Looking around on the Net for background information about Jordin Kare, who died last week at age 60 (see yesterday’s post), I realized how little is available on his SailBeam concept, described yesterday. SailBeam accelerates myriads of micro-sails and turns them into a plasma when they reach a departing starship, giving it the propulsion to reach one-tenth of lightspeed. Think of it as a cross between the ‘pellet propulsion’ ideas of Cliff Singer and the MagOrion concept explored by Dana Andrews.
So I thought this morning to offer you some thoughts about SailBeam and its genesis from the man himself. I interviewed Jordin back in early 2003 in a wide-ranging discussion that took in most aspects of his work. He was an easy interview — all I had to do was offer the occasional nudge and he would take off. I found him engaging and hugely likeable. What follows is a fraction of the entire interview, the part that focuses primarily on SailBeam and a bit on Kare himself. I’ve edited it but in general preferred to let Kare’s own voice come through. The images I use here are from Jordin’s “SailBeam Space Propulsion by Macroscopic Sail-type Projectiles,” a presentation he delivered at the 2001 NIAC workshop in Atlanta.
PG: Your work with NIAC on the SailBeam concept takes sail technologies down a new path. Tell me how SailBeam and the NIAC report came about.
JK: I am an astrophysicist by background. I worked at UC-Berkeley and got a doctorate there in 1984. For most of the time since, I’ve been an aerospace engineer, dividing my identity between physicist and engineer. A lot of what I’ve worked on in this area are advanced propulsion projects. So I’ve been involved in a community of people who do exotic propulsion things.
One of the things that’s always in my mind is doing advanced interstellar propulsion. In this case, I’d been aware of ideas for doing laser and microwave sails for interstellar propulsion. Bob Forward did prototypical work on that. I’d been involved in couple of workshops where he talked about the concept, one at the Jet Propulsion Laboratory a couple of years back.
Along those lines, I had realized that there’s a scaling law to how laser sails worked. If you took a laser sail and tried to get to a certain velocity with a certain size laser and certain size sail, and then you took that sail and cut it into several pieces and accelerated those one after another, you could get the same amount of mass to the same velocity in the same amount of time, but you could use a smaller laser because the sail doesn’t accelerate over as long a distance.
That was interesting but not very useful. And then I thought about work that Geoffrey Landis had done about using sails not made of metal foils. The trouble with small sails is that you’re pushing them harder, accelerating faster. And if you’re using metal sails, you can’t do much of that before they just melt. Landis had pointed out you could push harder on dielectric sails.
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PG insert: A brief bit of background on this. The problem with metal films is that they have low emissivity. A small sail made of such materials overheats under the beam. High emissivity materials with higher melting temperatures are needed. Dielectrics are non-conductive materials that will emit a lot but absorb little of the radiation impinging upon them. Silicon carbide is a dielectric, as is aluminum trioxide and, Kare’s favorite, diamond. But back to the interview.
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JK: Dielectric sails that are a thin layer of transparent material reflect better than metal foil sails because they have a different index of refraction [describing how light propagates through a particular medium], like the reflection off the surface of a piece of glass, or reflection off a metal film. Forward noted that dielectric sails could potentially have higher acceleration.
My mental light bulb went on and said I know from working in laser technology and other areas I work on, that you can make very low absorption dielectric materials. If I can make very high quality, very low absorption dielectrics, I could push them really hard. Now instead of thinking in terms of taking a sail and dividing it into ten pieces, I can divide it into a million pieces. I started doing calculations and realized that this made sense as a propulsion system.
I started pulling in pieces from other projects I’ve worked on. This got to the point where I could make a rough design of the system concept and started telling my associates about it. I did a quick presentation at a meeting of space people that we exotic propulsion people go to — the Space Technology and Applications International Forum every January in Albuquerque.
I was describing the SailBeam idea to Bob Forward and he was the one who said you should get some money out of NIAC to study this further; he’d been involved with reviewing stuff for NIAC. I knew other people who had worked with them, so I went to the next NIAC workshop and did a proposal for the following round. When I looked at the kinds of things NIAC was supporting, SailBeam fit with the tenor of their proposals.
PG: Of course the whole idea of sail technologies is changing.
JK: It is for sure, and we have to distinguish between solar and laser sails, or beamed energy sails. The idea of solar sails has been around for a long time. And there have been many changes of direction. They used to look primarily at metallic sails for solar sail missions, usually metallic sails coated on a plastic film. But people who were really aggressive thought in terms of free-standing metal sails. Just in the last couple or three years, carbon-carbon has emerged. Here we have carbon fibers fused together to make an open lattice material that is as lightweight as anything they were ever hoping for out of metal-coated plastics, and much easier to handle. Carbon-carbon also takes much higher temperatures than plastic film.
Suddenly the solar sail people began looking at Sundiver missions, where they fly a solar sail and let it drop close to the Sun, flying edge on until it gets well inside the orbit of Mercury and then turning it face-on to the Sun for that propulsive kick. This gives you much higher velocities than anything we’ve done today, something like 100 or 200 kilometers per second, which means this has application for missions far past Pluto. This kind of velocity lets you begin to talk about thousand astronomical unit missions, a true interstellar precursor.
PG: Missions to another star demand even more. A lot more.
JK: True. Bob Forward was working at Hughes on some of the earliest lasers back in the 1960s when he first came up with the beamed sail idea. His idea was that if you have a laser or microwave beam, you can focus much more energy over a longer distance than you can with sunlight. You can use the same light pressure that solar sail people are talking about to get much higher velocities. Forward came up with this thing called Starwisp [a microwave-driven wire-mesh sail about a kilometer in diameter with a flight time of 20 years to Alpha Centauri — see The Case for Beamed Sails].
Forward realized there were problems with the basic Starwisp concept. One that always bothered me was how Bob was going to get any useful information out of this Starwisp. He talked about having little sensors at the intersections of this fine wire mesh, magically having them turn into a large telescope aperture. I was never quite clear how that actually worked.
So that was his first notion of a very high velocity sail. Forward also came up with concepts for laser sails, in particular the multistage laser sail that would be able to decelerate at destination by splitting off part of the sail and using that to reflect the beam back onto a separate section. A lot of people were interested in that and the idea got used in a lot of science fiction, including Bob’s own writing.
I’ll mention there’s a paper Bob Forward wrote for a workshop I ran at Livermore in 1986, when we were looking at non-interstellar laser propulsion applications. His paper was “Laser Weapon Target Practice with GeeWhiz Targets.’ And in there he talked about a sail that was made of multiple layers of diamond film. I had almost forgotten about this when I came up with my notion of the SailBeam. He had the idea of using dielectric reflectors by way of getting to extremely high performance in a sail.
I use artificial diamond as the best material for my sail. So Bob, as was usually the case, had some of the same pieces considerably earlier than anyone else. But he also had much thicker sails with more layers and wasn’t trying for quite such high performance. He was talk about building something that could fly at perhaps 100 kilometers per second using the types of laser we were talking about building for strategic defense.
The problem with his interstellar propulsion scheme, and everyone agreed it was a problem, was the scale that was required. Because Bob Forward wrote about 10,000 kilometer diameter Fresnel zone plate lenses. He would show an artist’s conception of the lens hanging next to the Earth, and it was the same size as the Earth! The sail by itself would be a hundred or thousand kilometers in diameter, and the lasers were in the terawatt category. It was clear that in principle it would work, but it was, to say the least, a monumental engineering task.
