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Slingshot to the Stars

Back in the 1970s, Peter Glaser patented a solar power satellite that would supply energy from space to the Earth, one involving space platforms whose cost was one of many issues that put the brakes on the idea, although NASA did revisit the concept in the 1980’s and 90’s. But changing technologies may help us make space-based power more manageable, as John Mankins (Artemis Innovations) told his audience at the Tennessee Valley Interstellar Workshop.

What Mankins has in mind is SPS-ALPHA (Solar Power Satellite by means of Arbitrarily Large Phased Array), a system of his devising that uses modular and reconfigurable components to create large space systems in the same way that ants and bees form elegant and long-lived ecosystems on Earth. The goal is to harvest sunlight using thin-film reflector surfaces as part of an ambitious roadmap for solar power. Starting small — using small satellites and beginning with propulsion stablization modules — we begin scaling up, one step at a time, to full-sized solar power installations. The energies harvested are beamed to a receiver on the ground.


Image: An artist’s impression of SPS-ALPHA at work. Credit: John Mankins.

All this is quite a change from space-based solar power concepts from earlier decades, which demanded orbital factories to construct and later maintain the huge platforms needed to harvest sunlight. But since the late 1990s, intelligent modular systems have come to the fore as the tools of choice. Self-assembly involving modular 10 kg units possessed of their own artificial intelligence, Mankins believes, will one day allow us to create structures of sufficient size that can essentially maintain themselves. Thin-film mirrors to collect sunlight keep the mass down, as does the use of carbon nanotubes in composite structures.

There is no question that we need the energy if we’re thinking in terms of interstellar missions, though some would argue that fusion may eventually resolve the problem (I’m as dubious as ever on that idea). Mankins harked back to the Daedalus design, estimating its cost at $4 trillion and noting that it would require an in-space infrastructure of huge complexity. Likewise Starwisp, a Robert Forward beamed-sail design, which would need to power up beamers in close solar orbit to impart energy to the spacecraft. Distance and time translates into energy and power.

Growing out of the vast resources of space-based solar power is a Mankins idea called Star Sling, in which SPS-ALPHA feeds power to a huge maglev ring as a future starship accelerates. Unlike a fusion engine or a sail, the Star Sling allows acceleration times of weeks, months or even years, its primary limitation being the tensile strength of the material in the radial acceleration direction (a fraction of what would be needed in a space elevator, Mankins argues). The goal is not a single starship but a stream of 50 or 100 one to ten ton objects sent one after another to the same star, elements that could coalesce and self-assemble into a larger starship along the way.

Like SPS-ALPHA itself, Star Sling also scales up, beginning with an inner Solar System launcher that helps us build the infrastructure we’ll need. Also like SPS-ALPHA, a Star Sling can ultimately become self-sustaining, Mankins believes, perhaps within the century:

“As systems grow, they become more capable. Consider this a living mechanism, insect-class intelligences that recycle materials and print new versions of themselves as needed. The analog is a coral atoll in the South Pacific. Our systems are immortal as we hope our species will be.”

All of this draws from a 2011-2012 Phase 1 project for the NASA Innovative Advanced Concepts program on SPS-ALPHA, one that envisions “…the construction of huge platforms from tens of thousands of small elements that can deliver remotely and affordably 10s to 1000s of megawatts using wireless power transmission to markets on Earth and missions in space.” The NIAC report is available here. SPS-ALPHA is developed in much greater detail in Mankins’ book The Case for Space Solar Power.

Ultra-Lightweight Probes to the Stars

Knowing of John Rather’s background in interstellar technologies (he examined Robert Forward’s beamed sail concepts in important work in the 1970s, and has worked with laser ideas for travel and interstellar beacons in later papers), I was anxious to hear his current thoughts on deep space missions. I won’t go into the details of Rather’s long and highly productive career at Oak Ridge, Lawrence Livermore and the NRAO, but you can find a synopsis here, where you’ll also see how active this kind and energetic scientist remains.

Like Mankins, Rather (Rather Creative Innovations Group) is interested in structures that can build and sustain themselves. He invoked self-replicating von Neumann machines as a way we might work close to the Sun while building the laser installations needed for beamed sails. But of course self-replication plays out across the whole spectrum of space-based infrastructure. As Rather noted:

“Tiny von Neumann machines can beget giant projects. Our first generation projects can include asteroid capture and industrialization, giving us the materials to construct lunar colonies and expand to Mars and the Jovian satellites. We can see some of the implementing technologies now in the form of MEMS – micro electro-mechanical systems – along with 3D printers. As we continue to explore tiny devices that build subsequent machines, we can look toward expanding from colonization of our own Solar System into the problems of interstellar transfer.”

