orbit-gen

Periodic orbit family generation and continuation in the Circular Restricted Three-Body Problem (CR3BP).

Overview

orbit-gen generates families of periodic orbits in the CR3BP using single-shooting differential correction and pseudo-arclength continuation. It currently supports various families in the Earth-Moon and Sun-Earth systems.

Installation

pip install orbit-gen

Requires Python 3.12+.

Usage

From a config file

# orbit_config.yaml
system: SE                      # "EM" (Earth-Moon) or "SE" (Sun-Earth)
preset: SE_L2_Northern_Halo

max_iter: 1000                  # Maximum Newton iterations for a single differential correct attempt
eps: 1.0e-6                     # Differential-correction convergence tolerance on residual norm
step: 0.01                      # Pseudo-arclength continuation step size in canonical units
max_solutions: 20               # Maximum number of orbits to generate
max_period_days: 1000.0         # Maximum full orbit period in days

output_dir: results
show_plots: true                # Optional; for user use only

from orbit_gen import Family, RunConfig

cfg = RunConfig.from_yaml("orbit_config.yaml")
fam = Family(cfg)

fam.save()
if cfg.show_plots:
    fam.plot()

Note: constructing Family(cfg) runs the continuation, but does not save or plot results on its own — call .save() / .plot() explicitly, as shown above.

Programmatically

from orbit_gen import Family, RunConfig
from orbit_gen.setup.system_config import SE_SYSTEM
from orbit_gen.setup.presets import PRESETS

cfg = RunConfig(system=SE_SYSTEM, ic=PRESETS["SE_L2_Northern_Halo"])
fam = Family(cfg)
fam.save()

Available preset keys (see orbit_gen.setup.presets.PRESETS): EM_L2_Northern_Halo, EM_DRO_1, EM_DRO_2, EM_L1, EM_Butterfly, EM_Unnamed_1, SE_L2_Northern_Halo, SE_L1_Vertical, SE_L2_Lyapunov, SE_L2_Small_Lyapunov, SE_L2_Large_Lyapunov, SE_L2_Southern_Halo, SE_Butterfly, SE_Dragonfly.

Supported Systems

Output

fam.save() writes a .npz file (named after the preset, in cfg.output_dir) containing initial states, periods, and mu_star for all converged orbits:

import numpy as np
data = np.load("results/SE_L2_Northern_Halo.npz")
data["states"]              # (N, 6) containing initial [x, y, z, vx, vy, vz] in canonical units for each orbit
data["periods"]             # (N,) containing full period in canonical units for each orbit
data["mu_star"]             # System mass ratio

Contributing

Clone or download the repository and run

uv sync

to install the package in editable mode plus dev dependencies. Run the test suite with:

uv run pytest

The repo includes a driver script, scripts/crtbpModel.py, which reads orbit_config.yaml from the project root, runs continuation, saves results, and shows plots.

An example configuration file (orbit_config.example.yaml), is included in the repository, and orbit_config.yaml can be created and used by running:

cp orbit_config.example.yaml orbit_config.yaml   # edit as needed
uv run python scripts/crtbpModel.py

License

MIT — see LICENSE.