Solar System Simulator · What if? · gravity assist
How a gravity assist works
Relative to Jupiter, the spacecraft leaves exactly as fast as it arrived. Relative to the Sun, it leaves with kilometres per second it never had — paid for by the planet. Fly the 1977 flyby and read the numbers.
Borrowing the planet's motion
Picture a ball bouncing off the front of a moving train. Relative to the train it arrives at some speed and leaves at the same speed. Relative to the ground it comes away faster, because the train's motion was added on the way out. A planet does the same to a spacecraft, with gravity playing the part of the bounce: the probe falls in, is swung around, and climbs back out at the speed it came in with — as seen from the planet. Seen from the Sun, the planet's orbital motion has been added to the probe's.
Jupiter orbits the Sun at about 13 km/s. A flyby can hand a probe a good fraction of that. In the simulator's recreation of the 1977 flight, Voyager 1's path has it doing 12.3 km/s relative to the Sun at Jupiter's distance on the way in and 23.3 km/s on the way out — +11.1 km/s, with its engines off the whole time.
Behind the planet to speed up, ahead of it to slow down
Which side you pass on decides the sign. Cross behind the planet — its trailing side, where the planet is moving away from you — and you are dragged along and released faster. Cross in front, and the planet's pull works against your direction of travel: you leave slower. Missions to the inner solar system use exactly that in reverse, shedding speed at Venus or Mercury they could never afford to burn off with fuel.
Who pays?
Momentum is conserved, so Jupiter loses precisely what the probe gains. Divide Voyager's 825 kg by Jupiter's 1.9 × 10²⁷ kg and the planet's orbit slows by a few times 10⁻²⁰ m/s — an amount no instrument could ever detect, from a planet that will keep funding flybys for as long as there are spacecraft.
Where you measure decides what you see
This is the trap the lesson is built to avoid. Measure at closest approach and the probe is doing over 20 km/s — but that is speed borrowed from Jupiter's gravity well, and it is paid back on the climb out. Measure just before and just after at the same distance from Jupiter and the Sun's own well muddles the comparison. The honest number is the change in the probe's orbital energy about the Sun, quoted as its speed at Jupiter's distance before versus after. That is what the simulator's speedometer shows, and it is the figure the number tiles above use.
Then it happens again at Saturn
The same flight carries on to Saturn for a second assist — about +4.9 km/s in the simulator — which is what finally puts Voyager 1 above the Sun's escape speed. From there gravity can slow it but never stop it, and the mission plays out to today. The full trajectory, 1977 to now →
What the simulator does
- The giant planets take their 1977 positions and the probe launches on the real 1.5-year transfer, 10.1 km/s over Earth.
- It passes about six Jupiter radii out, on the trailing side; the camera switches to true scale and follows the planet through the encounter.
- The debrief reads the speed at Jupiter's distance before and after — the difference is the assist.
Questions people ask
Does a gravity assist break energy conservation?
No. The spacecraft gains exactly the energy the planet loses. Because the planet is about 10²⁴ times heavier, its share of the exchange is immeasurably small.
Can a gravity assist slow a spacecraft down?
Yes — pass ahead of the planet instead of behind it. Probes bound for Mercury and Venus use flybys to lose speed, since fuel to brake that much would be prohibitive.
How much can one flyby add?
In theory up to about twice the planet's orbital speed, in practice much less because the probe cannot turn all the way around the planet. Jupiter, at 13 km/s, is the best in the solar system: heavy enough to bend a path hard, fast enough to have plenty to give.
Why is Jupiter used so often?
Mass and speed. A small planet barely bends the path of a fast probe; Jupiter can swing one through a large angle and hand over a large share of its 13 km/s. Voyager, Galileo, Cassini, New Horizons and Juno all used it.
Keep going
Every number on this page comes from the simulator's own physics or the real mission record it is checked against. Where the simulation and reality differ, the page says so.