In order to get a feel for why using solar energy delivered from space is a good idea it helps if you use a little imagination and envisage where it will lead - the world humans will live in when SPS is a major energy source. Then it's easier to understand why it would obviously be a good direction for technological development to aim at today - instead of being almost completely ignored!

As said elsewhere, a simple way to think of the energy problem (or at least a large part of it - electricity) is that we have roughly 1 kW of electricity generation capacity per head in the rich countries. So it's easy to calculate what happens if everyone in the world plans to reach this level. Electricity demand is still growing, even in the richest countries, despite their efforts to save energy. But the poorer countries are industrializing fast, and the world population is also due to double over the next 50-100 years to about 10,000 million people. So total electricity capacity will grow to around 10,000 Gigawatts - about 10 times today's level. (In addition, of course, today's electricity plants only last a few decades, so the existing 1000 GW of capacity will all have to be replaced too.)

So in round figures this means that we'll have to build about 100 GW of CLEAN electricity plant per year on average through the next century. This is THE major challenge facing humans over the next century, since we don't yet know how to provide such vast amounts of electricity without destroying the environment. So all promising ideas need to be investigated.

Solar energy is certain to provide a growing portion of this energy. Despite minimal government funding to date - in total, perhaps 1/1000 of the funding given to nuclear power - the use of solar energy in many different forms continues to spread. Among others, the production and use of photo-voltaics (solar cells) also continues to spread as the technology improves year by year - faster than official energy departments ever predict(!) But of course solar energy is diffuse, intermittent, and unreliable in many parts of the world.

So back in 1968 the American engineer, Peter Glaser, explained the potential advantages of an additional way of using solar energy - collecting it in space and transmitting it to Earth as microwaves: it's more intense, it shines 24 hours a day, and could be delivered more or less anywhere - even when cloudy. So in the 1970s (yes, THAT long ago) the US Department of Energy ( DOE) spent about $20 million on some studies of " SPS" (short for Solar Power Satellites, Satellite Solar Power Stations and various other phrases). They designed a "Reference System" 5 km-by-10 km rectangular satellite using solar cells to generate DC, and 2.45 GHz microwave beams to transmit it to Earth.

The US DOE concluded that SPS would indeed be feasible - after all it's only what satellites do today, though on a scale a million times larger; it would be environmentally clean - converting microwaves to DC is highly efficient; but they calculated that it was much too expensive. So almost all work in the USA and Europe stopped. NASA even gave away their whole SPS archive! That was a mistake! It was like asking the Wright brothers to design an aeroplane to carry 300 passengers across the Atlantic - and then giving up aviation research because their design looks too expensive to compete with ships!

In 1910 the Wright brothers couldn't design a Boeing 747. Likewise, today we can't know what power-satellites will look like decades in the future. Particularly at a time when, as we all know, engineering capabilities are advancing faster than ever before. However, we do know a number of things for certain:

1. Photovoltaic cells are getting cheaper and more efficient all the time - they're part of the semi-conductor industry, the fastest-moving field of engineering. And just as the use of solar cells is growing continually on Earth, despite the huge subsidies given to other forms of energy, so the prospects of using solar cells in space for energy supply to Earth are getting better literally every day.

2. Though we don't know what SPS units will look like 50 years from now, we do know how big they'll be. It's easy to calculate: 1 sqkm (square kilometer - a million square meters) in orbit receives about 1 GW continuously (it's nearer 1.4 GW actually). So with solar cell efficiencies of 20% (more than today, but a very conservative estimate for "multi-bandgap cells" decades hence), and with efficiency of transmission to Earth of %50 (achievable today), we need 10 sqkm to generate 1 GW on Earth. So building 100 GW per year would need 1000 sqkm of solar cells. Supplying even 10% of that power from space will create mouth-watering business for the macro-electronics industry!

3. Environmentally, geo-politically and macro-economically SPS would be a very satisfactory energy source. The US DOE studied the potential environmental impacts of SPS in considerable detail, and found that it's very benign. In the intensities and quantity concerned, microwave beams would have no adverse impact on the environment (though continuing research is under way in Japan and elsewhere). And in particular SPS would produce far less CO2 than all fossil sources.

