Space-based solar power (SBSP) is the concept of collecting solar power in space for use on Earth. It has been in research since the early 1970s.

SBSP would differ from current solar collection methods in that the means used to collect energy would reside on an orbiting satellite instead of on Earth's surface. Some projected benefits of such a system are:

  • Higher collection rate: In space, transmission of solar energy is unaffected by the filtering effects of atmospheric gasses. Consequently, collection in orbit is approximately 144% of the maximum attainable on Earth's surface.
  • Longer collection period: Orbiting satellites can be exposed to a consistently high degree of solar radiation, generally for 24 hours per day, whereas surface panels can collect for 12 hours per day at most.[1]
  • Elimination of weather concerns, since the collecting satellite would reside well outside of any atmospheric gasses, cloud cover, wind, and other weather events.
  • Elimination of plant and wildlife interference.
  • Redirectable power transmission: A collecting satellite could possibly direct power on demand to different surface locations based on geographical baseload or peak load power needs.

SBSP also introduces several new hurdles, primarily the problem of transmitting energy from orbit to Earth's surface for use. Since wiresextending from Earth's surface to an orbiting satellite are neither practical nor feasible with current technology, SBSP designs generally include the use of some manner of wireless power transmission. The collecting satellite would convert solar energy into electrical energy on-board, powering a microwave transmitter or laser emitter, and focus its beam toward a collector (rectenna) on the Earth's surface. Radiation andmicrometeoroid damage could also become concerns for SBSP.





The SBSP concept, originally known as Satellite Solar Power System (SSPS), was first described in November 1968.[2] In 1973 Peter Glaser was granted U.S. patent number 3,781,647 for his method of transmitting power over long distances (e.g., from an SPS to Earth's surface) using microwaves from a very large antenna (up to one square kilometer) on the satellite to a much larger one, now known as a rectenna, on the ground.[3]

Glaser then was a vice president at Arthur D. Little, Inc. NASA signed a contract with ADL to lead four other companies in a broader study in 1974. They found that, while the concept had several major problems—chiefly the expense of putting the required materials in orbit and the lack of experience on projects of this scale in space, it showed enough promise to merit further investigation and research.[4]

Between 1978 and 1981, the Congress authorized the Department of Energy and NASA to jointly investigate the concept. They organized the Satellite Power System Concept Development and Evaluation Program.[5][6] The study remains the most extensive performed to date (budget 50 millions $).[7] Several reports were published investigating the engineering feasibility of such an engineering project. They include:

  • Resource Requirements (Critical Materials, Energy, and Land)[8]
Artist's concept of Solar Power Satellite in place. Shown is the assembly of a microwave transmission antenna. The solar power satellite was to be located in a geosynchronous orbit, 36,000 miles above the Earth's surface. NASA 1976
  • Financial/Management Scenarios[9][10]
  • Public Acceptance[11]
  • State and Local Regulations as Applied to Satellite Power System Microwave Receiving Antenna Facilities[12]
  • Student Participation[13]
  • Potential of Laser for SBSP Power Transmission[14]
  • International Agreements[15][16]
  • Centralization/Decentralization[17]
  • Mapping of Exclusion Areas For Rectenna Sites[18]
  • Economic and Demographic Issues Related to Deployment[19]
  • Some Questions and Answers[20]
  • Meteorological Effects on Laser Beam Propagation and Direct Solar Pumped Lasers[21]
  • Public Outreach Experiment[22]
  • Power Transmission and Reception Technical Summary and Assessment [23]
  • Space Transportation[24]

The project was not continued with the change in administrations after the 1980 US Federal elections.

The Office of Technology Assessment[25] concluded

Too little is currently known about the technical, economic, and environmental aspects of SPS to make a sound decision whether to proceed with its development and deployment. In addition, without further research an SPS demonstration or systems-engineering verification program would be a high-risk venture.

In 1997 NASA conducted its "Fresh Look" study to examine the modern state of SBSP feasibility.[26] In assessing "What has changed" since the DOE study, NASA asserted that:

US National Space Policy now calls for NASA to make significant investments in technology (not a particular vehicle) to drive the costs of ETO [Earth to Orbit]transportation down dramatically. This is, of course, an absolute requirement of space solar power.

