I once thought about putting together a collection of classic papers on interstellar flight. It would start with early work by the likes of Les Shephard, Eugen Sänger and Carl Wiley (whose groundbreaking paper on solar sails appeared not in a scientific journal but in Astounding Science Fiction). The book would proceed with the key papers of Forward, Bussard and Dyson and move into papers from the Project Daedalus report, then to Matloff and Mallove and up to the present day, with a long look at the Italian solar sail work of Vulpetti, Maccone and Genta. Especially later in this period there is abundant material to choose from, and there’s Alcubierre to consider, and Millis’ work with the Breakthrough Propulsion Physics project.
And then there’s Geoff Landis and Robert Frisbee and the closely reasoned sociological analyses of Michael Michaud and… Well, you can see what happens when you start pondering editing possibilities. The book is already growing to enormous size and I’ve done no more than sketch out the basic parameters. A book like this would also do well to include some science fiction, which has energized so many scientists as they ponder making their concepts real. A hybrid anthology — scientific papers mixed with science fiction — is a rare beast, though as we’ll see in a minute, it’s been done before.
I’m glad to see that Jim and Gregory Benford think it’s time to do it again, in the form of a new anthology based on the discussions at the 100 Year Starship Symposium in Orlando. Starship Century is to be the title, and the idea is to examine the ideas of current leading figures in interstellar studies — Robert Zubrin, Jill Tarter, Martin Rees, Paul Davies and many of those I’ve already listed above — in light of the latest developments. The 100 Year Starship Symposium is a natural trigger for this book, exploring as it did how a civilization expands into its solar system, developing the needed tools of interplanetary communications and propulsion as, in the words of Jim Benford, it “…focuses outward for its own evolution.” Into the mix will be science fiction from Steven Baxter, Nancy Kress, Allen Steele, Joe Haldeman and others.
“The anthology theme is the development of starships as a key inspiring goal for future interplanetary technologies, leading to the ability to build the first starships within a century,” says Gregory Benford, an award-winning science fiction author and physicist. “It addresses how future growth of an interplanetary economy can occur, leading to starships.”
Starship Century will mix the fiction and the science in equal measure in a volume slated to go on sale in about a year, with additional contributors to be announced in coming months. Unlike my solely hypothetical anthology, this one will collect not historical papers but state-of-the-art science. Those of us who had the privilege of being at the conference — and I think there were a couple of thousand all told — will look forward to seeing some of the remarkable talent that appeared there pushing the discussion forward in this new venue. Although what I once described as the ‘interstellar buzz’ of the symposium has cooled a bit since then, we’re still seeing more activity across the spectrum of interstellar studies than at any time in my memory.
I mentioned above how unusual it is to have anthologies that mix science with science fiction, but there are two exceptions that quickly prove the rule. The first is a volume edited by Arthur C. Clarke called Project Solar Sail (New York: Roc, 1990). This chunky paperback should be on your shelf, containing as it does fiction by David Brin, Clarke himself (“The Wind from the Sun”), Larry Niven and others as well as papers by Eric Drexler, Robert Forward and Louis Friedman. There’s the obligatory nod to Tennyson (“Saw the heavens filled with commerce / Argosies of magic sails”) and a longer attempt at sail poetry by Ray Bradbury and Jonathan V. Post.
We also have a book called Going Interstellar, edited by MSFC’s Les Johnson and science fiction writer Jack McDevitt, coming up for publication in May, and although I don’t have the contents list, I do know this one is also going to mix essays by space scientists and engineers with science fiction from a variety of authors. We have much to look forward to in the coming year or so as the echoes of the 100 Year Starship Symposium continue to resonate, and the hope here is that the synergy between science and science fiction that animates Starship Century reflects a growing public enthusiasm for our prospects in the Solar System and beyond.
I’ve been trying to figure out why exomoons — moons around planets that orbit stars other than our own — have such a fascination for me. On the purely scientific level, the sheer amazement of discovery probably carries the day, meaning that I grew up in a time long before we had confirmation of any exoplanets, and now we’re talking about getting data on their moons. But there’s also that sense of the exotic, for we can wonder whether gas giants in the habitable zone, which may be more plentiful than we realize, might have life on their own rocky moons.
David Kipping (Harvard-Smithsonian Center for Astrophysics) has been a key player in the exomoon hunt for some time now (search under his name in the archives here and you’ll retrieve articles going back for years). David is now working with a ‘crowd-funding’ source called Petridish.org to fund a new mini-supercomputer that will go to work on the Hunt for Exomoons with Kepler (HEK) project. The idea behind HEK is to use Kepler data to look for transit timing variations (TTV) and transit duration variations (TDV), perturbations in the motion of the host planet that should flag the presence of a large exomoon. The detection of exomoons down to 0.2 Earth masses seems feasible with these methods, as Kipping has determined in earlier work.