We all wondered if we could do this better somehow. At a workshop out at the Jet Propulsion Laboratory, Geoff Landis ran the session on laser sails and we looked at how you could make smaller sails, asking what was the smallest sail you could build and still do interesting missions. We were still looking at a single laser pushing a single sail.
It was hard to come up with something buildable and still interesting, but Landis had looked at optimizing sails in terms of choosing the best possible material. He was the one who pointed out there was this notion of designing not with multiple layers of dielectric that Bob Forward had put into his ‘gee whiz targets’ paper, but with a single layer of dielectric a quarter wavelength thick. That plus the scaling property that I had been thinking about were some of the ingredients that led to the SailBeam.
PG: SailBeam works by turning your micro-sails into plasma to push the departing spacecraft.
JK: This is where Dana Andrews’ work with magsails was so critical. The notion of putting magnetic coils on a spacecraft, essentially a magnetic loop, and making a magnetic field around it to deflect the solar wind, the stream of charged particles from the Sun. I had done some work for Dana on MagOrion, a notion of making the magnetic field strong enough that you could set off an atomic blast behind the spacecraft and deflect the plasma produced by the bomb.
PG: This was the Project Orion idea applied to magsails.
JK: Exactly. The magsail replaces what would have been a physical sail. The idea was designed originally for cruising around inside the Solar System. But magsails and all these other threads tie together — remember that the original invention of the magsail came when Dana Andrews and Bob Zubrin were trying to figure out if they could make the Bussard ramjet work. They wanted to see what you could do if you were trying to collect interstellar hydrogen with a magnetic scoop. And what they discovered is that they couldn’t make a Bussard ramjet work, because the magnetic fields always ended up deflecting the ionized hydrogen at high velocity. What that turns into is a very good drag brake.
Tweaking the numbers a bit, they could make it be a drag break against the solar wind, which is flying along at a pretty good velocity in the Solar System, 75 K per sec or so. So they could fly around on the solar wind. But all this originated from looking at another interstellar propulsion concept. These ideas build on each other; they’re hybrids.
So I had been working on MagOrion, had done designs of the field coils for Dana Andrews, and that was another piece, because I wondered if I can accelerate little bitty sails and do this scaling of launching a million little sails instead of one big sail, what do I do with them? They are too small to be useful individually. Well, I can use them like a MagOrion. I can turn them into blobs of ions and bounce them off a magnetic field at the vehicle. So I got to pull in yet another piece from things that other people had come up with that I adapted for my own design.
PG: You also applied magsail to deceleration in the target stellar system.
JK: Exactly. One of the things that Dana and Bob Zubrin had pointed out in the past is that a magsail worked as a way of decelerating interstellar spacecraft. I’m carrying a magsail anyway, so Dana and I collaborated on an IAF paper on slowing down a SailBeam vehicle at the far end. Now we had both a way of accelerating and reusing some of that hardware to stop at the far end.
PG: This seems like a more realistic way to do it than Forward’s ‘staged sail’ concept.
JK: I think it is. The one limit on it is that it is not a very fast braking system. It does take tens of years to stop. And it doesn’t bring you down to a full stop. That’s because the force you get to slow down varies with how fast you’re going. So the slower you’re going, the less you slow down. At some point, the time it takes to slow down from a tenth of the speed of light to one percent of the speed of light isn’t too bad, but it takes progressively longer to slow down the rest of the way. You can argue design details as to whether you can get down slow enough that you can then come to a stop by braking against the wind from whatever star you’re approaching. That gives you an extra 75 or 100 kilometers per second for the wind velocity to work against.
Or maybe you’re going to have to carry some system like nuclear electric to slow you down the last 100 kilometers per second. Forward’s sail in principle would let you come to a complete stop or reach any final velocity you wanted to. But it does seem like a very difficult thing to do. It’s in the category of ideal technology. It’s pretty hard to see how you’d actually build it.
PG: You talk about using relay lenses along the acceleration path for your micro-sails. How does this system improve the original design?
That was something i realized late in the process of doing the design. My little sails accelerate over short distances by comparison to Forward’s big sail concepts, a few tens of thousands of kilometers. The problem with pushing a big sail is that I have this one big lens that has to focus the light on the sail some large distance away. How about if I take a smaller lens and use it to focus light, but then I put another lens at a place where the beam spreads out again. And I put another lens out and focus the beam yet again. So I have this spaced series of lenses.
It’s pretty easy to show this is not a useful thing to do if you’re trying to accelerate a large sail over a light year. Partly because you have to put the intermediate lenses a large fraction of a lightyear away and partly because you don’t gain when the lens and the sail are about the same size. There’s no advantage to it; you end up having the same amount of material in multiple lenses as you would in one big lens. Geoff Landis did a paper to show why it doesn’t work.
With my situation, though, I was only accelerating things for a few tens of thousands of kilometers. I had been thinking I’ll do this with one big telescope, a 500 meter telescope. But at some point I realized I’m taking this 500 meter telescope and focusing the beam on this little tiny sail. If I were to try to focus on another lens, another telescope, i could do that easily. I’m only accelerating over a short distance, so I can physically put a telescope forty thousand kilometers away; it’s not like I have to put it half a light year away.
So I realized I could build a 50 meter telescope and have 10 of them. Because of the way the numbers work out, because I’m focusing on a very small object, it turns out I gain in terms of the total area of the telescopes. I can make ten telescopes each a tenth of the diameter and spaced one tenth of the way along the path. And end up using, since each telescope is a tenth of the diameter, a hundredth of the area. So I can have ten times the total material of the telescopes.
Now I had gotten the lens down from 10,000 kilometers to a few hundred meters. Which certainly helps. Look, Bob Forward figured out a way you could get to the stars using known physics. Cliff Singer talked about using particle accelerators for ‘pellet’ propulsion. Both these notions left us huge engineering problems. What Geoff Landis and I both did was to say, can we do better from an engineering standpoint. Can we make this something we can actually build.
What I like best about Sailbeam is that as far as I know right now, it is the most engineering-practical way to get up to a tenth of speed of light.
PG: These sails get up to speed, shall we say, quickly.
JK: Yes. In some of the designs they go from zero to light speed in about a tenth of a second. That’s pushing it and in the design that’s in the final proposal, they take about three seconds. I love showing that slide — it shows what the limiting acceleration would be for an ideal microsail and it’s like 30 million gravities, or zero to lightspeed in .97 seconds. But even backing up because of materials properties, you’re talking about accelerating at hundreds of thousands of gravities and getting up to a large fraction of lightspeed in a few seconds.
PG: And you’re pushing, in an ideal scenario, an interstellar probe of what size.
JK: The baseline is a one ton probe. There really is nothing, you probably can’t go a lot smaller than that, though i wouldn’t swear you couldn’t build a one kilogram probe. But even with sophisticated miniaturization, it becomes hard to make a useful probe that’s much smaller than a ton. So I tend to look at that size scale. On other hand, there isn’t an upper limit. You could build much bigger probes, but they would cost more to build the lasers to launch them. The laser power you need is proportional to the mass.
PG: Up to near lightspeed in a second! You seem to be somebody who enjoys pushing the boundaries.
