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.

Download the latest version of kalast here. You can directly run the executable and open an example script from the UI.

You can also run it in your terminal:

./kalast                                        # starts the kalast UI app
./kalast examples/crater_self_shadow/main.py    # load a Python example
./kalast examples/crater_self_shadow/main.rs    # load a Rust example
./kalast res/plane_crater_1024-5000_h=0.437.obj # load a mesh

More info on Running kalast for the first time. There are also additional resources to read, here or in the documentation tab of the UI app:

• res/README.md to get data
• docs/API.md python API
• docs/CONFIG.md config options
• docs/CONTROLS.md UI controls
• examples/README.md examples scripts

You can also write me an email at gregoireh@pm.me if you have any question or want any feature added.

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.

Running kalast for the first time

Kalast carries its own Python. Rust examples .rs 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/ next to the executable.

The bundles are not code-signed, so Windows warns once at the first launch of kalast.exe: More info, then Run anyway.

macOS refuses to run a bundle downloaded with a browser until it is flagged cleared, use:

xattr -cr /path/to/kalast-v*-*

Then double-click kalast.app in that folder to open the UI app; ./kalast runs it from a terminal.

Your own scripts belong in scripts/. The first start puts the examples in examples/, and each update replaces them, keeping any example you changed or added in scripts/examples-before-v<version>.

Kalast is also available as a PyPI package and as a crate.

pip install kalast          # Python -- do this in a venv, you can use astral uv
pip install "kalast[editor]"  # the same, with ty for the UI app's script editor
cargo add kalast            # Rust

Linux requirements

• glibc >= 2.35

If you want to clone and compile it yourself

• 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
• res/: resources folder.
• out/: default output directory for simulation results.

Create a virtual environment for the dependencies and the build. I recommend Astral's uv. Then, from within your venv, run the following to build kalast DLL for python kalast/_rs.abi3.so (its name on macOS):

maturin develop

Build in debug (the default) while implementing features or fixing bugs. But 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.

The UI app can also be started from the kalast python module:

python -m kalast examples/two_spheres/main.py

Credits

The UI app's icons are VS Code's Codicons (CC BY 4.0) and Catppuccin's icons for VS Code (MIT); their licences are in src/app/gui/assets/.