Building our system infrastructure requires cheap access to space. Rather’s concept is called StarTram, an electromagnetic accelerator that can launch unmanned payloads at Mach 10 (pulling 30 g’s at launch). The key here is to drop launch costs down from roughly $20,000 per kilogram to $100 per kilogram. Using these methods, we can turn our attention to asteroid materials that can, via self-replicating von Neumann technologies, build solar concentrators, lightsails and enormous telescope apertures (imagine a Forward-class lens 1000-meters in radius). 100-meter solar concentrators could change asteroid orbits for subsequent mining.

This is an expansive vision that comprises a blueprint for an eventual interstellar crossing. With reference to John Mankins’ Star Slinger, Rather mused that a superconducting magnetically inflated cable 50,000 kilometers in radius could be spun around the Earth, allowing the kind of solar power concentrator just described to power up the launcher. Taking its time to accelerate, a lightweight probe could reach three percent of lightspeed within 300 days, launching a 30 kg payload to the stars. The macro-engineering envisioned by Robert Forward still lives, to judge from both Rather’s and Mankins’ presentations, transformed by what may one day be our ability to create the largest of structures from tiny self-replicating machines.

The Solar Power Pipeline

Back when I was writing Centauri Dreams in 2004, I spent some time at Marshall Space Flight Center in Huntsville interviewing people like Les Johnson and Sandy Montgomery, who were both in the midst of the center’s work on advanced propulsion. A major player in the effort that brought us NanoSail-D, Sandy has been interstellar-minded all long, as I discovered the first time I talked to him. I had asked whether people would be willing to turn their back on everything they ever knew to embark on a journey to another star, and he reminded me of how many people had left their homes in our own history to voyage to and live at the other side of the world.


Image: Edward “Sandy” Montgomery, NanoSail-D payload manager at Marshall (in the red shirt) and Charlie Adams, NanoSail-D deputy payload manager, Gray Research, Huntsville, Ala. look on as Ron Burwell and Rocky Stephens, test engineers at Marshall, attach the NanoSail-D satellite to the vibration test table. In addition to characterizing the satellite’s structural dynamic behavior, a successful vibration test also verifies the structural integrity of the satellite, and gauges how the satellite will endure the harsh launch environment. Credit: NASA/MSFC/D. Higginbotham.

We’re a long way from making such decisions, of course, but Montgomery’s interest in Robert Forward’s work has stayed active, and in Oak Ridge he described a way to power up a departing starship that didn’t have to rely on Forward’s 1000-kilometer Fresnel lens in the outer Solar System. Instead, Montgomery points to building a power collector in Mercury orbit that would use optical and spatial filtering to turn sunlight into a coherent light source and stream it out into the Solar System through a series of relays built out of lightweight gossamer structures.

Work the calculations as Montgomery has and you wind up with 23 relays between Earth orbit and the Sun, with more extending deeper into the Solar System. Sandy calls this a ‘solar power pipeline’ that would give us maximum power for a departing sailcraft. The relaying of coherent light has been demonstrated already in experiments conducted by the Department of Defense, in a collector and re-transmitter system developed by Boeing and the US Air Force. Although some loss occurs because of jitter and imperfect coatings, the concept is robust enough to warrant further study. I suspect Forward would have been eager to run the calculations on this idea.

Wrapping Up TVIW

Les Johnson closed the formal proceedings at TVIW late on the afternoon of the 11th, and that night held a public outreach session, where I gave a talk running through the evolution of interstellar propulsion concepts in the last sixty years. Following that was a panel with science fiction writers Sarah Hoyt, Tony Daniel, Baen Books’ Toni Weisskopf and Les Johnson on which I, a hapless non-fiction writer, was allowed to have a seat. A book signing after the event made for good conversations with a number of Centauri Dreams readers.

All told, this was an enthusiastic and energizing conference. I’m looking forward to TVIW 2016 in Chattanooga. What a pleasure to spend time with these people.


Comments on this entry are closed.

  • Thomas Hair November 21, 2014, 17:46

    Hi Paul,

    I share your skepticism about fusion, but…
    Lockheed Martin is spending some good Air Force money to develop a small fusion reactor and them seem awfully confident. It’s something to consider.

  • Thomas Hair November 21, 2014, 17:48

    A space elevator and a small fusion reactor are all that is needed to open up the solar system…if you’re going to dream, dream big.

