A Project Plan for Space Based Solar Power

OVERVIEW OF A SPACE BASED SOLAR POWER PROJECT

In my post, Space Based Solar Power, I present an overview of SBSP and its value to humanity. I also propose the use of asteroids for raw materials to reduce the cost to profitable levels. Still, the construction of the first Solar Power Satellite (SPS) is special, as we must incur all of the up-front (bootstrap) costs in addition to building the SPS itself.  I will demonstrate that it still can be a profitable venture. However, this project plan targets the initial construction of a dozen SPS’s, because the slag by-product of that much steel is the correct mass of shielding needed for a Kalpana-One style habitat for the workers (see my post, designing a space habitat).

The approach I recommend is to capture an asteroid (see  Asteroid Capture into Earth Orbit) and use its resources to produce the steel and other materials needed to construct and operate a constellation of SPS’s.

This approach has a relatively high up-front investment and results in a relatively low per-SPS cost. The project has many phases:

1. Research & Development
2. Capture a suitable asteroid into Earth orbit
3. Launch Mining & Manufacturing Tools
4. Launch Construction Shacks & Workers
5. Construct & Deploy the SPS(s)
6. Construct the Kalpana-One style Habitat(s)
7. Repeat 5-6 as long as asteroid resources remain

For the purposes of this post, I make several assumptions:

• we have chosen the asteroid 99942 Apophis, which also locks us into a 2029 timeline (project start in 2027, launch capture mission in early 2028).
• use of SpaceX’s Falcon 9 Heavy launch vehicle (announced with pricing, but not yet built, yet alone flown).
• pricing in 2011 dollars which must be adjusted for inflation.
• Worker launch costs initially average $24M/person, but in a 2031 timeframe human-rated transports will be built for the Falcon 9 Heavy (or eventually Falcon X Heavy) that can transport 30 to 100 people per launch into LEO at a price of $1,000,000 per ton (also per person).
• I ignore in-space worker salaries (small compared to launch costs)
• For income, I only consider Solar Power Satellites, ignoring tourism, the sales/rentals of housing and retail space, sales of propellants to outside parties, product placement, tie-ins, naming rights, or any other potential revenue sources.

RESEARCH & DEVELOPMENT

It may be an unreal oversimplification, but I will ignore the R&D costs for this project although they are large and unknown. I believe that NASA, the various governments, and the network of universities should bear this cost as part of their normal operations (supported by tuitions, taxes, and donations), although the more directed research will be borne by the corporations that will profit from the sales of related equipment and supplies. Known areas of research & development include:

• Heavy lift launch vehicles
• Space taxies / trucks (using ion thrusters to cheaply change orbits and to capture the asteroid)
• Long-term life support (recycling & farming in space)
• Zero Gravity Mining, Smelting, Refining, and Manufacturing (note need to recycle reagents and to capture and utilize bi-products such as CO2).
• Manufacturing Solar Photovoltaic Panels from asteroid materials
• High-efficiency Muilti-Gigawatt Microwave Transmitters / Receivers
• Large-scale in-space construction techniques (of SPS and habitats) with limited resources

CAPTURE A SUITABLE ASTEROID

How to capture an asteroid is largely covered by my previous post, Asteroid Capture into Earth Orbit, and a future post will detail the components of the capture project, including a proposed project breakdown of the needed components, launches, delta-v budgets, and a project timeline.

The Asteroid Capture Project begins in 2027 with construction (in orbit) of a space tug. The tugship launch from LEO to intercept Apophis occurs in April of 2028, and the final Lunar slingshot of Apophis effecting capture into Earth orbit occurs 18 months later, in October of 2029. These dates are driven by orbital dynamics.

A simplified overview of the Apophis Capture Project (using current or near-term launch vehicles) is:

1. Launch tugship, fuel, solar panels to LEO: total 150 tons, needs 5 Falcon 9 Heavy launches ($500M); cost of tugship, etc. another $1.5B. This cost is lower than many space-based projects because of the extensive use of modular components assembled by humans in space(reducing the need for first-time perfection).
2. Launch in-orbit assembly crew: 1 @ $100M, probably another $1B for training, tools, supplies, support staff.
3. Launch mission crew & supplies: 1 @ $100M
4. Intercept mission uses ion thrusters and a lunar slingshot to enter Apophis intercept trajectory in April 2028.
5. 2 months to intercept; 3 months of thrust to change Apophis orbit; 1 month of contingency; 6 months of coasting. After the slingshot around the Earth in April of 2029 there is another 6 months of coasting (with likely trajectory tweaking) before the final drop into Earth orbit via a Lunar slingshot in October, 2029. Capture Mission duration is a little over 18 months; with recycling of water & oxygen, 600 days of food & water & hydrogen (to recycle CO2) for a crew of 4 masses four tons; a similar mass is needed for the associated equipment (triply redundant).

Total capture cost: $3.2B (ignoring R&D and ground support costs).

LAUNCH MINING & MANUFACTURING TOOLS

This is a big unknown, in that we don’t yet know how to best do mining, smelting, refining, and manufacturing in space (microgravity, necessity to recycle & save everything) from asteroid resources. We can make good guesses, however. Also, the facilities should grow over time, but the initial launches should suffice for starting manufacturing, especially since we are largely launching tools to make tools. After the initial launches, most of the needed additional equipment will be manufactured in space. These launches would begin in 2029; I expect the tugship will be used to transport these components from LEO to the asteroid now in Highly Eccentric Earth Orbit.