Conversely, Dr. Pete Worden claimed that space-based solar is about five orders of magnitude more expensive than solar power from the Arizona desert, with a major cost being the transportation of materials to orbit. Dr. Worden referred to possible solutions as speculative, and that would not be available for decades at the earliest.[27]

SERT sandwich concept.NASA


In 1999, NASA's Space Solar Power Exploratory Research and Technology program (SERT) (budget 22 millions $)[7] was initiated for the following purposes:

  • Perform design studies of selected flight demonstration concepts.
  • Evaluate studies of the general feasibility, design, and requirements.
  • Create conceptual designs of subsystems that make use of advanced SSP technologies to benefit future space or terrestrial applications.
  • Formulate a preliminary plan of action for the U.S. (working with international partners) to undertake an aggressive technology initiative.
  • Construct technology development and demonstration roadmaps for critical Space Solar Power (SSP) elements.

SERT went about developing a solar power satellite (SPS) concept for a future gigawatt space power system, to provide electrical power by converting the Sun’s energy and beaming it to Earth's surface, and provided a conceptual development path that would utilize current technologies. SERT proposed an inflatable photovoltaic gossamer structure with concentrator lenses or solar heat engines to convert sunlightinto electricity. The program looked both at systems in sun-synchronous orbit and geosynchronous orbit.

Some of SERT's conclusions:

  • The increasing global energy demand is likely to continue for many decades resulting in new power plants of all sizes being built.
  • The environmental impact of those plants and their impact on world energy supplies and geopolitical relationships can be problematic.
  • Renewable energy is a compelling approach, both philosophically and in engineering terms.
  • Many renewable energy sources are limited in their ability to affordably provide the base load power required for global industrial development and prosperity, because of inherent land and water requirements.
  • Based on their Concept Definition Study, space solar power concepts may be ready to reenter the discussion.
  • Solar power satellites should no longer be envisioned as requiring unimaginably large initial investments in fixed infrastructure before the emplacement of productive power plants can begin.
  • Space solar power systems appear to possess many significant environmental advantages when compared to alternative approaches.
  • The economic viability of space solar power systems depends on many factors and the successful development of various new technologies (not least of which is the availability of much lower cost access to space than has been available), however, the same can be said of many other advanced power technologies options.
  • Space solar power may well emerge as a serious candidate among the options for meeting the energy demands of the 21st century.[28]


The SBSP concept is attractive because space has several major advantages over the Earth's surface for the collection of solar power.

  • There is no air in space, so the collecting surfaces could receive much more intense sunlight, unobstructed by weather.
  • A satellite could be illuminated over 99% of the time, and be in Earth's shadow on only 75 minutes per night at the spring and fall equinoxes.[29]
  • Relatively quick redirecting of power directly to areas that need it most.
  • Higher collection rate: In space, transmission of solar energy is unaffected by the filtering effects of atmospheric gasses. Consequently, collection in orbit is approximately 144% of the maximum attainable on Earth's surface.
  • Longer collection period: Orbiting satellites can be exposed to a consistently high degree of solar radiation, generally for 24 hours per day, whereas surface panels can collect for 12 hours per day at most.[1]
  • Elimination of weather concerns, since the collecting satellite would reside well outside of any atmospheric gasses, cloud cover, wind, and other weather events.
  • Elimination of plant and wildlife interference.
  • Redirectable power transmission: A collecting satellite could possibly direct power on demand to different surface locations based on geographical baseload or peak load power needs.


The SBSP concept also has a number of problems.

  • The space environment is hostile; panels suffer about 10 times the degradation they would on Earth.[30] System lifetimes on the order of a decade would be expected, which makes it difficult to produce enough power to be economical.
  • Space debris are a major hazard to large objects in space, and SBSP systems have been singled out as a particularly hazardous activity.[31]
  • The broadcast frequency of the microwave downlink (if used) would require isolating the SBSP systems away from other satellites. GEO space is already well used and it is considered unlikely the ITU would allow an SPS to be launched.[32]
  • Only about half the power generated by the SSP would be delivered to the grid, once all losses are factored in. These losses are on the same order as modern fossil fuel plants.


Artist's concept of a solar disk on top of a LEO to GEO electricaly-powered space tug.

