01
Pass behind
Cross behind the planet along its orbit and you leave faster, having taken a slice of its orbital motion with you.
The physics
STEALING
FROM JUPITER
A gravity slingshot does not work the way it is usually drawn. The planet does not throw the spacecraft. Nothing pushes. And the speed a probe walks away with does not exist at all until you pick the right thing to measure it against.
This page is about the real manoeuvre the game is named after, and about which half of it survives when you put it in your hand.
01 · The manoeuvre
In the planet's frame, nothing is gained
Fly a spacecraft past a planet and gravity pulls it in, whips it around, and lets it go. Measure its speed relative to the planet on the way in and again on the way out, and the two numbers match. The planet gave it nothing. Gravity pulled just as hard on the approach as it did on the departure, and the books balanced exactly.
What changed is the direction. The craft came in on one heading and left on another, and how far it turned depends on how close it passed and how fast it was going when it got there. That deflection is free, it is large, and it is the entire mechanism.
02 · Where the speed comes from
The planet is moving, and that is the trick
Now measure the same flight against the Sun instead. Jupiter is not sitting still — it is running along its own orbit at about thirteen kilometres a second, dragging its gravity well with it. A probe that leaves the encounter pointed the same way Jupiter is travelling has that orbital speed added to its own.
The energy is real and it has to come from somewhere, so it comes from Jupiter. The planet is slowed in its orbit by exactly as much as the probe is sped up. Jupiter is around 1024 times the heavier of the two, so the price it pays is far too small to detect — but it is paid, and the sum is exact.
This is why the manoeuvre is properly called a gravity assist rather than a slingshot. Nothing is flung. The craft borrows a share of a planet's motion around the Sun by falling past it on the right side, and the only steering involved is choosing where to aim months in advance.
02
Pass in front
Cross ahead of it and the same physics runs backwards: you give motion back and leave slower. This is how a probe brakes into the inner system.
03
Pass close
The nearer the pass, the sharper the turn — until the atmosphere or the surface sets the limit on how near you dare go.
03 · In practice
Nothing reaches the outer planets without it
The manoeuvre is not a curiosity. It is the reason the outer solar system has been visited at all. No rocket that has ever flown could push a useful payload to Neptune directly; the fuel needed to do it directly does not fit on the vehicle that would have to carry it.
1977
Voyager 2
Jupiter, then Saturn, then Uranus, then Neptune — each encounter bending it onto the next, on a planetary alignment that comes round about every 175 years.
1997
Cassini
Venus twice, then Earth, then Jupiter, taking seven years and four assists to reach a Saturn orbit it could not have flown to directly.
The catch
Alignment
Assists are only available when the planets are where you need them. Miss the window and the next one may be decades out.
04 · The game
Which half you can actually hold
The speed half of a gravity assist is invisible. It only exists relative to a body you are not looking at, it accrues over months, and no player will ever feel it. The deflection half is the opposite: it happens in seconds, it is entirely visual, and it is the part that makes the manoeuvre worth watching. That is the half Starship Rescue is built on.
You get one launch — a direction and a strength — and then no thrust and no steering. The planets pull, the shot bends, and where it ends up is decided by a choice you made before anything moved. The grid behind the field bends toward mass so the pull is legible before you spend an attempt discovering it, and the aim line you drag out is a real trajectory from the same integrator that flies the ship, cut short so it hints rather than answers.
The one honest simplification is the moons. From sector nine they orbit the planets, and they block without ever pulling — an obstacle on a schedule rather than a second gravity well. Real moons pull, of course. But a field where everything attracts everything is a fog rather than a puzzle, and the thing worth keeping was the shape of the decision, not the completeness of the simulation.
05 · Asked about this
Common questions
Does a gravity slingshot break conservation of energy?
No. The spacecraft gains orbital energy and the planet loses precisely the same amount from its orbit around the Sun. The exchange only looks like something from nothing because the planet is so much heavier that its share of the loss is unmeasurable.
Can a slingshot slow a spacecraft down?
Yes, and it is used that way regularly. Passing in front of a planet along its orbit rather than behind it runs the same exchange in reverse, shedding speed. Missions heading for the inner solar system use it to lose energy they would otherwise have to burn fuel to remove.
Why does the speed depend on which frame you measure in?
Because speed is always relative to something. Relative to the planet, the craft arrives and departs at the same rate and gains nothing. Relative to the Sun, the planet's own orbital motion is added to the departure. Both descriptions are correct; they are measuring against different things.
Is a moon's gravity useful for an assist?
It can be — Cassini spent years being steered by repeated passes at Titan. It is a smaller effect than a planetary assist, and it is one of the things Starship Rescue leaves out: in the game the moons block but never pull.