  • JoeP November 21, 2014, 18:40

    Powerful beams of energy sent down from solar power stations…uh…too many problems, disasters, and misuse issues! Lets hope some of these small fusion efforts pan out.

  • Thomas Hair November 21, 2014, 18:59

    Agreed JoeP, a solar powersat in the wrong hands is a giga-watt death beam.

  • Adam November 21, 2014, 19:16

    That’s a common worry, but it’s incorrect. Diffraction means microwave beams always need large area rectennas on the ground to receive the energy. Tests in 1970s showed negligible effects on birds and no effect on aircraft.

  • Rob Henry November 21, 2014, 19:59

    That slingshot sounds a bit hopeful at those speeds and masses. Those 10 ton objects accelerated ‘slowly’ up to 3%c would experience one million g from the radial component of acceleration alone. A solid bock of steel with no internal cavities over 3mm tall would flow away under its own weight. Having components that massive must impose sever material limitations.

    I am also thinking of the energy content, let alone momentum of that wave that must travel round the ring and be confined to a small part of it. Even if it works, my guess is that an innocent software bug could pose a bigger threat than the potential of a nuclear weapon in the hands of terrorists.

  • rick November 22, 2014, 2:35

    Hi Thomas,
    I read the fascinating Lockheed compact fusion article. As an aging Bay Boomer, I sure would like to see Lockheed develop this “fusion : theenergy source of the future item” in less than 15 years!

  • Michael Michaud November 22, 2014, 11:23

    A historical note. Peter Glaser first published his idea in Science in 1968. Studies done during the 1970s identified a number of obstacles, including the “death ray” theory. As one of the people invited to comment, I suggested that some sort of international management would be required for an SSPS system.

  • Michael November 22, 2014, 13:13

    @Rob Henry November 21, 2014 at 19:59

    ‘A solid bock of steel with no internal cavities over 3mm tall would flow away under its own weight. Having components that massive must impose sever material limitations. ‘

    The steel will only flow at the base until the depth ‘pressure’ again equals the yield strength. This is what I found in my linear accelerator design, the g force would deform the material badly. Although we could use higher strength materials such as nanotubes even they deform. The other issue is the momentum in a circular path, what will support the craft components against the circular structure during acceleration as no magnet will have the strength to do it?

    I am very much in favour of the development of the moon, it has enormous resources and a very good base to build huge solar collectors. If we built these collectors on the moon we could transmit power to great ranges all over the solar system and to the L1 , L2, L4 and L5 points.

    To ignore the moon would be our greatest folly liken to mankind not starting our first fires.

  • Paul Gilster November 22, 2014, 15:13

    Thomas Hair writes:

    Agreed JoeP, a solar powersat in the wrong hands is a giga-watt death beam.

    Absolutely agreed. The question becomes dauntingly political — who is going to control the beamer apparatus to get the energy down to Earth? The same applies for any laser infrastructure, a huge issue.

    Tom, it’s great to see you back on Centauri Dreams!

  • Alex Tolley November 22, 2014, 16:49

    That was my understanding too. Use microwaves to beam energy to the Earth’s surface for efficiency, allowing wide beams to keep the energy levels low to prevent the problem of cooked wildlife (exactly the sort of problem the solar thermal plant at Ivanpah may be seeing with their “streakers”). Now maybe the beams could be focused, but I would have thought there were fairly simple engineering solutions to prevent this.

    For power beaming in space, we need this highly focused lasers or equivalent to reduce beam divergence to a minimum. Now having one of those turned to Earth. That would be bad.

  • Andrew Higgins November 22, 2014, 18:53

    A circular accelerator does not really make a lot of sense for launching macroscopic objects. The continuous centripetal acceleration that must be provided to the object being accelerated in a circular loop is (V^2)/R. The linear acceleration required in a linear launcher is (Vfinal^2)/(2 L),where L is the launcher length. So, if the object takes one trip around the loop, the centripetal acceleration need to keep the object in a circular path is greater than the linear acceleration required to just launch it directly. In the “star slinger” concept discussed here, the object would take a great many trips around the loop (“allows acceleration times of weeks, months or even years”), so the required force to keep it in a circular path would be many orders of magnitude greater than the tangential force accelerating it faster. What is the point of that? Note that techniques like superconducting levitation would not be able to generate the required normal force (if superconducting levitation even works at all at these speeds). You would need to generate an active retaining force electromagnetically via the same technique you are using to accelerate the object, so why not just use that force to accelerate it linearly?

