On Track for ‘Breakthrough Discuss’

by Paul Gilster on April 14, 2016

Expect no new posts until Monday and a slowdown in comment moderation as I attend the Breakthrough Initiatives event in Palo Alto. As has been my practice at previous such events, I won’t try to cover the talks and discussions live. While ‘live blogging’ has its place and seems to work for things like product introductions, I don’t find it useful for scientific conferences, where I need to take lots of notes and think about the implications of what I’ve written. But I’ll come back from Stanford with plenty of things to talk about, and we’ll use all of next week sorting them out. Dinner tonight with Jim and Greg Benford, my son Miles, and my old friend Claudio Maccone, with whom I’ll be talking SETI, the FOCAL mission (to the Sun’s gravity lens), and the best methods of data retrieval for a mission like Breakthrough Starshot. Many more good discussions ahead. SETI is a big part of this conference, but Alpha Centauri looms large.



The Odds on Starshot

by Paul Gilster on April 13, 2016

Yesterday’s announcement of Breakthrough Starshot brought an email from exoplanet hunter Greg Laughlin (UC-Santa Cruz), whose work has been an inspiration to me since the early days of Centauri Dreams. One of Greg’s new projects, working with Anthony Aguirre (Foundational Questions Institute) and several other colleagues, is a website called Metaculus, which bills itself as “a community dedicated to generating accurate predictions about future real-world events by aggregating the collective wisdom, insight, and intelligence of its participants.” In other words, this is a kind of prediction market space for science and tech issues.

Breakthrough Starshot fits the bill here exactly, because Metaculus is all about the probability of future events, some of which can be predicted to a high degree, while others are purely a matter of calculated odds. The site is open to all and contains the basic information about its methods, and any logged in user can propose a question for consideration. Starshot comes into play through a series of four questions now available under the category ‘To the stars!’:

  • Will the private investment in laser-sail extra-solar travel be matched by a comparable amount within 5 years?
  • Mountaintop lasers, rockets, optics and wafers? Or something else?
  • Will a first test of a high-power phased array laser system occur by 2018?
  • Do potentially habitable planets exist in orbit around Alpha Centauri A or B?

The questions are now active on the site, and I think you’ll enjoy having a look and perhaps getting involved with your own predictions. It’s interesting to cull through the other questions as well ranging from discoveries at the LHC to the Turing Test. As the site notes:

Like many mental capabilities, prediction is a talent that persists over time and is a skill that can be developed. By giving steady quantitative feedback and assessment, predictors can improve their skill and accuracy, as well as develop a quantified track record. Then, probabilities of future events can be reliably drawn by optimally aggregating predictions — counting more heavily those with domain expertise and a strong prediction track record.

An Interstellar Swarm

Dennis Overbye is one of our most incisive science writers. I’ve profited from his work ever since his Lonely Hearts of the Cosmos (HarperCollins, 1991) and continue to read him in the New York Times. And it doesn’t surprise me at all to find that Breakthrough Starshot has triggered his usual elegance in the essay Reaching for the Stars, Across 4.37 Light-Years:

If it all worked out — a cosmically big “if” that would occur decades and perhaps $10 billion from now — a rocket would deliver a “mother ship” carrying a thousand or so small probes to space. Once in orbit, the probes would unfold thin sails and then, propelled by powerful laser beams from Earth, set off one by one like a flock of migrating butterflies across the universe.

That lovely image reinforces what some of us believe, that swarms of spacecraft, offering crucial redundancy and working at the far edge of miniaturization, are the most promising technology for deep space exploration at this level of our development. Milner’s advisers, who include Avi Loeb (Harvard), Pete Worden (former director of NASA Ames), Saul Perlmutter (UC-Berkeley), Freeman Dyson (IAS, Princeton) and Ann Druyan (producer of Cosmos), clearly agree, despite costs that will eventually reach $5 billion to $10 billion.


Image: A phased laser array, perhaps in the high desert of Chile, propels sails on their journey. Credit: Breakthrough Initiatives.

Given that the phased laser array Milner contemplates would have to generate 100 gigawatts of power for the crucial two minutes of acceleration on the sails, the task ahead is daunting. Thousands of lasers, as Overbye notes, will have to fire in perfect unison, and because this is a beamer built on the Earth, adaptive optics will have to compensate for atmospheric distortion. Moreover, we have to design a sail that will ‘ride’ the beam rather than be blown off it, and one that will be so highly reflective that it will absorb less than 1/100,000th of the energy applied to it.

These are problems that Robert Forward faced with his Starwisp design, a kilometer-wide ‘spider web’ of a sail driven by microwaves, with sensors scattered throughout the sail itself. It was Geoffrey Landis who would go on to show that as described, Starwisp would likely vaporize under the powerful beam meant to drive it to Alpha Centauri, causing a flurry of re-thinking of sail materials and design. But leaving the fuel at home is a powerful technique, and advances in technology may get us to the kind of materials that can withstand the photon torrent.

Writing for The Atlantic, Ross Andersen describes the sail this way in Inside a Billionaire’s New Interstellar Mission:

Picture a thin disc about the size of a round picnic tabletop. It would have miniaturized electronics onboard, including a power source, cameras, photon thrusters for navigation, and a laser for communication. Some of this kit would be bundled into the disc’s center, and some would be distributed through the rest of the sail. But it would all be a single unit: If you saw it streaking by, it would look like a flat, round sheet of reflective material.

We’ve also got a problem in that concept, because Jim Benford has pointed out that a flat sail is not a good ‘beam-rider’ — we’ll likely have to look at the kind of curved sail designs both Jim and brother Gregory Benford have studied in lab work at the Jet Propulsion Laboratory. But get a sail under that beam successfully and it reaches Pluto the day after launch, as Andersen notes. Another 20 years and it’s streaking through the Alpha Centauri system.