• Solar Power generator: 1 launch, $200M
• Mining facility (ore movers, grinders, separators): 2 launches, $400M
• Solar smelter, gas collection & refining, metal purification: 5 launches, $1B
• Steel production: 5 launches, $1B
• Rolling mill (girders, rods, sheet metal): 3 launches, $600M
• Finished metal product plant (nuts, bolts, rivets, connectors, pipes, tanks, etc.): 2 launches, $400M
• Silicon refinery, solar panel manufacturing (3 launches, $600M)
• Slag processing, shaping, rock wool production, & slag handling (1 launch, $200M)

Total estimate is $4.4B for equipment and 22 launches, and this phase begins in 2029, ends in early 2030.

LAUNCH INITIAL WORKERS & THEIR CONSTRUCTION SHACKS

The initial workforce for all of the mining & manufacturing  equipment, even assuming significant automation, is considerable. I estimate 36 workers, needing 6 launches using Falcon 9’s and Dragon X capsules (total $600M) just to get started. Each crew member will likely require a ton or more of life support equipment and supplies.

The Construction Shacks are the living quarters, recycling equipment, supplies, and solar panels for the initial mining and manufacturing workforce, intended to be buried beneath asteroid regolith for radiation and meteorite shielding (alternately regolith may be bagged and packed around the shacks). This will need to be repeated as the workforce grows (until a permanent habitat is at least partially completed).

The Falcon 9 payload faring may be equipped to house up to 12, with room for supplies and equipment. Three of these would provide adequate housing and first-year supplies for the initial 36 workers, at a cost of $600M bringing the total cost to $1.2B ($33M/person).

Subsequent workers initially have similar high launch and support costs, ramping down to $15M per-person as capacity grows to produce living quarters, water, and oxygen in space. When the interim habitat is ready and larger capacity people launchers (32 workers/launch) become available, per-worker launch costs should plummet to roughly $3M per person (in 2031), and as low as $1M per worker in subsequent years (with use of dedicated people-carrying spacecraft ferrying 100 workers into space per launch). Note that a 1 ton per worker mass allowance includes some essentials that cannot be easily fabricated in space, such as high-efficiency LEDs for the farms,  some fertilizers, and medical supplies.

The workforce & construction shack launches could begin in late 2029.

This initial 36 worker contingent will be sufficient to prove the feasibility of mining, refining, and manufacturing in space, but will be inadequate to achieve sufficient volumes of production for a timely project completion.

A ramp up to 500 workers during 2030 should result in a steel production rate of roughly 100 tons per day, which in turn allows construction of the habitat pressure shell plus containers for the volatiles (oxygen, CO2, and H2O) in one year. At this point (an interim habitat consisting only of a spinning pressurized shell with partial radiation shielding), many of the workers can work without pressure suits, gravity prevents long-term health problems of zero-G living, and in-space production of food can begin. 

Doubling the population and production annually allows the first SPS to be completed by the end of 2031 (when steel production is 200 tons per day, comparable to a small mill on Earth). 2032 should see 2 more SPS’s completed, and 2033 results in 4 more and completion of the habitat structure (with room for 8,000). By the end of 2033 the habitat capacity is reached, and the associated manufacturing facilities should be capable of building 8 SPS’s per year. Remember, it takes all of the slag associated with building the first 12 SPS’s to fully shield the habitat, which occurs during 2034. By the end of 2034, the first permanent habitat (8,000 person capacity) is complete, along with 15 Solar Power Satellites.

The total worker cost through completion of the first SPS and the habitat shell would be $15B (by the end of 2031, a total of 1,500 workers). Adding 2,000 more workers in 2032 and 4,000 more workers in 2033 would cost only another $6B.

This analysis has ignored the cost of salaries for space workers (and the cost of ground support). One reason to ignore the salaries is that they are insignificant compared to launch and equipment cost in the early years.

Another reason to ignore them is that portions of each “salary” may be offset: workers may be charged for rent (or purchase of a condo), for food, water, recycling, even oxygen. The cost of living is very high in space, and salaries should reflect that, but the net cost of a worker is difficult to estimate. Many of the readers of this blog would likely volunteer to work for little more than the right to permanently live in space and be a part of humanity’s spacefaring future – I know I would. Very few of the workers would view these orbiting factories and habitats as temporary jobs and expect to return to Earth in a few years, especially if they were afforded the opportunity to buy a home and raise a family in the habitat.

CONSTRUCT & DEPLOY THE SPS

The construction of a Solar Power satellite requires multiple components, most of which will need to be built in orbit. For more details, see Solar Power Satellite Design Considerations. The components are:

• Solar Panels (from high-grade silicon, in thin sheets such as commonly manufactured today)
• Steel structures to frame and aim the panels.
• Steel structures and motors to maintain alignment of the panels with the sun, and the antenna with the target Earth station.
• Microwave transmitting antenna
• Microwave transmitter
• Shield mass to protect the electronics from meteorite damage.
• Fuel to insert the SPS into the target geosynchronous orbit

I expect that the motors and electronics (including the microwave transmitter) will be launched from Earth, for which I’ve budgeted $200M per SPS.

A considerable amount of fuel is needed to move each newly constructed SPS from the near-Lunar location of its birth and insert it into an appropriate geostationary orbit. The total delta-V needed is roughly 2 km/s, the total mass is 25,000 tons, and even using 5,000 ISP VASIMR ion thrusters, 1,000 tons of fuel is needed. Luckily, that is a tiny fraction of the 15,000 tons of oxygen produced as a by-product of the smelting of iron ore into steel for each SPS. Also luckily, we’ll have a lot of power available, so the time required for this orbit change is limited largely by the number of ion thrusters (and thus the fuel flow rate) we wish to dedicate to this task. We’ll have plenty of oxygen to spare, so our ion thrusters should be optimized for this most available rocket fuel.

