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The Sail Comes to Texas

Low clouds had descended upon Dallas when I landed at Love Field, and by the time I got to the Hilton Anatole for the Starship Congress being hosted by Icarus Interstellar, the city outside was swathed in mist. This was last Wednesday evening, and it was late enough by the time I had dinner that a quick stroll through the cavernous facility was about all I wanted to do before getting some sleep. The Anatole, though, was gorgeous, filled with paintings and sculpture, some of which (two statues of elephants) became helpful landmarks as I learned to navigate the place.

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Image: An atrium at the Hotel Anatole at night, one of two, connected so that inveterate walkers like myself could make figure-eight circuits by following the room corridors on any floor.

Starship Congress was a roaring success, the kind of thing that happens when you put people with passionate interests in the same place who usually know each other only through email or by reputation. Saving the day for me on day one was my friend Pat Galea, an Icarus Interstellar fixture, who had brought one of those little MiFi ‘hotspots’ with him. For we learned that the hotel wireless didn’t extend into the meeting room, but Pat, who is to computer issues what Itzhak Perlman is to violins, soon had four of us up and running with a slow but adequate connection. That allowed me to send out the occasional tweet and check various things on Google.

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I was glad that the Icarus Interstellar Kickstarter campaign had gone as well as it had, raising a good deal more than the initial goal, and thus funding the travel costs of a number of speakers. We’re all learning our way with crowdsourcing tools like Kickstarter, but they offer a level of public engagement that Richard Obousy, Icarus Interstellar’s president, acknowledged in his opening remarks. In the post-Apollo era, everyone is aware of the need to find ways to engage the public in the great issues of space exploration, and having a stake in an interstellar conference, even if the contributor can’t make the trip, is one way to get this done.

Riding the Sail

My conviction is that the first serious mission targeting another star will use sail technologies in one form or another. The first conference session was devoted to sails, leading off with Jim Benford’s keynote, followed by Les Johnson, who described current and near-term work. Right now the only propulsion method that will get us to interstellar velocities is the sail, and even then we’re talking no more than a couple of hundred kilometers per second, so it’s still a long trip. The Alpha Centauri crossing at 300 kilometers per second (perhaps realizable through a close pass by the Sun with deployment of the sail at perihelion) would still take 4300 years.

This is always a bit of a mind-bender because Voyager 1, the fastest probe we have leaving the system right now, moves at a ‘mere’ 17 kilometers per second, and while New Horizons topped that briefly in the early part of its journey, it will ultimately pass Pluto/Charon at about 13.9 kilometers per second. We need to find ways to ramp these numbers up, and that search begins in the lab. What Benford described in the opening session was a series of experiments he and his brother Greg made on beamed propulsion back in the year 2000. The researchers were working with a carbon fiber mat shaped into a small sail with a thickness of less than 1 millimeter.

Carbon fiber is ideal for sail work because when you put a microwave beam on the sail the material absorbs energy and begins to heat. A sail made of aluminum would begin to melt as you reach about 900 K, limiting possible accelerations, but carbon fiber has a low areal density (about 8 grams per square meter in the material the Benfords used) and a microwave reflectivity approaching 90 percent. The material is actually a carbon-carbon microtruss, meaning a core of carbon fibers is fused to a textured outer surface. With carbon nanotubes woven into the material, this microtruss is capable of temperatures up to 3000 K, at which point it doesn’t melt but sublimes, going from solid to gas with no intervening liquid state.

Working with a sail in an Earth-bound laboratory means you have to achieve an acceleration of one g just to lift off, but the Benfords were able to get up to 10 gs in these experiments, using a wavelength of about 3 centimeters and a pulse duration of 0.2 seconds. Although the tiny experimental sails began to heat up at higher beam powers and bounced off the ceiling of the lab, they survived and remained undamaged after the flight.

All this is provocative because a normal solar sail — think Japan’s IKAROS, for example — is pushed solely by sunlight, the fact being that while photons have no mass they do impart momentum. That’s fine when working in the inner Solar System but the effect of sunlight drops drastically as you move outward, dropping off by the inverse square of the sail’s distance from the Sun. In other words, a sail at Jupiter’s 5 AU from the Sun receives only 4 percent of the sunlight it would in Earth orbit, so an outbound sail is going to need a microwave or laser push if we want to keep it under acceleration all the way to system’s edge and beyond.

