At some point before 2050, satellites collecting solar power and beaming it back to Earth will become a primary energy source, streaming tera-watts of electricity continuously from space. That's if you believe a recent report from the US Pentagon's National Security Space Office (NSSO), which says confidently that we will see "a basic proof-of-concept within 4-6 years and a substantial power demonstration as early as 2017" (tinyurl.com/2lq8gx).
It's obvious in some ways: above the atmosphere, a solar cell receives about 40 times more energy per year than an equivalent site on the ground, due to the absence of atmospheric scattering and seasonal or nightly reductions in light.
The NSSO suggests that an orbiting spacecraft with solar panel arrays would be comparable to current ground-based installations spanning hectares. Then that energy can be sent to the ground using, the Pentagon suggests, a giant laser or microwave beam.
The report, Space Based Solar Power as an Opportunity for Strategic Security, suggests optimistically that one application will be the beaming of "energy aid" via satellite into conflict and disaster zones, minimizing the human cost of resource wars and catastrophic events caused by global warming.
"The technology has been in development for a while," says Joseph Rouge, associate director of the space office. "The truly hard and expensive part is going to be getting it into orbit. We'll need regular launches and on-orbit robotic assembly system. It's a US$10 billion program, but by 2050 it could deliver 10 percent of America's power needs."
The space office considers energy supply strategically important as oil supplies dwindle; according to a report by Germany's Energy Watch Group published last week, "peak oil" output occurred last year, and will fall by 7 percent annually to half its present levels by 2030. The space office notes that all remaining oil resources are estimated to contain 250 tera-watt-years of energy; but that a 1km band in orbit receives about 212 tera-watt-years of energy each year.
The first units to go up will generate between 10 megawatts and 25 megawatts of continuous power, enough for a town of 25,000 people. If the energy is transmitted by microwave, a surface array one-tenth of a square kilometer will be needed to pick it up. Larger beams will require larger collector arrays on earth. But wouldn't a microwave beam from space be equivalent to a deadly weapon? Unlike photovoltaic cells, these antenna arrays are practically transparent, so crops could be planted under them.
"If a 2.45 gigaherz beam drifted off its target and ended up over a town, the effect would be negligible," says Lieutenant-Colonel Damphousse of the space office. "By the time the microwave reaches the surface it has spread out considerably. The power density is one-sixth that of the noonday sun."
The US Army could also use such a device to deliver electricity to its troops. Military units in forward areas pay US$1 per kilowatt-hour, six times the UK domestic price. They pay a lot more to bring in fuel. Lives could be saved by cutting long and vulnerable logistics chains - though it would require large collectors.
The beam is most powerful near its source, and although at 40,000km up it would not pose a risk to astronauts in the International Space Station, it could be turned against communication or observation satellites in orbit. "Space Traffic Control would make sure the satellite is not tampered with before launch," says Damphousse. "They would also ensure the spacecraft do not interfere with each other."