by Paul Gilster | May 13, 2013 | Culture and Society |
Tau Zero founder Marc Millis and I will be among those interviewed on the upcoming History Channel show Star Trek: Secrets of the Universe, which will air this Wednesday at 10 PM Eastern US time (0200 UTC on Thursday). Being a part of this production was great fun, especially since it meant flying out to Oakland for a visit with my son Miles, who is now actively involved in interstellar matters. On long car trips when he was a boy, I would have Miles read Heinlein, Andre Norton and the like aloud while I drove — terrific memories — so you can imagine what a kick it is to see him as mesmerized by the human future in space as I am.
The idea of getting a payload to another star seemed closer in the days of those car trips. I’m sure that’s because we were coming off the successful Apollo program and assumed that a similarly directed effort could push us rapidly to the edge of the Solar System and beyond. It was in 1975, the year of the Apollo-Soyuz Test Project (the last time an Apollo flew) that Robert Forward introduced a singularly aggressive idea, an interstellar roadmap that he presented to the Subcommittee on Space Science and Applications of the House Committee on Science and Technology, which had requested proposals outlining possible future steps in space.
Forward called the proposal “A National Space Program for Interstellar Exploration.” Although it was published as a House document, it never received much by way of media attention and was never up for consideration by Congress. It says something about both Forward and the optimism of at least some in the space community that his proposal was so far-reaching. Consider this excerpt:
A national space program for interstellar exploration is proposed. The program envisions the launch of automated interstellar probes to nearby stellar systems around the turn of the century, with manned exploration commencing 25 years later. The program starts with a 15-year period of mission definition studies, automated probe payload definition studies, and development efforts on critical technology areas. The funding required during this initial phase of the program would be a few million dollars a year. As the automated probe design is finalized, work on the design and feasibility testing of ultra-high-velocity propulsion systems would be initiated.
Image: Interstellar theorist Robert Forward, wearing one of his trademark vests. Credit: UAH Library Robert L. Forward Collection.
Let’s pause there for a moment to consider that Forward was talking about the first launch of an automated interstellar probe somewhere around the year 2000, with the first manned missions beginning in 2025. The date reminds me of my friend Tibor Pacher, who heads up the Faces from Earth project, and who challenged me to take him up on our interstellar bet, which you can read about on the Long Bets site. The prediction is that ‘the first true interstellar mission, targeted at the closest star to the Sun or even farther, will be launched before or on December 6, 2025…’ Tibor supports that statement while I challenge it, but either way a good cause wins: If I’m right, the Tau Zero Foundation collects our $1000; if Tibor wins, it goes to SOS-Kinderdorf International, a charity dedicated to orphaned and abandoned children.
But back to Forward in Congress. Imagine the reaction of lawmakers reading on in the scientist’s proposal:
Five possibilities for interstellar propulsion systems are discussed that are based on 10- to 30 year projections of present-day technology development programs in controlled nuclear fusion, elementary particle physics, high-power lasers, and thermonuclear explosives. Annual funding for this phase of the program would climb into the multibillion dollar level to peak around 2000 AD with the launch of a number of automated interstellar probes to carry out an initial exploration of the nearest stellar systems.
And with the ‘damn the torpedoes’ optimism that always marked Bob Forward, he finishes this introduction with:
Development of man-rated propulsion systems would continue for 20 years while awaiting the return of the automated probe data. Assuming positive returns from the probes, a manned exploration starship would be launched in 2025 AD, arriving at Alpha Centauri 10 to 20 years later.
Now that’s moving — Alpha Centauri in 10 years would have to mean you’re getting pretty close to 50 percent of lightspeed at cruise, taking into account the necessary deceleration upon arrival. Forward’s own work would later include an Epsilon Eridani mission concept using a laser-beamed lightsail that would reach 30 percent of lightspeed, one that would be built in a ‘staged’ configuration so as to allow not only deceleration into the Epsilon Eridani system but eventual crew return to Earth. So the man thought big and his ideas were as bold as they come.
Michael Michaud, who would have been developing his ideas at roughly the same time, published a lengthy plan for moving out into the Solar System and beyond to the nearest stars in the Journal of the British Interplanetary Society in 1977, a program I want to talk about soon. But first, we need to address a slightly earlier program for interstellar flight written by G. Harry Stine and published in the science fiction magazine Analog in 1973. Stine not only had big ambitions, but he thought he knew just the kind of starship to propose for the job. We’ll talk about Stine’s “A Program for Interstellar Flight” tomorrow.
