astrodyn_gravity

Gravity computation for the astrodyn_bevy workspace — point-mass, spherical harmonics (Gottlieb algorithm), tides, and post-Newtonian relativistic corrections.

Ports models/environment/gravity/ from NASA JEOD v5.4.0. The core spherical- harmonics kernel is a faithful port of spherical_harmonics_calc_nonspherical.cc, a numerically stable normalized Legendre recursion that scales to high degree and order without the underflow / overflow problems of the classical formulation.

Layered architecture

astrodyn_bevy        (Bevy ECS adapter, mission code)
   ↓
astrodyn         (orchestration, recipes, single API surface)
   ↓
astrodyn_gravity     ←  this crate (pure Rust, zero Bevy)
   ↓
astrodyn_dynamics, astrodyn_math, astrodyn_quantities

astrodyn_gravity is part of the astrodyn_* physics layer — pure Rust with no Bevy dependency.

Public surface

• calc_spherical, gravitation, gravitation_with_scratch — point- mass and dispatched gravity computation.
• calc_nonspherical*, GottliebScratch — the Gottlieb spherical- harmonics kernel + reusable scratch buffers.
• GravitySource, GravityModel, SphericalHarmonicsData — per-body μ + coefficient payload.
• GravityControl, GravityControls — per-source selectors (degree / order, gradient, third-body / Battin / relativistic toggles).
• tides, relativistic — small-correction terms.

Coefficient files at JEOD source paths like earth_GGM05C.hh are parsed by astrodyn_gravity::jeod_cc into the binary fixtures committed under test_data/gravity/; production gravity does not parse JEOD source.

See also

• docs/JEOD_invariants.md — GV.* invariants this crate enforces.
• Project README and CLAUDE.md — workspace-level architecture.
• Rendered rustdoc: https://docs.rs/astrodyn_gravity