Enter the Beam Riders

The laboratory work that has been done thus far is highly encouraging. Having learned that a sail can indeed be pushed to high accelerations by using a microwave beam, the Benfords were also able to show that a sail of the right shape — concave and something like a parachute — will be stable and stay centered in the beam. In fact, the beam induces a sideways restoring force so that even assuming a certain amount of ‘jitter’ in the beam itself, the sail is capable of riding the beam. We’re a long way from the laboratory to the gigantic sails envisioned by Robert Forward, but we’re getting good indications that once we have the expertise to build the right kind of ‘beamer’ in space, the physics will allow sail missions that can reach interstellar velocities.

Transmitting angular momentum to a sail through a beam of photons has also been demonstrated and is, in Benford’s words, ‘a trivial process,’ so that sail deployment might become a matter of putting a large sail into space and inducing a spin to unfold it. Whatever deployment method might be used, the fact that a beam can carry angular momentum means that controllers can stabilize the sail against yaw and drift once deployed. Keep in mind, too, that a number of sail mission concepts actually call for lower power densities than the Benfords needed in the lab — remember, they were pushing the sail from deep within a gravity well. So the experimental case for beamed sails is solid and we can look forward to space-based testing.

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Image: Jim Benford discusses the experiments he and his brother performed on sail beaming at the Jet Propulsion Laboratory.

Half a million dollars produced the results Benford described, making the point that while we do have these initial results, we lack a large body of data to draw from not just with sails but most other interstellar propulsion proposals as well. “We have far too many concepts and far too little data,” Benford reminded the audience, adding that “Nature will produce the answers if we ask the correct questions.” Indeed. And these experiments tell us that useful work this early in the interstellar process can grow out of laboratory studies that aren’t hugely expensive. The interstellar community should be thinking about how it can support such ground-breaking work not only on sails but other proposed solutions to the interstellar propulsion conundrum.

One of the exciting thing about interstellar studies is the sheer number of questions it raises. With sails we are dealing with a subject that has moved out of the conceptual phase. We know the physics and are beginning to demonstrate sail methods here on Earth. We can now move in two directions, the first being to work on new materials that will be lighter and more responsive to the beam on the sail. The second step, an obvious one, is to continue with actual space deployments of sails and begin to do beaming experiments on them. More tomorrow as I look at other sail presentations from Starship Congress, beginning with Les Johnson’s overview on what we’ve accomplished so far in space and what we can expect in the next few years.

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Comments on this entry are closed.

  • Paul Titze August 19, 2013, 10:42

    Hi Paul,

    It was good that they got video online for this conference and watched some of the talks which I found very interesting. Just some comments:

    – Not much was said about the space radiation problem for humans (a human can only survive up to 3 years in space without passive and active shielding for the spacecraft. This problem will have to be solved before long human deep space missions are considered. Otherwise probes will continue to do most exploring for us for the foreseeable future. Even a trip to Mars several months long poses a radiation health issue for crew.

    – It is unlikely interstellar travel will kickoff unless a replacement for chemical rockets is found to place hardware from ground to Low Earth Orbit. The expected large amount of hardware required for interstellar travel makes the cost too prohibitive. Marc’s talk on Day 3 on Space Drive (Gravity Control Propulsion) was interesting.

    – Warp Drives and Wormholes (allowed by GR only but denied by Quantum Physics). Experiments related to these will continue to yield negative results.

    Cheers, Paul.

  • Bob August 19, 2013, 12:41

    I watched the streaming video of Sonny White’s keynote talk. I am confused about the state of research on his ‘Q thruster’ and hope someone can chime in.

    I thought he said they achived thrusts in the thousand micro Newton range already, was that real? I tried to check the talk again but the youtube video’s are so long they do not load on my Ipad.

    The other thing that bothers me is that I see the EXACT same slides used to describe his ‘Q thruster’ and Dr. Woodwards Mach Effect device. What is going on? Woodward only claims effects in the ten microNewton range. White’s papers and talk never actually say HOW the quantum vacuum is supposed to create thrust in the device while Woodward goes to great length explaining his Mach effect principles.