Forward’s presentation to Congress can be found in “A National Space Program for Interstellar Exploration,” Future Space Programs 1975, vol. VI, Subcommittee on Space Science and Applications, Committee on Science and Technology, U.S. House of Representatives, Serial M, 94th Congress (September, 1975).
by Paul Gilster | May 10, 2013 | Culture and Society |
The British Interplanetary Society was founded way back in 1933, and included such luminaries as Arthur C. Clarke and Val Cleaver among its early membership. The Institute for Interstellar Studies (I4IS), also based in London, was founded in 2012 and is not, to the best of my knowledge, yet incorporated. Between the two dates and mostly emerging in the first decade of the 21st Century are a number of organizations that in one way or another focus on what I call ‘interstellar studies,’ meaning science and engineering dedicated to interstellar flight.
The trick becomes to keep everything straight. When I started writing my Centauri Dreams book back in 2002, the BIS was a clear model for what a small group of dedicated workers could achieve. It had produced a Moon mission concept as early as the 1930s and went on to create the first fully realized design for an interstellar craft, Project Daedalus. The BIS used the ‘red issues’ of its journal to focus on interstellar work while JBIS was otherwise devoted to a wide span of subjects involving astronautics and rocketry.
What we didn’t have in 2002 was the influx of enthusiasm that new organizations with a specifically interstellar focus have brought. This is where the occasional confusion arises, as I can attest from reading email questions from readers. Consider the sheer number of conferences that are suddenly available. Early in the year we had the Tennessee Valley Interstellar Workshop. Fast approaching is Starship Century in San Diego. In August it’s time for the Starship Congress hosted by Icarus Interstellar, while the 100 Year Starship project looks at a conference to be held in September.
I’m also told there is to be a London conference, presumably under the auspices of the Institute for Interstellar Studies, some time in the fall, and the BIS continues to have smaller conclaves at its London headquarters. I’ll cover all of these in these pages, of course, but you should also be aware that we’ve set up a calendar feature on the Tau Zero Foundation’s site that will keep you posted on events you may not otherwise have known about. Right after Starship Century, for example, is the ISDC 2013 Global Collaboration in 21st Century Space event, from May 23-27 at the La Jolla Hyatt Regency in San Diego. The calendar is just up and we’re only now populating it, but the idea will be to track down the major events and make sure they’re visible.
Image: The ISV Venture Star from James Cameron’s Avatar, one of Hollywood’s best realizations of futuristic starship design. The proliferating number of interstellar organizations may have as one effect more attention by filmmakers to creating plausible designs. Credit: 20th Century Fox.
I’ll likewise call your attention to our listing of the various interstellar organizations, published by order of incorporation, on the Tau Zero site. Regular Centauri Dreams readers will already know about Icarus Interstellar and I4IS, but you may want to scroll through the organizations to learn more about groups like Space GAMBIT, which was incorporated in 2012 and is supported by a $500 K DARPA grant. This one focuses on research that can be accomplished by what the group calls ‘hackerspace’ groups, clusters of enthusiasts working with shared tools in venues ranging from garages to clubs and college campuses.
And while you’ve heard me talk at times about my friend Tibor Pacher and his group Faces from Earth (founded in 2006), you may not be as aware of the Global Starship Alliance (GSA), whose goal is “…to launch a manned starship to the nearest potentially habitable exoplanet.” Beyond this is an agenda for improving life for our descendants: “Pursuing the challenge of interstellar travel will enhance the groundwork for a host of technologies that society needs today to ensure the sustainability of the Earth.”
And I’m almost certain you won’t know about the Interstellar Propulsion Society, which surfaced as a predecessor of the Tau Zero Foundation in the mid-1990s, dissolving with the advent of NASA’s Breakthrough Propulsion Physics project, only to re-emerge as Tau Zero when BPP funding was withdrawn and its head, Marc Millis, took early retirement to devote himself to interstellar studies. Then too we can’t forget old stalwarts like the Planetary Society, which does occasionally probe into interstellar realms, and certainly the scientists at the SETI Institute.