JK: It’s definitely a lot of fun to do. The interesting part of my work is coming up with new schemes and combinations to see if they work. The flip side is that interstellar flight is such a hard problem that you don’t get the satisfaction of something you expect to see built.
PG: Interstellar flight is all about long time frames. Even mission durations of 50 to 100 years are wildly beyond our current capabilities. So how do you cope with this perspective — long-term thinking isn’t something our culture has much patience with.
JK: It’s certainly something that is pretty rare in our society. Although I am occasionally amazed because on the one hand people don’t think long term, and then, on the other hand, I see people worrying about things like Social Security going bankrupt in thirty years. We have no idea of what the economy is going to be like in thirty years. So there are a few places in our society where people do think long term, but most of them don’t seem to me to be. Actually this is an interesting phenomenon.
PG: The notion of working on projects where you won’t get results in a lifetime or more is rare indeed. But from talking to you, I get the idea that you would be pleased to think that something you did today would contribute to a mission that might not launch until you and I are both gone.
JK: That’s absolutely true. I would be delighted if when I am old and gray, I discover that people are just starting to work on building something like SailBeam and are referring to me as having come up with the idea, or part of the idea. It’s not that I can’t imagine this SailBeam concept actually being launched within my lifetime — it’s not impossible — but it’s as much as I can reasonably expect to hope that in my time on Earth we’ll maybe be getting started on it.
PG: You’re also a science fiction fan.
JK: Yes. No fiction of my own rather than the occasional song. But I do often point out that I write both science fiction and fantasy. It’s just that the science fiction is usually titled ‘technical proposal’ and the fantasy is titled ‘budget proposal.’ I have never turned pro like Geoff Landis.
Certainly I’ve been an SF reader since way back when. I will note in fact that if there was any single book that turned me onto the notion of engineering interstellar flight, it would be the book Tau Zero by Poul Anderson. That was the one that got me going, stimulating a lot of interest in interstellar flight as something that we might actually make happen.
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Jordin’s report on SailBeam concepts is “High-Acceleration Micro-Scale Laser Sails for Interstellar Propulsion,” Final Report, NIAC Research Grant #07600-070, revised February 15, 2002 and available here. And see Geoffrey Landis’ “Optics and Materials Considerations for a Laser-propelled Lightsail,” presented at the 40th International Astronautical Federation Congress, Málaga, Spain, Oct. 7-12, 1989 (full text).
When Greg Matloff’s “Solar Sail Starships: Clipper Ships of the Galaxy” appeared in JBIS in 1981, the science fictional treatments of interstellar sails I had been reading suddenly took on scientific plausibility. Later, I would read Robert Forward’s work, and realize that an interstellar community was growing in space agencies, universities and the pages of journals. Since those days, Matloff’s contributions to the field have kept coming at a prodigious rate, with valuable papers and books exploring not only how we might reach the stars but what we can do in our own Solar System to ensure a bright future for humanity. In today’s essay, Greg looks at interstellar propulsion candidates and ponders the context provided by Breakthrough Starshot, which envisions small sailcraft moving at 20 percent of the speed of light, bound for Proxima Centauri. What can we learn from the effort, and what alternatives should we consider as we ponder the conundrum of interstellar propulsion?
by Dr. Greg Matloff
Marc Millis, Paul Gilster and their associates of the Tau Zero Foundation are to be congratulated on the recent award of a $500,000 NASA grant to investigate the prospects for a near-term interstellar probe. As one of the co-authors of The Starlight Handbook, the author of Deep-Space Probes and many interstellar related papers, a former NASA consultant in this field and an Advisor to Project Starshot, I would like to offer some gentle and very personal suggestions about how to best spend this money. Since it is unlikely that I can attend this year’s Tennessee Valley Interstellar Workshop, I have elected to submit these concepts to Centauri Dreams.
Motivation
The basic reason for an early interstellar endeavor is knowledge acquisition. Data acquired by a star-probe en route to its destination includes in situ measurements of the interstellar medium including ions, neutral atoms, dust grains and cosmic rays. Of particular interest to designers of eventual human-carrying star arks is measurements of the directionality of high-Z cosmic rays. If these originate from discrete sources in and beyond our galaxy rather than being omni-directional, the problem of shielding a space ark will be more readily solved.
Another possible function of such a probe is extra-galactic astrometry. If the probe carries a telescope, the very-long baseline observations possible when pairing with solar-system instruments during interstellar cruise should yield valuable data regarding distances and kinematics of extra-galactic objects.
During the interstellar transfer after the probe’s distance from the Sun exceeds 550 AU, the Sun’s Gravitational Focus can be applied to obtain greatly amplified images of astrophysical objects occulted by the Sun. Trajectory deviations farther along the probe’s interstellar track might indicate the presence of elusive dark matter.
Upon arrival in the destination planetary system, investigation of planets within the target star’s habitable zone will be the highest priority. Does life evolve on any water-rich world within the liquid-water temperature range, if that world has an atmosphere? Or are special conditions such as a massive satellite a requisite?
If living planets are commonplace, do technology and civilization naturally evolve? Because we have received no unambiguous signals from hypothetical advanced extraterrestrial civilizations and intelligent ETs are apparently rare or non-existent in our solar system, our early interstellar robots should be configured to investigate the “Eerie Silence” (as Paul Davies has dubbed it) and Fermi’s Paradox (“where is everybody?”). Do advanced ETs perhaps evolve in a non-technological direction, or do they generally self-destruct? Or do they generally elect to remain radio silent and not engage in interstellar exploration and colonization?
Destination
I will next consider the probable destination for a probe that we might conceivably launch in the 2050-2100 time frame. Our early probes should almost certainly be directed towards the nearest stars—the Proxima/Alpha Centauri triple star system.
This system, which is estimated to be about 6 billion years old, consists of two central Sun-like stars (Alpha A and Alpha B) and a red dwarf companion (Proxima). Alpha A and B orbit their common center of mass in an elliptical orbit with a period of about 80 years. At their closest (periapsis), Alpha A and Alpha B are separated by about 9 Astronomical Units. At their farthest (apoapsis), their separation is in excess of 30 AU. Each of the central Centauri suns could have planets orbiting within their habitable zones. Alpha A/B Centauri is about 4.27 light years from the Sun.
Proxima Centauri is a bit closer at 4.24 light years from the Sun. It is quite possible (but not definite) that this star is gravitationally bound to the Alpha A/B even though its current separation from Alpha A/B is about 15,000 Astronomical Units.. During the summer of 2016, the discovery of a planet with a probable mass 30% greater than Earth orbiting Proxima Centauri within that star’s habitable zone was announced. A less-than-poetic designation for this planet is Proxima b Centauri.
Although several research teams are investigating the possibility of habitable worlds attending Alpha A or Alpha B Centauri, the discovery of Proxima b was totally unexpected. Since the nearest star to the Sun has a probable planet orbiting within its habitable zone, it is reasonable to conclude that such worlds are very common in our galaxy.
Achievable Interstellar Transit Duration
Our early extrasolar probes— Pioneer 10/11, Voyager 1/2, and New Horizons— don’t really count as starships. Yes, they have left or will eventually leave our solar system and move freely through the Milky Way galaxy. But their propulsion systems—chemical rockets combined with giant-planet gravity assists are not effective enough for true star voyaging. Even the fastest of these would require about 70,000 years to reach Proxima/Alpha Centauri if it happened to be pointing in the right direction (which it isn’t).