In any case, these are among the host of questions that the $100 million investment Milner is putting into the project will hope to answer, research extensive enough to offer proof of concept and finalize the design for a system that will eventually cost as much as the largest scientific projects we have built up to this time. A reusable facility may grow out of all this, one capable of sending fleets of small sails to our choice of nearby stars and returning imagery (the latter — data transmission at interstellar distances with craft this small — is another of the challenges that will have to be addressed. In a time of scaled-back thinking and low expectations, Breakthrough Starshot offers a sudden jolt of optimism that a new wave of research is on the horizon.



Breakthrough Starshot: Mission to Alpha Centauri

by Paul Gilster on April 12, 2016

Here on Centauri Dreams we often discuss interstellar flight in a long-term context. Will humans ever travel to another star? I’ve stated my view that if this happens, it will probably take several hundred years before we develop the necessary energy resources to make such a mission fit within the constraints of the world’s economy. This, of course, assumes the necessary technological development along the way — not only in propulsion but in closed-loop life support — to make such a mission scientifically plausible. I get a lot of pushback on that because nobody wants to wait that long. But overall, I’m an optimist. I think it will happen.

Let’s attack the question from another direction, though, and leave human passengers for a later date, as Yuri Milner’s Breakthrough Initiatives, aided by Stephen Hawking, is doing today in a New York news conference. What if we talk about unmanned missions? What if, in fact, the question is: How soon can we put a scientific payload past another star? Let’s not worry about decelerating — this will be a flyby mission. Let’s build it as soon as possible using every breakthrough technology we have at our disposal. How long would it take for that mission to be developed and flown?


Milner, a philanthropist and investor who was an early backer of Facebook, Twitter, Spotify and numerous Chinese tech companies, tells me his goal is to ‘give back to physics’ in developing just such a mission. Part of that giving back is the $100 million he has already put forward to support SETI, a ten-year project that will produce more telescope time for SETI than any other. Milner is also the founder of the Breakthrough Prize, issuing awards in physics, life sciences and mathematics. But in many respects this third Breakthrough Initiative is the most daring of all.

Time for the Stars

Breakthrough Starshot is an instrumented flyby of Alpha Centauri with an exceedingly short time-frame, assuming research and development proceed apace. Milner is putting $100 million into the mission concept, an amount that dwarfs what any individual, corporation or government has ever put into interstellar research. A discipline that has largely been the domain of specialist conferences — and in the scheme of things, not many of those — now moves into a research enterprise with serious backing.

Could an Alpha Centauri flyby mission be developed and launched within a single generation? I think it’s quite a stretch, but it’s the best-case scenario Milner mentioned in a phone conversation over the weekend. He’s enough of a realist (with a first-rate physics background) to know that the challenges are immense. Even so, he sees no deal-breakers.

Let’s walk through the case and see why he finds reason for optimism. “There are major advances that we can now turn to as we develop this proof of concept,” Milner says. “Twenty years ago, none of these things would have been available to far-thinking scientists like Robert Forward. But now we can put them to use and test their possibilities.”

If you’re thinking of an interstellar mission in the near-term, there is really only one choice of propulsion: The beamed sail. Sails have the advantage of known physics, laboratory experiment and actual deployment in space. We could talk about fusion for some indefinite point in the future, but at present, we don’t know how to do fusion even in massive installations on Earth, much less in the tight confines of a spacecraft engine. Interstellar ramjets are a far-future unknown — they may act more effectively as braking devices than engines, according to recent research. Antimatter is nowhere near readiness for propulsion, either in production methods or storage. Chemical rockets fall victim to the mass/ratio problem and are useless for fast interstellar journeys.

That leaves us with sails carrying very small payloads. To cross the 4.37 light years to the Centauri A and B system, Breakthrough Starshot proposes small spacecraft, taking advantage of advances in nanotechnology to reduce payload size. Think Moore’s Law and the reductions in size and cost that have accompanied the vast increases in micro-chip power. “Moore’s Law,” says Milner, “tells us that now is the time.”

StarChip is the Breakthrough Initiatives’ name for a payload measured not in kilograms but grams, a wafer that carries everything you would expect in a fully functional probe. ‘What was once a 300 gram instrument is is now available at three grams,” Milner continues. “What was 100 grams is now 0.5 grams. This is the trend we are riding.”

The StarChip payload includes cameras, power supply, communications equipment, navigation capabilities and photon thrusters. And it would be thrown across the interstellar gulf at 20 percent of the speed of light by a sail that is itself a miniaturized version of the sails Robert Forward used to discuss. Forget the thousand-kilometer sail (much less the continent-sized sails of the science fiction dreamer Cordwainer Smith). Milner’s team believes we can now talk in terms of a laser-driven lightsail that is no more than 4 meters across. This is actually smaller than the first deployed sail craft, the Japanese IKAROS, which boasts a sail measuring 14 meters to the side.

Advances in metamaterials and additional research should be able to produce, Milner believes, a 4 meter sail whose own weight is tallied in grams, and whose materials allow fabrication at a thickness of a few hundred atoms. A sail that small makes its own statement: Clearly, it’s not going to be under the beam for long, which means we need to focus a great deal of light on it for a very brief time. Lasers are another technology that benefits from rising power and falling cost. The trick here will be to create ‘phased arrays’ of lasers that can scale up to the 100 gigawatt level. A phased array involves not one but a group of emitters whose effective radiation pattern is reinforced in the desired direction by adjusting the phase of the signals feeding the antennae.