   The macro-economic pattern of evolution of the world economy is another (long) story, but in simple terms, unless the rich countries can continue to make things or provide services that poorer countries can't yet do, they can't maintain a higher standard of living. Once everyone's reached the current level of the rich countries, that's no problem - but there's a long way to go before then, and "old industries" are leaking to the rapidly developing countries FAST, bringing a real possibility of falling standards of living in the rich countries - possibly expressed in some countries as a growing unemployed population... As it happens, one industry in which the richer countries still have a considerable lead is aerospace - but apart from civil airliners, the only thing that makes serious money is weapons. (Space business is tiny, remember!) The rich countries are keen to export weapons, of course! (Especially the members of the UN "Security Council"!) But that's hardly a recipe for peaceful development of the world economy! It has a quite limited prospect, in fact!!

   So, using aerospace technology to "export" environmentally benign energy to the developing countries would be an extremely appropriate use of existing skills in such a way as to balance international trade while more and more basic manufacturing businesses move abroad. It just requires the will - and particularly the will to make low-cost launch systems.

4. In addition to these 3 reasons, there is a 4th reason why developing power from space will be beneficial. This is because building and operating SPS units in Earth orbit will lead on spontaneously, through purely commercial evolution, to a range of further developments in space - to our space future, in fact. It's easy to see this by considering the ways in which business typically grows - which is much easier to predict than political decisions.

Selling electricity from space to Earth will provide commercial companies with both the finance and the incentive to develop and use extra-terrestrial materials. The world electricity market is already the best part of $1 trillion per year, and due to grow by 10 times. So, without predicting the details, it's easy to see that if an electricity-from-space industry develops there will be a market in Earth orbit for hundreds of thousands of tons a year of a range of materials including aluminum, glass, silicon (for solar cells), oxygen, iron, titanium etc - all of which are of course available in Moon rock and other extra-terrestrial resources.

It's easy to see that the price of these materials in the orbital markets will be of the order of $100,000 per ton (based on the target of $100 / kg or so for reusable launch vehicles) - almost 100 times higher than on Earth. So companies are going to work out that if they could deliver these materials from the Moon (or from asteroids and comets) to Earth orbit at less than this price, they'll have ready customers. So they'll be able to raise finance to develop Moon mines, processing equipment,linear-motor launchers and so on, and will repay it by selling the materials they produce to the orbital power-satellite manufacturing companies. For just as soon as it becomes cheaper than buying materials launched from Earth (taking into account all the details of quality, types of alloy, etc), orbital SPS fabricators will buy these extra-terrestrial materials. And they'll pay for them out of the growing stream of commercial revenues that they earn from selling $billions per year of electricity into the insatiable energy markets of Earth.

Furthermore, not a penny would be needed from the taxpayer; not a politician need be lobbied or paid off. It'll happen in the ordinary course of competitive business evolution, just as soon as people can make a profit by it. And once that happens of course, the door to our space future will really be open - forever. And that's why an SPS pilot plant project like SPS 2000 is so important - to get started!

People sometimes say "But geo-stationary orbit will get crowded" or "We should build solar power systems on the Moon's surface" or some other pet project. Right, right - there are loads of different possibilities - and may the best ones win. But we can't now predict accurately which is going to be the best investment. Once we can just get started, so some companies are actually earning serious, genuinely commercial money in space, different companies will invest in different projects and fight it out commercially - to the advantage of the customers. And the scale of such business makes all current space activities trivial by comparison.

Which brings us back to one of the main themes of "Space Future" - of course in order for SPSs to be able to sell electricity at competitive prices, SPS needs re-usable, low-cost launch vehicles - and so work on them is an essential part of any plan to develop SPS. But herein also lies one of the problems for SPS. Transport is a totally different business from electricity supply. And electricity companies are not going to go into space transportation. And they can't be expected to understand the potential for reducing the cost of space transportation; they're outsiders to whom space transportation costs are clearly about 100 times too high. So the launch cost problem has to be cracked by other groups - and then the electricity companies will get interested in SPS.

Overall, there is no rational justification for not funding SPS research seriously. The reason why it is not being funded is the institutional hang-over from the cold war - excessive concentration of energy funding on the institutionally entrenched nuclear power industry, and domination of the launch industry by government organizations which have preserved their high-cost launch systems - thereby making it difficult for outsiders to understand the great potential for reducing launch costs. Indeed, some electricity companies even think that talk of SPS is crazy - because it's easy to calculate that at today's launch costs it's out of the question. Which brings us back to the central importance of developing suitable Vehicles; and of Space Tourism to help pay for their development.