More dumbness: How does 'gravity assist' work?

[Rum]

Was thinking of this as I was watching a Youtube on one of the probes out there.

They use the gravity of a planet to 'slingshot' around and give the probe a boost in speed.

BUT - where does the energy come from? You can't get something for nothing energy wise. To me it seems logical that the acceleration increase as the probe is 'pulled' into the planet should be lost as it escapes the gravity and is 'slowed'.

What am I missing - other than the brain cells for maffs?

[Seabass]

What am I missing?

That the planet is moving. Energy will be gained or lost depending on whether the probe moves with or against the planet's orbit.

[laklak]

You just ask gravity nicely for a little help.

[Rum]

What am I missing?

That the planet is moving. Energy will be gained or lost depending on whether the probe moves with or against the planet's orbit.

Really? That simply didn’t occur to me! Simples when you know!

[L'Emmerdeur]

Seabass covered it. The spacecraft 'steals' or adds a minuscule percentage of the planet's kinetic energy (orbital velocity) to either speed up or slow down. For a much more long-winded explanation, see 'Gravity assist'.

[pErvinalia]

What am I missing?

That the planet is moving. Energy will be gained or lost depending on whether the probe moves with or against the planet's orbit.

Cool! I've never actually bothered to think about how gravity assist might work.

[Clinton Huxley]

Gravity assist is theft. Like tax.

[laklak]

Illegal earth immigrants stealing Jupiter's gravity. They should build a wall.

[Hermit]

Seabass covered it. The spacecraft 'steals' or adds a minuscule percentage of the planet's kinetic energy (orbital velocity) to either speed up or slow down. For a much more long-winded explanation, see 'Gravity assist'.

And an illustration/explanation needing no words at all, courtesy the Wikipedia.

[pErvinalia]

That doesn't really answer his question, unless you can discern a speed difference after the object passes the planet.

[Rum]

That doesn't really answer his question, unless you can discern a speed difference after the object passes the planet.

I think the first one does, even if the increase in speed is not all that apparet - gives the idea well enough though.

[pErvinalia]

I can't see any speed increase. And I really want to push Hermit's buttons too.

[JimC]

Velocity includes both speed and direction. A change in velocity without a change in speed still counts - without a planet's gravity assist, a change in direction would require a fuel-using burn.

Speed increases, as described earlier, borrow some of a planet's kinetic energy.

[pErvinalia]

Velocity includes both speed and direction. A change in velocity without a change in speed still counts

Still counts as what? Rum's OP is talking about speed, not velocity.

[JimC]

Velocity includes both speed and direction. A change in velocity without a change in speed still counts

Still counts as what? Rum's OP is talking about speed, not velocity.

Velocity would be a better term in the OP - we are talking about rocket-powered spacecraft, and any change in velocity (speed and/or direction) without gravity assist uses up reaction mass. In finding a path from Earth to the future position of a planet or moon, one wants that path to use the minimum amount of reaction mass. Some paths of that nature use multiple fly-bys of planets, using each encounter to either change direction without using fuel, or to increase the speed (relative to the Sun) by stealing KE from a planet. I gather calculations of such paths are very intricate, taking much computer time...