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Solar Sails: The Interstellar Prospect

The vast laser-driven sails envisioned by Robert Forward have always fired my imagination. Hundreds of kilometers in diameter, they would rely upon a gigantic Fresnel lens in the outer Solar System to keep the critical laser beam tightly collimated over interstellar distances. Forward conceived of mission designs to stars as far away as Epsilon Eridani, journeys that could be achieved within a human lifetime. He even provided return capability through the use of a multi-part sail. You can read a fictional treatment of this in his novel Rocheworld.

But how do we get from here to there? As of today, we’re close enough to having an operational space sail that if we can talk SpaceX into lofting the NanoSail-D duplicate, we could be shaking out our first space sail within months. Assuming we do go operational before too many months (or years!) pass, the question then becomes, what kind of missions are possible between the laser-beamed lightsail of science fictional imagining and the practical workhorse sail that may well open up a space-based infrastructure for our use.

Such questions point to the pleasures of reading a new book on solar sails by three leading experts. Gregory Matloff has been examining the concept for the past thirty years, with seminal papers in the 1980s and continuing work on near-term concepts. His regular consulting at Marshall Space Flight Center keeps him in touch with co-author Les Johnson, NASA’s deputy manager of the Advanced Concepts office at Huntsville. The third author is Italian scientist Giovanni Vulpetti, who has spent most of his professional life on questions of interstellar propulsion ranging from antimatter annihilation to sail design, including the Aurora Project, a sail mission to the heliopause that grew out of earlier work at Italian aerospace firm Alenia Spazio and other European venues.

You would be hard pressed, in other words, to find a more knowledgable team to write a book titled Solar Sails: A Novel Approach to Interplanetary Travel (Copernicus, 2008), and it’s a pleasure to add that despite the sub-title, questions of interstellar significance receive solid treatment. Getting from here (a technology ready to fly for testing) to there (a genuine interstellar craft deploying sail technology) is a long haul, but near-term concepts for a Solar Polar Imager, putting a payload into a highly inclined orbit around the Sun to study its polar regions, are feasible. And so are missions like Heliostorm, which could use a sail to maintain a position between Earth and the Sun (at roughly 0.70 AU, but with a period of one year) to provide advance warning of solar storms.

We can add longer-term prospects that still fall well within our engineering capabilities, missions for comet rendezvous, Mars sample return and, via a Sundiver maneuver, a probe to the heliopause some 200 AU out. The latter, the authors note, could continue for a few decades more to study the environment at the Sun’s gravitational lens some 550 AU from Sol, providing a useful check on Einsteinian general relativity.

All of these concepts and more are discussed here, but it’s the longer-term missions (still using solar photons, not beamed lasers) that truly up the ante. I remember talking to Matloff about them during a Huntsville visit some years back, but the book lays them out sequentially, showing us the limits of the technology in terms of true interstellar missions, and pointing us toward the laser and other beaming options that may be necessary if we are to improve travel times significantly.

For in addition to the outer system work we’d like to perform, examining NEO deflection or developing mining strategies for interesting objects, we’d like to get all the way out to the Oort Cloud, where as many as a trillon comets may lurk. A specialized task indeed, as the authors note:

This is a task for the Oort cloud explorer, perhaps the ultimate sailcraft before a true starship. Imagine a sail 100 nanometers thick, perhaps a kilometer in radius, which is constructed of material capable of withstanding a perihelion pass of about 0.05 AU (about ten solar radii). Such a craft could perform a Sun dive and project its payload toward the stars at velocities in excess of 500 kilometers per second.

All of which gets us to a few thousand AU before the vehicle’s operational life ends, but we’re still — at 500 km/sec — talking about 2000 years to travel the 40 trillion kilometers to the Centauri stars. Can we do better? Matloff worked with Michael Mautner and Eugene Mallove in a series of papers in the Journal of the British Interplanetary Society back in the 1980s to examine such questions. An optimized interstellar sail would use a nanometers-thin monolayer of beryllium, aluminum or niobium, all highly reflective and temperature tolerant. It could be mounted partially unfurled behind an asteroid that would serve as a sunshade, then put into a parabolic solar orbit with a close pass by the Sun measured in the millions of kilometers.