Space-based solar power essentially consists of three elements:

  • a means of collecting solar power in space, for example via solar cells or a heat engine
  • a means of transmitting power to earth, for example via microwave or laser
  • a means of receiving power on earth, for example via a microwave antenna (rectenna)

The space-based portion will not need to support itself against gravity (other than relatively weak tidal stresses). It needs no protection from terrestrial wind or weather, but will have to cope with space hazards such as micrometeors and solar flares.

Two basic methods of conversion have been studied: photovoltaic (PV) and solar dynamic (SD). Most analyses of SBSP have focused on photovoltaic conversion (commonly known as “solar cells”). Photovoltaic conversion uses semiconductor cells to directly convert photons into electrical power.

Wireless power transmission was proposed early on as a means to transfer energy from collection to the Earth's surface, using either microwave or laser radiation at a variety of frequencies.

[edit]Microwave power transmission

William C. Brown demonstrated in 1964, during Walter Cronkite's CBS News program, a microwave-powered model helicopter that received all the power it needed for flight from a microwave beam. Between 1969 and 1975, Bill Brown was technical director of a JPL Raytheon program that beamed 30 kW of power over a distance of 1-mile (1.6 km) at 84% efficiency.[33]

Microwave power transmission of tens of kilowatts has been well proven by existing tests at Goldstone in California (1975) [33][34][35] and Grand Bassin on Reunion Island (1997).[36]

Comparison of laser and microwave power transmission.NASA diagram

More recently, microwave power transmission has been demonstrated, in conjunction with solar energy capture, between a mountain top in Maui and the main island of Hawaii (92 miles away), by a team under John C. Mankins.[37][38] Technological challenges in terms of array layout, single radiation element design, and overall efficiency, as well as the associated theoretical limits are presently a subject of research, as it is demonstrated by the upcoming Special Session on "Analysis of Electromagnetic Wireless Systems for Solar Power Transmission" to be held in the 2010 IEEE Symposium on Antennas and Propagation.[39]

[edit]Laser power beaming

Laser power beaming was envisioned by some at NASA as a stepping stone to further industrialization of space. In the 1980s, researchers at NASA worked on the potential use of lasers for space-to-space power beaming, focusing primarily on the development of a solar-powered laser. In 1989 it was suggested that power could also be usefully beamed by laser from Earth to space. In 1991 the SELENE project (SpacE Laser ENErgy) had begun, which included the study of laser power beaming for supplying power to a lunar base. The SELENE program was a two-year research effort, but the cost of taking the concept to operational status was too high, and the official project ended in 1993 before reaching a space-based demonstration.[40]

In 1988 the use of an Earth-based laser to power an electric thruster for space propulsion was proposed by Grant Logan, with technical details worked out in 1989. He proposed using diamond solar cells operating at 600 degrees to convert ultraviolet laser light, a technology that has yet to be demonstrated even in the laboratory.

[edit]Orbital location

The main advantage of locating a space power station in geostationary orbit is that the antenna geometry stays constant, and so keeping the antennas lined up is simpler. Another advantage is that nearly continuous power transmission is immediately available as soon as the first space power station is placed in orbit; other space-based power stations have much longer start-up times before they are producing nearly continuous power.

A collection of LEO (Low Earth Orbit) space power stations has been proposed as a precursor to GEO (Geostationary Orbit) space-based solar power.[41]

[edit]Earth-based receiver

The Earth-based rectenna would likely consist of many short dipole antennas connected via diodes. Microwaves broadcasts from the satellite would be received in the dipoles with about 85% efficiency.[42] With a conventional microwave antenna, the reception efficiency is better, but its cost and complexity is also considerably greater. Rectennas would likely be multiple kilometers across.

[edit]Dealing with launch costs

One problem for the SBSP concept is the cost of space launches and the amount of material that would need to be launched.

Reusable launch systems are predicted to provide lower launch costs to low Earth orbit (LEO).[43][44]

Much of the material launched need not be delivered to its eventual orbit immediately, which raises the possibility that high efficiency (but slower) engines could move SPS material from LEO to GEO at an acceptable cost. Examples include ion thrusters or nuclear propulsion.

