A Question About The Slingshot Effect (Not Related To Visiting The 1980's To Collect Whales)

[jotasbrane]

The stuff I've read about using gravity assists say that you fly in close to a planet, and if your angle is right, you'll swing around in a hyperbolic orbit and fly off in the opposite direction. And they also say that the reason you don't break any laws of physics doing this is because you're stealing some of that planet's kinetic energy from its

Comments

[ziz]

I believe you fling it off, for all intents and purposes, in the other direction, very very slowly. c.f. when a five-year-old runs at you, you grab his arms and swing him around 180 degrees, toss him into the bouncy castle, and fall over backwards.

If it is anchored to the universe, of course, you actually fling the universe off in the other direction. Warp drive is easy if you find something that's already stapled to the reference frame.

[jotasbrane]

But if you fling the planet away from you, and the planet flings you away, where does that energy to do the flinging come from if the planet was stationary?

[radiotelescope]

You don't have to bring potential energy in; kinetic energy *is* conserved in this process. (If you measure from when the spaceship is Far Away to when it's Far Away Again, that is. In the *middle* of the slingshot, the spaceship is speeding close to the planet, so potential has been temporarily turned to kinetic

[jotasbrane]

Hmm, OK. That makes sense. So the actual orbiting doesn't come into it; it's just that the spaceship gains the speed of the planet, in whatever frame of reference you're measuring the speed of the planet.