A human colony ship, often called an interstellar ark or world ship, could probably be designed using near-term technology such that it could survive a millennial journey to our nearest stellar neighbor. But such a long travel time for a robotic probe would be difficult to sell to the scientific community since most research participants would prefer to see some results within their lifetimes.
So the Breakthrough Initiatives project Breakthrough Starshot pushes technology to its limits on numerous fronts in order to design a starcraft capable of traversing the enormous distance between the Sun and Proxima/Alpha Centauri in about 20 years.
Everything about Starshot is enormously challenging. A hyperthin sail with dimensions up to a few meters on a side must be generated. It must have near perfect reflectivity, high emissivity, low areal mass thickness and very high melting point. This is necessary for it to survive a several minute exposure to a 50-100 GW laser beam without melting. By the way, it must also have enormous tensile strength in order to support the nano-payload during the acceleration process. The sail must also be configured to maintain stability within the beam.
The laser array would likely be mounted atop a Southern Hemisphere mountain, in order to point at Alpha Centauri. Adaptive optics must be used not only to compensate for the effects of Earth’s atmosphere but to insure that the beam completely fills the sail during the acceleration process at distances measured in millions of kilometers. Also, since a single continuous wave 50-100 GW laser is somewhat beyond current capabilities, thousands of smaller lasers must be synced together to produce the beam.
Assuming that the sail survives the acceleration process, it must possess ample on-board intelligence to perform several tasks independent of Mission Control. First, it should reorient itself to travel edge-on rather than broadside through interstellar space. This is necessary to reduce the effects of dust grain impacts. Although interstellar dust is rare in our galactic vicinity, even a single grain moving at 0.2c (60,000 kilometers per second) relative to the sail has an enormous wallop.
But we’re not done yet. Approaching Proxima/Alpha Centauri, the sail must reorient itself once again to allow its instrument suite to survey the environment of the destination stars and to send the results towards Earth. A very tall order indeed for a ~gram-massed nano payload.
None of the above challenges present physical impossibilities. The question is whether they can all be achieved in a single nano-spacecraft within the next few decades.
So any NASA-funded interstellar initiative intended for possible implementation within the next few decades should not attempt to duplicate the goals of Project Starshot. Rather than a 20-year travel duration, a 100-year flight time might be more realizable in the near term. Mission planners need to realize that even this is quite a challenge. A 100-200 year travel duration might be a reasonable goal.
Image: Artist’s concept of the Breakthrough Starshot sail under beamed acceleration. Credit: Breakthrough Initiatives.
Proposed Propulsion Systems
Many propulsion systems have been proposed to enable interstellar exploration and colonization. Only a few have any hope of being feasible in the near term. Before we get to the near-term possibilities, it might be nice to review some of the more exotic suggestions.
Space Warps, Wormholes, and Hyperdrives
It would indeed be lovely if one of these devices emerged from the realms of science fiction and Hollywood special effects into the real world. Then we could wander the star lanes with the same dispatch that we book a flight to Europe.
Unfortunately, all of these short-cuts through space-time require either enormous amounts of energy, exotic forms of matter or new physics. It seems wise to continue research in these possibilities. There is no telling when or if a breakthrough might occur. But it would be unwise to hold our collective breaths.
Thrust Machines
In the 1960’s, we were treated to the famous Dean Drive. Now engineers in several international locations are testing the Shawyer EM Drive. These and similar devices apparently violate one of the basic laws of classical mechanics: Conservation of Linear Momentum. Although excess unidirectional thrust seems to be generated by the EM Drive, Marc Millis has described in this blog numerous possible causes for this effect that do not violate this law.
Before any proposed thrust machine can be seriously considered for application to interplanetary or interstellar propulsion, it must demonstrate excess thrust in outer space conditions. Two venues for preliminary in-space tests are stratospheric balloons and sub-orbital rockets. If these succeed, a follow-on demonstration would be a dedicated cubesat containing the device deployed in Low Earth Orbit.
The Matter/Antimatter Rocket
This physically possible interstellar propulsion system utilizes total conversion of matter to energy in the reaction between matter and antimatter. Sadly, we are a very long way from the capability of creating the necessary mass of antimatter in a reasonable time frame. If we applied humanity’s best antimatter factory (the Large Hadron Collider) to the the task of full-time antimatter production, we might have a gram of the stuff after 100 million years.
Another problem is storing the antimatter. Charged sub-atomic particles can be stored in Penning Traps for periods of weeks. These devices use crossed electric and magnetic fields to contain the particles. If applied in space travel, how would the trap’s fields compensate for variable spacecraft acceleration? Also, might stray cosmic rays heat and divert the anti-ions so that they explosively interact with the walls of the containment vessel?
Perhaps it’s a good thing that application matter-antimatter technology does not seem a near-term possibility. Our security would be jeopardized enormously (and probably terminally) if terrorists could smuggle city-killing weapons in thimble-sized containers.
Ramjets and EM Sails
By far the most elegant of physically possible interstellar spacecraft is Robert Bussard’s fusion ramjet. This craft utilizes an electromagnetic (EM) scoop to collect interstellar hydrogen over a large area and redirect the plasma to a proton-proton fusion reactor. Energized fusion products (helium nuclei) are exhausted out the rear of the craft. An ideal ramjet, accelerating at 1g could reach near-optic velocities in about a year Earth time. Because of relativistic effects, the craft could cross the galaxy within the crew’s lifetime, according to on-board clocks.
Sadly, there are a few problems with the proton-fusing ramjet. First and most significant is the difficulty of igniting the proton-proton thermonuclear reaction. This reaction, which powers main sequence stars such as our Sun, is many orders of magnitude more difficult to ignite than the fusion reactions we currently experiment with. One way around this is to consider lower performance ramjet alternatives such as the ram-augmented interstellar rocket (RAIR) that carries on-board fusion fuel and uses scooped protons as additional reaction mass.
But even that approach is limited by the limitations of EM scoops that have been suggested to date. Most (including those considered by this author) function better as proton reflectors or drag sails—very good for interstellar deceleration but not too effective for achieving high velocities. The one exception to this is Brice Cassenti’s toroidal scoop, suggested in the late 1990’s. But because this scoop utilizes an array of superconducting wires projected in front of the spacecraft, only accelerations of the order 0.01 g are possible.
In the near future, the best we can likely hope for to apply ramjet technology is in-space experiments using electric and magnetic sails to reflect the solar wind. This might encourage the perfection of both an interplanetary propulsion option requiring no on-board fuel and experimental tests of an approach to interstellar deceleration.
Beamed Propulsion
It is unclear whether Project Starshot’s imaginative enterprise will be successful. Even if a beam projector is located on a high mountain, it is not known how rapidly it can be adjusted to compensate for atmospheric turbulence. Another unknown is whether the beam-steering mechanism will be efficient enough to keep the beam output directed at Alpha/Proxima Centauri for several minutes. Finally, much analysis is required to insure that the beam is centered on the sail and fills the sail during the acceleration process.
Any funded consideration of interstellar probes would be wise, however, to investigate terrestrial and in-space experiments to demonstrate the utility of beamed propulsion. These could be far less ambitious and expensive than the Project Starshot concepts.