Power beaming from geostationary orbit by microwaves carries the difficulty that the required 'optical aperture' sizes are very large. For example, the 1978 NASA SPS study required a 1-km diameter transmitting antenna, and a 10 km diameter receiving rectenna, for a microwave beam at 2.45 GHz. These sizes can be somewhat decreased by using shorter wavelengths, although they have increased atmospheric absorption and even potential beam blockage by rain or water droplets. Because of the thinned array curse, it is not possible to make a narrower beam by combining the beams of several smaller satellites. The large size of the transmitting and receiving antennas means that the minimum practical power level for an SPS will necessarily be high; small SPS systems will be possible, but uneconomic.

To give an idea of the scale of the problem, assuming a solar panel mass of 20 kg per kilowatt (without considering the mass of the supporting structure, antenna, or any significant mass reduction of any focusing mirrors) a 4 GW power station would weigh about 80,000 metric tons, all of which would, in current circumstances, be launched from the Earth. Very lightweight designs could likely achieve 1 kg/kW,[45] meaning 4,000 metric tons for the solar panels for the same 4 GW capacity station. This would be the equivalent of between 40 and 150 heavy-lift launch vehicle (HLLV) launches to send the material to low earth orbit, where it would likely be converted into subassembly solar arrays, which then could use high-efficiency ion-engine style rockets to (slowly) reach GEO (Geostationary orbit). With an estimated serial launch cost for shuttle-based HLLVs of $500 million to $800 million, and launch costs for alternative HLLVs at $78 million, total launch costs would range between $11 billion (low cost HLLV, low weight panels) and $320 billion ('expensive' HLLV, heavier panels).[citation needed]For comparison, the direct cost of a new coal [1] or nuclear power plant ranges from $3 billion to $6 billion dollars per GW (not including the full cost to the environment from CO2 emissions or storage of spent nuclear fuel, respectively); another example is the Apollo missions to the Moon cost a grand total of $24 billion (1970's dollars), taking inflation into account, would cost $140 billion today, more expensive than the construction of the International Space Station.

[edit]Non-conventional launch methods

SBSP costs might be reduced if a means of putting the materials into orbit were developed that did not rely on rockets. Some possible technologies include ground launch systems such as mass drivers or Lofstrom loops, which would launch using electrical power, or the geosynchronous orbit space elevator. However, these require technology that is yet to be developed.John Hunter of Quicklaunch is working on commercialising the 'Hydrogen Gun', a new form of mass driver which proposes to deliver unmanned payloads to orbit for around 5% of regular launch costs (or $500 per pound; US$1,000 per kilogram) and perform 5 launches per day.[46]

[edit]Building from space

[edit]From lunar materials launched in orbit

Gerard O'Neill, noting the problem of high launch costs in the early 1970s, proposed building the SPS's in orbit with materials from the Moon.[47] Launch costs from the Moon are potentially much lower than from Earth, due to the lower gravity. This 1970s proposal assumed the then-advertised future launch costing of NASA's space shuttle. This approach would require substantial up front capital investment to establish mass drivers on the Moon.[48]

Nevertheless, 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 Earth-based materials for a system of as few as thirty Solar Power Satellites of 10GW capacity each.[49]

In 1980, when it became obvious NASA's launch cost estimates for the space shuttle were grossly optimistic, O'Neill et al. published another route to manufacturing using lunar materials with much lower startup costs.[50] This 1980s SPS concept relied less on human presence in space and more on partially self-replicating systems on the lunar surface under remote control of workers stationed on Earth. The high Net energy gain of this proposal derives from the Moon's much shallower gravitational well.

Having a relatively cheap per pound source of raw materials from space would lessen the concern for low mass designs and result in a different sort of SPS being built. The low cost per pound of lunar materials in O'neill's vision would be supported by using lunar material to manufacture more facilities in orbit than just solar power satellites.

