Scale of the problem

(Transferred from where it did not get much discussion.)

At present, the human race uses about 18 TW of energy. One form of
energy can be substituted for another or used to synthesize another
form of energy (such as electrical energy being used to make synthetic
hydrocarbon fuels).

Energy growth rather than shrinkage seems more likely, but a
population crash would reduce consumption–i.e. an investment risk.

Taking the energy demand at 18 TW and acquiring somewhat over half
from power satellites is 10 TW. Mike Sneed makes somewhat different
assumptions but his numbers are in the same ballpark. A ten-year
construction of 10 TW requires building a TW/year (after several years
of ramp-up). Or more than ten years if you are not in a hurry to get
off fossil fuels.

At 5 GW per power satellite, a TW requires constructing 200 power
satellites. If we assume 6.5 kg/kW the mass is 32,500 tons
plus ~25,000 tons of reaction mass to move the power satellite to GEO,
call it 57,500 tons per satellite. 200 of them/yr would be 11,500,000
tons per year. Using SpaceX’s number of 100 tons to LEO per flight,
115,000 launches per year or ~315 per day.

(I don’t know if this traffic rate will cause excessive damage to the
ozone layer or not. NOAA found a million flights per year were
tolerable with Skylon, but it burns hydrogen, not methane.)

If the rockets last 100 flights, it will take a production rate of 10
every three days to keep up the fleet. At 1000 flights, the
production rate would fall to 1 every 3 days.

Years ago I worked out the energy payback time for using the
hydrogen-powered Skylon and got 3-4 months, which is excellent
compared to ground solar.

A power satellite would take 575 launches. The propellant for both
stages is around 4,500 tons of which 1/5 is methane.

CH4 + 2O2 → 2H2O + CO2
16 64 36 44

Methane at 55.5 MJ/kg is about 15.4 kWh/kg or 15.4 MWh/ton. 900 tons
would be 13.9 GWh, enough for a power satellite would be around 8000
GWh. Once the power satellite is operational at 5 GW, it would repay
the launch energy in 66 days or a little over 2 months.

This is leaving out the reaction mass needed to move the power
satellite to GEO and the energy that goes into the parts, but both are
small compared to the rocket fuel consumption.

Power satellites are, of course, not the only energy solution. A very
large build-out of fission plants would work. Fusion, if it comes
into existence, would do the job. Wind and solar plus a very large
amount of storage and long transmission lines might work.

But if you want to think seriously about power satellites at scale,
this should provide a starting place. Feel free to adjust the
assumptions as you wish.

Oh, and check the math, please.


It seems to me that any plan to deploy solar power satellites (SPS) on a scale to largely replace fossil fuels must seriously consider Gerard O’Neill’s strategy of building them largely from lunar resources instead of hauling all of that mass up from the bottom of Earth’s gravity well. I’d expect the first stages of SPS deployment to follow something like the roadmap in John Mankins’s 2014 The Case for Space Solar Power, which envisions a five stage program of design reference missions building successively larger satellites from eight kinds of identical modules, none massing more than 600 kg, starting with small demonstrators and building up to a pilot plant with a cost he estimates at US$ 5 billion, with a total budget to complete the first full-scale powersat at $20 billion, which is peanuts compared to what NASA is dumping into the ocean with SLS at US$ 4 billion per launch according to the NASA Inspector General. These figures assume all components built on and launched from Earth, with the full scale SPS requiring 500 to 1000 launches of payloads between 10 and 20 tonnes and a reusable launcher with cost to low Earth orbit between US$ 300 and 500 per kilogram. SpaceX is estimating figures for Starship much lower than this, but “we’ll see”.

But rather than hauling all of that mass (and the propellants needed to lift it) all the way from Earth, isn’t is more practical, when there’s, from an elemental composition standpoint, this huge ball of oxygen, silicon, iron, and aluminium just sitting there less than 4 km/sec delta-v from geostationary orbit (that’s lunar surface to GEO; low lunar orbit to GEO is just 2 km/sec). If, as O’Neill suggested, you launch regolith from the Moon’s surface with an electromagnetic mass driver and refine and fabricate in lunar orbit, propellant requirements to transport components to GEO are a fraction of bringing them from Earth (2 vs. ~13.2 km/sec). And, when O’Neill was proposing this, he didn’t know of water resources on the Moon which can be processed into hydrogen/oxygen propellant.

Now, building out the infrastructure to do this sounds fantastically ambitious in an age when it seems that in projects that actually involve atoms as opposed to bits, it takes almost forever to accomplish almost nothing, but so is launching and recovering 315 Starship-class vehicles per day, building the infrastructure and propellant production this would require, and building new Starships at a rate of one every three days or faster.

It would be interesting to work out the mass budget of lunar resource collecting, processing, and manufacturing required to build operational SPS at a given rate, then calculate how many Starship launches, including refuelling tanker flights, it would take to deliver that to the lunar surface and orbit, and then the rate of support launches that would be required after the infrastructure was in place (for repair and replacement parts, crews and supplies, and high-cost, low-mass components and materials [for example, microchips and rare earth metals]).


Nuclear fission power plants seem to be the plan that Russia & China – exempt from Western Political Correctness – are working on. Even Bill Gates is investing in new designs for fission power plants. France (not surprisingly) and the UK (somewhat surprisingly) are apparently getting serious about new nuclear power plants. The exception appears to be “Green” Germany, which is shutting down its nuclear power plants and building brown coal thermal plants to cover the inevitable deficiencies with unpredictably unreliable wind & solar.

