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Key Issues for Interstellar Sails

The evening after Jim Benford’s Starship Congress talk on his solar sail lab work at the Jet Propulsion Laboratory, a small group of sail advocates joined him in the Hilton Anatole’s top floor restaurant to talk over the issues. Benford is organizing Project Forward, named after the legendary Robert Forward, as an Icarus Interstellar effort to further refine the interstellar beamed sail concept. Asked to name the biggest problem areas for sails, the group came up with several, but at the top of the list was deceleration. How do you slow a beamed sail down when it arrives at its target?

A number of possibilities suggest themselves and at this point all of them are completely theoretical. Forward himself wrote up a ‘staged sail’ concept, in which the outer ring of the sail detaches as the star is approached, moving ahead of the inner ring and attached payload. The Earth-based beamer bounces the laser off the larger sail ring, which reflects it back to the smaller sail and slows it for orbital insertion. The maneuver is described in Forward’s 1984 novel Rocheworld, where a three-part sail is used to allow for crew return.

Here’s Forward’s description of a mission to Barnard’s Star on the sailship Prometheus as the sail staging has begun. The inner sail (and payload module) is being turned around so as to receive laser light from the outer ring segment, called the ring-sail:

As the central sail was almost halfway around, the ring-sail readjusted again and started to bring the rotation of the central sail and Prometheus to a halt. The teamwork of the four computers was perfect. The rotation stopped at the same instant the central sail was exactly one hundred and eighty degrees around. The central sail now had its back to the light coming from the solar system while it faced the focused energy coming from the ring-sail. Since the ring-sail had ten tines the surface area of the central sail, there was ten times as much light pressure coming from the ring-sail than from the solar system. The acceleration on the humans built up again, stronger than before, but now it was a deceleration that would ultimately bring them to a stop at Barnard.

Have a look at the diagram below to see the basic method. If the image looks familiar, it’s because this is one of the few I’ve found illustrating Forward’s idea, taken from his original paper on a mission to Epsilon Eridani. Remember that Forward was assuming a huge lens in the outer Solar System which would be used to keep the beam tightly collimated. You can see how complicated this is, and why a magsail also suggests itself as a somewhat more direct option, though braking against a star’s stellar wind brings up numerous problems of its own.


Image: Forward’s separable sail concept used for deceleration, from his paper “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails,” Journal of Spacecraft and Rockets 21 (1984), pp. 187-195. The ‘paralens’ in the image is a huge Fresnel lens made of concentric rings of lightweight, transparent material, with free space between the rings and spars to hold the vast structure together, all of this located between the orbits of Saturn and Uranus.

If you look at the Forward scheme, though, you can see why the second big problem for beamed sails is jitter. No matter how accurate your beamer, you’ve got huge issues trying to deliver a laser beam to a sail ten light years away, a kind of accuracy that’s just as breathtaking as the power requirements for driving the sail in the first place — Forward’s Epsilon Eridani mission calculation cited a power requirement of 7.2 terawatts. In fact, in a 2003 paper, Travis Taylor and Gregory Matloff took note of the jitter issue and how far we are away from solving it:

The analysis given here, which did not take into account pointing jitter, suggests a minimum of about 15 km in radius for the collector. If pointing error is considered, it appears that the current state-of-the-art of jitter control is many orders of magnitude from enabling a laser sailing mission. Beam control is the largest obstacle for laser sailing.

Building Large Structures in Space

But it’s interesting to see that useful work is being done on the matter of building and deploying large structures in space. Forward’s company Tethers Unlimited, which he founded in 1994 in partnership with Robert Hoyt, has just been awarded a $100,000 grant to develop its SpiderFab project, which will use 3D printing methods in orbit to create the kind of structures we need. You can see how critical this is: Right now a major part of the cost of engineering and launching space systems revolves around the demands of surviving the launch phase, not to mention the cost of the launch itself. Tethers Unlimited plans to find ways around the problem, as described in a report Hoyt wrote for NASA:

We propose to develop a process for automated on-orbit construction of very large structures and multifunctional components. The foundation of this process is a novel additive manufacturing technique called ‘SpiderFab’, which combines the techniques of fused deposition modeling (FDM) with methods derived from automated composite layup to enable rapid construction of very large, very high-strength-per-mass, lattice-like structures combining both compressive and tensile elements. This technique can integrate both high-strength structural materials and conducting materials to enable construction of multifunctional space system components such as antennas.