    Is NASA co-opting Woodwards work without credit? Are they hyping the results for publicity and funding?

    I hope someone with some inside knowledge can chime in. Thanks.

  • David Cummings August 19, 2013, 12:45

    When you read that 300km/sec equates to a 4300 year trip to AC, the tendency is to say “why bother, let’s just wait until we have something that can go faster”.

    But the point is that we won’t HAVE something that can go faster until we start having something that can go there. The sail is a good idea, a good place to start, a good way to develop interstellar travel technology, in my opinion.

  • Paul W August 19, 2013, 15:13

    4300 years, one way, to the NEAREST star is a tough act to introduce.

    I have a difficult time putting my eggs in that basket, even given that there are no other baskets.

    As long as, meanwhile, people are developing near-earth space technologies like SSTO, lunar bases, materials, evironmental systems, it doesn’t hurt anything to give the sailors all the canvas they can unreef.
    While we wait for physics to stumble on something no one thought of, let’s build better, higher resolution telescopes that also are maintainable, and find another earth to inspire the next Newton or Einstein. A real target would be useful. (NASA ended efforts to revive Kepler, sad)

  • Joy August 19, 2013, 15:44

    Three cheers for the Benfords! I had not heard the results of their carbon sail microwave power beaming experiments reported before. Looks like we have a winner, at least in the medium term for sending probes into the Kuiper belt.

    Unfortunately, the currently installed space electrical generating capacity (on the ISS) is 80 kilowatts. Power beaming for Forward’s Starwisp interstellar nanoprobe was to be 10 gigawatts. His crewed mission to Barnard’s Star required 1,500 terawatts. Our current space power system is 20 billion fold less than that. Yes, microwaves could be beamed from the surface of the Earth, but producing 1,500 Terawatts on the ground would cook the planet, so not a viable option.

    So, I must echo Paul Titze’s point, “It is unlikely interstellar travel will kickoff unless a replacement for chemical rockets is found to place hardware from ground to Low Earth Orbit.” – Amen.

    We need a cost cutting miracle method of getting to LEO on the order of a space elevator to make any of this possible.

  • David Cummings August 19, 2013, 15:50

    Paul W, I agree about telescopes and lunar bases. I’d like to see those 2 things discussed together, as in a telescope on the south pole of the moon.

  • Paul W August 19, 2013, 16:44

    If the sail had been launched during the Greek Golden Age, it would be arriving at AC in only another 1500 years or so!

    The Benfords deserve an audience and they fortunately have a deeper sense of time than many of us are able grasp.
    Think about Stonehenge…if the sail had been launched when those rocks were raised, it could be at AC by now!

  • William R. Alschuler August 19, 2013, 17:37

    I wonder if Jim Benford mentioned how complete the test sail surface was in his trials. Did he note that for any wavelength, a mirror at that wavelength can be highly reflective even if full of holes? If they are subwavelength than the mirror looks continuous to the waves.

    See for example: http://apl.aip.org/resource/1/applab/v102/i3/p031111_s1?isAuthorized=no

  • Paul Gilster August 19, 2013, 17:40

    Bill, I don’t recall if this came up specifically, but I can quote from my Centauri Dreams book on this issue when I interviewed Jim:

    “…a variety of even more exotic materials are under consideration for interstellar missions. But adding material to reflect photons also adds weight, which is one argument in favor of microwave methods. With pure carbon-carbon, light (with wavelengths clown to the io-micron level), goes straight through the sail. Whereas microwaves, with wavelengths tip to several inches, bounce off even this diaphanous material, giving it a push. “Think of it like a screen door with a big grid,” Benford told me. “The flies can come through but you can’t.”

  • Swage August 20, 2013, 10:09

    4300 years to Alpha Centauri, well that depends on a majority of factors. It depends on what kind of beam, how much energy the beam has, the material of the sail, the size of the sail, the mass of the spacecraft and so on.

    It seems strange to me to calculate 4300 years for a sailcraft with no on-board propulsionsystem, when a nuclear-powered spacecraft like Project Longshot with 264t deuterium fuel is calculated to achive it in a hundred.