This is classic Bob Forward thinking rotated according to the symmetries of our new era. Milner aims for a beamer technology that is modular and scalable. And it fits into a larger infrastructure. Breakthrough Initiatives talks about bringing a ‘Silicon Valley approach’ to the problem of interstellar flight. Build a StarChip that can eventually be mass-produced at no more than the cost of an iPhone. For the Alpha Centauri mission, whenever it flies, is itself a proof of concept that could lead to multiple destinations. And if the cost can be driven as low as Milner believes, then we can think in terms of redundancy, with StarChips sent in large numbers to return a full characterization of any destination system. Assemble the light beamer and, as the technology matures, the cost of each launch falls.

These are ideas that are at once familiar but also exotic, for while Forward talked about enormous power stations in close solar orbit to power up his banks of lasers (and a huge Fresnel lens in the outer system to focus the beam), Milner thinks we can build a ground-based beamer at kilometer scale right here on Earth. I was startled at the idea — surely efficiency favors a space-based installation — but Milner’s point is that he thinks we can begin to launch interstellar craft before we have the technology to build the kind of power station Forward envisioned. If you’re serious about a launch within a few decades (again, it’s a best case scenario, and a dramatic one), then you build an Earth-based beamer and use adaptive optics to cancel out atmospheric effects.


Image: A wide-field view obtained with an Hasselblad 2000 FC camera by Claus Madsen (ESO), of a region around the Southern Cross, seen in the right of the image (Kodak Ektachrome 200, 70 min exposure time). Alpha Centauri is the bright yellowish star seen at the middle left, one of the “Pointers” to the star at the top of the Southern Cross. Although it appears here as a single ‘star,’ it is actually comprised of the G-class Centauri A, K-class Centauri B, and the M-dwarf Proxima Centauri. Credit: ESO/Claus Madsen. Original here.

All this will be subject to tightly focused research, which is what the $100 million is for, but what Milner hopes to see are nano craft delivered to orbit and then boosted on their way with a 30 minute laser ‘burn’ that, reaching 60,000 g’s, drives the sail to 20 percent of the speed of light. That makes for roughly a twenty year crossing to Alpha Centauri. With a craft this small, data return is highly problematic, and in fact I think it’s one of the biggest unanswered questions Breakthrough Starshot will have to face (well, this and the challenge of interstellar dust, and key questions related to sail design and the sail’s ability to stay on thee beam during acceleration). The sail is itself the antenna on a craft of this design, and Jim Benford told me in conversation that it will have to be shaped to one-micron precision. Even so, powering up the system to send imagery and data to Earth is going to be tricky. It will be fascinating to see what kind of solutions emerge as this research gets underway, and what alternative methods may be suggested.

Even so, and granting the cost reductions digital technology makes possible, Breakthrough Starshot embarks upon a multi-year research and engineering phase that will focus on building a mission infrastructure. Creating the actual mission will demand a budget comparable to the largest scientific experiments of our time. These are no small aspirations, but what drives them is something that interstellar studies have never had at their disposal: A dedicated, enthusiastic, well-funded effort with the participation of major scientists.

“We have an advisory board of twenty, including Freeman Dyson and other top scientists,” Milner added. “$100 million will be spent in coming years as we look toward concept verification. Multiple grants should flow from this, research and experiments. We need to complete the initial study and see if building a prototype, perhaps at a scale of 1/100, is then the next step.”

At the very least, we can expect the research behind this project to spin off numerous useful technologies, all of which should be applicable not only to star missions but to in-system exploration, along with, potentially, a kilometer-scale beamer that can double as a large telescope for astronomical observations. And while I doubt we can look at interstellar missions within the next few decades (I am open to being convinced otherwise), I believe that the timing for a fast flyby of Alpha Centauri will be considerably advanced by this work.

There is much to be said about all aspects of the Breakthrough Starshot concept, and as you would imagine, I’ll be covering this closely, beginning with a trip later this week to the Breakthrough Initiatives meeting in California. That meeting will have a large SETI component growing out of Milner’s prior commitment of another $100 million, which is already being translated into active observations at the Green Bank observatory in West Virginia. But as you can imagine, the Alpha Centauri mission will be under discussion as well as the research effort begins to be assembled. What spins out of this will keep us talking for a long time to come.



The Snowbank Orbit, Redux

by Paul Gilster on April 8, 2016

We haven’t yet found Planet Nine, but the evidence for its existence is solid enough that we can start thinking about its possibilities as a mission target. That work falls in this essay to Adam Crowl, a Centauri Dreams regular whose comments on articles here began not long after I started the site. An active member of the Project Icarus attempt to re-design the 1970s Project Daedalus starship, Adam is also the author of Crowlspace, where his insights are a frequently consulted resource. Today he harkens back to a 1960s science fiction story that has given him notions about a way not only to reach Planet Nine but to establish orbit around it.

by Adam Crowl


Fritz Leiber is better known for his fantasy and SF-fantasy, but he could write hard-SF too. A fine example is his 1962 story, “The Snowbank Orbit”, the title of which alludes to World War II tales of pilots surviving bailouts without parachutes by plunging into snow-drifts. Five spacecraft, racing towards Uranus at 100 miles per second with empty tanks, intend a fiery plunge through the planet’s atmosphere to brake into orbit. The rest of that story I will leave to the interested reader [available here] but the idea of aerobraking into orbit around a distant Planet Nine is worth discussing.

Presently we know very little about Telisto – the mellifluous name suggested for Planet Nine by physicist Lorenzo Iorio [1] which I’ll use for convenience. Brown & Batygin [2] suggest an orbit averaging about 700 AU and a mass of at least 10 Earth masses. The mass could be somewhat higher, though certainly not of the order of a Saturn-mass as its infra-red glow would’ve been seen by earlier surveys. Modelling [3] suggests a 10 Earth-mass planet, with a substantial hydrogen-helium envelope, could be as ‘warm’ as 50 K – about 40 degrees warmer than the ~10 K from sunlight alone. A range of compositions were modelled. A Super-Earth, an Ice-Giant (like Uranus/Neptune) and a miniature version of Jupiter/Saturn are all possible. Some cosmogonic simulations [4] suggest a Neptune like object is likely to have been flung from amongst the other giant planets during their formation, so it seems the most likely option.

