Satellite Insertion
Two buttons: prograde and retrograde. Ten missions of real two-body orbital mechanics around a 3D Earth, from a simple circularisation up to geostationary.
About This Game
Orbital mechanics has a reputation for being difficult, and it mostly isn't. It is unintuitive, which is a different problem and a much more entertaining one. Satellite Insertion strips it down to the smallest thing that still teaches you something: one spacecraft, one planet, and two buttons.
Nothing here is approximated for gameplay. Gravity is a = −μr/|r|³ with Earth's actual gravitational parameter of 398,600 km³/s², the orbit drawn ahead of you is the exact conic section your current velocity puts you on, and the fuel a manoeuvre costs is the fuel it would really cost. Get to geostationary efficiently here and you have spent very close to the 3,900 m/s the real thing takes.
The integrator is symplectic leapfrog rather than a naive Euler step, which matters more than it sounds: it means your orbit does not slowly spiral into the planet through accumulated arithmetic error while you wait for apoapsis.
The One Thing To Understand
A burn changes the orbit on the opposite side. That is the entire game.
Speed up, and you do not immediately move outward — you keep going the way you were going, slightly too fast for the curve you were on, and you sail past the point where you would have come back down. The result is that the far side of your orbit rises. The side you are on barely moves at all.
So to raise your orbit you burn prograde at the bottom. To lower it you burn retrograde at the top. To make an ellipse into a circle you wait until you are at the highest point and speed up, which feels backwards right up until it works.
| You want to | Do this |
|---|---|
| Raise the far side | Burn prograde at periapsis |
| Lower the far side | Burn retrograde at periapsis |
| Circularise a high orbit | Burn prograde at apoapsis |
| Come home | Burn retrograde at apoapsis until periapsis is in the atmosphere |
Reading The Navball
The ball in the corner is the oldest instrument in spaceflight and the most useful thing on the screen once it clicks. Its brown half is the ground beneath you and its blue half is the sky above; the line between them is your local horizon, which tilts as you travel round the planet.
The marker sitting on it is where your engine is pointing. Above the horizon line means you are climbing away from the planet, below means you are falling back toward it, and exactly on the line means you are at apoapsis or periapsis — which is precisely where your burns are worth the most. When the marker crosses the horizon, that is the moment to be ready.
Hohmann Transfers
Moving between two circular orbits cheaply takes two burns and a lot of waiting. Burn prograde until the far side of your orbit just touches the height you want, coast the whole way round, then burn prograde again at the top to lift the near side up to meet it. That is a Hohmann transfer, and it is the most fuel-efficient two-burn route there is.
The waiting is not padding. Half an orbit at transfer altitude is genuinely hours, which is what the time warp is for — and why real mission planners talk about launch windows rather than just pointing at things.
How To Play
- Prograde speeds you up along your direction of travel. Retrograde slows you down. There is nothing else.
- Watch Ap and Pe in the telemetry. They are the altitudes of your highest and lowest points, and the countdown under each says how long until you get there.
- Use time warp to skip the coast. It caps automatically when you are close in, where the orbit changes fastest.
- Burns are locked above 10× warp, so that every metre per second is integrated properly rather than handed to you for free.
- Drag to swing the camera around, and View to cycle between the orbit overview, a chase camera behind the craft, and a low pass over the planet.
- On a computer: W or ↑ for prograde, S or ↓ for retrograde, , and . for warp, C for camera, R to reset.
Getting Good At It
- Burn in short taps and re-read the numbers. Holding the button through a whole manoeuvre is how people overshoot by four hundred kilometres.
- Aim slightly low on the first burn. Raising an orbit further is cheap; bringing an overshoot back down costs you the fuel twice.
- Burn at the extremes. A burn at apoapsis or periapsis changes only the opposite side. Burning halfway between changes both, usually in ways you did not want.
- The Oberth effect is real here. Fuel spent low and fast buys more orbital energy than the same fuel spent high and slow, which is why deorbiting is so cheap and escaping is not.
- Watch Pe on the way down. If it drops below about 120 km the atmosphere starts taking energy out of your orbit whether you wanted it to or not.
Why This Exists
Almost everyone's mental model of orbit is "far enough up that gravity stops," which is wrong in a way that is hard to fix by being told. Orbit is not about height. It is about going sideways so fast that you keep missing.
Ten minutes of watching a burn on this side lift the orbit on that side does more for that intuition than any amount of explanation, because you are not being told the rule — you are being handed the consequences of it and left to work backwards.
Frequently Asked Questions
What is Satellite Insertion?
Satellite Insertion is a free 3D browser game about getting a spacecraft into a particular orbit using only two controls, a prograde burn and a retrograde burn. It runs genuine two-body gravity on real Earth constants, so the orbits, transfer times and fuel costs are the ones you would actually get.
Why does burning here change the orbit over there?
Because a burn changes your speed at the point you are, and that decides how far out you coast on the opposite side. Speeding up raises the far side of the orbit while leaving the near side almost untouched, and slowing down lowers it. This is the single most useful fact in orbital mechanics and it takes about two missions to become second nature.
What is the ball in the corner?
That is a navball. The brown half is the ground below you and the blue half is the sky above, so the line between them is your local horizon. The marker shows where your engine is pointing relative to that horizon, which tells you at a glance whether you are climbing away from the planet or falling back toward it.
How do I move the camera?
Drag anywhere on the view to swing around, and use the zoom buttons or a pinch to move in and out. The view button cycles between an overview that frames your whole orbit, a chase camera riding just behind the spacecraft, and a close pass low over the planet.
What do the numbers on the display mean?
Ap and Pe are the highest and lowest points of your current orbit, measured as altitude above the surface. The countdown next to each is how long until you arrive there. Ecc is eccentricity, where zero is a perfect circle. Delta-v is the total change in speed your remaining fuel can still buy you.
Does it work on mobile?
Yes. It works on modern mobile browsers including Chrome, Safari and Firefox. There are only two burn buttons and a time warp control, so it is comfortable with one thumb, and you can drag the view around with the other.
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