The sail, of course, would be unfurled at the right moment to maximize acceleration. The results:

Analysis revealed that acceleration times measured in hours or days were possible. By the time the ship reaches the orbit of Jupiter, the sail could be furled, since acceleration has fallen to a negligible value. The sail could be used as cosmic ray shielding and later unfurled for deceleration. Flight times to Alpha Centauri, even for massive payloads that could carry human crews, could approximate a millennium. Of course the hyperthin sail sheets required to ‘tow’ such large, multimillion-kilogram payloads would be enormous — in the vicinity of 100 kilometers.

A thousand years to Centauri — this is the figure I recall from my Huntsville discussion with Matloff, and it stuck with me as a kind of insurance policy. If we were to learn, for example, that we had for reasons of survival to exit our Solar System, the prospect of getting at least a fraction of humanity to another, although demanding a lengthy, multi-generational voyage, would not be beyond the reach of a technology we could conceive of developing within this century, using the laws of physics as currently understood. All of which confirms what Robert Forward used to say, “Travel to the stars is difficult but not impossible,” a real reversal of many pre-Forward opinions.

There is more to be said about laser-beaming and other sail concepts (not to mention sail design, construction, deployment and trajectories), but we’ll look at the book’s treatment of these in a later post. Consider this simply a heads-up to alert you to a title that needs to be on your shelf if you’re seriously interested in the next step as we move beyond rocketry. And move beyond it we must, to explore the advantages of leaving the propellant behind to maximize payload for missions that may one day take us deep into the Solar System and beyond.

Comments on this entry are closed.

  • James M. Essig September 4, 2008, 21:50

    Hi Paul;

    This is an excellent article.

    One possibility for a vary large sail with high mass specific area might be one made of metalized or metal filled carbon nanotubes wherein the tubes would be spaced in a net-like fabric wherein the average separation of the tube threads would be about 200 nanometers. The net would act like a grid-like antenna which can reflect microwaves with a wavelength of 1 1/2 to 2 + times the spacing between the electrically conducting grid elements.

    Because carbon nanotubes might be at least almost superconducting, their electromagnetic skin depth might be adequate to reflect the optical and IR components of sun light. Perhaps a space craft using such a net could be shielded by a carbon reflective shield and pass within 0.02 AU from the sun or even closer, although for such a close approach, the space craft would need a darn good extremely refractive and reflective shield.

    Such a carbon nanotube net might have such a mass specific area that it might be operable as far as 1/2 light-years from the sun. Since the mass specific area of the sail would be so low, sails of huge area could be deployed once the craft was well away from the planetary solar system.

    As for Fresnel lens concentrated solar radiation, using a concentrated beam to push a carbon nanotube based sail might permit 1 or more Gs of acceleration, even at distances of several light years. I do not remember the title of the source in which I read that beam ships might effectively continually accelerate for distances as far as 1,000 light-years, but 1,000 light-years seems to be the maximum distance I have seen quoted for beam ship acceleration from beaming stations in solar orbit. In reality, the first beam ships would probably be much less capable.

    Anyway to increase the mass specific area of solar sails or beam sails is fine with me. With a power output of 4 x 10 EXP 26 Watts, I hate to see all that energy go to waste.



  • Adam Crowl September 5, 2008, 1:44

    The elegance of star-sailing versus the flash-bang of pulse drives – I know which one I’d prefer. Still a high-gee solar fry-by with a millennial chill out is not undertaken lightly. I hope we develop something a bit speedier and bit less arduous… but then per aspera, ad astra.

  • Bernd Henschenmacher September 5, 2008, 13:14

    It is a great article about a wonderfull and well written. I have already it and I can highly recommend it. I hope that solar sails will soon be found outside book pages. Even if they are a little slow. Well they are faster than ordinary chemical rocketry or ion engines and they are feasible.