A Neptune-like Telisto would then be an ice-wrapped rocky core wrapped in a layer of captured hydrogen/helium mixture. It’s likely that hydrogen will be depleted from its atmosphere by some fraction being chemically bound and mixed with its core, so helium will be a higher fraction of the atmosphere, as appears to be the case for Neptune. If the atmosphere is a small fraction of Telisto’s mass, then it’s possible it will have an icy surface or even a liquid water ocean under a hydrogen atmosphere via its greenhouse effect trapping the planet’s internal heat. In that case Telisto will be very interesting from an astrobiological perspective, though the energy sources available to sustain life are impossible to quantify at present.

Telisto, at 700 AU, would be in interstellar space, well beyond the moving boundary of the Sun’s magnetosphere, so its intrinsic magnetic field would dominate over a vast volume of space. The raw flux of impinging cosmic rays might allow enhanced creation and trapping of antimatter, as suggested occurs around the planet Saturn and the Earth. Any moons of Telisto would also provide a ready source of materials, if we chose to build starships there.


Image: Artist’s impression of Planet Nine as an ice giant eclipsing the central Milky Way, with a star-like Sun in the distance. Neptune’s orbit is shown as a small ellipse around the Sun. The sky view and appearance are based on the conjectures of its co-proposer, Mike Brown. Credit: Tomruen, nagualdesign; background taken from File:ESO – Milky Way.jpg (Own work) CC BY-SA 4.0, via Wikimedia Commons.

Deep Space Propulsion

Given sufficient motivation we’ll send a probe and eventually follow in person. Getting there will be a challenge. At the present 3.5 AU/year of “Voyager 1” the journey would take 200 years. Leiber’s 100 miles per second would get a probe there in 20 years, which might be acceptable if the probe has a compelling secondary mission it can pursue during the long cruise phase. Long baseline telescopic observations might be sufficiently attractive to combine the two. A flyby at 100 miles per second is probably too quick to provide sufficient science return for the investment, so stopping will be required.

Conventional propulsion, such as nuclear powered ion drives, are unlikely to be up to the task. In 1987 the Thousand AU (TAU) probe to was studied as a first interstellar mission [6]. The eventual design chosen used a nuclear reactor that was technically not far removed from the SNAP reactors that had been tested in the 1960s. The ion-drive would run for a decade and the probe would take 50 years to reach 1,000 AU – without stopping. Ion drives have improved significantly since then and could bring the mission time down to 20 years. The chief performance limitation is power supply. Fissioning a kilogram of uranium produces about 90 trillion joules of energy, but the rate at which it can be released is limited by the maximum temperature at which the reactor can operate. Typically a power reactor runs at less than the melting point of the fuel elements and its components, especially when required to operate reliably for years at a time. Then waste heat has to be ejected into space, which requires heavy radiators. Minimising radiator size means the reactor’s power production cycle must convert raw heat into power at less than 25% efficiency, so 75% of the energy of fissioning uranium has to be dumped to space. Due to these limitations solid-core power reactors can supply power with a specific power of at most 50 to 100 watts per kilogram (W/kg) of reactor power-system.

To reach 700 AU in 20 years requires increasing the vehicle’s kinetic energy at the rate of about 470 watts per kilogram of vehicle. If the reactor power-system is a hefty 75% of the vehicle’s mass, then it must supply power at over 600 W/kg of total vehicle mass. Advanced ion drives typically can convert raw electricity into kinetic energy with an efficiency of between 75%-85%, so the total power supply from the reactor needs to be over 700 W/kg. No solid core reactor can run hot enough to achieve this. Reactors, in theory, can run hotter – much hotter. Liquid, gas or plasma core reactors have been researched, but require several decades of development to bring to operational readiness. As yet theoretical, Fission fragment reactors might also push performance beyond this level, though with similar development times.

An incredible nuclear fusion power source already exists in space – the Sun. Two torrents of momentum and energy stream out from the Sun, in the form of photons and the Solar-Wind, with a total power of 400 trillion trillion watts. Tapping just a tiny fraction of that torrent would allow a quick trip to Telisto – and the stars beyond. Doing so is the challenge. Solar-sail propulsion taps the photon torrent and is the option being vigorously tested by the Planetary Society, NASA and JAXA. Another option is the Electric-Sail, or E-Sail, which uses a multitude of long, thin wires that are charged so they reflect the charged particles of the Solar-Wind. E-Sails are being developed by a Finnish team led by Pekka Janhunen [5], with some NASA involvement.

A less well-developed option is the Magnetic-Sail (Mag-Sail), which uses a loop of superconducting wire to form a miniature magnetosphere to ride the Solar-Wind. Unfortunately the need for cooling systems for the superconducting wire makes the Magnetic-Sail less attractive for operation near the Sun. The Solar Wind itself is quite turbulent magnetically on the size-and-time scales relevant to interplanetary Mag-Sail applications, so considerable applied research into Mag-Sailing the Solar-Wind is needed before it can be used with confidence.

All Sail types will require a probe to closely approach the Sun to intercept sufficient photons or Solar-Wind (mostly protons and alpha particles). If the probe were to drop to 0.2 AU – half the orbit of Mercury – it would need to be pushed outwards with a force about 7 times greater than the Sun’s gravitational attraction at that distance to reach a final speed of 35 AU per year (i.e. 100 miles per second.) For a total mass of 500 kilograms the probe would need an E-Sail about 48 km across or a Solar-Sail about 700 metres across. Increasing the total mass will require proportionally larger sails of both kinds, though the exact final mass will depend on power sources and payloads chosen.