    I wonder about the specs for the sailcraft. Whats its mass? We can accelerate particles to 80% lightspeed in our cyclotrons. I’d say a nano-probe could make it in less then ten years (heat resistance will be a problem). The problem is, of course, getting it back. But i guess if we could focus our beams better we could even bouce it back with a detatchable smaller sail for beam-reversal.

  • Paul Gilster August 20, 2013, 10:44

    Swage, the 4300 years is for a sail moving at 300 kilometers per second. That’s an unassisted sail; i.e., without a beam on it. Laser and microwave concepts can get us up to 10 percent of the speed of light for much faster crossings. Project Longshot was a student project at the US Naval Academy. They did a good job on preliminaries but it was by no means a finished design study.

  • william f collins August 20, 2013, 15:31

    Here is the sweet irony: the interstellar sail-craft as a super-higher direct continuation of the Kon-Tiki sail-craft in which the Polynesians set out over the vast unknown Pacific Ocean.

  • Paul Gilster August 20, 2013, 16:21

    Bob writes:

    I watched the streaming video of Sonny White’s keynote talk. I am confused about the state of research on his ‘Q thruster’ and hope someone can chime in.

    I thought he said they achived thrusts in the thousand micro Newton range already, was that real? I tried to check the talk again but the youtube video’s are so long they do not load on my Ipad.

    I checked with Dr. White and learned that thrusters his team has tested in Houston generate hundreds to thousands of micro Newtons. As to Woodward, he is a believer in Mach’s Principle whereas the Houston team supports the quantum vacuum. The path to a realizable technology is different, though we are still very early in exploring all this.

    As to the funding issue, presenting this material in Houston does nothing to fund the work despite press hype that ignores Sonny’s continued statements that these investigations are a work in progress and that much work lies ahead to see what’s possible.

  • Bob August 20, 2013, 21:34

    Thanks Paul.

    I just got a copy of Woodward’s book in which he mentions White and seems to suggests that White simply believes that the devices he (Woodward) has built and tested work by the quantum vacuum and not by Mach’s principle as best I can decipher. That would explain why White shows slides of Woodward’s work as evidence that Q thrusters work.

    Regardless of whether it works by the quantum vacuum or is a Mach effect, if the data holds up, propellent-less propulsion should be a really big deal.

  • Eniac August 26, 2013, 0:46

    Bob:

    Regardless of whether it works by the quantum vacuum or is a Mach effect, if the data holds up, propellent-less propulsion should be a really big deal.

    Alas, unless it also includes free energy, it is not as big of a deal as it may seem. A rocket uses its fuel (source of energy) also as propellant (source of momentum), in a sort of “dual-use” arrangement. A “propellant-less” drive would, presumably, still need that same amount of fuel to generate the energy required for acceleration. That includes the rocket-equation, with the dreaded exponential stemming from having to accelerate the fuel along with the payload. Nothing gained, or at least not much.

    Of course, doing actual math on these schemes is impossible, since without conservation of momentum nothing else makes sense anymore.

  • Eniac August 26, 2013, 0:59

    Paul:

    My conviction is that the first serious mission targeting another star will use sail technologies in one form or another. The first conference session was devoted to sails, leading off with Jim Benford’s keynote, followed by Les Johnson, who described current and near-term work. Right now the only propulsion method that will get us to interstellar velocities is the sail, and even then we’re talking no more than a couple of hundred kilometers per second, so it’s still a long trip.

    I think you are unfairly ignoring the option of ion-drive, here. They are comparably ready technology, with comparable capabilities, and when combined with a nuclear reactor can function independently from the sun, like a beamed sail.

    I share your enthusiasm about solar sails for interplanetary transport, but would not count out the nuclear-electric alternative so easily.

  • Bob August 26, 2013, 20:29

    Eniac,

    I was thinking more of local travel in our solar system where free solar energy abounds. With regards to the Q-thruster and Woodward’s Mach device, I think momentum is conserved. For the Q-thruster, momentum is conserved between the ship and the virtual plasma and in Woodward’s scenario, momentum is conserved with the entire universe through Mach’s principle.