Each SPS will require a matching ground receiving station consisting of a large (10+ kilometer) rectenna plus power conversion and distribution. These costs are only a guess, but I’d estimate that $1B each is more than enough. I have not included this portion of the per-SPS cost in my summary, as this cost would be born by the receiving local electric utility.

CONSTRUCT THE HABITAT

Another post details the Design of a minimum Kalpana-One style Habitat: a 100-meter radius cylinder, 130 meters wide, spinning at 3rpm for 1G along the outer rim, lined with 3 meters of slag which provides excellent shielding against radiation and meteorites.

The outer pressure shell is built using average strength (easily welded) steel plates nearly 1 inch thick, and masses approximately 28,000 tons. Completed first, once spun up it provides a shirt-sleeve environment with gravity, boosting the productivity and health of the workers.

Over time, blocks of slag are brought inside to line the exterior walls, and interior structures are built as housing, workspaces, and farmland. Most of the volume is left open to eventually become a 12 acre cylindrical park. The core will be available as a low-gravity industrial and research space. Agriculture space (primarily for crops) is more than 5.5 million square feet; residential space is over 2.5 million square feet, there is 1.5 million square feet of offices / light industrial and a million square feet of storage and overhead space. The total internal structures including the outer 15 meters along the endcaps and perhaps 6 stories of rooms lining the cylinder rim, plus a 40 meter diameter core structure, masses 126,000 tons (a generous allowance). This is less tonnage than an Oasis Class Cruise Ship (225,000 tons carrying 8,000  passengers and crew) yet much more spacious.

Power (6 kw/inhabitant or 50 megawatts total) is provided by solar panels lining the exterior surface of one endcap, plus a ring extending 100+ meters beyond the cylinder (effectively a 425-meter diameter solar panel array).  Additional power needed for exterior factories (such as the smelter) is included in their structures and allowances. The spin axis of the habitat points to the sun, requiring a massive gyroscope at the center of the habitat to slowly precess the spin axis at one revolution per year.

Speaking of spin, it takes a lot of fuel and energy to spin up the habitat to 3rpm: about 1150 tons of fuel (oxygen) at an ISP of 5,000. Using 25 megawatts (half of the habitat’s available power), the spin up takes nearly 2 years. However, the empty shell (no shield mass) spins up very quickly; the majority of the energy is needed to maintain the spin as roughly a million tons of shield mass is gradually added to the habitat’s periphery. 

Once complete, this habitat houses 8,000 workers / colonists, and provides all life-support needs. All of the steel (154,000 tons) and shield mass (1,600,000 tons of slag) are from the asteroid, along with the oxygen, water, and carbon needed for the life support system. The known components to be launched from Earth include the LED’s and other light sources for the farms and interior (see Lighting our Space Habitats), various electronics, seeds, and perhaps 50 kilograms of nitrogen (per inhabitant) needed as fertilizer.

There are no additional launch or equipment costs not included elsewhere (possibly excluding the ion thrusters needed for spin-up).

REPEAT

This project is an eight-year plan (5 years to the first SPS) culminating in the deployment of a permanent, self-sustaining habitat with a population of 8,000 and fifteen 5-gigawatt Solar Power Satellites in geostationary Earth orbit. By completion, the project will have consumed approximately 10% of Apophis, leaving  90% available to build additional habitats and Solar Power Satellites.

The capacity will be in place to build additional Solar Power Satellites at the rate of 8 per year, but note that demand is much higher: current global electric demand would need 400 five-gigawatt SPS’s to fill, and total projected energy demand might require more than 4,000 by 2050 (most energy is used for heating, generally by burning fossil fuels). Apophis alone has sufficient resources to build up to 150 Solar Power Satellites (while building 10 habitats accommodating 8,000 people each), but in the long run additional asteroids will be needed.

We should build additional habitats, including larger ones, as part of an ongoing project. We’ll likely need a “mining town” for each asteroid we capture, and an SPS construction / maintenance habitat just outside of geostationary orbit, and even a large habitat in low Earth orbit (as low as possible without serious atmospheric drag) as an Earth to LEO launch target and tourist destination. Good arguments can be made for additional habitats in various Lagrange points such as L5, and for transition orbits such as LEO to Geostationary, or to the Lagrange points. There may even be a 3:1 Lunar synchronous stable Earth orbit that visits L3, L4, and L5 in succession, 9.1 days apart – an ideal platform from which to host interplanetary missions, including to Mars.

SUMMARY

It all sounds time consuming and expensive: $22.8 Billion before that first SPS is operational. But remember, in 2032 that first SPS will generate $1.3B in cash flow (at a wholesale price of $0.03/kwh), in 2033 the 3 deployed SPS’s generate another $3.9B, in 2034 the 7 deployed SPS’s generate $9.1B, and subsequently the 15 SPS’s generate nearly $20B/year. The steel production rate is 800 tons per day, allowing 8 or more additional SPS’s to be built each year. The peak cash outlay is $25B with all investment repaid in 2035 – and huge net profits after that.

Even considering only that first SPS, the total cost is comparable to the $5 billion per gigawatt that my local power company may spend on a nuclear power plant, without any need for radioactive waste disposal or refueling, and without quite as much local objection to the installation.