The Challenge of Deceleration


After cruising for 20 years, the probe then needs to stop, a non-trivial task. Telisto certainly isn’t radiating enough photons to slow a Solar-Sail, but the Interstellar Medium that it is embedded in might provide some drag for an E-Sail. Past the orbit of Saturn, the Mag-Sail’s superconducting wires would no longer need active cooling and the Interstellar Medium (ISM) is a calmer medium for Mag-Sailing. Thus a combination of E-Sail and Mag-Sail can be used. A Mag-Sail could form the outer ring to support the E-Sail and would only be powered up once the ambient temperature was low enough. For years there have been hints of high temperature superconductivity materials, so there might be a breakthrough at any time which would allow a purely Mag-Sail system, but it’s unnecessary at present. Before Aero-Capture at Telisto – the probe’s “Snowbank Orbit” maneuver – the Mag-Sail/E-Sail will need to be packed away or detached.

Image: Fritz Leiber’s “The Snowbank Orbit” involved aerobraking in the atmosphere of Uranus. Can we adapt these methods to Planet Nine, achieving a stable orbit for our probe?

Some speed will need to be shed in the atmosphere, but how much? The fastest re-entry ever survived was by the Galileo mission’s Descent Probe in 1996, which re-entered at a speed over 47 kilometres per second, surviving more than 200 gees of peak deceleration. That probe’s Thermal Protection System – an aerodynamic cone of material designed to ablate away absorbed re-entry heat – lost almost half its mass during its fiery plunge. An Orbiter doesn’t want to plunge all the way into Telisto, but shed sufficient speed to go into orbit. An initial capture can be a highly elliptical almost-escape orbit, with a short burn needed at its highest point (apoapsis) to raise the low-point (periapsis) of the orbit away from the planet into something more circular that doesn’t plunge back into the atmosphere.

A 10 Earth-mass Super-Earth Telisto with a relatively thin atmosphere – less than 0.1% of its mass – would need a capture speed of about 25 km/s. If a probe arrived at 50 km/s it would need to shed 75% of its kinetic energy to brake to 25 km/s. If the planet is rotating relatively quickly the relative arrival velocity will be reduced if the maneuver happens close to the equator – Galileo’s descent probe arrived at nearly 60 km/s, but Jupiter’s cloud tops rotate at 12.5 km/s, thus reducing the relative velocity to a more manageable 47.3 km/s. If Telisto were a mini-Neptune (mostly ices) or mini-Jupiter (mostly hydrogen/helium) then the planet will be significantly larger. Both compositions in this mass-range have been modelled. A Neptune-like Telisto would have a radius of about 4 times Earth, while a Jupiter-like Telisto would be more like 8 Earth radii. Required capture orbital velocity would be significantly lower – 18 km/s and 12 km/s respectively – but would have higher cloud-top rotation speeds for the same rotational period. A top re-entry speed of 50 km/s seems likely, but there is some research into magnetic braking in an outer atmosphere environment that may change that figure once matured.

A journey to Telisto is a stretch-goal for our aerospace technologies. Any suite of techniques that can place an orbiter there in 20 years, or less, is a breakthrough for all missions to the Outer Planets and beyond. In the wider historical context, we can liken Telisto to an over the horizon island that’s hinted at by flights of birds, a trick that ancient mariners used to guide them successfully to new lands. Star formation models suggest that the ISM is home to thousands of planet-like objects that formed from tidally disrupted proto-stars, a prospect strongly supported by microlensing data, though as yet only from the further reaches of the Galaxy. As microlensing surveys and techniques become more refined we will discover closer islands in the Dark Ocean between the stars. Getting beyond the noisy, obscuring Heliosphere will give us new means of detecting such worlds via their radio emissions and other disturbances of the ISM. Even long-range gravitational detectors could be deployed, away from perturbations from the flotsam and jetsam of the solar system. Colonizing Telisto’s likely system of moons would thus be the first outpost in a cosmic Ocean dotted by interstellar planets.

Reference Links

[1] http://adsabs.harvard.edu/abs/2015arXiv151205288I

[2] http://adsabs.harvard.edu/abs/2016AJ….151…22B

[3] http://adsabs.harvard.edu/abs/2016arXiv160207465L

[4] http://adsabs.harvard.edu/abs/2012ApJ…744L…3B

[5] http://www.electric-sailing.com/

[6] https://en.wikipedia.org/wiki/TAU_(spacecraft)



A Young, Free-Floating Jupiter Analog in TW Hydrae

by Paul Gilster on April 7, 2016

A stellar association is a loose grouping of stars of similar spectral type and age that share a common motion. About 90 percent of all stars are thought to originate as members of associations. The TW Hydrae association (TWA) is a case in point: The group is made up of about thirty young stars, each thought to be roughly ten million years old. This is the youngest grouping of stars in the neighborhood of the Sun. You may recall 2M1207, which has turned up in these pages before, a brown dwarf member of the TWA that has a companion of planetary mass. Now we learn of another exotic find, a young, bright free-floating planet-like object.

Jonathan Gagné (Carnegie Institution for Science) used the FIRE spectrograph on Carnegie’s Baade 6.5-m telescope in Chile to measure the line-of-sight velocity of the object, known as 2MASS J1119–1137. This along with the sky motion of the object allowed researchers to make the definitive call that 2MASS J1119–1137 is indeed a member of the TW Hydrae association. The identification allowed the team to peg the object’s age at no more than 10 million years.

Between four and eight times Jupiter’s mass, 2MASS J1119–1137 is somewhere between large planet and small brown dwarf. Carnegie’s Jacqueline Faherty notes the challenge of distinguishing such free-floating objects in nearby space from more distant counterparts:

“Much more commonly, distant old and red stars residing in the far corners of our galaxy can display the same characteristics as nearby planet-like objects,” says Faherty. “When the light from the distant stars passes through the large expanses of dust in our galaxy on its way to our telescopes, the light gets reddened so these stars can pose as potentially exciting nearby young planet-like objects in our data, when they actually are not that at all.”

