kalast

Kalast is a thermophysical model (TPM) for binary asteroids; it applies to other airless bodies as well. Kalast is also an image simulator for spacecraft cameras, in the visible and in the infrared. Its renderer serves several other uses — viewing and interacting with meshes, generating lightcurves — see the examples/ folder.

TPM

Several solvers of the heat conduction equation are implemented, all on a variable-spacing depth grid:

• explicit: forward Euler, plus Runge–Kutta–Chebyshev super-time-stepping for problems where the depth system is no longer tridiagonal — lateral conduction, FEM, or a GPU port
• implicit: backward Euler, Crank–Nicolson, BDF2

Validated against analytical solutions (damped thermal wave, slab relaxation) in examples/analytical/, and pinned by tests: error budgets, amplitude decay and phase lag, and the observed order of accuracy — 1 for backward Euler, 2 for Crank–Nicolson and BDF2.

The surface boundary condition includes:

• solar radiation
• self- and mutual heating (thermal re-emission and reflected sunlight)

Depth BC:

• adiabatic
• internal flux for larger bodies

Shadows, eclipses and occultations are computed by shadow mapping on the GPU (a CPU ray-tracing path exists, and is slower).

View factors for self- and mutual heating are computed by the hemicube method on the GPU: occlusion comes free from the depth test, one render yields a whole row, and the result is stored sparse (0.3 % dense on the Didymos pair). Radiosity is first order by default, with more bounces on request. Validated to 0.07 % against the closed form for perpendicular squares.

Once surface temperatures are known, the infrared image is rendered from the observer: emitted plus reflected flux, through a camera's resolution, field of view, filters and spectral response function.

Surface roughness is treated twice, once per wavelength range. In the infrared, the Kuehrt spherical-crater model corrects the emitted flux — beaming, which makes a rough surface read hotter and flatter at low phase — with multiple scattering after Lagerros (1996) and Mueller (2007). In the visible, Hapke's macroscopic roughness (θ̄, 1984) enters the photometry and the lightcurves.

Renderer

Kalast's renderer was first written to watch TPM results — surface temperatures evolving as the bodies spin. Matplotlib and MATLAB had served before, and both slowed the CPU simulation down.

The renderer runs on the GPU through wgpu, a pure-Rust implementation of the WebGPU standard, natively on every platform.

Shadow mapping uses PCF and fits its light frustum to the bounding boxes of the bodies in the scene automatically.

Kalast is also a UI app for editing scripts and interacting with the rendered scene, in the spirit of Blender or Unity.

Shape models

Shape models represent a body's surface as triangular facets. Of the many formats, kalast reads Wavefront .obj only.

Getting started

Grab a release from https://github.com/GregoireHENRY/kalast/releases, unpack it, and run it from inside the folder.

./kalast                                       # starts the kalast UI app
./kalast examples/two_spheres/main.py          # loads a Python example
./kalast examples/crater_self_shadow/step.rs   # a Rust one
./kalast some/shape.obj                        # a mesh

There is nothing to install and nothing is written outside the folder. The archive carries its own Python, with kalast and its dependencies already in it, and the .rs examples come pre-compiled. Editing one, or opening a .rs of your own, means compiling it: if the machine has no cargo the kalast UI app fetches a minimal toolchain into toolchain/ beside the executable and reuses it afterwards. On macOS that also wants Apple's command line tools for the linker (xcode-select --install).

Packages

You can also install kalast as a package, in a Python virtual environment or a Rust project:

pip install kalast          # Python
cargo add kalast            # Rust

Repo structure

• src/: Rust core. Written to be usable standalone by Rust users, independent of Python — the Python wrapper must not compromise its speed.
• kalast/: Python wrapper. Provides Pythonic usage of Kalast (e.g. object references) for users less familiar with Rust. Built with maturin.
• shaders/: wgpu shaders (.wgsl) used by the rendering pipeline.
• examples/: Examples of usage of Kalast. Scripts under examples/old/ are earlier/superseded versions kept for reference, not maintained as user-facing examples.
• res/: resources folder.
• out/: default output directory for simulation results.

If you want to clone and compile it yourself

Create a virtual environment for the dependencies and the build — I recommend Astral's uv for Python. Then, from within your venv, run the following.

Build the kalast Rust extension, kalast/_rs.abi3.so (its name on macOS):

maturin develop

Build in debug (the default) while implementing features or fixing bugs. Use --release for benchmarks, and once a feature works.

maturin develop --release

Beyond the default opt-level = 3 there is nothing worth adding: lto = "fat" + codegen-units = 1 were measured on this project and gave no improvement. Recorded in Cargo.toml so it is not retried blindly.

The UI app can also be started from the Python module, here loading an example:

python -m kalast examples/two_spheres/main.py

Then import kalast from Python (or add the crate from Rust) and write your own scripts.

import kalast