That’s if you believe a recent report from the 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 2017-2020”.

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 and, eventually, a few square kilometres. 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, minimising the human cost of resource wars and catastrophies caused by global warming.

“The technology has been in development for a while,” says Joseph Rouge, associate director of the space office. “By 2050, it could deliver 10 pc of America’s power needs.” The space office sees energy supply as one of strategic importance as oil supplies dwindle. They notes that all remaining oil resources are estimated to contain 250 terawatt-years of energy; but that a one-kilometre wide band in geosynchronous orbit receives about 212 TW-years of energy each year.