  • Andrew Higgins November 22, 2014, 19:00

    Also, “Star Sling allows acceleration times of weeks, months or even years, its primary limitation being the tensile strength of the material in the radial acceleration direction (a fraction of what would be needed in a space elevator, Mankins argues).” At 3% of lightspeed, an object moving in a circular path with a radius equal to that of, say, the earth (R = 6,300 km) is undergoing a centripetal acceleration of
    a_ centripetal = V^2/R = (0.03*3e8 m/s)^2/(6.3e6 m) = 12.9e6 m/s2 = 1.3 Mgee

    The pressure required to keep a 1 ton cubic object (w = 1 m) with density 1 g/cc would be:
    P = F/A = m a/A = density*w*a_ centripetal = 1000 kg/m3 * 1 m * 12.9e6 m/s2 = 12.9e9 Pa = 12.9 GPa

    This pressure exceeds the ultimate strength of known engineering materials. This pressure would be exerted both upwards on the object being accelerated and outward on the track along which the object moves. Presumably, this is what motivates Rather’s suggestion to make the thing 50,000 km in radius, thus lowering the loading on payload and structure by a factor of ten from this value.

  • Brett Bellmore November 23, 2014, 9:27

    “Now maybe the beams could be focused, but I would have thought there were fairly simple engineering solutions to prevent this.”

    The Glaser solution was to send a reference beam up from the ground, which was used by each of the transmitters to achieve phase lock. Without it, the beam would be hopelessly defocused.

    But, frankly, with modern technology, you’d have to expect that a work-around could be designed in covertly. Maybe just a nearby satellite sending a spoof reference beam to drive the focus onto a specific target. There really is no way to get around it, these things would be potential death beams, you’d have to trust the operators not to use them that way.

  • Tom November 23, 2014, 13:19

    I’m currently enjoying everything that is coming from the Rosetta mission; Humans are placing our machines around and on an active comet.

    One point made by one of the mission directors was that the optic systems that were ‘state of the art’ 10 years ago have been surpassed by the common cellphone camera? This is causing some technical difficulties with the mission.
    ‘Moore’s Law’ is impacting space mission mangers.

    There is a need to get hardware around the Solar system faster before the built designs become obsolete. Sure, things work; however when your current missions are 1 generation behind the stay home end of your infrastructure… this would be fatal in a commercial business model.

    All the technical arguments may boil down to… could any of these plans be built before they become obsolete?

    I miss Dr. Forward… in principle his lasersail ships could reach anywhere within our galaxy.
    It’s not hard to speculate that the SSPS and ‘Star Sling’ system would mature into ‘Dysonspheres’ and some kind of Petawatt super-assembler that would ‘farm’ entire solar systems into habitable realms for futurekind?

    Plenitudes and hyperbola aside, we got a lot of work ahead of us… but the next 10 kiloyears look awesome! I think I would brave 10 kilocycles around a ‘star sling’ if the gamma factor went high enough to place me in that future?
    The dominant lifeform might be cybersapiens?

  • Eniac November 23, 2014, 17:30

    Adam, Alex: Sure, a weak beam makes it less dangerous, but it also makes it less desirable. A rectenna may be cheaper to build than an equivalently sized field of terrestrial solar panels, but presumably the space based equipment would be much more expensive than either. Consequently, a terrestrial solar farm will blow an SPSS plus equivalently sized rectenna out of the water, economically, at least until you can make space based hardware as cheap as the terrestrial kind.

  • Eniac November 23, 2014, 17:38

    As others have pointed out, the Star Sling has some serious problems in the details on how you are going to keep the payload on track against a centripetal acceleration of mega gees. I am pretty sure that wheels won’t do it, magnets won’t do it, and I’d be interested in what Rather thinks might do it?

  • Brett Bellmore November 24, 2014, 6:57

    The argument for the weak beam microwaves is that you can dual use: The rectenna will stop the microwaves, while letting through light, so you could quite easily farm under one. So, in theory, you mostly don’t have to pay for the land.

    From a heat balance standpoint, a sole use rectenna *could* be better than terrestrial solar power, because you could paint it white, and reflect back all the solar energy, while very efficiently converting the microwaves. While terrestrial solar panels tend to turn most of the light they don’t convert into electricity into heat, and then the electricity gets turned into heat: Covering an area with solar panels is roughly equivalent to painting it black. Unless you float them on the deep ocean, or site them some other place that’s comparably dark, you’re going to increase the net heat takeup of the Earth. While the rectenna can represent a net heat savings if not dual used.