Advanced techniques for launching from the Moon may reduce the cost of building a solar power satellite from lunar materials. Some proposed techniques include the lunar mass driver and the lunar space elevator, first described by Jerome Pearson.[51] It would require establishing silicon mining and solar cell manufacturing facilities on the Moon.[citation needed]

[edit]On the Moon

David Criswell suggests the Moon is the optimum location for solar power stations, and promotes lunar solar power.[52][53] The main advantage he envisions is construction largely from locally available lunar materials, using in-situ resource utilization, with a teleoperated mobile factory, a crane to assemble the microwave reflectors, and rovers to assemble solar cells, which would significantly reduce launch costs compared to SBSP designs.Power relay satellites orbiting around earth and the Moon reflecting the microwave beam are also part of the project.[54] Another design fusion the rovers with the factory and directly pave the Moon with a thin film solar cell.[55]The Shimizu Corporation proposed using combination of lasers and microwave for the lunar ring concept, along with power relay satellites.[56][57]

[edit]From an asteroid

Asteroid mining has also been seriously considered. A NASA design study[58] evaluated a 10,000 ton mining vehicle (to be assembled in orbit) that would return a 500,000 ton asteroid fragment to geostationary orbit. Only about 3,000 tons of the mining ship would be traditional aerospace-grade payload. The rest would be reaction mass for the mass-driver engine, which could be arranged to be the spent rocket stages used to launch the payload. Assuming that 100% of the returned asteroid was useful, and that the asteroid miner itself couldn't be reused, that represents nearly a 95% reduction in launch costs. However, the true merits of such a method would depend on a thorough mineral survey of the candidate asteroids; thus far, we have only estimates of their composition.[59]


[edit]Counter arguments


The use of microwave transmission of power has been the most controversial issue in considering any SPS design.

At the Earth's surface, a suggested microwave beam would have a maximum intensity at its center, of 23 mW/cm2 (less than 1/4 the solar irradiation constant), and an intensity of less than 1 mW/cm2 outside of the rectenna fenceline (the receiver's perimeter).[60] These compare with current United States Occupational Safety and Health Act (OSHA) workplace exposure limits for microwaves, which are 10 mW/cm2,[61] - the limit itself being expressed in voluntary terms and ruled unenforceable for Federal OSHA enforcement purposes.[citation needed] A beam of this intensity is therefore at its center, of a similar magnitude to current safe workplace levels, even for long term or indefinite exposure. Outside the receiver, it is far less than the OSHA long-term levels[62] Over 95% of the beam energy will fall on the rectenna. The remaining microwave energy will be absorbed and dispersed well within standards currently imposed upon microwave emissions around the world.[63] It is important for system efficiency that as much of the microwave radiation as possible be focused on the rectenna. Outside of the rectenna, microwave intensities rapidly decrease, so nearby towns or other human activity should be completely unaffected.[64]

Exposure to the beam is able to be minimized in other ways. On the ground, physical access is controllable (e.g., via fencing), and typical aircraft flying through the beam provide passengers with a protective metal shell (i.e., a Faraday Cage), which will intercept the microwaves. Other aircraft (balloonsultralight, etc.) can avoid exposure by observing airflight control spaces, as is currently done for military and other controlled airspace.

The microwave beam intensity at ground level in the center of the beam would be designed and physically built into the system; simply, the transmitter would be too far away and too small to be able to increase the intensity to unsafe levels, even in principle.

In addition, a design constraint is that the microwave beam must not be so intense as to injure wildlife, particularly birds. Experiments with deliberate microwave irradiation at reasonable levels have failed to show negative effects even over multiple generations.[65]

Some have suggested locating rectennas offshore,[66][67] but this presents serious problems, including corrosion, mechanical stresses, and biological contamination.

A commonly proposed approach to ensuring fail-safe beam targeting is to use a retrodirective phased array antenna/rectenna. A "pilot" microwave beam emitted from the center of the rectenna on the ground establishes a phase front at the transmitting antenna. There, circuits in each of the antenna's subarrays compare the pilot beam's phase front with an internal clock phase to control the phase of the outgoing signal. This forces the transmitted beam to be centered precisely on the rectenna and to have a high degree of phase uniformity; if the pilot beam is lost for any reason (if the transmitting antenna is turned away from the rectenna, for example) the phase control value fails and the microwave power beam is automatically defocused.[64] Such a system would be physically incapable of focusing its power beam anywhere that did not have a pilot beam transmitter.

The long-term effects of beaming power through the ionosphere in the form of microwaves has yet to be studied, but nothing has been suggested which might lead to any significant effect.