  • george scaglione September 5, 2008, 14:43

    jim and adam -yes i am back as i said i would be earlier and as expected did find that now the comments are posted. and as jim said,yes this is a good article. i am not usually all that excited by solar sails but when it comes to space exploration,probably as far out as the oort cloud – i will not quibble too much! hope we soon see more about this and frankly all other advanced means of propulsion. i know that nasas breakthrough propulsion physics team is pretty much out of business and that the esa is running an advaced team like it that is exploring some good ideas.not to mention naturally our group here. sure hope alot of folks will jump on board soon with whatever is on their minds visavie going to the stars! but it is good to know that a start has been made,sincerely all, your friend george

  • David Portree September 5, 2008, 16:21

    I ordered my copy of the book as soon as I finished your article, Paul. Thanks for the heads up.

  • John Hunt September 5, 2008, 17:40

    I always appreciate articles addressing near/intermediate-term concepts.

    constructed of material capable of withstanding a perihelion pass of about 0.05 AU

    SOHO routinely finds sun grazing commetary fragments all coming from the same direction. It has observed probably 350 in the last 14 years. Likewise, Comet C/2006 P1 McNaught came to within 0.17 AU of the Sun which is fairly close to the 0.05 AU needed. My point is that finding a comet to follow that close to the sun might be fairly easy to accomplish. However, I’m not sure what challenges there might be having to deal with the off-gassing.

    Flight times to Alpha Centauri, even for massive payloads that could carry human crews, could approximate a millennium. Of course the hyperthin sail sheets required to ‘tow’ such large, multimillion-kilogram payloads would be enormous — in the vicinity of 100 kilometers.

    Again, we don’t have to worry about multimillion-kilogram payloads if our human crews are frozen embryos. True, the frozen embryo strategy involves very challenging automated systems of gestation, lifesupport, and rearing at destination. But I’m betting that we could solve those technical challenges earlier and at lower cost than dealing with a multimillion-kilogram payload and 100 kilometer sails.

    Incidentally, do we get much better performance by combining solar sail and magsail?

  • Adam Crowl September 5, 2008, 20:00

    Hi Bernd

    Solar-sailing doesn’t have to be slow compared to ion-drives. One Newton thrust – which is impressive performance for current ion-drives – only needs a reflective sail about 100,000 square metres in area. That’s not a huge stretch of current abilities.

    One day immense sails will be plying the space-ways between the planets for the same reasons they dominated the oceans for so long – the energy is FREE. And, unlike ocean travel, the solar-sails already have an exhaust velocity that can’t be exceeded. Thus, as a reaction drive, sails will never be out of fashion. A sustainable interstellar laser-sail system will require solar-power – all the uranium and thorium in the asteroid belt would be depleted rapidly if we powered the lasers with nucleonics.

    Or so I see it. Nukes might be preferred in the early days, but they’re only an option of “an economics of scarcity”. In the long run – at least to 100 trillion AD – only the stars can sustainably power an interstellar civilization.

  • dad2059 September 5, 2008, 20:33

    One must not forget Cordwainer Smith’s imagery in “The Lady Who Sailed The Soul” where one lone volunteer doped with time ‘slowing’ drugs managed light sails hundreds of miles in area.

    The ‘passengers’ were in hibernation pods strung hundreds of miles behind the ship. Plus the sails helped cut down on dangerous cosmic radiation.

    Classic ideas are the best at times!

  • James M. Essig September 5, 2008, 21:41

    I am a fan of solar/stellar sailing as well. Since the stellar epoch will last at least for 10 EXP 14 years, we can travel very far indeed by star light which is free. In fact, at the current rate of universe expansion and assuming an eventual 1/2 C average propagation rate of human colonization throughout intra-galactic and intergalactic space, in only one trillion years, our species remotest outposts will be traveling at least a couple orders of magnitude greater than C from the Milky Way Galaxy, assuming stable and constant time averaged space time expansion rate.

