Microwave or laser: trading wireless power paths for space solar power

What the work claims

The paper asserts that wireless power transmission (WPT) is the enabling link for space solar power (SSP) and that two basic options, radio waves (microwaves) and light waves (lasers), each have credible but different system-level advantages.¹ For far-term Earth baseload from geostationary orbit (GEO), microwave WPT offers higher end-to-end efficiency and all-weather transmission, while laser-photovoltaic WPT offers smaller apertures, a dual-use terrestrial receiver, and a power profile that can better match daily electricity demand. For the near term, the authors propose a 100 kWe-class "Power Plug" (or LAMP) spacecraft and a 10 kWe-class lunar polar solar power outpost as flight demonstrations that would mature WPT before committing to gigawatt-class systems.

How it works

Both options convert collected sunlight into an electromagnetic beam and recover it as electricity at a receiver. The paper gives conceptual sizing for comparable 1.2 GWe Earth-delivered systems to make the trade concrete.

For the microwave case, the source describes a 5.8 GHz Industrial, Scientific and Medical (ISM) band transmitter: a 500 meter diameter phased array in GEO sends radio waves to a 7.5 kilometer diameter rectifying antenna (rectenna) on Earth across roughly 40,000 km.¹ A 15 kilometer long gravity-gradient-stabilized backbone carries 2.8 GWe from the solar arrays to the transmitter; after pointing, side-band, filtering, and power-conversion losses, the design delivers 1.2 GWe of electrical power on Earth. The efficiency boundary here is the assembled system from solar-array output to grid electricity, not a measured link.

For the laser-photovoltaic case, the same 1.2 GWe peak is delivered by a 55 kilometer long satellite (about half as wide as the microwave concept) carrying a series of small laser transmitters along its length.¹ Each beam is small and diverges quickly, but their combined energy fills a roughly 6 kilometer diameter dual-use photovoltaic receiver on Earth with about 500 W/m2 of monochromatic light. That intensity is roughly half the peak sunlight flux of about 1 kW/m2, but because the laser wavelength can be matched to the cell band-gap, receiver cell efficiency can be much higher (the source gives >40% for laser light versus about 20% for sunlight). The paper notes that laser-photovoltaic WPT is currently less efficient by about 50% at the system level, so a larger solar array is needed for the same delivered power.

The near-term concepts exploit these differences. The Power Plug/LAMP spacecraft would upgrade the International Space Station's Integrated Energy Assembly (IEA) arrays with state-of-the-art photovoltaic cells to roughly double their output, then add a WPT payload.¹ A derivative could go to Mars stationary orbit at about 17,038 km altitude and, because of the shorter distance, use laser WPT with a beam diameter on the order of meters to deliver an average of about 10 kWe to the surface. The lunar polar outpost would land a solar generation station on a sunlit mountaintop near the Moon's pole and beam power by laser to a robot operating in permanently shadowed craters.

Steelman: why the trade still matters

The strongest part of the paper is that it frames WPT as a system-level choice, not a physics contest. Microwave WPT builds on decades of radar and communications experience, can penetrate clouds and light rain, and offers higher projected end-to-end efficiency. If the goal is baseload power to Earth from GEO, those are first-order advantages.

The laser case is compelling for a different reason: it relaxes the aperture scaling that makes microwave systems so physically large. Smaller apertures mean smaller initial systems and a smoother commercial ramp. A dual-use receiver that also works as a terrestrial solar farm lowers the dedicated infrastructure cost. The paper also argues that gravity-gradient "Sun Tower" satellites pair better with laser systems because the terrestrial receiver can collect ambient sunlight as well as laser light, partially offsetting the cosine losses that reduce satellite output around local mid-day.

Where a skeptic should push

The most load-bearing assumption is that the conceptual numbers are internally consistent designs rather than measured results. The 1.2 GWe delivered, 500 meter transmitter, 7.5 kilometer rectenna, 55 kilometer satellite, and 6 kilometer receiver are all sizing estimates. None of them has been demonstrated at scale, and the efficiency chain for each option contains links that were assumed, not measured.

For microwave WPT, the source acknowledges that spectrum allocations and RF interference would have to be renegotiated, and public perception of microwave beams is a real risk. For laser WPT, the source explicitly notes that efficiency is "rapidly improving" but "it is not clear whether it will ever approach the efficiency of radio frequency transmission."¹ Atmospheric transmission, weather outages, and the need for multiple desert receiver sites are all flagged as open issues. The near-term Power Plug and lunar polar concepts are described as concepts building on the ISS IEA, not as approved or funded missions.

What it means for beamed power in space

Implication: The paper makes clear that the WPT choice is driven by the application boundary, not by a single efficiency figure. For space-to-space or space-to-surface links over thousands of kilometers, the laser option's smaller beam and aperture may be more practical than microwave, even at lower efficiency. The Mars stationary orbit example with a meter-scale beam is the non-obvious takeaway: beamed power does not have to start with Earth-to-GEO baseload to be useful.

Threat: The efficiency gap is real and unforgiving for power-limited spacecraft. If laser-photovoltaic WPT remains roughly half as efficient as microwave at the system level, the solar array and thermal rejection mass needed for the same delivered power grow in direct proportion. For a surface habitat where every kilogram counts, that mass penalty can erase the aperture advantage.

Opportunity: The dual-use terrestrial receiver concept, combined with rigid non-gimbaled arrays whose front and back sides both collect light, offers a way to align SSP with existing solar infrastructure rather than competing with it. If laser transmitter efficiency improves and receiver sites are diversified to handle weather, the laser path could lower the entry threshold for space-to-Earth power.

The bottom line

This 2002 preprint is a conceptual trade study, not a test report. Its value is in identifying the system-level drivers that still shape SSP architecture today: aperture size, atmospheric transmission, receiver dual use, spectrum regulation, and public perception. The microwave path is more mature and efficient on paper; the laser path offers smaller hardware and a more flexible deployment curve. The near-term demonstrators sketched in the paper have not flown, and the efficiency numbers are design estimates bounded to the authors' assumptions. A calibrated reading: the trade remains open, and the paper is best read as a durable checklist of what any serious SSP WPT system must reconcile.