[edit]Atmospheric damage due to launches

When hot rocket exhaust reacts with atmospheric nitrogen, it can form nitrogen compounds.[citation needed] These nitrogen compounds are problematic when they form in the stratosphere, as they can damage the ozone layer. However, the environmental effect of rocket launches is negligible compared to higher volume polluters, such as airplanes and automobiles.[citation needed]


  • 1968: Dr. Peter Glaser introduces the concept of a "solar power satellite" system with square miles of solar collectors in high geosynchronous orbit for collection and conversion of sun's energy into a microwave beam to transmit usable energy to large receiving antennas (rectennas) on Earth for distribution.
  • 1973: Dr. Peter Glaser is granted United States patent number 3,781,647 for his method of transmitting power over long distances using microwaves from a large (one square kilometer) antenna on the satellite to a much larger one on the ground, now known as a rectenna.[3]
  • 2000: John Mankins of NASA testifies in the U.S. House of Representatives, saying "Large-scale SSP is a very complex integrated system of systems that requires numerous significant advances in current technology and capabilities. A technology roadmap has been developed that lays out potential paths for achieving all needed advances — albeit over several decades.[7]
  • 2001: Dr. Neville Marzwell of NASA states, "We now have the technology to convert the sun's energy at the rate of 42 to 56 percent... We have made tremendous progress. ...If you can concentrate the sun's rays through the use of large mirrors or lenses you get more for your money because most of the cost is in the PV arrays... There is a risk element but you can reduce it... You can put these small receivers in the desert or in the mountains away from populated areas. ...We believe that in 15 to 25 years we can lower that cost to 7 to 10 cents per kilowatt hour. ...We offer an advantage. You don't need cables, pipes, gas or copper wires. We can send it to you like a cell phone call—where you want it and when you want it, in real time."[69]
  • 2001NASDA (Japan's national space agency) announces plans to perform additional research and prototyping by launching an experimental satellite with 10 kilowatts and 1 megawatt of power.[70][71]
  • 2010: Professors Andrea Massa and Giorgio Franceschetti announce a special session on the "Analysis of Electromagnetic Wireless Systems for Solar Power Transmission" at the 2010 Institute of Electrical and Electronics Engineers International Symposium on Antennas and Propagation.[86]

[edit]In fiction

  • Space stations transmitting solar power have appeared in science-fiction works like Isaac Asimov's Reason (1941), that centers around the troubles caused by the robots operating the station. Asimov's short story "The Last Question" also features the use of SBSP to provide limitless energy for use on Earth.
  • In the novel "Skyfall" (1976) by Harry Harrison an attempt to launch the core of powersat from Cape Canaveral ends in disaster when the launch vehicle fails trapping the payload in a decaying orbit.
  • Solar Power Satellites have also been seen in the work of author Ben Bova's novels "Powersat" and "Colony".
  • In Sid Meier's Alpha Centauri, an endgame 'building' that fulfills the same function as an SPS is the 'Orbital Power Transmitter' which provides every city that you own with a unit of energy per satellite launched, providing the city has an Aerospace Command building or your faction controls the space elevator. Building multiple Orbital Power Transmitters provides massive bonuses to energy generation and soon pay for themselves many times over.
  • In a 1981 storyline from the Iron Man comic book (issues #142-144), a rogue microwave transmission from a secret Solar Power Satellite is responsible for numerous deaths in Allentown, Iowa.
  • In the computer games SimCity 2000 and 3000, plants that implemented solar satellite technology called microwave powerplants were available in the future. One disaster scenario involved the beam missing the receiver and hitting the city's infrastructure. The plant was discontinued in SimCity 4 but several fan-made microwave powerplants were available on various SimCity 4 fan sites.
  • In the film Die Another Day, a satellite weapon is disguised as a solar power satellite.
  • In Mobile Suit Gundam 00, a solar power satellite array is constructed around the Earth and is used to harness solar energy for use. They play a critical plot role in the superpowers'power balance.
  • In After War Gundam X, a solar power station is built on the Moon, and is used to supply energy via microwave to various mobile suits, to energize their powerful "Satellite Cannons".
  • In Cadillacs and Dinosaurs (TV series), space solar power is one of the losts technologys of the ancients.


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