Help us find the first exomoon is getting plenty of attention. The beauty of Petridish.org is that it lets individuals become a part of science one project at a time, playing an important role in the kind of things that get funded. Have a look at the site and you’ll see a wide range of projects ranging from a study of wolf populations on Isle Royale National Park (Lake Superior) to the collection of rock samples in Antarctica. Each project has a short video explaining the work at hand and the funding goal, along with the rewards for donors, which could be souvenirs of some kind or, for large donations, having the project named after the donor. Needless to say, backers are also on the fast track for updates on the research.
With over 1000 new planetary candidates just released by Kepler, the exomoon possibilities are getting more and more interesting, but Kipping points out that hunting for a single moon takes about 6 years of computer time:
Searching for moons requires the most sophisticated statistical techniques, many of which we have borrowed from cosmologists studying the Big Bang and dark energy. The systems we model have complex dynamical interactions and produce strange, asymmetric light curves requiring a lot of computer power. But we are *almost* there. A mini-supercomputer would have a huge impact on our search, so please do consider supporting us!
The fund-raising project still has eleven days to run and is making excellent progress. But faster computer processors would bump up the speed for HEK’s work, and with almost two weeks to go, Kipping is hoping the project can not only acquire the needed machine but upgrade it to state-of-the-art standards. Have a look at what HEK is doing with crowd-funding, and be aware, too, of Kipping’s paper “The Hunt for Exomoons with Kepler (HEK): I. Description of a New Observational Project,” available on the arXiv site and now accepted for publication in The Astrophysical Journal. For more on HEK, see the Centauri Dreams post New Exomoon Project Will Use Kepler Data.
We’ve been looking at slowing down a starship, pondering ways the interstellar medium itself might be of use, and seeing how the stellar wind produced by the destination star could slow a magsail. A large solar sail could use stellar photons, but the advantage of the magsail is that it’s going to be effective at a greater distance, and we can also consider other trajectory-bending effects like the Lorentz turning studied by Robert Forward and P.C. Norem. But if you take a look at the relevant papers on magsails and other uses of the medium, you’ll find that they all assume the interstellar medium is more or less uniform. We know, of course, that it is not.
For one thing, the Sun itself seems to be near the boundary of the Local Interstellar Cloud, and there are a number of such clouds within about 5 parsecs of the Solar System. In fact, we’re not exactly sure whether the Sun is just outside the LIC or barely within it. In any case, as Ian Crawford has pointed out, Centauri A and B appear to be outside of the LIC in the direction of the G cloud, yet another denser region of the local interstellar medium. Although Robert Bussard assumed densities of about 1 hydrogen atom per cubic centimeter for his ramjet, a starship between denser clouds may encounter far less, perhaps 0.01 hydrogen atoms in the same volume.
The other wildcard is the fact that leaving and approaching a stellar system, we encounter the kind of interesting effects shown in the image below. This is anything but a uniform interstellar background. The bubble created by the solar wind is called the heliosphere, at the outer boundary of which is the heliopause (here the solar wind is balanced by inward pressure from the interstellar medium), and as you can see in the diagram, the bow shock forms on the outer edge as the star moves through the ionized gases of the medium. Still within the heliosphere is the region called the termination shock, where the speed of the solar wind is abruptly reduced — between the termination shock and the heliopause is the area known as the heliosheath.
Image: The complicated interactions between the Sun and the local interstellar medium. Credit: NASA/JPL.
Physicist and writer Gregory Benford calls the bow shock, that bumper of plasma and higher density gas that forms 100-200 AU from the star, “the obvious place to decelerate.” Obvious it may be, but I haven’t encountered the idea in the literature before, and it’s an ingenious enough notion that I suspect we’ll be seeing a paper or two on the matter before long. The suddenly higher density and plasma content available here should allow interesting maneuverability along the lines of the Lorentz force turning that Forward and Norem studied for course correction and round-trip missions. The bow shock should also offer prime ground for deceleration.
We have early data on the termination shock from Voyager 2, which crossed it at 84 AU back in 2007, while Voyager 1 entered the heliosheath at 94 AU in 2004, and Benford figures the plasma density increase at the bow shock should be one to two orders of magnitude above the interstellar density, and that means one or two orders of magnitude more deceleration. I want to quote him on this from a recent email:
My main point is that these are 3D structures, so a starship could navigate through them using the Forward I x B torque model which steers without decelerating. Each of the bow shock, heliopause and termination shocks are surfaces one can sail on and in, maximizing the deceleration.
