Orekit-grade mission analysis in your browser.
No install, no server, no account.
Design an orbit around Earth, the Moon or Mars. Get ground tracks, eclipse geometry and orbital period computed from real measured gravity fields — and send someone a link to exactly what you are looking at.
Free. Nothing to install. Nothing is sent anywhere.
Why it exists
Professional mission-analysis tools are excellent and expensive, and they need an install, a licence and an administrator. Free tools are either shallow orbit viewers or, like the good ones, something you must deploy and host yourself.
Deep analysis with zero install was unoccupied. Octant runs Orekit — the astrodynamics library used across European space engineering — compiled to JavaScript and executed in your tab. The physics is not a re-implementation or an approximation of Orekit. It is Orekit.
What it does today
- Earth, Moon and Mars
Each with its own measured spherical-harmonic gravity field to degree and order 20 — EIGEN-6S, GRGM660PRIM and jgm85f01 — not a point mass with a borrowed J2. - Ground tracks
In the body-fixed frame, so the orbit visibly walks west with the rotating body and the track agrees with it. - Eclipse
Conical umbra with the Sun as a disc. Lit and shadowed arcs drawn separately; fraction reported. - Shareable links
The whole scenario lives in the URL. No account, no saved state, no database — a link that still works years from now.
Models
Every number Octant reports comes from one of the models below. The tool names the one it used, on screen, every time — because which model ran is part of the answer.
Propagators
Analytical mean-element theories, chosen automatically by orbit regime. None is general: each is derived under assumptions, and outside them Orekit refuses rather than returning something plausible and wrong.
| Theory | Used when | Includes | Breaks down |
|---|---|---|---|
| Eckstein-Hechler | e ≤ 0.05, non-equatorial | J2–J6 zonal | Refuses past e ≈ 0.1; singular at i ≈ 0 |
| Brouwer-Lyddane | e > 0.05, non-equatorial | J2–J5 zonal | Singular at i ≈ 0 and near the critical inclination |
| Keplerian | i < 1° (equatorial) | Two-body only | No perturbations at all |
None of them model drag, solar radiation pressure or third-body attraction. Against a full numerical force model, the analytical solution drifts roughly 3.3 km over 24 hours in low Earth orbit. That is the price of an answer in milliseconds instead of seconds, and it is the right trade for a slider — but it is a real error, not a rounding one.
For a geostationary orbit the Keplerian fallback loses less than it appears to: GEO longitude drift is driven by the tesseral J22 term, which neither mean-element theory models either.
Gravity fields
| Body | Model | Degree/order | Source |
|---|---|---|---|
| Earth | EIGEN-6S | 20 × 20 | GFZ / GRGS via ICGEM |
| Moon | GRGM660PRIM | 20 × 20 | NASA GSFC via ICGEM (GL0660B) |
| Mars | jgm85f01 | 20 × 20 | ICGEM |
Real measured fields, trimmed from models that go far higher — the lunar one is degree 660 upstream. Reference radius and GM are read from each model's own header, so the globe drawn and the gravity felt cannot disagree.
Frames and time
- Earth: ITRF via IERS 2010 conventions, using real Earth-orientation parameters — precession, nutation, polar motion and UT1.
- Moon and Mars: body-fixed frames from the IAU rotation elements. Libration terms are omitted — roughly 600 m on the lunar surface.
- Time: UTC with the real leap-second table; TAI, UT1 and TT via Orekit.
Sun and eclipse
- Solar position from Orekit's analytical model, which is what removes any need for the 15 MB JPL ephemerides.
- Eclipse geometry is a conical umbra with the Sun treated as a disc. Penumbra counts as shadow — for a power budget that distinction matters and this does not yet make it.
- For the Moon and Mars this is shadow cast by that body. A real lunar mission also cares about Earth eclipses of the Sun, which needs ephemerides Octant does not ship.
Not modelled
Atmospheric drag · solar radiation pressure · third-body attraction · tesseral resonance · manoeuvres · attitude dynamics · orbital lifetime and decay · station keeping · collision screening. Several are planned; none are present.
Launchers. Octant does not model launch-vehicle ascent, and will not. Trajectory optimisation — solving for the steering law that maximises payload — is deliberately out of scope on dual-use grounds, permanently. What is in scope, and planned, is the geometry around a launch rather than the flight itself: launch windows and launch azimuth from a given site to a target orbit, payload capability read from published launcher user manuals, and injection accuracy from a separation state. Those answer the Pre-Phase-A question — can this launcher put my spacecraft where I want it — without going anywhere near guidance design.
What it does not do
Interactive propagation is analytical (Eckstein-Hechler, mean elements, J2–J6). It omits atmospheric drag, solar radiation pressure and third-body attraction. This is a deliberate trade: a full day of analytical propagation costs milliseconds where the numerical force model costs seconds, so the slider stays instant and the tool always states which model produced a number.
Octant is a preliminary analysis tool. It is built for Pre-Phase-A trade studies and for teaching. It is not a flight dynamics system, and no number it produces should be the sole basis for a design decision.
Scope. Octant models passive bodies only — objects that cannot steer. It does not model, and will not model, guided atmospheric entry, targeting, or launch-vehicle trajectory optimisation.
Privacy
Your orbits never leave your machine. There is no API and no account, because there is no server to send anything to — every number is computed in your browser.
Octant counts page views and referrers so it can tell whether anyone is finding it. No cookies, no fingerprinting, no personal data, no third-party trackers, nothing sold or shared. Selecting a NASA imagery layer fetches map tiles from NASA; the orbit maths still runs locally.
Get in touch
Octant is early and actively built. Three things are especially worth an email:
- Using it for work? Commercial use needs a licence — it is not prohibited, it is not yet priced. Get in touch.
- Teaching with it? Course material and classroom use are being built now — tell me what you need.
- Need more than this? Custom studies, tool customisation and training.