We don't yet know what satellite solar power stations will look like fifty years from now. But we do know how big they'll be: they'll be hundreds of square kilometers in area - otherwise they won't generate enough power to have a significant impact on humans' growing energy needs.

Consequently the design and construction of orbital structures kilometers in dimensions will be required for delivering power from space to Earth, as well as for other purposes such as large-scale orbiting resort hotels and sports stadiums, macro-electronics manufacturing plants, non-terrestrial materials processing plants and others (see publications of the American Society of Civil Engineers).

The design, construction and operation of large structures in space will create whole new fields of employment. In zero-G it's possible to build much larger structures than on Earth, and to manoeuver much larger and more massive objects. Riggers and robots will connect up hotel sections to make structures hundreds of meters long, and assemble 500 meter-long stadia from hundreds of separate panels delivered to orbit.

For SPS construction there will be even bigger things - tugs and cranes manoeuvring kilometer-sized structures, electrical engineering of Gigawatt cables, orbit-raising of power-satellite sections 10 square kilometers or more in area, orbital manufacturing plants using materials from comets and asteroids. And there's no limit to the innovative technological employment that will be created by these new and profitable commercial activities in space.

This will be just the same as the way that the amazing contemporary technologies of deep-sea oil-rigs that operate in stormy seas hundreds of meters deep, or wide-body jets that carry hundreds of passengers non-stop half-way round the world, have created many new career-paths, from diving to air-traffic control, from geological analysis to aircraft leasing, from underwater welding to international ticket sales.

And note that these jobs have all been created by two industries on Earth - energy and tourism - which represent $trillion segments of the mass consumer markets that power economic growth world-wide. These two industries can and will power the economic development of space, through the use of large-scale solar energy generating systems for electricity supply to Earth, and tourism activities in Earth orbit and beyond.

Some people think this is fantasy, because they're unable to imagine it. But the people who migrated to California last century had no idea that their descendants would become air traffic controllers and computer programmers, film-makers and radio DJs - how could they?

But once you stop thinking of space as a place where governments spend a lot of taxpayers' money to carry out "missions", and consider instead what many people will voluntarily pay to do there, tourism and energy supply quickly become very important. And with the sophistication of modern engineering and financial markets, a genuine space development boom is on the cards in the near future, once the first reusable launch systems get operating. Roll on, roll on!