Yes, I’ve been accused of being an optimist. Yes, it is likely to take longer than this project plan, due to unforeseen setbacks. Yes, I’ve ignored the cost of money in this analysis. I’ve even ignored the cost of bringing all these workers back to Earth (assuming they’d want to return). But even if it takes another year or two before breakeven, even if the costs might run 20% (or 100%) higher, even if you factor in the up-front R&D costs, even if you add a generous salary for every in-space worker plus free room & board, it is still a wildly profitable venture.

And did I mention that Apophis has enough raw materials to repeat this project another nine times, at a fraction of the cost each (and completely funded by free cash flow)?

Is there an entrepreneur listening that likes the sound of $200 Billion per year of free cash flow for a measly $30 billion investment? And with the bonus of earning a permanent place in the history books as the party responsible for bootstrapping humanity’s move into the Solar System?!

Solar Power Satellite Design Considerations

The major considerations driving solar power satellite design decisions are:

• location (geostationary, LEO, other)
• energy delivery method to Earth
• solar panel photovoltaic versus turbine generator (efficiency/cost tradeoff)
• size of an SPS (dimensions, mass, power)

LOCATION

Most studies recommend a geostationary orbit for Solar Power Satellites. This choice simplifies designs, and is the only choice that can deliver continuous power from a single SPS – the others require a constellation of satellites with any given ground station receiving power from a sequence of satellites. The disadvantages are that a relatively large transmitting antenna is required, and it takes more energy to reach those geostationary orbits.

Another disadvantage is that it is inefficient to deliver power to high latitudes; multiple satellites with Molniya orbits are one possible alternative.

An SPS constellation in medium Earth orbit has been proposed, because of the lower cost to launch from Earth and the smaller size of the transmitting antenna. The disadvantages are that both the transmitting and receiving antennas may need to be dynamically aimed, plus the “dark” time due to passing through the Earth’s shadow is much greater, requiring still more satellites to deliver continuous power.

The remainder of this post will focus on geostationary locations, which have been studied more thoroughly.

MICROWAVE ENERGY DELIVERY TO EARTH

The minimum size of an SPS is driven by the size of the energy transmitter and the Earthside receiving station. While several alternatives have been considered, the best option appears to be beaming microwaves to Earth receiving stations from geosynchronous orbit. To achieve sufficiently narrow beams that arrive at sufficiently low energy densities (so as not to cook anything in their path), we’ll need a transmitting antenna one kilometer wide.

The receiving station is larger at 10 kilometers east-to-west and as long north-to-south as necessary to appear circular from the SPS (a 10 kilometer circle at the equator, a 10 by 15 kilometer oval at 30 degrees latitude, 10 by 20 kilometers at 45 degrees latitude, perhaps 10 by 30 kilometers at 60 degrees). Note that 60 degrees latitude is approximately the limit of feasibility due to both the size of the receiving array and the amount of atmosphere that must be traversed. Luckily, this supports serving the vast majority of the global population, excluding only the Artic regions, Alaska (U.S.A.), northern Canada, and the European Nordic countries and northern Russia. Note that St. Petersburg (Russia), Helsinki (Finland), Stockholm (Sweden), and Oslo (Norway) are near the limits of servable destinations.

An Earth-side energy density of 23 mW/cm² (chosen as safe for all life forms) equates to 5 gigawatts as a minimum power limit near the equator and proportionally more at higher latitudes. Building smaller SPS systems can be done, but the antenna arrays would not be smaller (or cheaper), and thus would be less cost-effective. Note that some sources suggest a minimum size of 4 gigawatts. In any event, at 2.45 Ghz (the highest frequency with negligible absorption by rain, snow, clouds, and people), a one-kilometer transmitter could not possibly focus higher intensities upon the Earth, making this a safe and non-military technology.

The likely technology for the receiving station is a rectenna array, a network of many simple wires with diodes. It can be deployed above cropland (and herds of cattle, even forests) with only minor loss of productivity of the farm. Receiver losses will be of the order of 10%, much less than typical long distance power transmission.

A side point: direct-to-home power transmission is perfectly feasible. A home rooftop rectenna could easily capture 5 or 10 kilowatts of power, several times the average household demand. The problem is all the power wasted between rooftops, including undesirable electric currents induced in most metals and wiring. But perhaps this is not a problem in a military environment, where an energy beam could be directed to the conflict zone, not as a weapon, but for providing electrical power wherever needed. This also has direct military applications: SPS powered drones could fly indefinitely.

HOW BIG IS A SOLAR POWER SATELLITE?

So how big is the SPS itself? That depends critically on the efficiency of solar energy capture and thus the technology used. There are two likely technologies: solar cells (photovoltaic) and solar dynamic (mirrors and gas or steam turbine powered generators). Both methods take advantage of the near-constant sunlight (only shadowed by the Earth for a few total hours near midnight twice a year, and never for more than 75 minutes at a time) – a net 99% availability. Also, the intensity of solar energy in space is greater than on the Earth’s surface due to the lack of air to absorb energy. About 950w/m² strikes the Earth’s surface at noon on the equator on a clear day, while in orbit the solar energy density is 1367w/m² all of the time.

Solar Photovoltaic versus Solar Dynamic

Solar cells vary hugely in cost and efficiency. Common cells convert about 10% of the incoming energy to electricity. But modern cells convert 20% (thin film) to 40% (optimized) of the incoming energy. Caltech claimed to achieve 85% in a highly experimental demonstration in March of 2010. We don’t know what technology will be the most cost-efficient. Note that 25% efficiency implies a 4km by 4km photovoltaic array would generate 5 gigawatts in orbit, although it would take a 4.25km square array to deliver 5 gigawatts to the Earth assuming an 80% efficiency in the transmitter / receiver.

