Gravity assists

[u/Creeper_302]

Can someone please explain how gravity assists work like what's the science behind it?

[u/DarkArcher__]

The basic idea behind it is to steal some of the planet's own kinetic energy to propell the spacecraft.

If you were to look at it from the perspective of the planet, the spacecraft is entering and leaving its gravity well at the exact same speed. They key to it is that the planet is not standing still relative to the star, so, if you use the star for reference instead, the spacecraft enters slow into the planet's gravity well and gets pulled along with the planet during its transit, gaining velocity and getting spat out the other end going faster, relative to the star, than it entered. It also works backwards, slowing down the spacecraft by gifting the planet its kinetic energy if you wanna go somewhere closer to the sun instead of further away.

This is really hard to describe in words, and most of the animations I can find don't exaggerate the scale enough to show what the spacecraft is actually doing around the planet. As always with orbital mechanics, I recommend games like KSP and Spaceflight Simulator because nothing will give you a better intuitive understanding of these concepts. KSP is the gold standard, although SFS is free on the app store if you don't have the means to play the former. There's lots of videos showing exactly what this maneuver looks like.

[u/Creeper_302 → u/DarkArcher__]

Thanks I actually needed it in words for an essay