Lead author Kendra Kellogg (Western University, Ontario) says that the object’s strong infrared signature was an early clue to its youth. Follow-up work using the Gemini South instrument in Chile allowed the team to confirm the object’s proximity to the solar neighborhood. It is thought to be about 95 light years from the Earth. The linkage to the TW Hydrae association nailed down the object’s age, exposing its position as the nearest isolated member of the TWA.

Video: An animated view of the TW Hydrae association with the young object depicted within it. Credit: David Rodriguez, using visualization software Uniview by SCISS and the American Museum of Natural History’s Digital Universe data.

Finding free-floating objects like these can be a significant help to our studies of exoplanets, as the paper explains:

Young brown dwarfs, especially at the latest spectral types, have masses and atmospheres similar to those of directly imaged gas giant exoplanets. Isolated young brown dwarfs offer a way to study cool, low-pressure atmospheres of exoplanets without the inherent difficulties of isolating the planet flux from that of a brighter host star. Most of the known isolated planetary-mass brown dwarfs have been found through their unusually red optical and near-infrared colors, often in the regions of young stellar associations… Over the past few years, targeted searches have also encompassed the position-velocity phase spaces of nearby young stellar associations… These have helped recognize or discover the lowest-mass isolated brown dwarfs in the solar neighborhood…

No bright star overwhelming the light of 2MASS J1119–1137. No wonder such objects are useful.

The paper is Kellogg et al., “The Nearest Isolated Member of the TW Hydrae Association is a Giant Planet Analog,” accepted for publication at the Astrophysical Journal Letters (abstract). A Carnegie news release is also available.



Supernova at Twilight

by Paul Gilster on April 6, 2016


In his novel The Twilight of Briareus (John Day, 1974), Richard Cowper, who in reality was John Middleton Murry, Jr., wrote about a fictitious star called Delta Briareus that goes supernova (true, there is no constellation called Briareus, but bear with me). Because it is only 130 light years out, the supernova showers the Earth with radiation, with consequences that are in some cases obvious, in others imaginative in the extreme. It’s a good read, one that at least one critic, Brian Stableford, has compared to J. G. Ballard’s early disaster novels.

The novel contrasts an earthy domesticity with the celestial display that soon shatters it. It’s worth quoting a patch of the book:

It so happened that I, in common, no doubt, with several million others — was among the first in England to observe that ‘majestic effulgence’ within seconds of its arrival. At about twenty past nine on the Tuesday evening I switched off the telly and suggested to Laura that we could do worse than saunter down to The Three Foxes for some fresh air and a gin and tonic. Ten minutes later we were strolling pubwards when she suddenly gripped my arm and yelped: ‘Hey, look at that!’

We stood stock still and gaped up into the heavens.

‘It’s a magnesium flare,’ I said. ‘They used to drop them during the war. There must be some sort of RAF exercise.’

‘Well, why isn’t it moving then?’

‘It is. Only slowly. They have parachutes.’

‘But it’s so bright!’ exclaimed Laura. ‘Look at the shadows it’s given us!’

She was quite right. There on the road beside us were two distinct silhouettes. I contemplated them for a moment and then looked up again. The flare was still there, completely outshining every other thing in the sky with its eye-aching bluish-white brilliance.

And there we are, a supernova in progress, with results no one at this point in the tale can imagine. Cowper, who also wrote as Colin Murry, is a personal favorite. His short story collection Out There Where the Big Ships Go (Pocket Books, 1980) is a good introduction.

Image: The cover of the US paperback of The Twilight of Briareus, the edition I read when it came out.

Supernovae in the Pliocene?

Putting humans under the torch of a supernova makes for exciting fiction, but new work from an international team of researchers now suggests a very real supernova — and probably a series of them — exploded in the Pliocene epoch and later, with evidence of radioactive debris indicating a window between 3.2 to 1.7 million years ago. That would place these events at the boundary between the Pliocene and the Pleistocene (the latter beginning 2.58 million years ago and ending 11,700 years ago).

“We were very surprised that there was debris clearly spread across 1.5 million years,” said team leader Anton Wallner (Australian National University). “It suggests there were a series of supernovae, one after another. It’s an interesting coincidence that they correspond with when the Earth cooled and moved from the Pliocene into the Pleistocene period.”

We’re talking about supernovae no more than 300 light years away, which would make them comparable in brightness to the Moon, and certainly visible during daylight hours. Appearing in Nature, the work argues that iron-60 found in sediment and crust samples from the Pacific, Atlantic and Indian Oceans show Earth’s exposure to cosmic ray bombardment, but at levels that would have been too weak to cause major biological damage, much less extinctions.

The team searched for interstellar dust by examining 120 samples of the ocean floor spanning the past eleven million years. All iron had to be extracted from the ocean cores, work performed at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Germany and the University of Tokyo, and the minute amounts of iron-60 had to be separated from terrestrial isotopes using the Heavy Ion Accelerator at ANU. The decay of the radioactive isotopes beryllium-10 and aluminum-26 was used to determine the age of the cores.

Iron-60 itself has a half-life of 2.6 million years, unlike the stable iron-56, and as Wallner explains, is ‘a million-billion times less abundant than the iron that exists naturally on Earth.’ Interestingly, fallout shows up not only in the 3.2 to 1.7 million year window, but also at about 8 million years back. The researchers suggest that the supernovae responsible were probably found in a star cluster that has subsequently moved away from the Earth.

A Writer’s Choice


What would the much closer supernova of the fictional Delta Briareus do to the Earth? In The Twilight of Briareus, the effect is largely meteorological. At first.