For example, imagine two cubesats launched simultaneously into Low Earth Orbit. One contains a wafer sail. Its neighbor deploys a very low-power laser or maser projector. The beam is focused on the unfurled sail. It should be possible to monitor both sail acceleration and stability in the beam.
Another possibility is to repeat an experiment originally planned for the failed Planetary Society Cosmos-1 Earth-orbiting solar-photon sail. After the sail is unfurled, a microwave beam from a terrestrial radio telescope could be focused on the sail. If sail stability and acceleration can be demonstrated, this will advance the possibility of Earth-escape by low-orbit photon sails as well as furthering the cause of interstellar travel.
Theoretical researchers might also expand the concept of particle-beam propulsion. Because electrically charged sub-atomic particles carry significantly more linear momentum than photons, it would be interesting to develop an understanding of particle-beam collimation over interplanetary and interstellar distances.
But there is a geopolitical obstacle to the construction of a ~gigawatt laser-, maser-, or particle-beam projector in space. Such a device could be applied to accelerating a starship or diverting an Earth-threatening asteroid; it could also be construed as a weapon.
If such an enormous beam projector could be constructed in space and could maintain its aim for decades, a hybrid interstellar propulsion system might ultimately become feasible. This is the laser ramjet. In such a vehicle, interstellar ions collected by a Cassenti EM scoop could be accelerated by energy beamed from the solar system.
Fission-Electric Propulsion
Nuclear fission has been an available energy source for more than 70 years. The solar-electric rocket (or ion drive) has been used successfully on several interplanetary probes. One reasonable approach to interstellar travel is to remove the solar panels and connect the ion drive’s thruster to a nuclear-fission reactor. In such a device, the reactor energy output would ionize propellant atoms (or molecules) and accelerate the resulting ions out the rear of the spacecraft.
There are at least three factors limiting interstellar application of fission-electric propulsion. One is propellant availability. To reduce thruster erosion, the inert gas xenon is used as propellant in most current solar-electric drives. Applying this approach to the much more massive fuel requirement of an interstellar probe would likely far exceed the annual terrestrial production rate of xenon. Alternative propellants should be investigated.
Then there is the matter of geopolitics. Many citizens of our planet would be somewhat unnerved if one of the major space powers began to store the large amount of fissionable material required in Low Earth Orbit during construction of the massive probe. One way around this is to construct the probe as an international project, similar to that applied to creation and operation of the International Space Station.
Technology is another limitation. Present day ion thrusters are limited to exhaust velocities of about 100 kilometers per second. So a nuclear-electric rocket launched using current technology might require 10,000 years to reach Alpha/Proxima Centauri.
Exhaust velocity must be raised to at least 1000 kilometers/second to propel a “1000-year ark”, as discussed by Les Shepherd in his 1952-vintage JBIS paper on interstellar travel. To reduce probe flight time to 100 years or so, the ion-exhaust velocity must be increased by another order of magnitude.
Another required improvement to implement ion-propelled interstellar travel is the reduction of the propulsion system’s specific mass (kilograms/kilowatts). As my late friend, the UK propulsion expert Dr. David Fearn once told me, such a reduction is challenging but ultimately not impossible.
Thermonuclear Fusion Rockets
There are two major types of fusion under development. Magnetic fusion, which confines the reacting plasma in EM fields, seems to always be a few decades in the future. Some have quipped that it is the energy source and the propulsion system of the future and always will be.
Small scale inertial fusion confines and compresses micropellets using crossed electron or laser beams. Large scale inertial fusion—the hydrogen bomb—accomplishes confinement and heating reactants using fission charges, and has of course been operational for more than 60 years.
Large scale inertial-fusion propulsion was first investigated during the early space age by NASA and the US Department of Defense in the original Project Orion. The first demonstration in a scientific journal of the near-term feasibility of large-scale interstellar travel was Freeman Dyson’s original paper on an interstellar Orion in the October 1968 issue of Physics Today. Assuming propulsion by exploding hydrogen bombs, Dyson demonstrated that the US and USSR Cold War nuclear arsenals were sufficient to dispatch thousands of migrants on colonization ships. The estimated duration of one-way voyages to Alpha/Proxima Centauri was 130-1,300 years.
In an ideal world, the former Cold War adversaries would be glad to donate their now-obsolete thermonuclear arsenals to the worthy cause of promoting an interstellar diaspora. Sadly, we do not live in such a world.
Even if nuclear “devices” would be donated to the worthy cause of interstellar exploration/colonization, there are a few technical difficulties to contend with. Unless we can master aneutronic fusion reactions such as the boron-proton scheme, it must be demonstrated that spacecraft structures can survive periodic high-energy thermal-neutron doses.
Application of fusion micro pellets also has a number of technical issues. First, there is the problem of fuel availability. To reduce neutron irradiation on ship structures, the Daedalus study of the British Interplanetary Society (BIS) considered a Deuterium-Helium3 fusion fuel cycle. The problem is that Helium3 is very rare on Earth. To construct a Daedalus craft, cosmic helium sources must be tapped—perhaps the lunar regolith, atmospheres of giant planets or the solar wind.
The BIS follow-up to Daedalus, called Icarus, uses a Deuterium-Tritium fuel cycle. Here, it might be necessary to breed Tritium in nuclear fission reactors.
Some engineering issues must be addressed before Daedalus/Icarus-type pulsed fusion ships can become operational. What are the acoustic effects of repeated fusion ignitions within the reaction chamber? Will the walls of the reaction chamber be damaged if laser- or electron-beams miss a fuel pellet?
Another significant issue is the enormous size of inertial fusion ships. Even if payload mass can be drastically reduced, the beam projectors, reaction chamber and associated gear are massive.
One suggestion to reduce the mass of an inertial-fusion propelled spacecraft is worthy of future study. That is Johndale Solem’s Medusa concept. In Medusa, the massive reaction chamber is replaced by a hyper-thin, high-melting-point, radiation-tolerant sail. Fusion charges are ignited within this flexible canopy, which is connected to the payload by strong cables.
The Solar-Photon Sail
There are several reasons why photon sails have emerged as the near-term interstellar propulsion system of choice. First, small photon sails have been unfurled and operated in Earth orbit and interplanetary space.
Second, the photon sail can be scaled with the payload. A payload-on-a-chip requires a small sail. If the payload is small enough, sail and payload can be deployed from a small cubesat. Sail deployment and integration with payload can therefore be based upon current operational experience.
But today’s multi-layer solar-photon sails are not really capable of interstellar travel. Even if sail acceleration is combined with giant-planet gravity assists, it seems clear that Alpha/Proxima Centauri travel times less than 10,000 years will be difficult to achieve.
The best we can expect from current solar-photon sails is exploration of the heliopause at around 550 AU, the Sun’s gravity focus at >550 AU, and the inner reaches of the Sun’s Oort Comet Cloud.
In all likelihood, interstellar probes launched by solar-photon sails will never be as fast as those launched by laser-photon or maser-photon sails. The reason for this is that solar irradiance is an inverse square phenomenon—acceleration at Jupiter is 1/25 that at Earth’s solar orbit. A collimated and accurately aimed beam could maintain sail acceleration over much greater distances.
But the advantage of solar-photon over beam-photon sails is that mission designers need not concern themselves with the beam-projection system. The solar constant should not vary too much for the foreseeable future.