Solar dynamic may be more efficient, as large, low-cost mirror arrays focus the sun’s heat on a boiler driving a turbine electric generator. The mirrors are relatively inexpensive, and the cost will be dominated by the need to remove the waste heat using large cooling arrays. But the efficiencies are quite high, typically 40% to 50% (potentially even higher, depending upon the fluid used). Ordinary steam turbines can achieve 40% efficiency. Hybrid designs with a gas turbine followed by a steam turbine can achieve 60% or higher efficiency. The problem with solar dynamic is that moving parts imply mechanical failures. These units would likely require a nearby crew for maintenance – something easily provided from a space-based habitat. Note that at 60% efficiency, a circular mirror of 2.8 km diameter (or a 2.5 km square array) is sufficient for a 5 gigawatt SPS. 

Solar dynamic has other advantages: lightweight flexible mirrors (such as ordinary aluminum foil) are not significantly affected by radiation, ultraviolet light, or micrometeor punctures. Disadvantages include that the turbines and generators are heavy, high-tech components that may have to be launched from the Earth. Also, the working fluid in the gas/steam turbines may not be cheaply available. The only gas available in large quantities is oxygen, which is very corrosive to many metal surfaces, at least until we capture the resources of a live or extinct comet which is likely to contain enormous quantities of water and ammonia.

PHYSICAL SIZE OF A SOLAR POWER SATELLITE

Using assumptions listed above, the width of an SPS might range from 2.5km to 4.25km, or as large as 6.75km if we use the cheapest solar cell technologies. But 4.25km looks like a reasonable target. Note that at geosync orbital distances, these would be as visible as the larger planets, and easily resolved with binoculars. Once hundreds were in orbit, they would appear as an arc of lights to anyone who looked up on a clear night.

There is also a nearby 1.0km microwave transmitting antenna, visible using binoculars. One problem is that the solar cells must always point at the sun while the transmitting antenna must always point to the Earth side receiving station. Proposed solutions include:

• use the same structure with phased array techniques to steer the beam (always pointed at the sun), difficult for certain orientations;
• use separate structures with some connecting mechanism that rotates one or both arrays;
• use a smaller integrated array of a 1km transmitter (always pointing to the Earth) backed by a (fixed) 1km photovoltaic array, and a separate large mirror to track the sun and focus its light on the back of the transmitter. While appealing in several respects, this does have its own problems: dissipating roughly 15kw of waste heat per square meter, which implies a blackbody temperature of 240^oC if there is no additional radiating surface (it couldn’t be a manned structure, and would have to cool off prior to maintenance). The mirror and the transceiver would have different orbits – one would require constant adjustments. Lastly, it would be difficult to focus sufficient light on the transceiver near local noon each day. But some clever engineer may find a solution to all these problems.

MASS OF A SOLAR POWER SATELLITE

Using modestly optimistic assumptions for solar photovoltaic panels, supporting structure, power collection, and for the microwave transmitter and its own antenna, I estimate a total mass of 25,000 tons for a five-gigawatt SPS. Early estimates were closer to five times higher, and some optimists currently estimate that thin film panels would weigh one-fifth as much, although a supporting structure and the transmitter must also be considered. Note that if the solar panel array is 4km on a side, then 1.0 kg/m² of panels, structure, and wiring results in a total mass of 16,000 tons, leaving the remainder of my mass allowance for the transmitter, its antenna, and other support structures.

A 1.0kg/m² mass allowance solar cells, wiring, and structure seems like a significant challenge, requiring a substrate similar to a sheet of paper in thickness. Note that a silicon wafer sliced 1.0mm thick weighs 2.3kg/m². However, the typical thickness of photovoltaic silicon wafers today is only a tenth that, and thus weighs 0.23kg/m2, leaving adequate allowance for structural components and wiring.

When using the resources of an asteroid to build the Solar Power Satellite, the mass of the photovoltaic panels is only a minor concern, as there is more than enough silicon and iron in even a small asteroid to build them as crudely as desired. Indeed, using a convenient asteroid allows the construction of large and inefficient but presumably simple and reliable solar power satellites. The final technology choice may be one of minimum labor costs or total time-to-market. Those tradeoffs are not simple, because using a less efficient technology (that is simpler to produce) implies a significantly larger structure with more materials and thus more time and effort to build.

But by using an asteroid for raw materials, we at least have choices to make beyond “is it even feasible?” Launch costs of people and tools to bootstrap the process will be high enough as it is, but having the cheap and convenient asteroidal resources in orbit where we need them gives us the flexibility to try not just one approach, but several approaches and learn which is the best way to deliver clean and cheap energy to the inhabitants of Earth.

Space-Based Solar Power

Many people have written pages, papers, even entire books on the subject of Space Based Solar Power. Certainly many of the authors are better qualified than I am. What my posts add to the mix is the cost reduction and simplicity gained by using an asteroid captured into Earth orbit for the bulk of the materials.

Also, some of my sources are more recent than many older papers – which makes a huge difference. For example, my earlier posts used a figure of 125,000 tons for a 4 gigawatt SPS, based upon references quoting 20,000 tons per gigawatt, plus structure and the transmitter. But more recent sources propose a very lightweight design requiring only 1,000 tons per gigawatt – a 20-fold reduction. For these posts, I’ll be using an intermediate figure of 5,000 tons per gigawatt, or a total of 25,000 tons for a 5 gigawatt SPS (measuring 4 km on a side).

Even with current high launch costs, at least one company believes they can build a cost-effective SBSP system using all Earth-sourced materials. I don’t see how, since fifty to a hundred or more 100-ton payload launches would be required. But the people with their money on the line have put more thought into justifying the ROI, and I wish them well.