The loss of ‘Tiros’ and the rest of the observation satellites hamstrung the world’s long-range weather forecasters, but there was still sufficient evidence of cataclysmic upheaval in the upper atmosphere for a hundred assorted professors to chill humanity’s blood with their doom-laden warnings. These ranged from an ice age at one end of the scale to a slow roasting at the other. The best we could hope for, apparently, was a period of tempests of unprecedented severity. We listened, felt appropriately chastened, and then cheered up again, either from endemic atrophy of the imagination or for no better reason than that the human psyche cannot exist for long on a diet of undiluted pessimism.

There is a compelling lyricism in Cowper’s fiction that eschews irony; in that sense he’s at odds with many of his contemporaries. What an interesting man. He gave up writing of any kind in 1986 and put his effort into painting and antiques, a kind of escape that makes him more akin to William Morris and his circle than, say, Anthony Burgess or Martin Amis (the latter detested his work). I’ve always been taken with Cowper’s elegance, and wonder what he would have done with ancient supernovae blossoming over one of his finely wrought landscapes. If only we knew. Cowper was devastated by the death of his wife Ruth, and died shortly after her in 2002.

Today’s paper is Wallner et al., “Recent near-Earth supernovae probed by global deposition of interstellar radioactive 60Fe,” Nature 532 (07 April 2016), 69–72 (abstract). See also Breitschwerdt et al., “The locations of recent supernovae near the Sun from modelling 60Fe transport,” Nature 532 (07 April 2016), 73–76 (abstract). From the latter:

The Local Bubble of hot, diffuse plasma, in which the Solar System is embedded, originated from 14 to 20 supernovae within a moving group, whose surviving members are now in the Scorpius–Centaurus stellar association7, 8. Here we report calculations of the most probable trajectories and masses of the supernova progenitors, and hence their explosion times and sites.



SETI: A New Kind of ‘Water Hole’

by Paul Gilster on April 5, 2016

Some of you may recall an episode of Star Trek: The Next Generation in which the inhabitants of a planet called Aldea use a planetary defense system that includes a cloaking device. The episode, “When the Bough Breaks,” at one point shows the view from the Enterprise’s screens as the entire planet swims into view. My vague recollection of that show was triggered by the paper we looked at yesterday, in which David Kipping and Alex Teachey discuss transit light curves and the ability of a civilization to alter them.

After all, if an extraterrestrial culture would prefer not to be seen, a natural thought would be to conceal its transits from worlds that should be able to detect them along the plane of the ecliptic. Light curves could be manipulated by lasers, and as we saw yesterday, the method could serve either to enhance a transit, thus creating a form of METI signaling, or to conceal one. In the latter case, the civilization would want to create a change in brightness that would essentially cancel out the transit light curve. It’s not exactly a ‘cloaking device,’ but it ought to work.


Image: The Next-Generation Transit Survey (NGTS) telescopes operating at ESO Paranal, Chile. Transit observations have SETI implications we are only beginning to explore. Credit: ESO/ G. Lambert.

A Galaxy of Xenophobes?

As I said yesterday, I’m not here to reignite the METI debate as much as to acknowledge that what an alien culture might do is unknown. Rather than asking whether any civilization should try to conceal itself, let’s simply ask what it could do if it made the attempt.

The idea has a brief history, with Eric Korpela (UC-Berkeley) and Shauna Sallmen (University of Wisconsin-La Crosse) suggesting in 2015 that ETI could effectively hide a planetary signature through the use of orbiting mirrors. This would, like the geometric masks envisioned by Luc Arnold, require engineering on a huge scale, and would also demand elaborate tuning for each target. Kipping and Teachey argue for a more affordable alternative using a directed laser beam:

In our scheme… the advanced civilization emits a laser directed towards the other planetary system at precisely the instant when the other system would be able to observe a transit. The power profile of the laser would need to be the inverse of the expected transit profile, leading to a nullified flat line eliminating the transit signature.

Screenshot from 2016-04-05 09:24:41

Image: Top: The unaltered light curve of the Earth transiting the Sun, as viewed by different broadband optical photometers (offset by 5 ppm). Middle: The power profile of a 600 nm laser array designed to cloak the Earth. An array of lasers producing a peak power of ∼ 30 MW over 13 hours nullifies the transit. Bottom: Residual light curve, as seen by the different photometers. Credit: David Kipping/Alex Teachey.

The Kepler mission has produced the vast majority of recent exoplanet discoveries, and we have upcoming transit surveys in the works including TESS (Transiting Exoplanet Survey Satellite), PLATO (PLAnetary Transits and Oscillations of stars) and NGTS (Next-Generation Transit Survey). If a civilization wanted to shield itself from this kind of broadband optical survey, a monochromatic optical laser should do the trick. The paper estimates that the Earth could be ‘cloaked’ — hidden from view from a particular star system by having its transit nullified — with a 600 nm laser array emitting a peak power of ~30 MW over 13 hours.

The power requirements are interesting because they are relatively low for a specific target, but the paper adds the obvious point that if we are trying to cloak a planet from a large number of targets, we would require larger power production. Nonetheless, we routinely use much larger numbers when talking about laser lightsails in the configurations that could enable interstellar flight. Kipping and Teachey point out that for a culture that develops those kinds of technologies, cloaking could become a secondary function of the laser arrays used primarily for propulsion.