One of the foundational documents for the “Peak Oil” movement (if can you remember that far back) was geologist M. King Hubbert’s 1956 paper presented at the American Petroleum Institute titled: “Nuclear Energy and the Fossil Fuels”. Hubbert concluded:

"Attention is accordingly invited to Figure 30 which covers the time span from 5000 years ago – the dawn of recorded history – to 5000 years in the future. On such a time scale the discovery, exploitation, and exhaustion of the fossil fuels will be seen to be but an ephemeral event in the span of recorded history. There is promise, however, … that we may at last have found an energy supply [nuclear fission] adequate for our needs for at least the next few centuries of the “foreseeable future”. "

1956! Just before the International Geophysical Year, when the world seemed hopeful and mankind’s horizons extended to the stars!


Where should we set our sights? We know that the standard of living (even the quality of life) is inextricably linked to energy supply. We also know that billions of human beings today are living in energy poverty. And we know that the population of humans on Earth will grow from the current 7 Billion to 9 or 10 billion.

Put it all together, and perhaps we should be thinking about how do we supply a 100 TW world.

If power transmitted from space through the atmosphere is part of the answer, why do it via satellites? Would it be more feasible to build the infrastructure on the surface of the Moon itself, using Moon-mined materials to the fullest extent possible?


Hi John. Having founded the L5 Society, I was deep into moon mining technology. The mass driver turned out to be much more difficult than originally thought, though now it might be relatively easy.

The problem is velocity scatter from the ejection end of the mass driver. I don’t remember the exact numbers, but they would not be hard to find in old papers by Tom Heppenheimer. The source of the problem was the switching speed and jitter of the SCRs that controlled the mass driver coils. Over the last 40 years, this has become perhaps 1000 times better with things like GaN fast power transistors.

In the days when Heppenheimer was working on the problem, the only solution was “achromatic” orbits where (by analogy to lenses that bring different wavelengths to the same focus) mass driver launches from a particular point on the moon with a little variation in velocity wind up (with some scatter) at the same place, the “catcher” out near L2.

I spent close to ten years looking into power satellites built from the earth simply because the complications of a moon mine, mass driver, and all the processing needed to fabricate SPS parts were beyond me. (And an estimate of the cost was in the trillion-dollar range.) Power satellites make economic sense if you can get the cost to lift parts to GEO under $200/kg. At high flight rates, Skylon and StarShip should be able to get the cost to LEO down to $100/kg. If electric propulsion with Ve of 20 km/s is feasible, the cost of the reaction mass to get to GEO is another $100/kg.

I gave up because the space junk will not let power satellites be constructed in LEO and if you build them above the junk, the radiation is lethal in hours. That means teleoperation. The cost and time to develop robots or teleoperation were out of my skill range.


People have proposed to build power plants on the moon and transmit the energy to earth. There are a bunch of problems that make this impractical and unecoonomic.


Assuming this can be done technically, what becomes of the scores of TW’s of energy beamed to the Earth’s surface. Even if directly converted to electricity, energy is conseerved. Will this not eventually yield heat, i.e. global warming?

It eventually gets degraded to heat and represents an increment to the Earth’s energy budget or, more precisely, the solar component of energy input, since it is just the capture of solar energy which would otherwise miss the Earth. The main natural energy inputs are:

  • Solar: 174,000 TW
  • Geothermal: 44 TW
  • Tidal: 3 TW
  • Total: 174,047

Now, any form of energy produced by humans which does not come from the Sun (for example, fossil fuels or nuclear energy) adds to this total, as would additional solar energy captured and delivered by solar power satellites. If total human energy consumption is 18 TW, this represents around 0.01% of the natural energy input from the Sun. This would produce, by itself, a negligible increase in the Earth’s temperature and is around one tenth the variability in solar input over the 11 year solar activity cycle. The main human drivers of changes in the Earth’s temperature are not energy consumption by itself, but side-effects such as release of greenhouse gases from burning of fossil fuels and agricultural and industrial activities.

If produced from sources that do not create these other consequences, human energy production could probably increase by a factor of ten or more without harmfully warming the Earth. Energy production from solar power (directly by Earth-based photovoltaic panels or indirectly by means such as wind turbines or hydroelectric power) do not affect the energy balance, since that is solar energy which would have ended up heating the Earth one way or another anyway.


You need to consider the alternatives. Nuclear power for example dumps about 2/3rds as waste heat making the efficiency around 33%. Rectenna losses are around 15% so 85% of the power is useful.

The best fossil (combined cycle) is around 60%, but the long term CO2 heat-trapping is a problem.


“ended up heating the Earth one way or another anyway.”

Wind and hydro yes, but PV is blacker ground it covers. So even PV can add to the heat problem.


This is not the topic to discuss the hypothesis of Anthropogenic Global Warming – suffice it to say that anyone who looks into the claims begins to have serious doubts about its validity. Water vapor is also a Radiatively Active Gas (to give tne mis-named “greenhouse gases” their proper name) – and it is present in the atmosphere at concentrations orders of magnitude higher than carbon dioxide; liquid water also forms 2/3 of the surface of the planet, providing a huge area for continued evaporation. Anthropogenic CO2 is minor – and as King Hubbert pointed out, it will go away in a time frame that is geologically insignificant.

That is not to say there are no anthropogenic influences on climate. Agriculture may be a big one. There have even been claims that large scale use of wind turbines will alter wind patterns with consequent effects on climate. However, we need some humility about all these claims about human impacts on climate – variation in solar activity likely dwarves anything we could do, even if we tried.