So here is the method for the star sailors of the far future:
I imagine that any trial of a starship in, say 100 years, will begin with expeditions into the several hundred AU shock environment, have a look at distant iceteroids and maybe dwarf stars. Then turn back and try to decelerate using magsail skills on the shock surfaces available. (I surf, and this is like inverse surfing, using natural wave phenomena to slow.) Develop the tech and skills to sail the interstellar seas!
As a starship approaches a star, sensing the shock structures will be like having a good eye for the tides, currents and reefs of a harbor.
Image: Spitzer image and artists conception of the bow shock around R Hya. Credit: NASA/JPL, Toshiya Ueta.
Now we can look at certain astronomical images in a new light, as witness the Spitzer imagery and subsequent artist’s concept above. This is the star R. Hydrae in infrared, showing the bow shock about as well defined as I have ever seen it. Approaching a star using these decelereation methods would involve a long period of braking moving through and along the bow shock, heliopause and termination shock, staying within the high density plasma to take advantage of the increased densities there with the starship’s magsail fully deployed. After the long spiral into the inner system, continued magsail braking or perhaps inner system braking using a solar sail would allow the vehicle to maneuver and explore the new solar system.
One of the first things we need to do in terms of interstellar exploration is to get a spacecraft built for the purpose to travel outside the heliosphere and give us solid measurements on the interstellar medium. The Voyagers are doing their best but they were never designed for what has become their interstellar mission, and while we can marvel at their longevity, it’s with the knowledge that their resources are few and their years of useful data gradually drawing to an end. Something along the lines of Ralph McNutt’s Innovative Interstellar Explorer would do the job nicely, allowing us to sample the environment that much longer missions will have to work in.
Lorentz Force Turning
The interstellar medium (ISM) is important not just because we have to learn about things like shielding a fast-moving spacecraft and cosmic ray flux but also because we may be able to use some aspects of the medium for deceleration. Yesterday’s discussion of magsails reminded me of a 1969 paper by P. C. Norem that took a truly round-about way to stop a laser-beamed lightsail at a destination star. Norem was interested in what we can call ‘thrustless turning,’ an idea Robert Forward explored in a 1964 paper (thanks to Gregory Benford for sending the Forward document, my copy of which had disappeared somewhere in the wilds of my office).
Norem’s notion was to send his spacecraft on a trajectory taking it far beyond the target star, using long wires and an electrical charge induced on the spacecraft to allow interactions with the interstellar magnetic field to cause it to turn. The vehicle would actually approach the star from behind (as seen from Earth), allowing a laser beam from Earth to slow it for system entry and exploration. The idea takes advantage of the fact that a charged object moving through a magnetic field experiences a Lorentz force at right angles to its direction of motion and the magnetic field itself. You can see why this would be attractive to those hoping to use the local interstellar medium to accomplish what would otherwise require massive propulsion systems.
Norem was able to extend the idea into a concept for a round-trip mission, because he realized that he could once again accelerate his laser-sail by turning on the beam from Earth. Coming up to cruise, the craft (now moving away from Earth) would again use Lorentz turning to make the needed 180 degree maneuver that would put it on a trajectory back to our planet. Final deceleration into the home system would be with the sail deployed against the same laser beam.
Image: An early concept for the Innovative Interstellar Explorer probe to the interstellar medium. Credit: JHU/APL.
Forward hadn’t worked out the laser sail ramifications as early as 1964, but he thought thrustless turning was a workable mechanism, one powerful enough, by his calculations, to allow not only for mid-course corrections but in some cases to return a small probe to Earth after its journey, in which case we have the odd situation in which the energy required to launch a flyby probe to a star is also the energy needed to fly a round-trip probe. Inspired by Norem, he might have considered deploying a thrustless turning system on some of his own designs, but I imagine that the idea of tripling the mission time, which is what would have happened, for instance, on one of his hypothetical Barnard’s Star laser sail missions, may have led him to drop the idea.
