|Fig. 1: An illustration of the SPS-ALPHA (Solar Power Satellite via Arbitrarily Large Phased Array) [ (Courtesy of J. C. Mankins)|
Reason, a science fiction short story published in April 1941, depicts a future world where robot-manned space stations collect energy from the sun and feed them to the Earth as well as other planets using energy beams.  Although the notion of harvesting energy from the sun using large satellite arrays in space sounded far-fetched back then, today that may not be very far from reality. A concept first formally proposed by Peter Glaser in 1968, space-based solar represents a promising method of energy generation with the potential to meet all of Earth's energy needs without causing any adverse environmental impacts.
Space based solar energy essential has three elements. First, satellites in geosynchronous Earth orbit (GEO) about 22,000 miles above the Earth's surface use giant mirrors to reflect the sun's rays onto solar collectors to generate electricity. Next, the electric current is converted to an electromagnetic radiation that is then wirelessly transmitted from the satellite to the Earth. Finally, the radiation is collected by large receivers on Earth and converted back to electricity for use. An illustration of a NASA concept for such a space-based solar satellite, SPS-ALPHA (Solar Power Satellite via Arbitrarily Large Phased Array), is shown in Fig. 1.
Similar to regular solar energy generation on Earth, space-based solar power does not emit any greenhouse gases unlike traditional oil, gas and coal power plants. It does not compete with farm land for food production like bio-fuels, nor does it cause environmental destruction like hydroelectric power or carry health risks like nuclear power.
What makes space-based solar power more promising than solar farms on Earth is that it is not subjected to irregular solar irradiation due to fluctuating weather conditions and the cycle of day and night, and thus can receive light form the sun nearly all the time. In addition, as the light from the sun passes through the Earth's atmosphere, a significant fraction of it reflected back into space. A space-based solar farm will not experience such problems, and as such can deliver up to 40 times as much energy each year as its counterpart on Earth.  The power beam of the radiowaves can also be sent to different locations on the Earth through a method called "retrodirective beam steering", thus allowing energy to be delivered to various places according to need. 
The main obstacle to SBSP is the high costs involved in, particularly with getting the infrastructure into space. According to a report by Discover magazine, it costs about $4600/kg to launch things into low orbit, making SBSP economically uncompetitive with alternative forms of renewable energy unless the costs can be reduced ten-fold.  Another problem is transmitting the energy to Earth in an efficient manner. Microwaves are currently believed to be the method of choice as it is a well-established technology can be transmitted through the atmosphere without much loss and can provide gigawatts of power, however producing a focused beam and directing it from the satellite to receivers on Earth remains a challenge. 
In recent times, companies and countries around the world have been leading efforts to lower costs. Elon Musk's commercial space company SpaceX has been working on developing rockets that can be reused, a move that is projected to reduce the costs of launches by a hundred times.  In the UK, there has been the proposal of a new design named the Highly Elliptical Solar Power Satellite (HESPeruS) which purportedly has the potential to achieve cost parity with nuclear power.  HESPeruS uses a highly elliptical 12-hour orbit 40,000 km over the northern hemisphere that allows it to cut costs by half compared to using a geostationary orbit while being capable of generating an average power density of 58 W per square meter, which is six times better than a terrestrial solar farm.  Meanwhile the Japanese Aerospace Exploration Agency (JEAE) is also working on several models for solar-collecting satellites. At the same time, China has plans to launch a space station around 2020, which could lead to an experimental space-based solar power station by 2030, followed by a commercial version by 2050. 
Space-based solar power is undoubtedly a challenging venture both technologically as well as economically, however if it is successful the potential payoffs are massive. With more and more resources being devoted towards its development, the future for space-based solar power is bright.
© Chor Seng Tan. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 I. Asimov, "Reason" in I Robot (Doubleday, 1963).
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 J. Mankins, "SPS-ALPHA: The First Practical Solar Power Satellite via Arbitrarily Large Phased Array," Artemis Innovation Management Solutions LLC, September 2012.