NAPLES, Italy — Advocates of space solar power (SSP) continue to refine their ideas for harnessing the Sun’s energy, beaming it to Earth and plugging it into the power grid. Papers presented at the 63rd International Astronautical Congress in Naples, Italy, this month indicate some very good minds are at work on clearing the hurdles to SSP, with some interesting results.

“The major problem associated with [SSP] is to apply the technologies to the huge system at [gigawatt] level in power, [kilometer] level in size, and several ten [of] thousands of tons in weight,” writes Susumu Sasaki of Japan’s Institute of Science and Astronautical Science, in a technical paper presented in Naples. “Also it is [necessary] to make its power price be competitive with that of existing power generation systems on the ground.”

Reviewing the technical readiness level of SSP components, Sasaki reports that to begin deploying commercial SSP spacecraft in the 2030s, “large advances” in power transmission will be needed in the next 10 years, followed by significant advances in large space structures in the 10 years after that, and a final five-year push in space transportation to kick off service. Photovoltaic cells, for example, need to move from 15-30% conversion efficiency to 35-40% in the Japanese model, with specific weight dropping from 10-100 grams/Watt to 1 g/W and service life in space growing from 10 years to 30-40. Today’s cost of $4-6/Watt need to drop to $1-0.50.

To meet the transportation needs, Sasaki sees reusable launch vehicles as the ultimate solution for SSP, and he finds developments in the space tourism field encouraging.

“Another important trend in the space transportation system is the suborbital RLV for space tourism, such as SpaceShipTwo,” he writes. “The technology gap from the suborbital flight to the orbital flight is considered very large, but the suborbital flight technologies could lead to breakthrough in the orbital RLV.”

John Mankins, a longtime U.S. SSP advocate, presented an update on an advanced concept under study with NASA funding known as Solar Power Satellite by means of Arbitrarily Large Phased Array (SPS-Alpha). The idea, he writes, “represents a very different architecture for SPS, using a hyper-modular approach in which all platform elements can be mass produced, and none are larger than a ‘smallsat.’ This could enable significantly lower development time/cost, much greater ease of manufacturing at lower cost, and significantly higher reliability.”

Basically, mass-produced “intelligent” spacecraft weighing 100-300 kg would assemble themselves into a constellation shaped to collect solar energy, convert it and transmit it through the “hive” of other spacecraft to a transmitter array assembled in the same fashion. Mankins says the idea is based on the behavior of bees or ants.

“SPS-Alpha incorporates the concept of the retrodirective phased array, which allows a large number of individual RF elements to be controlled and their transmissions made coherent through the use of a ‘pilot signal’ transmitted from the site of the planned receiver. This technology (co-invented by professor Nobuyuki Kaya of Kobe University) allows the large microwave transmitter required for the concept to be assembled from modular elements via an RF version of adaptive optics.”

The SPS-Alpha study, conducted for the NASA Institute for Advanced Concepts, aims at establishing an analytical proof of concept (technology readiness level 3) for its “technical and economic viability,” and is directed toward a near-term road map for development like the one Sasaki provided for the more conventional approach he studied.