Powering Up the Spacecraft
But the notion persists that the interstellar medium is supple and useful if we can learn how to take advantage of it. We also need a lot more data — Forward noted that his 1964 calculations could be carried out only to 20 percent accuracy because parameters like the strength of the interstellar magnetic field were not yet known. But the physics of thrustless turning as applied to an interstellar mission are well worth considering as we continue working on future missions to the ISM. Here’s Forward’s overview of the idea in terms of technology:
In order to use this force in space effectively, it is necessary to find an efficient lightweight method of maintaining a substantial charge on a space vehicle in spite of the discharging effects due to field emission and ion capture from the surrounding regions. It is shown in the following sections that the concept is quite feasible for probes or vehicles in interstellar space, whereas it would not work in interplanetary space because of the high ion densities near the sun. By using a long, thin quartz fiber to increase the capacitance of the probe, the charge-to-mass ratio can be made very large without having to use high voltages. This, in turn, means that the necessary voltage and current can be obtained from a few grams of a suitable radioisotope or a very small charged particle acceleration.
Gregory Matloff considers thrustless turning in his book Deep Space Probes (2nd edition, Springer, 2005), where the charge carried by the spacecraft is generated by the decay of radioactive isotopes. He notes that a starship of any substantial size would demand an enormous electrostatic charge to make the turning maneuver feasible within decades. But in a 2005 paper with Les Johnson (also kindly sent by Gregory Benford), Matloff examined the use of an electrodynamic tether (EDT) to supply power to an Alpha Centauri expedition that would take 1433 years to reach its destination. In the conclusion of that paper, the authors make the case:
Electrodynamic tethers have a number of applications to interstellar travel. Consideration of a model for a sample world-ship mission through the local interstellar medium reveals that the interaction between an EDT and the interstellar magnetic field can satisfy on-board starship power requirements without an inordinate amount of starship deceleration [in other words, magnetic braking induced by the tether is found to be a minimal consideration].
Thrustless turning using an EDT’s interaction with the interstellar magnetic field will allow for course correction and rendezvous of solar sail-launched modules in interstellar space. It will not, however, allow rapid thrustless circling to allow a starship to re-enter a power beam or make numerous solar passes.
Lorentz force turning turns out to be slow and power-demanding, and maintaining the charge is also an issue because interstellar ions of opposite charge will be attracted to the spacecraft, thus reducing the effective charge. But the work of Forward, Norem, Matloff and Johnson on thrustless turning reminds us that interactions with the medium itself may become a component of starship design, just as the magnetic sail idea — braking against a stellar wind — uses the ambient environment to do something that would otherwise demand onboard fuel. Tomorrow we’ll look at a novel way of taking advantage of a star’s own interactions with the interstellar medium to slow a starship, a kind of solar sailing that may reduce overall travel times.
The Norem paper is “Interstellar Travel: A Round Trip Propulsion System with Relativistic Capabilities,” AAS 69-388 (June, 1969). Robert Forward’s paper on Lorentz force turning is “Zero-Thrust Velocity Vector Control for Interstellar Probes: Lorentz Force Navigation and Circling,” AIAA Journal 2 (1964), pp. 885-889. Matloff and Johnson write about electrodynamic tether possibilities in “Applications of the Electrodynamic Tether to Interstellar Travel,” JBIS 58 (June, 2005), pp. 398-402.
This morning I want to pick up on the ‘problem of arrival’ theme I began writing about on Friday, and we’ll look at interstellar deceleration issues a good bit this week. But I can’t let Monday start without reference to the Icarus results from Gran Sasso that finds neutrinos traveling at precisely the speed of light. All of this adds credence to the growing belief that the earlier Opera experiment was compromised by equipment problems. The news is all over the place (you might begin with this BBC account) and while we’ll keep an eye on it, I don’t plan to spend much time this week on neutrinos. We still have much to get done on the subject of slowing down.
Magsails and Local Resources
When you begin to unlock the deceleration issue, the options quickly multiply, and you find yourself looking into areas that weren’t remotely the subject of your earlier research. As we saw on Friday, the concept of magnetic sails grew organically out of Robert Bussard’s idea of an interstellar ramjet. Bussard didn’t want to slow down — he wanted to go very fast indeed. Read the comments on that post and you’ll find Al Jackson’s entertaining reminiscences of a dinner with Bussard (Tau Zero author Poul Anderson was present too), and a reminder that the scientist always claimed to have come upon the ramjet idea because of an encounter with Mexican food. The usual story has it that it was a burrito which, bitten down upon, suddenly opened for Bussard the splendors of matter being forced into a cylinder at high speeds.