We’ve just gotten the list up and will add to it as we go, but I wanted you to be aware of it given the confusion I’m hearing from some readers about who is doing what at which conference. Over time, the hope here is that these things will settle down into a more sustainable schedule. But we can also enjoy the flurry of activity that accompanies the rise of new organizations, and hope that this enthusiasm burns bright as we attract a larger gathering to the fascinating study of starflight.
by Paul Gilster | May 9, 2013 | Sail Concepts |
What we hope to learn from early experiments with the electric sail is whether keeping a steady electric potential on long tethers will give us enough interaction with the solar wind to make for viable propulsion. ESTCube-1, launched earlier this week, is a step in that direction. Even though it uses but a single 10-meter wire, its rotation rate should change once the tether is fully extended and powered up. Bear in mind that ESTCube-1 is deep within the Earth’s magnetosphere, so the charged particles it will be interacting with are not from the solar wind, but a proof of principle is sought here that could make electric sailing a candidate for outer system-bound spacecraft.
It’s important to distinguish between solar sails and their electric counterparts. The Japanese IKAROS sail, successfully tested, showed that solar photons could impart momentum to a thin sail, as our experience with early satellites had already demonstrated. The beauty of sailing in any form is that we leave the propellant behind. The biggest problem with the rocket equation is that it tells us that as speed increases linearly, propellant mass increases exponentially. That’s why chemical rockets can’t take us to the stars, and why finding a way around carrying propellant has inspired concepts from lightsails to ramscoops and forms of pellet propulsion.
Pekka Janhunen’s work suggests that a fully developed electric sail might deploy 100 tethers by using its rotational motion, while an electron gun with beam sent along the spin axis would power up the system. In a sense, then, the electric sail does use electric power as part of producing thrust — in this way it’s similar to an ion engine. And in the sense that it hitches a ride on the solar wind, it could also be compared to the magnetic sail, which grew out of work by Dana Andrews and Robert Zubrin on creating a magnetic scoop to collect interstellar hydrogen. The magnetic scoop turned out to create more drag than the engine it fed could overcome, at which point the idea of using a magnetic sail for deceleration came into the picture.
An electric sail rides the solar wind using different principles, depending upon Coulomb interaction — the attraction or repulsion of particles caused by the electric charge — with the solar wind to get the work done. Positively charged solar wind protons are repelled by the positive voltage they encounter in the charged tethers. At the same time, captured electrons are ejected by an onboard electron emitter to avoid neutralizing the charge built up in the tether system.
Image: A full-scale electric sail consists of a number (50-100) of long (e.g., 20 km), thin (e.g., 25 microns) conducting tethers (wires). The spacecraft contains a solar-powered electron gun (typical power a few hundred watts) which is used to keep the spacecraft and the wires in a high (typically 20 kV) positive potential. The electric field of the wires extends a few tens of metres into the surrounding solar wind plasma. Therefore the solar wind ions “see” the wires as rather thick, about 100 m wide obstacles. A technical concept exists for deploying (opening) the wires in a relatively simple way and guiding or “flying” the resulting spacecraft electrically. Credit: Pekka Janhunen.
What worries some scientists about schemes that use the solar wind, though, is its variability. We’re dealing with a continuous stream of plasma flowing outward from the Sun. While a solar sail works with a steady source of photons, an electric sail will have to contend with solar wind particles that can vary between 400 and 800 kilometers per second. In their book Solar Sails: A Novel Approach to Interplanetary Travel, Gregory Matloff, Les Johnson and Giovanni Vulpetti compared riding the solar wind to putting a message into a bottle at high tide and throwing it out to see whether the currents will take it to the right destination.
But here the electric sail design may have an edge on magnetic concepts, because the spacecraft may be able to control the electric field that surrounds it. Would this be sufficient to produce a controlled level of thrust despite a rapidly shifting solar wind flowing past the vehicle? The question is unanswered, which is why we need early experiments like ESTCube-1 and the upcoming Aalto-1 to tell us more. But as he is the authority on electric sails, it seems pertinent to quote Pekka Janhunen on the matter. He believes that the thrust can be controlled by adjusting the electron gun current or voltage. This is from a 2009 paper delivered at the Aosta interstellar conference in Italy:
…there are two mechanisms that efficiently damp the variations of the electric sail thrust even when the solar wind parameters (density and speed) vary a lot. The first mechanism is due to the fact that the electron sheath width has an inverse square root dependence on the solar wind electron density. Thus when the solar wind density drops, the thrust becomes lower because the dynamic pressure decreases, but the simultaneous increase of the effect sail area (sheath width) partly compensates for the decrease.
