How a Spacecraft Steals Speed from a Planet
🛰️ Orbital mechanics
Question
How can a spacecraft speed up without burning fuel?
A gravity assist (or 'slingshot') is when a spacecraft flies by a planet and uses the planet's gravity to change its speed or direction. In the planet's frame, the spacecraft's speed is the same before and after the flyby - gravity just bends the path. But the planet is itself moving around the Sun, so in the Sun's frame the spacecraft leaves with a different speed: it has 'stolen' a tiny bit of the planet's momentum. That's how Voyager got fast enough to reach the outer planets, and how Lucy gets to the Trojans. The planet barely notices - it's so much more massive that its orbit is unchanged.
The math
Max speed change = 2 × the planet's orbital speed (for a perfect alignment). Jupiter orbits at v = √(GM_sun/a) = √(1.327×10²⁰ ÷ 7.785×10¹¹) ≈ 13.1 km/s, so the theoretical max is 2 × 13.1 = 26.1 km/s. Real flybys get a fraction of this, depending on the approach angle.
Why it matters
Gravity assists are the reason we can reach the outer planets at all - a direct trip to Neptune would take decades, but a string of assists (Voyager's Jupiter-Saturn-Uranus-Neptune chain) cut the journey in half. It's the same trick Lucy uses on the way to the Trojans.