More commonly, pundits believe that Lunar materials (typically using a launch-rail system to lower the cost of launching materials from the Moon) might reduce the total cost to an acceptable level for as few as 50 to 100 SPSs (to amortize the costs of landing a great deal of equipment and people on the Moon).

Asteroidal resources are much more readily available, and several sources have proposed robotic mining missions to various asteroids (chosen for a low net delta-V, especially for returning material to Earth orbit). Some proposals return raw material to Earth orbit for processing; others return materials processed to some degree (such as iron). Robotic missions are often assumed to eliminate life-support and radiation shielding costs.

Note that a 100-meter radius Kalpana One style habitat (housing and feeding up to 8,000 residents) can be built from a single 120-meter asteroid, along with 12 five-gigawatt Solar Power Satellites, using the slag from iron smelting as radiation shielding.

My proposal (see Capturing an Asteroid) is to use gravitational slingshot maneuvers (around the Earth, the Moon, or even Mars or Venus when appropriate) to capture one or more asteroids into Highly Eccentric Earth Orbit (HEEO), then to launch the tools needed to mine the asteroid, smelt it into valuable materials, and build the SPS network in orbit. Of course, this is simply a logical extension of the proposals to return asteroid raw materials to Earth orbit. I propose to send the whole asteroid (which we can do given the extraordinary special circumstance of an existing close approach of an asteroid).

This approach requires large numbers of people in space, because people are good at problem solving. Using robotic approaches requires that the engineers anticipate all possible contingencies, and, speaking as an experienced computer programmer, I absolutely guarantee that we will never succeed at anticipating every possibility.

But people are resourceful, and their ingenuity will solve all of the little problems, and likely the big ones that crop up, as well. The number of people needed is huge, because each SPS is itself huge (kilometers wide), and there is a lot of work required. We’ll need to smelt ore into steel at impressive rates (but typical of a foundry on Earth). We’ll need to form that steel into plates, girders, pipes, tanks, cables, etc.. We’ll also need tables, and chairs, and sinks, and toilets. It’s expensive to bring those things up from Earth.

We’ll need to use part of that steel (and much of the slag) to build habitats for space workers (see Designing a Space Habitat). We’ll need to produce magnesium for mirrors (assuming turbine-driven electric generators) or silicon for solar voltaic cells (or both – we won’t know what is best until we try). We’ll need to weld or bolt all the pieces together into immense structures, and then maintain them (because parts fail, regardless of how well-built they are). We are likely speaking of thousands of people to build that first SPS, and thousands more to build and maintain a network of them.

Carbonaceous chondrite asteroids and extinct comets certainly contain everything needed. These comprise at least 75% of all asteroids, although they are relatively rare among Earth-crossing ones. However, any undifferentiated asteroid (such as ordinary chondrites) should suffice. All of these contain vast quantities of iron, oxygen, and magnesium. Most asteroids will contain small but significant quantities of carbon and hydrogen, easily extracted by simple heating. The asteroids to avoid are those from parent bodies that melted and differentiated, since that would isolate most metals into an iron core, and deplete the volatiles, leaving ore as poor and dry as moon rocks.

HOWEVER, there are significant ground-based costs to consider as well. In addition to building an SPS with a microwave transmitter to deliver power to an Earth station, we must build that ground station, which (in principle) is easy and low-tech, but is still large (and thus expensive, partly because of land-use costs), plus we must build the power distribution network to get the electricity to the consumer. I do not have a good handle on this cost, which I am certain will vary tremendously from site to site, as it should be much cheaper where land is cheap (and power is not needed), and will be much more expensive close to major cities where the power is needed but land is expensive plus there is always resistance to putting the receiving rectenna in our back yards.

Note that the receiver may typically require an oval roughly three miles wide and six long, but it is sparse and may be placed over a farm with little impact on crops or livestock below. It may also be place in a forest just above the treetops, where it would not even be all that visible. As a very rough estimate, I will assume that the ground-side costs will equal the in-orbit costs per SPS, although I will also plan that all of the up-front capital costs are for the SPS itself (a simplistic approach that ignores the cost of lawyers and politicians).

My next post will discuss specific design considerations for a Solar Power Satellite.

Economics of Solar Power Satellites

Reading my local paper yesterday, I learned that the cost of building new, modern, nuclear power plants was much higher than I had thought. The Jacksonville Electric Authority (JEA) is considering a 20% share of a new 2 Gigawatt plant – at a cost of $2 Billion. The total cost of the plant is $10 Billion – or $5 Billion per gigawatt.

This seems horribly expensive, but the resulting steady, cheap, zero-carbon electricity apparently makes the capital investment worth while.

The cost is indeed high, which is good news for Space Based Solar Power: it may be possible to profitably launch the components directly from Earth to build a Solar Power Satellite (just barely).

But as I have argued in previous posts, there is a much less expensive way to build Solar Power Satellites: build them in orbit using asteroids which provide all the raw materials we need (just add tools and workers).

As a side benefit of great importance to most of the readers of this blog, my plan requires the building of permanent orbiting space habitats, self-sufficient, rotating for gravity, and using the slag from iron smelting as radiation shielding – a topic I’ve discussed before.

My next several posts will discuss various aspects of Space Based Solar Power built using asteroid materials, from some high-level design considerations to construction techniques and ultimately to revenue generation and return on investment.

I am more convinced than ever that we can expand humanity into space using ground-based profit motive, with benefits of lower energy costs, near-zero carbon emissions, and, did I mention, HUGE PROFITS?

