Initial commit. Scaffolding, license, readme

.gitignore

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+/target
+/Cargo.lock
+
+# Kernels and reference data are fetched by script, not committed: they are
+# large, and `scripts/fetch-kernels.sh` pins their URLs and checksums.
+/data/kernels

Cargo.toml

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+[package]
+name = "astrokit"
+version = "0.1.0"
+edition = "2021"
+authors = ["Connor Johnstone"]
+description = "A highly performant astrodynamics engine"
+readme = "readme.md"
+repository = "https://git.rcjohnstone.com/connor/astrokit"
+license = "MIT"
+keywords = ["astrodynamics", "orbit", "propagation", "spaceflight"]
+categories = ["science", "aerospace"]
+
+[features]
+default = ["autodiff"]
+autodiff = ["ordinary-diffeq/autodiff", "dep:num-dual"]
+
+[dependencies]
+# We handle the kernel fetching, so we don't need anise's default features
+anise = { version = "0.10.6", default-features = false }
+hifitime = "4.3"
+nalgebra = { version = "0.35", features = ["serde-serialize"] }
+num-dual = { version = "0.15", optional = true }
+num-traits = "0.2"
+ordinary-diffeq = { version = "0.4", default-features = false }
+serde = { version = "1.0", features = ["derive"] }
+thiserror = "2.0"
+
+[dev-dependencies]
+approx = "0.5"
+criterion = "0.7"
+num-dual = "0.15"

LICENSE

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+MIT License
+
+Copyright (c) 2026 Connor Johnstone
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

readme.md

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+# astrokit
+
+A highly-performant astrodynamics engine, written in Rust. Force models, propagation, reference
+frames, spacecraft geometry and measurement models — built on
+[`ordinary-diffeq`](https://crates.io/crates/ordinary-diffeq) for integration,
+and generic over the scalar type so that any propagation you can solve you can
+also differentiate.
+
+## Status
+
+The crate is still being built to a milestone plan:
+
+- **M1** — the study-critical subset: two-body and spherical-harmonic gravity,
+  drag, third-body, cannonball SRP, GCRF/ITRF frames, ground stations and the
+  four measurement types, with a validation suite against Orekit reference
+  trajectories.
+- **M2** — general-purpose completion: higher-degree gravity fields, more
+  density models, box-wing SRP and eclipses, manoeuvres, covariance propagation,
+  relative motion, TLE and CCSDS I/O.
+
+The tables under [Validation](#validation) and [Performance](#performance) are
+the acceptance criteria.
+
+## Conventions
+
+- Everything is SI
+- Time is typed, using `hifitime`
+
+## Foundations
+
+These are depended on, not reimplemented.
+
+| Crate | Licence | Provides |
+|---|---|---|
+| [`hifitime`](https://crates.io/crates/hifitime) 4.3 | MPL-2.0 | TAI/TT/ET/TDB/UTC/GPS time scales, leap seconds |
+| [`anise`](https://crates.io/crates/anise) 0.10 | MPL-2.0 | Reference frames, planetary ephemerides, SPICE-format kernels |
+| [`ordinary-diffeq`](https://crates.io/crates/ordinary-diffeq) 0.4 | MIT | Adaptive integration, dense output, callbacks, autodiff |
+
+## Validation
+
+Before we can make a highly-performant propagator, we have to make a *correct* propagator.
+
+1. **Orekit reference fixtures.** A committed, rerunnable generation script
+   produces reference trajectories and observations; the data lands under
+   `tests/fixtures/` with the Orekit version, force-model configuration and
+   generation date recorded beside it; Rust tests assert agreement within
+   declared tolerances.
+2. **Analytic invariants.**: ie, "does a two-body orbit close back in on itself after one-period?" and similar conservation-tests, meant to run fast as unit tests to ensure that nothing has broken.
+3. **ANISE cross-checks.** Frame transformations and ephemeris lookups against
+   ANISE's own validated values, confirming that we are calling it correctly.
+
+Every fixture comparison matches gravity degree and order, density model,
+third-body ephemeris source and integrator tolerances; anything that cannot be
+matched is documented and its contribution bounded.
+
+| Case | Target | Achieved | Test |
+|---|---|---|---|
+| 1-day LEO arc, full matched force model | 1 m position, 1 mm/s velocity | not yet measured | — |
+| 6-day LEO arc | to be set from the 1-day result | not yet measured | — |
+| GEO arc, including a time-varying-area change | to be set | not yet measured | — |
+| Two-body energy and angular momentum | integrator tolerance | not yet measured | — |
+| Two-body closure over N periods | integrator tolerance | not yet measured | — |
+| J2 secular nodal regression and apsidal rotation | to be set | not yet measured | — |
+| RA/DEC, range, range-rate, az/el against Orekit | to be set | not yet measured | — |
+
+## Performance
+
+Measured against `nyx-space`, `astrolabe` and Orekit on the same benchmarks with
+matched force models, on the same hardware. None of the three publishes usable
+figures, so the references are measured here rather than cited.
+
+| Benchmark | astrokit | nyx-space | astrolabe | Orekit |
+|---|---|---|---|---|
+| Two-body throughput (state-steps/s) | not yet measured | — | — | — |
+| Full-force LEO, 1 day | not yet measured | — | — | — |
+| Ensemble of N states, scaling vs. cores | not yet measured | — | — | — |
+
+## Licence
+
+MIT. See [LICENSE](./LICENSE).

src/lib.rs

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+//! An astrodynamics engine built on [`ordinary_diffeq`].
+//!
+//! # Conventions
+//!
+//! - Everything is SI: metres, seconds, kilograms, radians.
+//! - Times are always typed using hifitime
+
+// Needed to build docs
+#![cfg_attr(feature = "autodiff", doc = include_str!("../readme.md"))]
+#![warn(missing_docs)]
+
+/// The types you want in scope to set up and run a propagation.
+pub mod prelude {}