A gravitational slingshot (or gravity assist) lets a spacecraft steal a small amount of a planet's orbital momentum to change its speed and direction without burning any propellant. This simulation numerically integrates real Newtonian gravity to show a spacecraft's trajectory bend and accelerate as it swings past a moving planet.
The spacecraft's path is computed frame by frame from Newton's law of gravitation, F = GMm/r². Because the planet itself is moving, the encounter is not symmetric in the Sun's frame: in the planet's own reference frame the spacecraft's speed is unchanged, but transformed back into the surrounding frame it can gain (or lose) speed equal to roughly twice the planet's velocity component along the flyby direction.
Set the approach speed, the impact parameter (how far off-centre the spacecraft aims — sign flips which side it passes), the planet's mass and its own orbital velocity, then press Launch flyby. Watch the trajectory curve around the planet and compare the "Speed before" and "Speed after" readouts to see the gravity assist in action.
NASA's Voyager 2 used gravity assists from Jupiter, Saturn and Uranus to reach Neptune and beyond — a trip that would have taken far longer, or been altogether impossible, on a direct trajectory with the chemical rockets of the 1970s.
This gravitational slingshot simulation integrates Newton's law of universal gravitation step by step to move a spacecraft through the gravity field of a moving planet. Unlike a purely analytical formula, the spacecraft's velocity and position are updated every animation frame from the instantaneous gravitational acceleration, so the curved hyperbolic-like path and the resulting speed change emerge from the physics itself rather than being scripted in advance.
A gravity assist (or "slingshot") manoeuvre: a spacecraft flies close to a moving planet, its path bending under gravity. Because the planet is not stationary, the spacecraft can leave the encounter faster (or slower) than it arrived — momentum is conserved between spacecraft and planet, but since the planet is vastly more massive, its own velocity barely changes while the spacecraft's changes a lot.
Tune approach speed, impact parameter (miss distance and side of approach), planet mass and planet velocity, then click Launch flyby. The info pill reports the current phase, and the readouts show the spacecraft's speed measured well before and well after the encounter, plus the net speed gained.
A gravity assist does not violate conservation of energy: the spacecraft's kinetic energy gain is balanced by an immeasurably tiny loss in the planet's own orbital energy, since the planet is enormously more massive than any spacecraft.
A gravitational slingshot, or gravity assist, is a manoeuvre where a spacecraft passes close to a moving planet or moon and uses its gravity to change speed and direction, borrowing a tiny amount of the body's orbital momentum in the process.
In the planet's own reference frame, the spacecraft's speed does not change at all — only its direction does, exactly like an elastic collision. But because the planet itself is moving relative to the Sun, transforming that same encounter back into the Sun's frame shows the spacecraft's speed can increase or decrease. The energy comes from the planet's orbital motion, which loses an immeasurably small amount in return.
The impact parameter is the perpendicular offset between the spacecraft's undeflected straight-line path and the planet's centre. A larger impact parameter means a more distant, gentler flyby with a smaller deflection; flipping its sign sends the spacecraft past the opposite side of the planet, which can turn a speed boost into a speed reduction.
A more massive planet produces a stronger gravitational pull at any given distance, bending the spacecraft's path more sharply and allowing a bigger change in speed and direction for the same approach distance.
Yes, extensively. Mariner 10 used Venus to reach Mercury, the Voyager probes used Jupiter and Saturn to reach the outer planets, and Cassini used multiple flybys of Venus, Earth and Jupiter to reach Saturn on a fraction of the fuel a direct trajectory would need.