    A high end output O class super giant star can put out a luminosity perhaps as great as 10 million suns. This amounts to 40 trillion metric tons of mass converted into energy per second. 400 million Nimitz Class Aircraft Carriers could be accelerated to 0.86 C in one second by this power if it was used with 100 percent efficiency. 400 Nimitz Class Aircraft Carriers could be accelerated to a gamma factor of 1 million in just one second assuming such power output. The LHC will only accelerate protons to a gamma factor of only about 10,000.

    Star power is just too valuable of a resource to waste.

    It is really kind of ironic that as we develop advanced interstellar manned vessel propulsion systems, we might go back to sailing; this time instead of “sailing the ocean blue”, we may be sailing the great void black.


    Your Friend Jim

  • andy September 6, 2008, 14:06

    One of the issues I’ve never been entirely clear on regarding the laser-propelled sails idea is how to slow the craft down in an unexplored target system. If the craft only has a solar sail as a braking mechanism how does the requirement to be able to rendezvous with a planet in the target constrain the maximum velocities?

    (Laser-sailing would probably result in a whole bunch of acceleration/deceleration taxes on spacecraft entering or leaving a system)

    O/T: why is the “recent comments” section in the sidebar gone? I personally found this very useful to follow the various discussions that go on…

  • Administrator September 6, 2008, 14:13

    Andy, the only methods I’ve seen for decelerating a solar sail are for the payload to include a magsail for braking that can be deployed upon approaching the destination system, or Robert Forward’s idea of a multi-part sail that detaches so that laser light is reflected off one part of the sail back onto the other, payload-bearing part. More about the latter in a coming post. Forward fictionalizes this in Rocheworld.

    Re the ‘recent comments’ function, the software was causing server problems whenever deployed and I had to turn it off. This may not be permanent.

  • ljk September 6, 2008, 14:20

    MESSENGER Sails on Sun’s Fire for Second Flyby of Mercury


    “On September 4, the MESSENGER team announced that it would
    not need to implement a scheduled maneuver to adjust the probe’s
    trajectory. This is the fourth time this year that such a maneuver
    has been called off.”

  • forrest noble September 6, 2008, 14:23

    It’s true that sails don’t sound exciting but man’s first trip around the world was accomplished by sail. Like mentioned above, it was more than 300 years before we had a better means of sea locomotion. Just like the wind is plentiful and free, so solar power, or later maybe stellar power, might be the easiest way to get there and back until other systems are perfected engineering-wise, and designed to be cost effective.

    Large lensing systems in space strategically placed like orbiting Earth and Venus, both focusing and reflecting sunlight, and conversions to micro-waves for beaming, might be cost justifiable as a power source for earth. Such a system could second as a focusing or beaming source for solar sailed space craft, also supplying micro-waves for electricity, laser beams, ionization power, heat for combustion systems, focused heat, light and micro-waves for Martian colonies etc. when needed.

    Such a system around Venus was proposed maybe 25 years ago as I recall but they were considered not to be cost effective and too risky as far as potential loss of a large lensing satellite.

    The Russians already have sent up a small lens to detect the heat difference in focused areas on the ground. As I vaguely recall it did have a measurable desired effect on temperatures as it traced a path across Russia.

  • Peter September 7, 2008, 17:03

    Solar sails should be pursued as a means for interplanetary transport, but (as I’ve related here before) it doesn’t make sense to use them to embark on interstellar missions. To be honest I’m still trying to figure out why these “millenia- long missions” are even being discussed. Even science fiction authors generally don’t bother with them- not just because they’re boring; they’re boringly impractical. Before you advocate such missions, first ask yourself whether you’d be willing to spend the rest of your life inside a tin can with little to do, in the depths of interstellar space….It’s almost a self-defeating proposition, since the kind of people who’d be willing to do this probably aren’t right stuff astronaut material- they’re the kind of people who belong in psyche wards.

    And please, don’t underestimate the value and relevance of the Incessant Obsolescence Postulate.

