# Gravity assist as energy source

The Gravity Assist allows for a spacecraft to gain more speed and energy ("stealing" it from the planet).

Can this approach be used for creating power plants that generate "cheap" energy for use on Earth?

Yes, you can.

It is entirely possible to create tidal power plants, stealing energy from the Earth-Moon system.

Eventually this source runs out of energy, but it would be a much longer duration than the duration until we run out of fossil fuels (or out of room to dump the carbon dioxide somewhere). So it could be considered renewable, like solar power is considered renewable (which it isn't since the sun runs out of fuel someday).

• "eventually" not just in geologic time, but literally an astronomically long period of time.
– Móż
Commented Mar 22, 2023 at 21:58
• But think of all the problems we're having now because we stole energy from dinosaurs. If we're stealing energy from the moon's orbit, we could bring it down on our heads! (but it does beg the question of whether we could push the moon away by having giant propulsors assist the tides). Commented Mar 27, 2023 at 2:17
• Actually, due to virial theorem if you steal energy from the Moon, it will cause Moon to become more distant. There are two forms of energy, gravitational energy and kinetic energy. Due to virial theorem, kinetic energy is twice the negative gravitational energy. You are stealing from Moon's kinetic energy, giving a bit back to its gravitational energy, and the total energy of the Moon reduces as it becomes more distant from Earth. Commented Mar 28, 2023 at 15:26
• Such power plant already exist, for example: en.wikipedia.org/wiki/Rance_Tidal_Power_Station Commented Apr 5, 2023 at 8:38

The spacecraft gain energy mainly because it do not "return" back in the place where the gravitational effect is applied, otherwise the "slingshot effect" would not be effective. So, in the earth, you can use "potential energy" when you have a differential height, but if you want to return to the initial position you must spend the same amount of energy to close the loop. Tidal energy is an exception, because the system sun-moon-earth is creating and subtracting energy to the sea in form of "waves", and you can steal energy on each front (rising or falling) of the waves, giving the sun-moon-earth is not returning on the same exact position as before after each wave cycle.

• I'm not sure I'm following, because it's known that the gravity assist can be used multiple times against the same planet. For instance, the Rosetta spacecraft used three assists from Earth: sci.esa.int/web/rosetta/-/58336-rosetta-by-numbers Commented Apr 7, 2023 at 11:08
• @emaxx, may be this picture will give you a better understanding of the concept: cdn.mos.cms.futurecdn.net/…. Though Rosetta used 3 Earth slingshots, it was part of an extended trajectory where it also used other planets and asteroids in between and their positions in their respective orbits at specific times to make it to its final destination. Commented Apr 7, 2023 at 18:47