The Economics of Life In Space

For mankind to move into Space, it must be

• Affordable in the short term
• Profitable in the mid term
• Self-sustaining in the long term

Each of these should be analyzed in more detail, even if they are self-evident to the optimists among us. Even the definition of short, mid, and long term are subject to discussion, but for these purposes the points above are self-defining.

The long term will begin when Earthbound civilization is no longer necessary for a space faring humanity. By not necessary, I don’t mean not useful: I expect that the cradle of mankind will always be an important part of humanity’s heritage. But at some point the continued expansion of humanity will no longer depend upon Earth resources. This has happened to every expansion of humanity (or a branch of civilization) at some point or another. For example, when Europeans colonized the Americas, the new territories may have initially profited by sending goods to the home country and depended upon tools and technologies created there, but eventually the continued expansion of the frontier no longer depended upon the Motherland. This may take longer (perhaps much longer) in space than on Earth, because the environment is hostile, a high level of technology is needed to survive there, and technological civilizations are complex. It may take tens of thousands of people living in space, or it may take tens of millions to replicate all of our technology. But it will happen.

The mid term will be the period when Earth profits from investments in space, and in some sense this will be a Golden Age of immense profits, rapid growth, unbridled enthusiasm and optimism. Many people have proposed many different potential sources of profit, but two stand out: tourism and Solar Power Satellites (SPSs).

Space Tourism is perhaps an indirect Earth profit generator. As long as launch costs are high (even as cheap as $500/pound), it will only be affordable by the wealthy. But most of the expenses are Earth-bound, and every million dollars spent on Space Tourism will contribute perhaps $2.5M to the Earth’s economy, supporting 25 to 50 families on Earth.  Remember, you can’t spend money in space; every dollar spent on the space program is ultimately spent on Earth, and will continue to be until a space civilization can thrive on its own.

Space-Based Solar Power (SBSP) would directly benefit civilization on Earth, in multiple ways. Not only through stable, low-cost, zero pollution electric power, but also since the construction of Solar Power Satellites and the construction of Earth receiving stations would stimulate the Earth’s economy. Note that SBSP can provide cheap power to remote areas, including many of the poorest nations on Earth. Note that, as with space tourism, every dollar spent on SBSP (both construction and operation) is a dollar spent on Earth. The fact that a large-scale SBSP network would be enormously profitable for some corporations or nations in no way reduces its value to the Earth’s economy. And low-cost reliable power directly contributes to the wealth of the recipient. In a sense, reducing the use of fossil fuels (and the resulting global warming) is only an indirect benefit of SBSP.

Also note that SBSP would be expensive to build and launch from Earth; it will likely be affordable only when we can use space-based resources to build Solar Power Satellites (see my post, Capturing Apophis). But once built, Earth’s civilization benefits for the indefinite future.

The short term is the period – however long – when Earthbound civilization must invest in space. This includes the period when we are beginning to build a network of Solar Power Satellites, or space habitats for living, or space hotels for tourism. While any long-term space-based habitat is likely to produce its own power, food, water, and oxygen (recycling wastes in a closed cycle), most other needs must be met using tools and technologies imported from Earth, including LED’s for lighting the farms, computers, communication equipment, high-technology space suits, VASIMR rocket motors, vitamins, pharmaceuticals, medical equipment. The list is endless, although the relative need and value tails off rather quickly.

More importantly, the Short Term is the period while the costs of launching people, tools, and bootstrap resources into space exceed the profit derived from space-based enterprises, primarily SBSP. But the cost of bootstrapping a space-based civilization is an investment, pure and simple, yielding enormous profits for those clever and resourceful enough to make that investment.

The revenue from a single SPS is of the order of $1 Billion per year, suggesting that an investment of even $100 Billion to build and deploy a hundred SPS’s would be wildly profitable. Yet it would cost a fraction of that to capture an asteroid such as Apophis into Earth orbit, and to launch sufficient people and tools to turn that asteroid into a habitat and a factory to build Solar Power Satellites. Note that Apophis is too small to build more than about a dozen SPSs (assuming half of its mass is reserved for habitats). Yet it is more than large enough to bootstrap the process and support the ongoing space-based resources needed to capture additional asteroids to build thousands of SPSs and habitats for millions of people.

Let’s estimate some numbers:

• $2 Billion: Commercialize the technologies to capture an asteroid (large-scale VASIMR, long-duration space flight)
• $2 Billion: Launch the capture equipment and team. This will result in the capture of an asteroid such as Apophis into a highly-eccentric Earth orbit after a period of a year or two.
• $2 Billion: Develop the processes and tools needed to mine, smelt, and process asteroid material into steel, oxygen, and hopefully CO2 and water. Other valuable materials are a bi-product. There are many unknowns, including the raw materials themselves, and zero-gravity smelting, and recycling of effluent gases such as carbon dioxide. Nothing should be vented / wasted.
• $2 Billion: Launch the solar smelters, mining equipment, and tools to process iron ore into steel plates, girders, cables, etc.. Part of this is launching a small fleet of VASIMR tugs, fueled by excess oxygen from the smelters and using solar power for energy, to boost cargo and people from LEO to the HEEO of the captured asteroid. To a degree, this is launching the tools to build the tools to build the tools….
• $2 Billion: Launch the people and habitat resources (LED lights for farms, solar panels for power, initial supplies of oxygen, food, and water, pumps and recycling equipment, ….)

Okay, so I used nice round numbers to get the total cost around $10 Billion. It may even be accurate to within a factor of two. In reality, I’d expect on-going costs of continuing launches of additional people and resources, perhaps $2 Billion per year for the 5 years I expect it would take to build the infrastructure and that first Solar Power Satellite, but then you get another one built every year, and the continuing influx of people and resources builds additional SPSs every year after that.