So a number of researchers have evaluated the possibility of all-metal sails, dielectric sails, carbon nanotube sails and mesh sails. But the ultimate sail material might be a molecular monolayer such as graphene.
Graphene is a hyper-strong layer of carbon, one molecule thick. Its melting point is in excess of 4,000 K and it is impermeable to many gases. In the visible spectral range, graphene is essentially transparent. Its fractional visible absorption is 0.023. As I describe in a 2012 JBIS paper, combination with other materials can increase reflectivity to about 0.05 and absorption to ~0.4. Graphene sails carrying robotic payloads and unfurled near the Sun seem capable of reaching Alpha/Proxima Centauri in a few centuries. Because human-carrying arks are limited to ~3g accelerations, these larger ships require about 1,000 years to reach these stars if they are propelled by graphene sails.
But here is where Project Starshot can play a very major role. In order to reach ~0.2c in a ~50 GW laser beam without melting, the sail reflectivity to laser light must be very high. Perhaps this can be achieved with an appropriate mesh-like meta material. Or perhaps the reflectivity of molecular monolayers such as graphene can be greatly increased.
After the Project Starshot workshop last August, participants produced draft Requests For Proposals (RFPs). I have discussed the possibility of increasing graphene reflectivity with theoretical condensed-matter researchers at my home institution (CUNY). It is quite possible that they will submit a proposal in response to the RFP when it is issued.
If monolayer reflectivity can be greatly increased, it will be necessary to demonstrate that this action does not adversely affect monolayer tensile strength so that the wafer sail is strong enough to support the payload during a very close solar approach. It will also be necessary to demonstrate that sail and payload can survive the very hostile environment encountered near the Sun.
A solar-photon sail will likely never achieve the ~0.2c interstellar velocity of the laser-boosted Project Starshot sail. But, just possibly, solar-photon-sail terminal velocities capable of making the journey to Alpha/Proxima Centauri in a century or so may not be totally infeasible.
One of the worst things we can do is to get so wedded to a concept that we fail to see conflicting information. That’s true whether the people involved are scientists, or stock brokers, or writers. It’s all too easy to distort the surrounding facts because we want to get a particular result, a process that is often subtle enough that we don’t notice it. Thus I was interested in what Rodrigo Luger said about his recent work on the outermost planet of TRAPPIST-1:
“It had me worried for a while that we were seeing what we wanted to see. Things are almost never exactly as you expect in this field — there are usually surprises around every corner, but theory and observation matched perfectly in this case.”
And that’s just it — in exoplanet research, we’ve come to expect the unexpected. So when Luger (a doctoral student at the University of Washington) went to work on this intriguing star some 40 light years from Earth, and its seven now famous planets, he was understandably edgy. Would TRAPPIST-1h turn out to orbit just where his team had projected?
Image: A size comparison of the planets of the TRAPPIST-1 system, lined up in order of increasing distance from their host star. The planetary surfaces are portrayed with an artist’s impression of their potential surface features, including water, ice, and atmospheres. Credit: NASA/R. Hurt/T. Pyle.
You’ll recall that it was the original survey led by Michaël Gillon (University of Liège, Belgium) that identified three planets around TRAPPIST-1 in 2016, a number that jumped to seven in a 2017 paper that drew worldwide attention, especially because several of the planets appeared to be within the star’s habitable zone. TRAPPIST-1h was a problem because Gillon and team were only able to observe a single transit. What Luger proceeded to do, working with Gillon as one of his co-authors, was to follow up on that work with an international effort that studied 79 days of K2 data, snaring four transits of TRAPPIST-1h in the process.
The bit about seeing what you want to see comes from the fact that the planets of TRAPPIST-1 are locked into an orbital resonance, making their orbital periods mathematically related. Luger and colleagues wanted to use the orbital velocities of the better understood TRAPPIST-1 planets to make a prediction about the orbital velocity and period of TRAPPIST-1h. Six possible resonant periods for the planet came out of these calculations, but subsequent data ruled out all but one. Would resonance unlock the problem?
The answer was yes. The K2 data confirmed the prediction, showing us that TRAPPIST-1h is in a frigid 18.77 day orbit around the ultracool dwarf star. At this distance from the host, the planet receives about as much energy per unit area from its star as Ceres receives from the Sun.
Image: The animation shows a simulation of the planets of TRAPPIST-1 orbiting for 90 Earth-days. After 15 Earth-days, the animation focuses only on the outer three planets: TRAPPIST-1f, TRAPPIST-1g, TRAPPIST-1h. The motion freezes each time two adjacent planets pass each other; an arrow appears pointing to the location of the third planet. This complex but predictable pattern, called an orbital resonance, occurs when planets exert a regular, periodic gravitational tug on each other as they orbit their star. The three-body resonance of the outer three planets causes the planets to repeat the same relative positions, and expecting such a resonance was used to predict the orbital period of TRAPPIST-1h. Credit: Daniel Fabrycky/University of Chicago.
So now we have a seven-planet chain of resonances. We’ve seen other multi-planet resonances — four-planet resonances exist at Kepler-80 and Kepler-223 — but TRAPPIST-1, as in so many things, ups the ante considerably. The resonance picture here also gives us ideas about the history of this system, which is thought to be somewhere between 3 billion and 8 billion years old (and if that range of possibilities isn’t a reminder of how much we have to learn about dating cool stars like this, I don’t know what is).
From the paper:
The resonant structure of the system suggests that orbital migration may have played a role in its formation. Embedded in gaseous planet-forming disks, planets growing above ? 1 MMars create density perturbations that torque the planets’ orbits and trigger radial migration. One model for the origin of low-mass planets found very close to their stars proposes that Mars- to Earth-sized planetary embryos form far from their stars and migrate inward. The inner edge of the disk provides a migration barrier such that planets pile up into chains of mean motion resonances.
Thus we’re looking at possible migration inward of a set of planets that migrated in what Luger calls ‘lock-step.’ Suddenly TRAPPIST-1 becomes an excellent test case not only for planet formation but planet migration theories. The paper continues:
This model matches the observed period ratio distribution of adjacent super-Earths if the vast majority (? 90%) of resonant chains become unstable and undergo a phase of giant impacts. Some resonant chains do survive, and a handful of multiple-resonant super-Earth systems have indeed been characterized. The TRAPPIST-1 system may thus represent a pristine surviving chain of mean motion resonances.
This would have been, the authors believe, a slow migration given the low mass of the TRAPPIST-1 planet-forming disk, and the fact that the planets themselves are low in mass. Perhaps this explains why the TRAPPIST-1 resonant chain is less compact than in systems with more massive planets, and why its unique stability has survived.
And what more can I say about K2? Despite everything going against the TRAPPIST-1h work, including not just the drift and jitter of the spacecraft in its less than optimum state but the faintness of the occasionally flaring target, K2 nonetheless delivered the goods. It’s a real testimony to those working the mission that we’re still pulling useful data out of the K2 observations, and testimony as well to the quality of the team working TRAPPIST-1h.
The paper is Luger et al.,”A seven-planet resonant chain in TRAPPIST-1,” Nature Astronomy 1 (2017), 0129 (abstract / preprint).