Image: SpiderFab combines techniques evolved from terrestrial additive manufacturing and composite layup with robotic assembly to enable on-orbit construction of large spacecraft components optimized for the zero-g environment. Credit: Tethers Unlimited/NASA.

Rob Adams (NASA MSFC) described current work on 3D printing in his session at Starship Congress, a technique that would make it possible to create parts on the fly, and one that could even be used for printing out various kinds of foods — NASA has been looking at 3D treatments of pizza (I kid you not) as an experiment in lowering food waste and varying what astronauts eat to go beyond the typical pre-packaged fare. SpiderFab is a step further, the kind of robotic technique that could one day be used not only to build kilometer-scale sails but also key parts of the beamer.

We’re in the early days of sail design but we’re making progress, both in the lab and in space through missions like IKAROS and NanoSail-D. I think too about the two sails that NASA deployed at its Plum Brook facility in Sandusky, Ohio in 2005. These were demonstrators built within a vacuum chamber that have paved the way for Sunjammer, a mission Les Johnson described to the Starship Congress audience. Sunjammer folds up into something the size of a dishwasher, but when deployed in space it will have spread to 1200 square meters, seven times the area of the IKAROS sail, while weighing a scant 32 kilograms (ten times less than IKAROS).

Launch of Sunjammer is currently planned for next year. The mission stirs fond memories of Arthur C. Clarke, whose story “Sunjammer” inspired its name. We should also acknowledge Poul Anderson, who confusingly enough published a story of the same name in Analog a month after Clarke’s story appeared in a 1964 issue of Boy’s Life. You’ll find the Clarke story retitled “The Wind from the Sun” in many anthologies, but whatever its name, the story of a solar sail race to the Moon firmly established the sail concept as a player in the minds of science fiction readers and helped its acceptance by the public.

Johnson, whose recent book Going Interstellar (co-edited with Jack McDevitt) should be on the shelf of any interstellar advocate, went on to describe the Sunjammer mission and a number of other sail concepts currently under development. I had intended to get to all of these today but in my enthusiasm I’ve run out of time, so we’ll continue with more of Les Johnson and talk of where sails are going tomorrow. I was glad to see that Starship Congress was heavy on sail technologies, including the uses of nanotechnology and an interesting idea for a beamed laser infrastructure that we’ll be examining as I continue sorting out my notes.

The Taylor and Matloff paper referred to above is “Space Based Energy Beaming Requirements for Interstellar Laser Sailing,” CP664, Beamed Energy Propulsion: First International Symposium on Beamed Energy Propulsion, ed. By A.V. Pakhomov (2003), American Institute of Physics 0-7354-0126-8.


Comments on this entry are closed.

  • Paul W August 20, 2013, 10:17

    I do appreciate the technical feasibility discussion.

    But they seem to all require “beaming” from earth.
    I would suggest that sailers think in terms of a self-sustaining ship that can take care of it’s own needs.

    After 4 thousand years, who is going to remember that anyone is out there needing a beam? It’s a big assumption.

  • Paul Gilster August 20, 2013, 10:46

    Paul, do keep in mind that the 4300 years was for a sail without a beam on it; in other words, a simple solar sail with a close approach to the Sun and subsequent acceleration. If we put a laser or microwave beam on the sail, according to many studies, we can, depending on the power and the sail materials, achieve speeds up to 10 percent of the speed of light. I use the 4300 year figure to illustrate the value of beaming power to a sail that would otherwise be much more constrained in velocity.

  • John H August 20, 2013, 12:00

    Optical tweezer technology holds objects with the requisite optical properties in the focal point of the beam. A variable focal length can control acceleration and deceleration by controlling the time rate of change of the focal plane. The intrinsic restoring forces keep the beam in the waist and provide immunity to a limited amount of jitter. A lens with a focal length of light years is a bit of a problem.


  • Vasili August 20, 2013, 12:31

    Pointing accurracy is indeed a huge problem, as is the issue of a slightly off-centre laser beam shredding the delicate sail to pieces. Not to speak of the huge apertures and sails required …

    All of this can be avoided however, if the laser is used to accelerate microscopic sails towards a target spacecraft (Kare’s SailBeam). Another option would be particle-beam accelerated micropellets deflected by a magnetic mirror spacecraft (Nordley’s Beamriders). All of these concepts seem much more feasible than Forward-esque laser sails and deserve more attention by the interstellar community.