Or maybe he was eating huevos rancheros — the story seems to have varied a bit over the years. Whatever the case, the idea of scooping up interstellar hydrogen and fusing it turned into a 1960 paper for Acta Astronautica and, along the way, into a critique by Robert Zubrin and Dana Andrews that showed just how much drag an electromagnetic scoop could generate. Andrews was working for Boeing at the time, and had grown interested in using Bussard concepts right here in the Solar System, thinking that a big enough scoop could gather hydrogen for use in an ion engine that could be powered up by an onboard nuclear reactor. A self-fueling ion drive might not be adaptable for interstellar missions, but for interplanetary work it seemed worth a look.
But the numbers were intractable. The magnetic scoop Andrews hoped to deploy created more drag than the ion engines produced thrust. The two researchers quickly found that the scoop’s best function was as a magnetic sail, and their work on the idea appeared in the literature in the early 1990s. In his 1999 book Entering Space, Zubrin recalls that the time was right for the magsail given that Paul Chu (University of Houston) had just invented the first high-temperature superconductors, which a magsail could theoretically use to create the magnetic field that would allow it to ride on the solar wind. Practical high-temperature superconducting wire born out of this work might one day allow magsails to achieve higher thrust-to-weight ratios than solar sails.
Magsails have clear propulsion implications, but Zubrin states the obvious about their most effective uses:
…the most interesting and important thing about the magsail is not what it can do to speed up a spacecraft — what’s important is its capability for slowing one down. The magsail is the ideal interstellar mission brake! No matter how fast a spaceship is going, all it has to do to stop is deploy and turn on a magsail, and the drag generated against the interstellar plasma will do the rest. Just as in the case of a parachute deployed by a drag racer, the faster the ship is going, the more ‘wind’ is felt, and the better it works.
Which takes us to the idea of using in-situ resources to tackle the deceleration problem. If your goal is to launch a starship that can decelerate in the destination system to explore it, the magsail lets you do the job without carrying the deceleration fuel aboard the vehicle. Play around with the numbers long enough and you’ll see what a huge boost this would be, for otherwise you’re carrying all the fuel needed to slow down a starship (moving, perhaps, at .10 c!), and that means you’ve got to get all of that fuel up to cruise in the first place. The idea of creating drag against the interstellar medium and a destination stellar wind thus has a powerful appeal.
Rise of the Superconductor
When Bussard studied how his ramjet could operate in a region of interstellar space where the density of hydrogen was roughly 1 hydrogen atom per cubic centimeter, he saw that he would need a collecting area of 10,000 square kilometers. This is so vast that even if it were made of 0.1-centimeter mylar, a physical scoop would weigh something on the order of 250,000 tons. But a much smaller collector generating a magnetic field seems practical given the advances in superconducting alluded to above, with a loop of superconducting wire deployed from the spacecraft, the current applied to it cycling continuously to generate the magnetic field. Here’s how Zubrin and Andrews described it in a paper based on their presentation at the 1990 Vision-21 symposium at NASA’s Lewis Research Center (now Glenn Research Center):
The magnetic sail, or Magsail, is a device which can be used to accelerate or decelerate a spacecraft by using a magnetic field to accelerate/deflect the plasma naturally found in the solar wind and interstellar medium. Its principle of operation is as follows: A loop of superconducting cable hundreds of kilometers in diameter is stored on a drum attached to a payload spacecraft. When the time comes for operation the cable is played out into space and a current is initiated in the loop. This current once initiated, will be maintained indefinitely in the superconductor without further power. The magnetic field created by the current will impart a hoop stress to the loop aiding the deployment and eventually forcing it to a rigid circular shape.
Image: A space probe surrounded by a magnetic sail. Early work on these concepts has taken place at the University of Washington under Robert Winglee, with reports available at NASA’s Institute for Advanced Concepts site. Credit: NASA/University of Washington.
Thus the hybrid concept Andrews and Zubrin came up with in the Vision-21 work, extending ideas they had first presented in a 1988 paper: Use laser beaming technology to push a sail to interstellar cruise speeds, then deploy a magsail upon arrival to reduce deceleration time. The authors looked at the numbers and worked out 0.8 years for acceleration, 17.4 years of coasting at almost half the speed of light, and 18.8 years for deceleration. This gets you about 10 light years out in around 37 years, a mind-bending pace that uses a huge sail and some generous assumptions about laser power that we’ll look at tomorrow. For there are other ways to use lasers, even for deceleration, and other ways, too, to exploit the local interstellar medium.
Zubrin and Andrews’ paper from Vision-21 is “Use of Magnetic Sails for Advanced Exploration Missions,” in the proceedings of Vision-21: Space Travel for the Next Millennium” (NASA Conference Publication 10059. The citation for their 1988 work is given in yesterday’s Centauri Dreams post.
In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).
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