  • Eniac November 26, 2014, 13:48

    Brett, I see the dual use argument, but I do not see circumstances where it is benefit enough to justify the expense of generating the beam in space, when equally dense solar energy can be picked up directly on the ground. Real estate is just not that expensive, especially if restricted to whatever use it is still fit for when constantly irradiated by microwaves at solar power levels. Perhaps the best use might be ranch land in cold latitudes: Any microwaves not picked up by the rectenna could help keep the cattle warm.

    Heat added to the Earth, either by the blackness of solar collectors or by microwave beams, is insignificant until a substantial fraction of the Earth is covered with solar panels or microwave beams.

  • Brett Bellmore November 27, 2014, 10:39

    “Heat added to the Earth, either by the blackness of solar collectors or by microwave beams, is insignificant until a substantial fraction of the Earth is covered with solar panels or microwave beams.”

    That is only to say that heat from any energy source is insignificant so long as that energy source is still insignificant. Smaller differences in albedo than between desert and solar panels drive the urban heat island effect.

    The brute fact is that our industrial society is using energy at levels approaching some natural energy flows in the environment, and so that energy use effects the environment. Concern about injecting extra energy into the Earth system is as reasonable as concern about enhancing it’s retention. (The greenhouse effect.)

  • Alex Tolley November 27, 2014, 12:34

    In extremis one could imagine the Earth completely enclosed within a rectenna. If high enough off the ground, it would be invisible, have an extremely low footprint with the support towers, allowing the biosphere to work without disruption while supplying power to civilization. This would be far more viable than covering the planet with solar collectors displacing the biosphere.

    Alternatively, to avoid the frying of wildlife with high power beams, collectors could be placed on platforms high in the atmosphere and the power transmitted to the ground via conductors. This would allow high power beams from space to be collected with environmental safety. Tethered balloons might make good platforms in the stratosphere.

  • Eniac November 28, 2014, 19:33

    I don’t see how putting the rectennas higher up would keep the beams from hitting the ground. Even the most efficient rectenna will let a significant fraction of a beam through, which would travel straight through any remaining distance to irradiate the ground, whether it be meters or kilometers away. I don’t think you’d want people to live or work there (although it could really help save on heating cost), so full coverage of the Earth would be pretty much out of the question.

  • LocalFluff November 28, 2014, 20:41

    What if we had the luck to make gravity sling shot by one of these semi relativistic hyper velocity stars, moving at up to 100 000 km/s, that is 0.33c!

  • Alex Tolley November 28, 2014, 23:03

    @Eniac – what is the leakage of the rectenna? My domestic microwave has a very low leakage rate. It must be possible to build something with that low a leakage rate, a rectenna with extra shielding?. The original O’Neill suggestion of rectenna farms over pasture suggested not only almost no effect on the cows, but that birds could fly through the low power beams above the rectennas with very little heating (probably incorrect). Using those assumptions, a high altitude rectenna would be safe to live under, be invisible, and have no biospheric effects other than very low levels of microwave heating, much less than that of PV panels. I would have thought that was a very nice way to get power from a SPS with minimal impact on the biosphere. However it would be one very large cage.


  • Eniac November 29, 2014, 21:00

    Above the antenna the power levels should definitely be not much less than sunshine, and as such clearly affect the birds. If it were much less than sunshine, solar cells would be preferrable. Below a very efficient rectenna, you might get 10%, or even close to zero if you add a metal mesh underneath. You will never get people to live there, though. You’d be lucky if you got them to eat the “irradiated” agricultural products, I think.

  • Alex Tolley November 30, 2014, 0:57

    Birds cannot fly in the stratosphere. So the rectenna would be above them and the rest of the biosphere. People might get used to living under invisible cages. After all we got used to all sorts of strange structures. Make the rectenna reflective and you also increase albedo, reducing warming. A two-fer!

  • Mark Zambelli December 16, 2014, 11:30

    The ‘orbital circular accelerator’ mentioned above seems like a fantastic idea but I must be missing something as I fail to see how this would even be possible. Like others here, I calculate that with a ring radius of 50,000km (safely above geostationary orbit) anything moving at 3% of c will be pulling 165,300 g. While this is half the g-force experienced by the bottom of a high speed lab-centrifuge, is it do-able?

  • Andrew Palfreyman January 3, 2015, 3:44

    The StarSlinger is a turkey as it stands, unless the radius be made large enough. We can build electronics (including cameras) to withstand 1,000 gees, and in order for that to be the maximum acceleration experienced by the payload, the radius of this beast for 3% c final velocity would need to be at least 8.2 million kilometres, which calls for a maglev of track length of 52 million kilometres. Still a very silly idea, then.