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If I by some mean threw lunar soil in the earths direction, and used it to power some turbines, would the turbines generate more energy than it took to get the lunar soil out of the moons orbit?

For example:
Assuming I can convert all the kinetic energy from the moon dust to electricity.
If I had an x amount of explosives (enough to produce debris that escape the moons orbit towards the earth) and I could convert the explosives energy to electricity, would I get more energy from the explosives or from using them to blow up the moon and using the debris to power my turbine?

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  • $\begingroup$ You realize there's still the gravity of the moon to overcome if you want to get energy out of the Earth's gravity, right? $\endgroup$ – JMac Feb 9 '17 at 19:35
  • $\begingroup$ @JMac I was assuming that the energy from the explosion was enough for the debris to escape the moons gravity. I will add this to the question, $\endgroup$ – Beothorn Feb 9 '17 at 19:39
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    $\begingroup$ There is nothing wrong with your way of generating energy by using Gravitational Potential Energy, but the amount of variables(Thermodynamics, Aerodynamics, Stress-Strain curve of lunar soil etc.) are so many that makes the trajectory extremely capricious, because lunar soil isn't made of expensive materials and alloys like spaceships which reduces the effects of variables to a much higher extent, and we can calculate the position/trajectory of spaceship at any given instant of time. $\endgroup$ – Tanishq Jaiswal Feb 9 '17 at 20:57
  • $\begingroup$ I know it is a silly question, but it is something that came up on a casual conversation and some of my coworkers argued that discarding the engineering problems that there must be something fundemental that would make generating energy like this not possible. I couldn't thing on anything and that's why I'm posing this question here. $\endgroup$ – Beothorn Feb 10 '17 at 14:07
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At a pure physics level, you're correct. The potential energy of the earth-moon system is decreased by moving a small mass from the moon to the earth. Therefore you can construct some mechanism to extract the energy from that different state.

But at an engineering level, this is very difficult. There's no cable or conveyor connecting the earth and the moon. So pulling mass from the moon requires expensive and fragile machines. The energy gained from this process is much less than what is required to put the machines on the moon to start the process.

Without a space elevator, we don't have a method to easily extract energy from bringing objects from space to earth. Instead objects speed up as they approach the earth, and this speed causes problems either penetrating the atmosphere or on reaching the surface. So just "throwing" doesn't create useful energy. It just heats the atmosphere as the objects burn up on re-entry. You mention turbines, but I don't see how the turbines would be fed.

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It's not a scheme that I would invest in.

Energy Expenditure

  1. Get the digging gear to the moon. To make the process efficient, you need lots of equipment, that's lot of energy required.

  2. Energy required to dig up the regolith, where will that come from?

  3. Energy required to get a large amount of material out of the Moon's gravity well.

  4. Some (unknown- railgun?) method of directing it to the place on Earth where the (unspecified mechanism) of the turbine is located.

  5. Lunar regolith will burn up on the way in through Earth's atmosphere.

To me, this would not be in any way efficient.

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