  • Alex Tolley August 20, 2013, 13:20

    Flight testing another solar sail. Who would have even thought this might happen just a few years ago? So what path needs to be followed for the Benford’s microwave beamed sails to be flight tested? It seems to me that a test on even a very small sail using a ground based beam should be quite feasible, perhaps being deployed from a cub sat? Could a beamed sail even be used as a final stage to orbit if launched from a suborbital rock at the edge of space? (deployment might be very tricky and payloads small).

    Whether sails are ever going to be useful for interstellar flight or not, they do seem to offer a clear, scalable technology for solar system flight for science and cargo missions.

    Is there any data on the Isp equivalent for the beamed carbon sail experiment?

  • Mark S August 20, 2013, 17:37

    Since you made a reference to some scifi, I think one of the stories that turned me off to this idea pointed out that the real problems are political rather than technical. Would a government keep the laser fully funded for the duration of the mission? Would they decided the return mission is too expensive? How would you handle politicians that decide to curtain funding in the middle of the mission or decide that they could just double the laser power later in the mission to make up for the time they had it turned off or operating at reduced power?

  • Paul W August 20, 2013, 22:30

    Ok, so an accelerated sail moving at a tenth of the speed of light.

    So 50 years for a voyage to AC. Obviously another time scale than 4000.
    50 years for the travelers.

    That’s doable, assuming all other factors can be controlled. I’m for it.

    I still think someone in deep space shouldn’t need to rely on what’s happening back on earth.
    The “ship” I fly through the air likewise relies on tech outside of itself for guidance. But not for power. If all the GPS sats failed and the radios all failed, I could still get it back down in one piece.

  • Peterh August 20, 2013, 23:06

    How would an electric or magnetic drag sail compare for slowing down? Depending on politicians back home decades after departure sounds risky.

  • SWage August 21, 2013, 9:38

    Hmm… and if we use lots of small solar deflectors orbiting the sun wich are programed to direct the sun’s light to a focus station, wich is relatively stationary relative to the sun? So we can create a continous beam directly twoards the target star. The sailcraft would ride the beam, instead fo being actively targeted. There is of course beam drift, as well as the target is ultimately not stationary as well as the sun isn’t staionary. Detatchable deflectors could be used to reroute the propulsion beam for course corrections, instead of retargeting the propulsion beam itself and to refocus the beam as repeater. Do we need to power the sailcraft all the way to the target system? What if we use a detatchabele magsail wich is aimed directly at the target sun, braking on solar wind and then launches the payload in the opposite direction before impact, taking most of the crafts mass with it? The payload could never return, of course, but it could establish long range communications with an on board beam with a few years “lag”. We could launch a number of those probes focusing a bundled communication beam back to Earth.

  • galacsi August 21, 2013, 16:56

    I am hightly sceptical about these sails. Because the interstellar medium , even if very thin, will oppose a resistance to the sail. The faster the sail will move, the bigger will be the resistance.

  • Mark Wakely August 22, 2013, 15:07

    Have to agree with galacsi, although with the sun in a local bubble or chimney that’s hundreds of light years across, the danger of dust and gas resistance is 100 to 1,000 times less that outside our current “safety zone”:


    Clear sailing still isn’t guaranteed, of course, but it’s fortunate that the bubble exists and makes sailing within its confines less risky to a significant degree.

  • Paul D. August 23, 2013, 11:48

    The problem from the ISM is not resistance, but rather erosion. An incoming particle is not going to deposit much of its momentum in the sail (the sail is far too thin to stop energetic particles), but can easily dislodge atoms from the sail.

  • Andrew Palfreyman August 29, 2013, 18:34

    JWST is spec’d at 25 nRad (5 mas) pointing accuracy, made possible by using SiC. I’m taking that as a baseline capability when working up my spreadsheet.

    An idea that may be new is this – rather than having a sharp cutoff in beam power at a few AU due to the pointing accuracy limit, it would be smarter for the beamer to progressively widen the beam over time. That way we keep useful power on the sail for longer, and obviate potential mis-steering problems. This refinement turns out to make a significant difference to the final achievable velocity, and thus to overall trip time.