So in at least one sense we have the possibility of a self-correcting system. Janhunen goes on:
The second mechanism arises from the natural desire to run the electric sail electron gun with the maximum available power. When the solar wind electron density drops, so does the electron current gathered by the tethers, so that one may increase the tether voltage (electron gun voltage) without increasing the power consumption. Both mechanisms combined imply that if applying the strategy of running the electric sail with the maximum available power, the thrust depends only on power ? of the solar wind density.
Janhunen goes on to say that the navigability of the electric sail is “almost as good as that of any other propulsion system such as an ion engine.” His results so far have shown that variations in the thrust are much weaker than variations in the solar wind itself, and careful juggling of power margins should allow the craft to correct for the unevenness of the flow. All this, of course, needs to be tested out in space, and not just through small, preliminary satellites but larger deployments that build upon what we learn. It’s good to see with the launch of ESTCube-1 that this process has begun.
The paper is Janhunen, “Status report of the Electric Sail: a revolutionary near term propulsion technique in the solar system,” in Proceedings of the 6th IAA Symposium on Realistic Near-Term Advanced Scientific Space Missions: Missions to the outer solar system and beyond, G. Genta (ed.), Aosta, Italy, 6-9 July 2009, 49-54, 2009.
by Paul Gilster | May 8, 2013 | Sail Concepts |
Some years back at the Aosta interstellar conference I had the pleasure of being on a bus making its way at night through the Italian alps with Pekka Janhunen sitting immediately in front of me. Janhunen (Finnish Meteorological Institute) is the developer of the electric sail concept soon to be tested by the ESTCube-1 satellite, which launched last night aboard a Vega rocket from the Kourou spaceport in French Guiana. Our group had been talking about interstellar issues all day at the conference and now, headed back to the hotel following a memorable dinner at high elevation, I was curious whether an electric sail had interstellar applications.
The immediate answer seemed to be no, given that the highest velocities Janhunen had been talking about for the idea were about 100 kilometers per second, much faster than Voyager 1’s 17 kilometers per second, but a long way short of what we would like to see on an interstellar flight. But the ever thoughtful Pekka pointed out to me that as a means of deceleration, electric sails might have a future, braking against the stellar wind from a destination star. Deceleration being a huge problem for any interstellar probe, the idea has stuck with me ever since.
Image: The electric sail is a space propulsion concept that uses the momentum of the solar wind to produce thrust. Credit: Alexandre Szames.
What an electric sail would do is to ride the stream of charged particles flowing out from the Sun, and fast missions to the outer system could thus be implemented if we get the system into full gear. The ESTCube-1 satellite, the work of Estonian students testing out Janhunen’s ideas, uses a long wire that maintains a steady electric potential as its means of interacting with the solar wind. Janhunen, in an article in IEEE Spectrum, calls ESTCube-1 “…the first attempted experiment to measure the Coulomb drag experienced by a charged wire or tether in moving plasma.”
ESTCube-1’s tether is a 50 micrometer wide, 10 meter long wire made out of four strands of aluminum that will gradually be deployed from the satellite in a process that could take as much as a week. Once deployed, the tether will be charged and variations in the satellite’s rotation rate will, if all goes well, reveal the interactions between it and atmospheric ions. But future electric sails will soon be deploying longer wires. A follow-up to ESTCube-1 called Aalto-1 is designed around a 100-meter tether. This one is also a student project, built at Aalto University in Finland and designed in part to test charged tethers as a means of deorbiting small satellites.
Assuming the concept passes its initial muster, we can look forward to upsized missions using tethers up to 20 kilometers long, deploying as many as a hundred of these from a single spacecraft. This is the design that, in computer simulations, yields potential speeds of 100 kilometers per second, fast enough to get a payload into the nearby interstellar medium in about fifteen years. With a spacecraft like this, keeping the sail’s wires in a 20 kV positive potential allows the sail to ride the solar wind ions while making issues of deployment relatively simple.
A sail like this is surprisingly efficient. From a page on the concept maintained by Pekka Janhunen:
The solar wind dynamic pressure varies but is on average about 2 nPa at Earth distance from the Sun. This is about 5000 times weaker than the solar radiation pressure. Due to the very large effective area and very low weight per unit length of a thin metal wire, the electric sail is still efficient, however. A 20-km long electric sail wire weighs only a few hundred grams and fits in a small reel, but when opened in space and connected to the spacecraft’s electron gun, it can produce several square kilometre effective solar wind sail area which is capable of extracting about 10 millinewton force from the solar wind.