    Just imagine how it must feel, to be at year 546 of your one way journey, and look out the window to see a starship 10 times faster than yours whiz by…

    Interstellar flight is like nuclear power- it’s a goal that not any civilization should undertake- civilizations such as ours who are still dependent on fossil feuls aren’t yet ready for it on an engineering level (which is why we shouldn’t even be talking about solar sails for interstellar missions; compared to the other options, it’s simply not appropriate for interstellar flight). If we had already harnessed fusion or antimatter power, then we would have orders of magnitude more credibility when discussing interstellar flight on an engineering level, since these technologies would at least make interstellar voyages conceivably practical, if only to the intensely misanthropic and slightly demented. (that is, 40 years one way, instead of millenia one way, with little chance for a return voyage).

    But even fusion and antimatter are, as it turns out, impractical for interstellar flight, and I will continue to defend this position for many reasons. Since most of us who frequent this site would agree that we’re ready for at least the experimentation if not the engineering stage for interstellar flight, we are at that time where we need to filter out the dead ends, so we can better focus our energies. When it comes to interstellar prospects, I’ve come to the decision that this means filtering out not just solar sails, but fission, fusion, and even antimatter. These make awesome interplanetary engines, no doubt- we could even mass-produce them, but the crucial question is: will they ever be practical for interstellar flight?

    Let’s face it- the whole idea of intense, concentrated power and reaction mass is probably the wrong approach. It’s as though the universe were a horse that we need to break in- but the best way may not be brute force; we need to understand it better first.

  • James M. Essig September 7, 2008, 22:35

    Hi Folks;

    I read a website of Karl Schroeder about a fascinating concept of an interstellar space craft that would harness energy or mattergy from stellar natural resources and which would gradually accelerate to velocities better than 1/2 C. As the space craft accelerated, it would make ever larger loops or curvilinear circuits wherein it would pass by known sources of energy such as magnetic fields around planets, brown dwarfs etc repeatedly, each time receiving a boost in relativistic velocity. The craft could also find new sources of energy as its curvilinear itinerary bought it by these additional sources.

    I am not sure of all of the ways that Schroeder has contemplated extracting energy from the stellar or interstellar medium and I make absolutely no claim to be the originator of the concepts I am about to mention, but one can imagine deploying a solar sail each time the craft cycled by a given one or more stars along its curvilinear itinerary. According to Schroeder, the stellar cycle would maintain its curvilinear circuitous path by towing behind at least one affixed electrically charged cable wherein the Lorentz force acting on the charged cable by the interstellar magnetic field would cause the craft to perform a loop like orbital motion gradually picking up speed along the way in batch mode.

    The stellar cycle by Schroeder is another elegantly brilliant application of reflective-to-electromagnetic energy sails.

    I could not champion the elegance of solar or beam sailing more, short of the development of really exotic travel methods such as wormhole travel or warp drive superluminal travel and the like.

    One can read about this wonderful concept by Schroeder at the following link.




  • Tibor September 8, 2008, 15:59

    “…prospects that still fall well within our engineering capabilities, mission…via a Sundiver maneuver, a probe to the heliopause some 200 AU out.”

    The mission profile of the Interstellar Heliopause Probe ( an ESA Technology Reference Study) “requires the spacecraft to travel a distance of 200 AU from the Sun within 25 years”.

    A good summary is here:


  • Tibor September 8, 2008, 17:08

    Some numbers and ideas to play with, starting at “…the Oort Cloud Explorer …(which) could…project its payload toward the stars at velocities in excess of 500 kilometers per second.“:

    With a velocity a bit larger than that of such an Oort Cloud Explorer we enter, of course, our Long Bet`s arena (http://www.longbets.org/395). Just a bit simple number crunching: 500 km/s = 105.4 AU / year (for comparison: Voyager 1 has a velocity about 3.6 AU/year) For a Centauri Mission (4.24 light years) this would mean 2.544 years cruising time, neglecting any acceleration and perhaps deceleration phases.