When to launch a starship, given that improvements in technology could lead to a much faster ship passing yours enroute? As we saw yesterday, the problem has been attacked anew by René Heller (Max Planck Institute for Solar System Research), who re-examined a 2006 paper from Andrew Kennedy on the matter. Heller defines what he calls ‘the incentive trap’ this way:
The time to reach interstellar targets is potentially larger than a human lifetime, and so the question arises of whether it is currently reasonable to develop the required technology and to launch the probe. Alternatively, one could effectively save time and wait for technological improvements that enable gains in the interstellar travel speed, which could ultimately result in a later launch with an earlier arrival.
All this reminds me of a conversation I had with Greg Matloff, author of the indispensable The Starflight Handbook (Wiley, 1989) about this matter. We were at Marshall Space Flight Center in 2003 and I was compiling notes for my Centauri Dreams book. I had mentioned A. E. van Vogt’s story “Far Centaurus,” originally published in 1944, in which a crew arrives at Alpha Centauri only to find its system inhabited by humans who launched from Earth centuries later. I alluded to this story yesterday.
Calling it a ‘terrific story,’ Matloff discussed it in terms of Robert Forward’s thinking:
“Bob had a couple of concepts of technological advancement. He had a famous plot of the velocity of human beings versus time. And he said if this is true, and you launch a thousand-year ship today, in a century somebody could fly the same mission in a hundred years. Theyre going to be passed and will probably have to go through customs when they get to Alpha Centauri A-2.”
Customs! Clearly, we’d rather not be on the slow starship that is superseded by new technologies. What Heller and Kennedy before him want to do is to figure out a rational way to decide when to launch. If we make assumptions about the exponential growth in speed over time, we can address the question by adding the time we spend waiting for better technology to the time of the actual journey. We can then calculate a minimum value for this figure based on the growth rates we find in our historical data.
This is how Kennedy came up with a minimum figure of 712 years (from 2006) to reach Barnard’s Star, which is about 6 light years away. The figure would include a long period of waiting for technological improvement as well as the time of the journey itself. Kennedy used a 1.4 percent annual growth in speed in arriving at this figure but, examining 211 years of data on historical speed records, Heller finds a higher annual growth, some 4.72 percent.
From the Penydarren steam locomotive of 1804 to Voyager 1, we see a speed growth of about four orders of magnitude. Growth like this maintained for another 112 years leads to 1 percent of lightspeed.
Image: A Bussard ramjet in flight, as imagined for ESA’s Innovative Technologies from Science Fiction project. Credit: ESA/Manchu.
But how consistent should we expect the growth in speed over time to be? Heller points out that the introduction of new technologies invariably leads to jumps in speed. We are now in the early stages of conceptualizing the Breakthrough Starshot project, which could create exactly this kind of disruption in the trend. Starshot aims at reaching 20 percent of lightspeed.
Working with the exponential speed doubling law we began with, we would expect that a speed of 20 percent of c would not be achieved until the year 2191. But if Starshot achieves its goal in the anticipated time frame of several decades, its success would see us reaching interstellar speeds much faster than the trends indicate. Starshot, or a project like it, would if successful exert a transformative effect as a driver for interstellar exploration.
We know that speed doubling laws cannot go on forever as we push toward relativistic speeds (we can’t double values higher than 0.5 c). But as we move toward substantial percentages of the speed of light, we see powerful gains in speed as we increase the kinetic energy beamed to a small lightsail like Starshot’s. Thus Heller also presents a model based on the growth of kinetic energy, noting that today the Three Gorges Dam in China can reach power outputs of 22.5 GW. 100 seconds exposure to a beam this powerful would take a small sail probe to speeds of 7.1 percent of c. Further kinetic energy increases could allow relativistic speeds for at least gram-to-kilogram sized probes within a matter of decades.
Usefully, Heller’s calculations also show when we can stop worrying about wait times altogether. The minimum value for the wait plus travel time disappears for targets that we can reach earlier than a critical travel time which he calls the ‘incentive travel time.’ Considered in both relativistic and non-relativistic models, this figure (assuming a doubling of speed every 15 years) works out to be 21.6 years. In Heller’s words, “…targets that we can reach within about 22 yr of travel are not worth waiting for further speed improvements if speed doubles every 15 yr.”
Thus already short travel times mean there is little point in waiting for future speed improvements. And in terms of current thinking about Alpha Centauri missions, Heller notes that there is a critical interstellar speed above which gains in kinetic energy beamed to the probe would not result in smaller wait plus travel times. His equations result in a value of 19.6 percent of c, an interesting number given that Breakthrough Starshot’s baseline is a probe moving at 20 percent of c, for a 20-year travel time. Thus:
In terms of the optimal interstellar velocity for launch, the most nearby interstellar target α Cen will be worthy of sending a space probe as soon as about 20 % c can be achieved because future technological developments will not reduce the travel time by as much as the waiting time increases. This value is in agreement with the 20 % c proposed by Starshot for a journey to α Cen.
We can push this result into an analysis of stars beyond Alpha Centauri. Heller looks at speeds beyond which further speed improvements would not result in reduced wait times for ten of the nearest bright stars. The assumption here would be that Starshot or alternative technologies would be continuously upgraded according to historical trends. Plugging in that assumption, we wind up with speeds as high as 57 percent of lightspeed for 70 Ophiuchi at 16.6 light years.
Thus the conclusion: If something like Breakthrough Starshot’s beaming capabilities become available within 45 years — and assuming that the kinetic energy transferred to the probes it pushes could be increased at the historical rates traced here — then we can reach all ten of the nearest star systems with an interstellar probe within 100 years from today.
Just for fun let me conclude with a snippet from “Far Centaurus.” Here a ship is approaching the ‘slowboat’ that has just discovered that Alpha Centauri has been reached by humans long before. The crew has just puzzled out what happened:
I was sitting in the control chair an hour later when I saw the glint in the darkness. There was a flash of bright silver, that exploded into size. The next instant, an enormous spaceship had matched our velocity less than a mile away.
Blake and I looked at each other. “Did they say,” I said shakily, “that that ship left its hangar ten minutes ago?”
Blake nodded. ‘They can make the trip from Earth to Centauri in three hours,” he said.
I hadn’t heard that before. Something happened inside my brain. “What!” I shouted. “Why, it’s taken us five hund… ” I stopped. I sat there.
“Three hours!” I whispered. “How could we have forgotten human progress?”
The René Heller paper discussed in the last two posts is “Relativistic Generalization of the Incentive Trap of Interstellar Travel with Application to Breakthrough Starshot” (preprint).
NASA has awarded a $500,000 grant to the Tau Zero Foundation for a 3-year study titled “Interstellar Propulsion Review.” Unlike prior studies, which were based on a specific mission concept, this study is an overall comparison between the different motivations, challenges, and approaches to interstellar flight. The work is split into three major 1-year phases:
1. Create an interstellar work breakdown structure (WBS) tailored to the divergent challenges and potentially disruptive prospects of interstellar flight in a manner that will allow for ‘level-playing-field’ comparisons. Prior mission and project information will be used to populate this first WBS.
2. Identify and work with subject matter experts to populate the WBS with their most recent reliable data.
3. Analyze uploaded data to identify (1) the most consequential knowledge gaps and (2) recommend research. Once all these phases are completed, the tools and methods are available to repeat the assessments as needed.