As with any sail, the effect is small but cumulative and yields serious velocities over time:
…by equipping a 1000 kg spacecraft with 100 such wires, one may produce acceleration of about 1 mm/s2. After acting for one year, this acceleration would produce a significant final speed of 30 km/s. Smaller payloads could be moved quite fast in space using the electric sail, a Pluto flyby could occur in less than five years, for example. Alternatively, one might choose to move medium size payloads at ordinary 5-10 km/s speed, but with lowered propulsion costs because the mass that has to launched from Earth is small in the electric sail.
ESTCube-1 will help us measure the forces exerted on its single tether by the ionospheric ram flow acting on the satellite, a flow that substitutes for the solar wind in the case of this small CubeSat mission. The Aalto-1 test will occur next year if all goes well, and we will then have to see how the electric sail stands up compared to its solar sail competition. More on electric sail concepts tomorrow, when I want to look at important questions of stability.
A key paper on electric sails is Janhunen and Sandroos, “Simulation study of solar wind push on a charged wire: solar wind electric sail propulsion,” Annales Geophysicae 25, (2007), pp. 755-767.
by Paul Gilster | May 7, 2013 | Culture and Society |
We had a successful launch last night of the ESTCube-1 satellite from Kourou, about which more tomorrow when I’ll be talking about electric sails and their uses both interplanetary and interstellar. But this morning, with the Starship Century Symposium rapidly approaching, I wanted to run this overview, which corrects and updates several things in the post I published a couple of weeks ago. Seats are still available for those of you in range. Thanks to Jim Benford for the following:
The Starship Century Symposium is the inaugural event at the new Arthur C. Clarke Center for Human Imagination at UC San Diego, Tuesday Wednesday, May 21-22. The program is located here. The symposium celebrates the publication of the Benfords’ anthology, Starship Century. Jon Lomberg, the artist who collaborated extensively with Carl Sagan, has read the book and has this comment:
Starship Century is the definitive document of this moment in humanity’s long climb to the stars. Here you can find the physics, the astronomy, the engineering, and the vision that provides the surest guideposts to our future and destiny.
A number of luminaries will discuss a wide variety of starship-related topics derived from the book. The gathering features thinkers from a variety of disciplines including scientists, futurists, space advocates and science fiction writers. The program includes Freeman Dyson, Paul Davies, Robert Zubrin, Peter Schwartz, Geoffrey Landis, Ian Crawford, James Benford and John Cramer. Science fiction writers included are Neal Stephenson, Gregory Benford, Allen Steele, Joe Haldeman and David Brin. Other writers attending are Jerry Pournelle, Larry Niven and Vernor Vinge.
The book will be available for sale for the first time on Tuesday the 21st at a book signing immediately following the first day of the Symposium. There many of the authors in the anthology will be available for signing. Following the first day of the Symposium there will be a reception featuring an exhibition of Arthur C. Clarke artifacts in the Giesel Library of UCSD.
In addition to the speakers, there are a number of panels. One, about the development of the Solar System, is ‘The Future of New Space’. Another is a panel on ‘Getting to the Target Stars,’ moderated by SETI celebrity Jill Tarter. The conclusion is a science fiction writers panel, ‘Envisioning the Starship Era,’ moderated by Gregory Benford and featuring Joe Haldeman, David Brin, Vernor Vinge and Jon Lomberg. At the conclusion of the Symposium there will be a book signing for other books of the authors present. There will also be a later book signing at Mysterious Galaxy bookstore a few miles from the University. It will feature Starship Century and the works of the other writers present.
The Symposium will be webcast and then archived. The webcast, which activates at the time of the event, is here.
The Benfords will donate the profits from sale of the book to interstellar research activities. They are currently working to establish a research committee that will award research contracts. The edition available at the symposium will be unique, a collectors item. The book will then go into general distribution in the summer. The Benfords recommend purchasing through a link that will soon appear on the Starship Century website.
This route is optimal because it maximizes the percentage profit, thus maximizing the money available for research. As we all know, research dollars have been greatly lacking in the interstellar area, which is one reason why the interstellar organizations such as Icarus Interstellar, Tau Zero and the Institute for Interstellar Studies are volunteer organizations. The Benfords are planning a second symposium to be held in London in the fall.
