How to play
This game separates aiming from reading physics. You control one thing: the velocity vector at the moment of release. After that, the field takes over. So the real question is not where to shoot, but which star the parcel should pass beside, and when — a near miss bends a straight line into an arc, while a direct hit ends the attempt.
The preview line took the most work. It had to use the very same integration code as the flight, because the moment players learn not to trust it, the puzzle falls apart. Preview and flight now share one step function, both at a fixed 1/120 of a second. What you see is what flies.
- Levels
- 24 / 4 chapters x 6
- Body types
- 4
- Fuel per level
- 3 cells (3 launches)
- Gravity constant
- G = 2400 (game units)
- Integration step
- 1/120 s, semi-implicit Euler
- Trajectory preview
- 120 steps
- Max stars
- 72
Controls
After the launch
Press near the start, drag out to a direction and distance, and the power bar shows how hard the throw will be. Release, and the parcel leaves with that velocity. It has no thruster and cannot be steered mid-flight. The only thing you can change is the next launch — and each level gives you three.
Flight advances in fixed steps: each step sums the acceleration from every massive body, updates velocity, then updates position. The step is locked at 1/120 of a second and does not depend on the display refresh rate, so the same shot lands in the same place on a 60 Hz and a 120 Hz screen.
How gravity is computed
Every massive body adds an acceleration pointing towards itself, with magnitude G times its mass divided by the square of the distance. G is 2400 in game units. The closer you pass, the harder it pulls, which is why a graze and a head-on approach are entirely different things: one bends a line into a curve, the other is usually a crash.
Moons travel on circular orbits, so their centre of attraction moves every frame. Black holes are much heavier and have an event horizon you do not come back from. Both make a single well-aimed shot insufficient — you have to account for time as well.
- G = 2400, acceleration falls off with r squared;
- Fixed 1/120 s semi-implicit Euler step; frame rate cannot change a trajectory;
- Contributions from all bodies are summed each step, with no shortcuts;
- Leaving the bounds, hitting a surface, or slowing to a near stop all fail the shot.
Six bodies, six rules
Each body changes exactly one rule, so the shape tells you what to worry about. They arrive chapter by chapter: the first chapter has only planets and moons; the fourth is where portals and black holes share the same map.
- Planet: attracts the parcel and destroys it on contact — the everyday tool for bending a path;
- Moon: orbits on a circle, so its pull moves with time and your launch timing matters;
- Black hole: far stronger gravity, with an event horizon that ends the shot — usually worth going around;
- Asteroid belt: a ring of collisions you can cross, but only at the right angle;
- Wind field: a rectangle applying a constant force, slowly pushing the path off course;
- Portal: a pair; enter A and leave B with the same velocity and the same direction.
The 120-step dotted line
While aiming, a dotted line runs from the start for up to 120 steps. It is not a parabola drawn for show: the preview feeds your current velocity into the same step function used by the flight, runs 120 steps, and stops early at a surface, an event horizon or the edge of the map. The moon’s orbital position advances on the same clock, so the line moves with it.
That turns trial and error into reasoning. Hold a direction, watch which side of a planet the line grazes, then nudge the power until the end of the arc sits inside the delivery ring. After a failed shot, its path stays on screen as a faint ghost so you can see exactly how far off you were.
Three cells, three stars
Every level allows three launches. Deliver on the first for three stars, the second for two, the third for one. Miss all three and you stay on that level and can restart. Twenty-four levels at three stars each is 72, shown on the title screen and in the level grid.
Undo rolls the state back to just before your last launch and removes that ghost trail too, which makes it easy to try the same shot at a slightly different angle. It does not cost anything extra: you are taking back a shot, not spending another one.
How the levels are arranged
The 24 levels are generated from fixed seeds, six per chapter, and a given level has the same layout every time you open it. Chapters are not reskins: the first teaches bending around one planet, the second adds the timing of moons, the third stacks wind fields and asteroid belts, and the fourth puts portals and black holes on one map.
Each level stores its stars, whether it has been cleared, and the fewest launches you used. All of that stays in this browser — nothing is uploaded and nothing syncs.
Questions
- Does the preview ever disagree with the flight?
- No. Both call the same integrator at the same 1/120 s step. The only moving part is an orbiting moon, and the preview advances it on the same clock.
- Why do I keep hitting the planet?
- Gravity grows with the square of the distance, so a close pass pulls much harder. Aim a few degrees wider and let the parcel graze the side — that usually produces the arc you wanted.
- Does undo cost a shot?
- No. Undo returns to the moment before your last launch; that shot is treated as if it never happened, and its ghost trail is cleared as well.
- Does progress sync?
- No. There is no account and no server. Stars, cleared levels and best launch counts live only in this browser.