My expected cost to get that first habitat and first solar power satellite operational is of the order of $20 Billion. But then the investment starts to multiply, and by the time you’ve invested $30 Billion, you’d have 30 Solar Power Satellites in production and your investment ROI is 100% per year (ignoring ground-based costs of receiving and distributing the power, which might be as much as another billion per satellite). Actually, by the time you have two SPSs (ignoring ground costs) or four SPSs (assuming $1B/satellite in ground costs) in operation the operation is self-sustaining and doesn’t require additional capital investments, yet the profits continue to grow.

Assuming the chosen asteroid is Apophis (but see A Choice of Asteroids), the first $10 billion would be spent by 2030, the first SPS operational in 2035 (after spending another $10 Billion), and the entire operation is wildly profitable by 2040 (by which time you’ve invested $30 Billion but your satellites are earning you $30B/year). It sounds like a great investment for my IRA.

A lot of research is needed, and a lot of talent. We need to solve these problems:

• Farming in Space (total closed-system recycling)
• micro-gravity mining
• zero-gravity smelting of ores using recycled reducing agents and probably direct solar power
• zero-gravity refining (separation of metals, slags, and effluent gases into valuable component parts)
• zero-gravity rapid capture and separation of gases from iron and steel production (we can’t afford to waste that carbon dioxide).
• zero-gravity metal forming (turning steel into girders, rods, plates, cables, etc.)
• Welding of large structures in space.
• Low-cost, human-friendly space suits (ie, skin suits) for hard-working people.
• VASIMR (or similar rocket technologies) to use the excess oxygen from the production of iron as a rocket fuel for in-orbit shuttles and to capture asteroids. Oxygen is the primary bi-product of steel production from ore (other than slag, and assuming recycling of carbon), with a ton of oxygen freed for every three tons of iron produced. Thus the 75,000 tons of steel needed for a habitat for the first 8,000 people yields 25,000 tons of oxygen. Building each 180,000 ton SPS (4 km on a side) yields 60,000 tons of excess oxygen. That’s a lot of rocket fuel.
• Low-cost launch to LEO. Part of this may be the economy of scale, as very large heavy-lift rockets are much cheaper per ton to orbit than smaller rockets. I believe this entire operation is highly profitable and sustainable if the launch cost to LEO is $1 million per ton or less. While NASA and the Space Shuttle (or its proposed replacements) can’t approach this cost, commercial private-sector efforts can. And the scale of this project is large enough to justify those investments.

There are a myriad other problems to be solved, but most of them are engineering efforts, not R&D projects. They will still require a lot of talented people, and many more people will be needed to work in space – thousands of them, of every persuasion. Miners. Steel workers. Welders. Electricians. Plumbers. Mechanics. Farmers (lots of farmers). Doctors and nurses. Pharmacists. Cooks. Wait staff. Bartenders. Construction workers. Janitors. Barbers and cosmetologists. Massage therapists. Truck drivers & bus drivers (but we’ll call them space ship pilots). Clerks. Accountants. I suspect a lot of movies might be made in space, so add actors and all those people listed in the credits for your favorite movie. And where lots of people go, families happen. So we’ll also need day care workers. Teachers. Playgrounds. Schools. Police. We might even need a manager or two. Counselors. And a divorce lawyer.

If you have a skill, you’re probably needed in space. Welcome to the future.

Near-future Space Industries

Many people have written about commercial opportunities in space. The big ones are power satellites (beaming zero-carbon-footprint energy to earth), zero-G industrial processes (things that can't be cheaply made in a gravity field, such as foamed steel), and tourism (I'm looking forward to Disneymoon, and that first Hyatt with an out-of-this-world view).

Another significant opportunity exists in communication satellites and research. It is much cheaper to maintain / repair / service satellites from an orbit near them. It's even cheaper to build them there. Send the expensive components to low Earth orbit, assemble them in space, and launch to a higher geo-synchronous orbit using in-space resources (fuel made from asteroids & comets). It is much cheaper. An asteroid-based satellite assembly factory in a thousand-mile-high orbit could easily perform those functions. Another asteroid near geo-synch orbit could perform maintenance functions.

Astronomers take note: such a space-based satellite assembly factory could also build a really huge space telescope by assembling a collage of launchable mirror segments. Imagine the resolving power and light-gathering capabilities of a fifty-meter version of the Hubble Space Telescope. Add the convenience of a nearby maintenance crew that could swap out new instruments for old, replace failing gyroscopes, perform routine repairs. If desired, the maintenance crew could be positioned permanently between the sun and the telescope to shade it from those pesky thermal cycles due to the contrast between the sun's heat and the cold of space.

In the long run, the biggest space industry is likely to be the same as on Earth: people, their entertainment, their housing, their food and water (and air), and information. As mankind expands into the cosmos, there is no need to make money by sending products home to Earth, just as the economy of the USA is not entirely dependent upon sending products back to mother Europe. An expanding population creates its own wealth as there are always opportunities for us to help one another (and make a buck in the process).

In the relatively near term, supporting Earth's needs will be paramount and will fund the expendables and technologies needed to thrive in space. Soon after, mining, housing, and food (recycling) will be the major industries. But after the space population exceeds some critical threshold (I don't know if it is ten thousand, or a million, or even tens of millions), it will become completely self-sustaining. Expanding humanity's presence in space will become the fundamental driving force of the space-based economy. And from then on, there's no looking back.