    What kind of reasonable things could such a “Long Bet 395” craft do during the long journey? Here a sort of milestones list:

    Year 1: LB 395 passes through the Kuipert belt and reaches the heliosheath about 100 AE
    Year 2: she leaves the heliosphere about 200 AE
    Year 6: she passes Sun’s gravitational focus about 550 AE
    Year 20 – 50: LB 395 now approaches the Oort Cloud (> 2.000 AE)
    All the time: gather tracking data for investigation of the Pioneer-Anomaly

    Imagine that during the long cruising time the craft will be sent into hibernation phase, but will be awake maybe every 100 years and sends a life signal, amended with some data about the interstellar environment. The signals will be detected by probes located at the opposite direction to Alpha Centauri in a distance about 600 AE from the Sun, exploiting the Sun’s gravitational lens. A good job for the Long Now folks to gather all these data.

    Year 2000: Long Bet 395 reaches the Centauri System!

    The Long Now Foundation aims to exist around 04025; so we should not forget the possibility to have a time capsule on board – like the Rosetta craft (https://centauri-dreams.org/?p=2734), or the planned KEO (www.keo.org) satellite with an envisioned 50 thousands (!) years mission (Paul, I believe, re long term stories a report about KEO would be interesting – it is a wonderful idea).

    These are anything else than boring perspectives, I believe.

  • Lubo September 10, 2008, 5:30

    Hello Paul,

    I strongly disagree with these technology. When such solar sail exits Sol with that speed, it’ll be destroyed by micrometeorites, stardust and anything that goes on it’s way. That’s why you, and all others here in this forum, should change the point of view and understanding how stuff works. At least this is Tau Zero Foundation forum, not NASA. Because NASA and it’s employees are so fundamental and with so backwards thinking, that by listening to them, even 500 years won’t be enought to colonize our own Solar System.

    What deserves attention is a project about building a spaceship with Electrostatic High Voltage Field propulsion, powered by magnetic motor producing ZPE, or Quantum Energy. Anything else is for trash ;)

  • Hiro September 10, 2008, 14:28

    Yeah, I think interstellar flight should be divided into 2 stages:

    The first one should be somewhere around our galaxy, we have to use fusion or antimatter propulsion instead of a giant solar sail which will easily hit the interstellar-dust without any protection at a fraction of c.

    The second one should be FTL. If we want to visit some supercluster of galaxies, then we have to use something else. Solar sail is completely useless when the travel distance is several billion light years.

    Anything takes longer 500 years to reach the nearest star is not an option.

  • Lubo September 12, 2008, 17:56

    No space vessel with organic life aboard can travel much above 1/6 light speed, regardless of its propulsion method, due to
    incident interstellar radiation and microparticles (like stray hydrogen

  • ljk September 19, 2008, 11:44
  • ljk October 7, 2008, 12:39

    Review: Solar Sails

    by Jeff Foust

    Monday, October 6, 2008

    Solar Sails: A Novel Approach to Interplanetary Travel
    By Giovanni Vulpetti, Les Johnson, and Gregory L. Matloff

    Copernicus Books, 2008

    Hardcover, 256 pp., illus.

    ISBN 978-0-387-34404-1


    Solar sails have for decades promised to revolutionize in-space transportation. However, while simple on paper—using large but lightweight sails to reflect solar photons, transferring their momentum to an attached spacecraft—the concept has proven difficult to actually implement, be it because of technological issues or funding problems or just bad luck.

    Regardless of those setbacks, solar sails still attract interest, primarily because of their potential to maneuver spacecraft through much of the solar system without requiring any fuel. That promise is at the heart of a new book by three scientists who explore how solar sails work and how they can be used.

    Full review here:


  • Tom June 15, 2012, 4:49

    I have a favorite imagined application for ‘Interstellar light sail craft’. Why not build one for the sole purpose to function as an advertising billboard to the Galaxy? Using the laser sail as a means of propulsion, you get better numbers with increased scale; what to do with it then? Put holographic film on the forward surface with the Pioneer plaque image! Get it up to a good fraction of lightspeed and let it fly into the galaxy. If it should get pulverized, you’d have a cloud of ‘holographic’ smoke… I’m sure if scanned by an advanced civilization it would be determined this was an intelligent artifact. Give it some angular torque and it would look like a bright flashing beacon in the interstellar darkness…. visible up to how many years it was ‘laser-backlit’.