Solar Power Satellites

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The concept of Solar Power Satellites (SPS) was invented and first described, in November 1968 by Dr.Peter Glaser of Arthur D. Little Corporation

Ref: Glaser, Peter E. "Power from the Sun: Its Future". Science Magazine, 22 November 1968 Vol 162, Issue 3856, Pages 857-861.

The concept of SPS are solar arrays in geosynchronous orbit around Earth, beaming power to the ground via microwaves. The receiving antenna (rectenna) is quite large, several square miles. The conversion efficiency of a rectenna is about 95%, compared to 20% or less for photovotaic cells. So SPS rectennas would require a lot less land area than conventional solar cells. According to wikipedia a solar panel in the contiguous United States on average delivers 19 to 56 W/m². By comparison an SPS rectenna would deliver continuously about 1,000 W/m², hence the size of the rectenna required per watt would be about 1.9% to 5.6% that of a terrestrial solar panel. Some have proposed beaming down power via lasers instead of microwaves.

Using Lunar Resources

The late Dr.Gerard K. O'Neill determined that SPS could most cheaply be built from lunar materials. On 30 April 1979 the Final Report "LUNAR RESOURCES UTILIZATION FOR SPACE CONSTRUCTION" by General Dynamics Convair Division under NASA contract NAS9-15560 concluded that use of lunar resources would be cheaper than terrestrial materials for a system comprising as few as thirty Solar Power Satellites of 10GW capacity each.

Radio Frequency Issues

The inverse square law does not apply to a focused beam.

The equation of beam spreading is a function of the transmitting antenna aperture versus the frequency. The bigger the aperture, the tighter the beam. Beam spreading does not cause significant power loss. The size of the antenna is set to precisely match the dimensions of the beam as it intersects the Earth's surface.

A little bit of energy is lost due to sidelobes which are caused by diffraction. It is typically not worth it to make the receiving antenna large enough to catch all the sidelobes.

As for problems of radio interference: that has little to do with the size of the rectenna, but is certainly an issue which needs to be addressed in the system design. The problem of radio interference is solvable.

SPSes normally would be about 2.4 GHz. This would not affect Ku band at all. There would have to be some review of the effect of harmonics. But since the SPS sends a narrow beam, the Ku band downlink receiving station would have to be within a few kilometres of the rectenna to even notice the harmonics within the sidelobes.

Interference between comm-sats of the same frequencies is a much bigger problem than interference between them and the SPS.

The biggest problem is that the best frequencies for SPS have now been allocated to cell phone services.


As for maintenance: very little maintenance is required for a rectenna system, it is essentially passive with no moving parts.

Next steps

Lunar L-1 is the best place to put an initial solar power satellite demonstrator. We can place the rectenna on the Moon and the solar PV arrays at L-1. The distance from L-1 to the Moon (50,000 km) is similar to the distance from GEO to Earth (40,000 km), so it will validate the engineering design well, and prove that useful power can be beamed over that distance. This is also the cheapest way to deliver large scale power to the lunar surface, as rectennas are light weight and PV cells area heavy. Soft landing hardware on the Moon from Earth is very expensive.


Solar power Satellites (SPS) will not compete head to head on price alone in the foreseeable future. This is because deceptively cheap (subsidized) energy continues to be readily available using nuclear power and fossil fuels, and could continue for a couple of centuries or more.

On the other hand....

If we assume, hypothetically, that the world decides that "Fossil Fuels Are Bad", and mandates zero emission of greenhouse gases then what forms of power will be used? Does this mean a widespread increase in the use of nuclear power? Is this a good thing or a bad thing?

Is SPS better than nuclear power?

The collateral damage caused by fossil fuels and nuclear fission far outweighs their deceptively low price. The real price for these fuels is very high when you consider these factors:

Alternatives to Solar power Satellites

Fossil fuel (coal, oil, gas):

- military cost of securing sources of supply and supply channels, with associated geopolitical problems and trouble with the local insurgents - Bush budget hikes war funding
- cost of the war on terror (unfriendly regimes and terrorists funded by oil revenues)
- huge trade deficits from importing fossil fuels US deficit heading towards record
- air pollution - reduced life expectancy / healthcare costs
- global warming exacerbated by greenhouse gases
- water pollution (e.g. Mercury from coal) - reduced life expectancy / healthcare costs

