Solar Power Satellite. This is an idea popularised by Gerard O'Neill. The idea behind SPS is that a satellite could be placed in Geostationary Orbit that has a huge solar power collector, that collects the suns rays, and turns them into electricity. The electricity can then be turned into microwave radiation and beamed back down to the earth, where it is collected using a rectenna, and turned back into electricity and put into the electrical power grid; thus saving lots of fossil fuels.

There's a number of common questions:

1. Is is safe?

- yes very. The intensity of the microwaves would only be the same as a cell phone. No significant evidence of damage is known at this intensity.

2. Wouldn't it be better to build the solar panels on the ground where we can maintain them?

- no, the solar panels in space get 3-6x as much sunlight, due to lack of weather and because there's no 'night' in GEO (basically, there's 2 hours per year).

3. What would it cost?

- About 1 trillion dollars; however the cost can be brought down in a number of ways, for example lower launch costs, or mining the moon.

4. that's a lot. would it pay for itself?

- Probably but the electricitiy would be fairly expensive, but not ridiculously.

5. How big do the antennas have to be?

- Absolutely enormous. The size of the antennas are diffraction limited. The rectennas have to be 100s or even 1000s of meters across; however the rectennas are dirt cheap to build, a few dollars per square meter.

6. Wouldn't it be better to put it on the moon?

- No, doing that is popular, but in all fairness, an extremely stupid idea.

- Even though the moon keeps the same face to the earth, the earth doesn't keep the same face to the moon; the earth rotates every day. This means that no point on the earth gets continuous power from the moon; which is very inconvenient. By way of contrast the Solar Power Satellites at geosychronous orbit maintains the same point in the sky, day or night, and can supply continuous energy to any single point on earth that can see it; much like a TV satellite with a very much more focused beam.

- From the distance of the moon, microwave energy transmission probably can't be used (the antennas would be inconceivably big due to the extreme distance), lasers could be used, but weather systems cause far more attenuation to laser light than microwave light; and the device for converting light to electricity are costly over large areas, and the lasers could be dangerous.

- The surface of the moon is in shadow for half the lunar month. Therefore, you would only actually get energy available for at most 1/4 of the day, on average, due to the rotation of the earth.