ATHENS

ATHENS is an open-source C++17 ray-tracing code for modelling the aerodynamic behaviour of Resident Space Objects in low Earth orbit under free-molecular, hyperthermal flow conditions. It is intended for orbital-aerodynamics applications where intermolecular gas collisions are negligible on the scale of the spacecraft, making a ray-tracing treatment more efficient than full DSMC or TPMC simulation.

The code represents the atmosphere through gas rays travelling through a cuboidal computational domain. These rays are generated from the domain boundaries according to the prescribed free-stream velocity, gas temperature, and molecular species. Each ray carries a molecular mass flux and has a velocity composed of a bulk flow component and a thermal component sampled from a Maxwellian distribution.

Spacecraft geometries are imported from STL files and represented as triangular surface meshes. ATHENS detects ray-surface intersections with the geometry, computes the transferred momentum, and generates reflected rays according to the selected gas-surface interaction model. The code supports repeated reflections, making it suitable for both convex and concave geometries.

The currently implemented gas-surface interaction models are:

DRIA: Diffuse Re-emission with Incomplete AccommodationCLL: Cercignani-Lampis-Lord scattering kernel

ATHENS computes global aerodynamic force and moment coefficients, including drag, lift, and side-force components. It can also compute surface-resolved pressure and shear coefficient distributions over the triangular mesh.