Your Inputs Sought
Tau Zero invites the participation of the broader interstellar community to affect this grant, with this call for papers for the next Tennessee Valley Interstellar Workshop (TVIW). This call is in addition to the more general call for abstracts issued from the TVIW hosts, whose topics and conditions can be found here.
Seeding Infrastructure
Many interstellar mission concepts rely on substantial infrastructure in our solar system to build, power, and launch their vehicles. What is seldom addressed, however, is how to begin to build that infrastructure, incrementally and affordably. Abstracts are invited that address that gap, with an emphasis on defining the first infrastructure missions that (a) can be launched with existing spacecraft, (b) provide an immediate utility in space, and (c) are part of a larger plan to extend that capability. This encompasses power production and distribution, mining, construction material processing, in-space construction, and propellant harvesting and delivery.
Exoplanet Science Instruments
What scientific instruments should an interstellar probe carry to collect meaningful information about an exoplanet – information that cannot be obtained from Earth-based astronomy alone? How close would such a probe need to get to an exoplanet to collect this information and how much time will it take within that distance to collect enough data to reach meaningful conclusions? What volume of data would need to be communicated back to Earth? What are projected mass and power requirements for such instrumentation? Abstracts are sought that discuss these instrumentation requirements, characteristics, and the trade-offs between minimizing instrumentation and maximizing information. Papers can be as basic as compiling a list of existing, relevant instrumentation for baseline comparisons, all the way to projections of the minimal mass, power, and computational ability for basic observations. Abstracts are also welcome that discus trends in the abilities of Earth-based exoplanet science and how this affects the instrumentation requirements of interstellar probes.
Foundationally Consistent Baselines
Different mission/vehicle concepts often use different projected performances for common functions such as: (a) heat rejection, (b) energy storage, (c) power management and distribution (PMAD), (d) magnetic nozzles, (e) communication with Earth, (f) equipment longevity, (g) structural mass {if built in space}, and (h) guidance, navigation and control (GNC). Fair comparisons of mission-vehicle concepts are difficult when different values are used for such baseline technologies. Presentations are invited that can credibly delineate reasonable performance estimates for such common functionalities so that future mission-vehicle studies can use common baselines for comparison (e.g. efficiencies, specific masses, readiness levels, etc).
Consistent Comparison Measures
It is difficult to objectively compare different interstellar propulsion and power concepts that use different fundamental methods with method-specific performance measures (e.g. rocket specific impulse, laser pointing accuracy, etc). Abstracts are sought for suggested alternatives to compare both the abilities and resource requirements of diverse interstellar mission concepts – measures that are consistent across all modalities (perhaps in terms of energy, power, mass, mission time, etc.).
Humanities – Interstellar Prerequisite of a Mature Humanity
The energy levels required for interstellar flight are large enough to have the potential to become weapons of mass destruction. Hence, a key prerequisite for achieving interstellar flight is not technical, but societal. Human civilization must mature to where it can wield these energy levels for the greater good instead of on each other. Abstracts are invited that explore these issues in rigorous, academic depth, or suggest how to begin such studies.
Humanities – World Ships as a Crucible of Cultural Study
In addition to the physical life support that has to function reliably for centuries aboard world ships, the culture of the on-board colony will also require a sustainably peaceful governance system along with a culture where the individual citizens live meaningful lives. Abstracts are invited that explore these issues in rigorous, academic depth, or suggest how to begin such studies.
Breakthrough Propulsion Physics
In addition to propulsion and power concepts based on known physics, it is prudent to also consider the possibility that new physics discoveries will lead to breakthrough propulsion, such as faster-than-light transport or propellant-less space drives. Abstracts are sought that identify relevant open questions in physics and then how to further investigate those unknowns. The connection between the open question in physics and its propulsion or power relevance must be explicit. Note, this is not an invitation for new theories or speculations about propulsion devices. Instead, this is a call to identify credible lines of inquiry that might lead to testable, relevant hypotheses. This invitation includes seeking experimental proposals for testing critical relevant questions in physics.
About the Workshop
The TVIW is a scientific and educational association that promotes interstellar exploration, travel, and communications. The TVIW provides an opportunity for relaxed sharing of ideas in directions that will stimulate and encourage interstellar exploration including propulsion, communications, and research. The ‘Workshop’ theme suggests that the direction should go beyond that of a ‘conference’. Attendees are encouraged not only to present intellectual concepts but to develop these concepts to suggest projects, collaboration, active research and mission planning. It should be a time for engaging discussions, thought provoking ideas, and boundless optimism contemplating a future that may one day be within the reach of humanity. Though the TVIW concept was intended to be regional (viz., the American Southeast), it is now, in fact, an internationally recognized event.
Presentation and Publication Requirements
Abstracts should describe content that can be introduced in a 20 minute presentation, followed by 5 minutes for Q&A. Though not a firm requirement, it is desirable that the author prepare a manuscript suitable for submission to a peer-reviewed journal (such as the Journal of the British Interplanetary Society for general papers). The workshop organizers plan to video and stream the presentations, and share the presentation charts with participants.
Submission Requirements
Submit 1-2 page PDF including the following information (file size cannot exceed 3 MB):
– Title
– Presenting author & affiliation.
– Coauthors and respective affiliations.
– Abstract text between 300 to 500 words in length
– Outline for the body of the report
– Cite at least 3 references upon which the work is based
– Cite the most recent publication by the presenting author that relates to the invited topics.
Where to Submit
The Tau Zero specific interests are in addition to a more general call for papers from TVIW. If you are submitting an abstract for the more general coverage of the TVIW, then visit this page. For the Tau Zero specific topics of interest, email your PDF to: Info@TauZero.aero for submissions.
Due Dates
10-May-2017 Submission deadline for TZF Paper Abstracts
31-May-2017 Accepted TZF papers announced
30-Jun-2017 Last day for early registration
30-Sep-2017 Deadline for electronic submittal of all final presentation materials
4-Oct-2017 TVIW 2017 begins
Selections Process
Tau Zero reserves the right to reject any abstract it deems as out of scope or not satisfactorily substantive. It is expected, as a minimum, that abstracts:
– Address the requested topics
– Adhere to the submission requirements
– Reflect that the presentation will be based on sound, credible information instead of speculative or subjective assertions.
Tau Zero is a 501(c) non-profit organization dedicated to accelerating progress toward the scientific breakthroughs required to support interstellar flight. The Foundation’s efforts, driven by the experts most capable of addressing the formidable challenges of interstellar flight, include fundamental scientific research, encouraging and supporting academic involvement in sciences related to its goals, empowering youth in this quest, forging collaborations for cross-fertilization, and engaging governmental and industry support on a global scale.
Tau Zero’s motto is “Ad Astra Incrementis” – to the stars in ever-expanding steps.
PG Note: To prevent redundancy, I’ve closed comments on this post so that comments can flow to Tau Zero at the address above.
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).
If you'd like to submit a comment for possible publication on Centauri Dreams, I will be glad to consider it. The primary criterion is that comments contribute meaningfully to the debate. Among other criteria for selection: Comments must be on topic, directly related to the post in question, must use appropriate language, and must not be abusive to others. Civility counts. In addition, a valid email address is required for a comment to be considered. Centauri Dreams is emphatically not a soapbox for political or religious views submitted by individuals or organizations. A long form of the policy can be viewed on the Administrative page. The short form is this: If your comment is not on topic and respectful to others, I'm probably not going to run it.
