacts on a representative spacecraft area over a representative mission duration, in civilian engineering terms. Using only published input values and transparent formulae, the calculation obtains the following headline results: cumulative_flux_per_m2_s_millimetre_class = 3.8e-11; expected_impacts_per_m2_per_year = 0.001199; expected_impacts_10m2_10yr = 0.1199; probability_at_least_one_impact_percent = 11.3; mean_years_between_impacts_on_10m2 = 83.4. A ten-square-metre spacecraft in interplanetary space expects of order one tenth of a millimetre-class impact per decade, so such impacts are a design case to accommodate rather than a routine occurrence. The method, inputs and limitations are stated in full so that any reader can reproduce or rescale every number from the deposited calculation script. Agreement and disagreement with independent published estimates are reported at their actual size, and the conclusions are limited to what the budget arithmetic supports.