Global Warming

On 2nd February 2007, Working Group I of the Intergovernmental Panel on Climate Change (IPCC) published IPCC Working Group I Fourth Assessment Report Summary for Policymakers (SPM)

Later in 2007 the IPCC will publish the complete version of the most strongly worded report so far "Fourth Assessment Report (AR4)", confirming that human emissions of greenhouse gases is causing the temperature of the Earth to rise, which is resulting in increasing changes to the planet's climate. The consequences of this include disruption to agriculture and global food supply, extinction of species, rising sea levels, loss of human habitat, increased erosion, increased disease from insect habitat expansion, slowing or shutting down the thermohaline circulation which keeps western Europe warm, and property damage due to more violent storms.

SPS is a potential solution to global warming. SPS will reduce heat pollution, not increase it.

Assuming the world is supplied by 200 SPS at 5 GW each. Each SPS loses 1 % into the atmosphere, a total of 10 GW of atmospheric heating caused by all the world's SPSes.

10 / 1.2 x 10E14 = 8 x 10e8 GW

So the entire losses of all the world's SPSes would be 8 parts in a hundred million. The present power stations of the world are injecting about thirty times as much into the atmosphere right now even as we type for a total of 2000 GW.

And even that is a drop in the bucket compared to global warming. According to the NASA GSFC website (in 2002), the imbalance due to greenhouses gases is 2.45 W/m2, which the Earth is absorbing and not radiating to space. Of the greenhouse gases, 1.56 W/m2 is due to CO2, 0.47 to methane and 0.14 to N2O. This equates to an energy absorption rate of 12 million GW.

SPS will reduce the problem of global warming, because it will replace the 12 million GW due to greenhouse gases, and the 2000 GW due to nuclear and fossil fuels, and replace it with a more tolerable 10 GW (worst case) of direct atmospheric absorption and 100 GW of waste heat at ground level.

Mercury contamination from burning coal

It is now official, seafood is becoming unsafe because of Mercury contamination from burning coal.

Oil and Natural gas will be running out in a few decades and the world will then rely essentially on coal for its primary source of energy.

Seafood contamination will get worse.

Nuclear fission

- military cost of securing supply chains against theft
- military cost of securing waste sites against theft
- cleanup cost of decommissioning power stations
- cost of meltdown - reduced life expectancy / healthcare costs
- cost of waste leakage - reduced life expectancy / healthcare costs
- cost of insuring and banking prohibitively high and primarially possible through government subsidy
- social cost of draconian global security regimes (big brother)

Terrestrial Solar power

The sun angle across PV arrays constantly changes, and is usually less than the 1,360 w/m^2 maximum. To maintain constant max power the PV array must have expensive and heavy steering equipment. Whereas a SPS rectenna does not need to be steered, and always gets maximum power.

Lack of 24 hour coverage (ignoring weather) means that terrestrial solar power systems need some means of supplying consumers during the night time. Night time load is usually less than daytime load, but it is not zero, far from it. In winter time especially, the working day extends substantially into dark time. Many industries and transportation systems need to operate on a 24/7 basis.

This means that one of two systems are needed, either

a) a global power grid to pass power from the daylit side to the night side, or

b) power storage systems.

Both solutions exceed the cost of the solar cells themselves and are conveniently ignored by most proponents of terrestrial solar power.

Solution a) also suffers from political problems, similar to the international wrangling going on about the Tengiz oil field pipeline. Long range transmission of power across the planet will also cause significant losses of power over transmission lines.

Pipelines and power lines are political hot potatoes, nobody wants a hostile neighbor to have the ability to cut off their power or their oil.

Another issue: Ironically, terrestrial solar power has a more severe impact on terrestrial ecosystems and land usage than SPS rectennas. Permanent shadowing of the soil from a solar panel kills the local flora and results in a dustbowl. But agriculture can continue unabated beneath an SPS rectenna and soil erosion is thus mitigated. SPS rectennas can be sited on prime agricultural land, terrestrial solar panels cannot.


After oil is gone, ethanol will compete with hydrogen as fuel for motor vehicles and aircraft.

However, it will not compete (on price) with coal or nuclear for grid electrical power. But neither will SPS.

The benefit of SPS is that it has about the lowest collateral damage cost, when compared with the environmental damage of coal and nuclear, including the cost of nuclear station decommissioning.

Though ethanol has a lower collateral cost than coal there are other concerns with ethanol. Does the world have enough agricultural capacity (or waste straw) to create the necessary quantities of ethanol? If not, what about the quantities of fertilizers required and their environmental impact (production and runoff)? And what about the waste products of ethanol production? New studies have also indicated that ethanol and biofuels lead to a worsening of global warming. The increasing commodities prices of corn and soybean have lead to increased virgin acreage being converted to corn, soybean and other food crops. (As other foods have gone up in price due to more acreage going to fuel crops and because feed grain prices have also risen for livestock.) Virgin land, whether forest or grasslands are far better for soaking up CO2 then farmland.

Interestingly, ethanol and SPS can coexist rather nicely. We can grow fields of cereal crops beneath the SPS rectennas, at least where the soil and climate allow it.

Other Renewables (wind, tidal, hydro, geothermal)

Other Renewables (e.g. wind, tidal, hydro, geothermal) only have the capacity to supply a tiny fraction of the global demand for energy. The limitation is geography, there simply are very few sites in the world where generating systems of these types can be built. Though theoretically wind does have some potential above ground level. By harnessing the wind at around 3,000 feet, essentially tethering a large wing to the ground, the geographic reach of wind grows dramatically, covering most of the United States. Ocean based windpower is one possibility, but that is dominated by the high cost of long distance power transmission, in which case SPS would be highly competitive.

Nuclear Fusion

For the past few decades humanity has been "ten years " away from achieving nuclear fusion breakthrough. At this time there is no credible timeline for when nuclear fusion power plants will come on line. So until then, nuclear fusion is not a credible competitor to solar power satellites. Nevertheless, many countries are pouring billions of dollars and euros annually into nuclear fusion research, but SPS R&D is receiving no funding at all, except via tiny discretionary accounts.


If you include the cost of maintaining a military presence in the middle east, and the cost of global warming, then the cost of oil would probably quadruple.

Nuclear power might appear cheaper than SPS at first sight, until you factor in the cost of disposing of nuclear waste, decommissioning the reactors and the insurance and banking rates for nuclear plants, then it suddenly becomes horrendously expensive and SPS becomes attractive. But so far, nuclear decommissioning costs have been ignored, so that is a problem that future generations will have to figure out how to pay for.

If society ever reaches the conclusion that fossil fuels and nuclear fuels are undesirable for the above reasons, then there is the remaining alternative of SPS for clean inexhaustible power on a global scale. When compared to space projects to date, SPS is very grand and ambitious, and much bigger than anything ever attempted in space before. But when compared to the activities of the energy industry, it is in the same ball park. If we start thinking of SPS as an ENERGY project, instead of a SPACE project, then it starts to become a lot more feasible. SPS is really no more expensive than the energy projects which are under way today.

Energy is big business, it involves big money. Much bigger than the space program. NASA's budget is a tiny insect when compared to the oil empires of today. And SPS has so many advantages over oil.

But as long as society is willing to continue subsidizing fossil fuels and nuclear systems, then SPS is not an option.

U.S. Budgetary Footnote

Notice how tiny the US Dept of Energy research budget is.

Out of a total 2003 budget of about $23 billion, less than half was for energy research. This is less than the NASA budget.

Yet in the same year, the US spent over $100 billion on military activities to defend sources of oil.

Current Developments

In 2007 the US Department of Defense expressed interest in studying the concept[1].

On 10/10/2007 The National Security Space Office of the US Department of Defense, published an assessment report [2]. The report was released at a press conference which simultaneously announced the formation of the Space Solar Alliance for Future Energy which intends to pursue the recommendations of the NSSO-Led Study.


  1. Pentagon Considering Study on Space-Based Solar Power Thursday, April 12, 2007, By Jeremy Singer
  2. SpaceBased Solar Power As an Opportunity for Strategic Security - Phase 0 Architecture Feasibility Study - Report to the Director, National Security Space Office - Interim Assessment, Release 0.1, 10 October 2007

See Also

Solar Power

NASA TM-2004-212743 - "Reinventing the Solar Power Satellite" and "Peak Power Markets for Satellite Solar Power" from the Houston IAF Congress (International Astronautical Federation).

Author: Geoffrey A. Landis

Basic Principles of Beamed Microwave Power [ ]

Center for Space Power · A NASA Resarch Partnership Center