Chromatic cloaking (across all wavelengths) could be achieved by using a large number of beams (although with an order of magnitude higher energy cost), while tunable (‘supercontinuum’) lasers may emerge that can simulate any spectrum. But even with these capabilities, is cloaking an entire planet the most efficient choice for a civilization trying to hide itself? Perhaps a better course from the standpoint of economics and efficiency is to cloak the biosignatures that announce life’s presence. Let me quote from the paper on this:

It is straightforward to use a chromatic laser array to cancel out the absorption features in the planet’s transmission spectrum, assuming laser emission can be produced at any desired wavelength. Indeed, the presence of an atmosphere could be cloaked altogether if the effective height changes of the planet as a function of wavelength are canceled out by lasers. The planet might then resemble a dead world totally devoid of any atmosphere and appear almost certainly hostile to life. Not only would this approach require a significantly smaller power output, it would also have the benefit of producing self-consistent observations insomuch as the presence of the planet might still be inferred by other means (i.e. through radial velocity analysis).

What SETI Can Learn

Kipping and Teachey refer to these methods as a ‘biocloak,’ and suggest that cloaking can be selective indeed, perhaps focusing on the absorption features of molecular hydrogen and ozone. In this case we are dealing with peak laser power of just ∼160 kW per transit. But the authors are clear about the limitations of these methods. Radial velocity methods can find a planet otherwise hidden by a chromatic transit cloak, and given technologies not so far advanced over what we have today, direct imaging can reveal atmospheric features of a planet even when a ‘biocloak’ is in place. “For these reasons” write the authors, “perhaps the most effective use of laser enabled transit distortion would be for broadcasting rather than cloaking.”

And it was on that note that I began yesterday’s look at these possibilities. If we have based fifty years-plus of SETI on the notion that another civilization may choose to contact us, we have to acknowledge what Kipping and Teachey make clear: There are ways to alter transit signatures that make it obvious we are dealing with an advanced technology. And you can make the argument, as the authors do, that transits offer a different kind of ‘water hole’ for SETI, comparable in its own way to the ‘water hole’ frequencies we monitor in radio SETI.

Thus while the cloaking aspects of this paper have received the most attention, I think the SETI implications are its strongest takeaway. It is a very short step from existing optical SETI to archival searches of transit signatures already in our files. Knowing what these signatures would look like is a step forward as we continue to probe for civilizations around nearby stars.

Addendum: This email from Dr. Kipping, excerpted below, further explains the authors’ thinking about cloaking possibilities:

…we never intended to solve cloaking from all detection methods in one paper (that would be a tall order to demand of any research paper). Rather, we started with the simplest and most successful technique, transits, and showed that it is energetically and technologically quite feasible for even our current level of technology to build an effective cloak. Whilst we acknowledge that there are ways to defeat the proposed cloak (e.g. polarization of laser beams, direct imaging), we see these as problems which are likely to be solved by more advanced civilizations than ourselves, or indeed in future work (by humans!). What we are trying to do on the cloaking side is stimulate a conversation- that it is surprisingly easy to hide planets. Given that many notable scientists are opposed to METI, it is not unreasonable that other civilizations may choose to do this. The scenario could be that they would have long ago observed the Earth as an inhabited planet, and then turned on a cloak as a insurance policy, buying them time to reveal their presence when they choose to, rather than our increasingly penetrating telescopes finding them before they wish.

The paper is Kipping and Teachey, “A Cloaking Device for Transiting Planets,” accepted at Monthly Notices of the Royal Astronomical Society (preprint), The Korpela and Sallmen paper is “Modeling Indications of Technology in Planetary Transit Light Curves – Dark Side Illumination,” Astrophysical Journal Vol. 809, No. 2 (abstract).



A Transit Signature for SETI?

by Paul Gilster on April 4, 2016

David Kipping and Alex Teachey have a new paper out on the possibility of ‘cloaking’ a planetary signature. The researchers, both at Columbia University, make the case that any civilization anxious to conceal its existence — for whatever reason — would surely become aware that all stars lying along its ecliptic plane would see transits of the home world, just as its own scientists pursued transit studies of planets around other stars. And it turns out there are ways to make sure this signal is masked by adjusting the shape of the planet’s transit light curves.

Now this is a fascinating scenario as presented by the head of the Hunt for Exomoons with Kepler, whose business it is to know about the slightest of variations in light curves because they may contain information about exomoons or rings. Thus Kipping is a natural to look into the artificial manipulation of light curves, a study with definite SETI implications. Because methods like these work in two directions — a civilization that does want to communicate could also alter its light curve in ways that would be unambiguously artificial. Today I want to focus on the latter idea, reserving cloaking methods for tomorrow’s post.

This veers into the METI debate, but that’s not my purpose. Messaging to Extraterrestrial Intelligence is highly controversial, triggering arguments in these pages for the last decade. But let’s hold METI at one remove. The paper, delightfully titled “A Cloaking Device for Transiting Planets,” allows us to imagine how the manipulation of transit signatures could change a distant planet’s visibility. We may well decide not to brighten the Earth’s visibility through intentional transmissions, while understanding that an extraterrestrial culture might choose differently. Knowing what is possible by way of cloaking or enhancing a planetary signature, then, gives us plenty of food for thought for SETI as we consider what might turn up in our data.

Modifying the Light Curve

Here’s the notion in a nutshell, as drawn from the paper.


Image: Figure 1. Illustration (not to scale) of the transit cloaking/broadcasting device. A laser beam (orange) is fired from the night side of inhabited planet (blue) towards a target star whilst the planet appears to transit the star, as seen from the receiver. In the case of the Earth, the planet could be cloaked by generating an inverse transit-like signal of peak power 60 MW. Credit: Kipping and Teachey.

Controlled laser emissions are the key, effectively distorting the shape of a transit light curve. Just how this could be done is something we’ll discuss in the next post. But to begin, let’s think about how visible we are to any advanced civilization studying us. Forget about our radio and television signature, which is infinitesimal, and focus instead on the things we are doing right now to study other stars. I often write about transmission spectroscopy, which is how we search for the constituent molecules in a planetary atmosphere. The transiting planet, as it moves in front of its host star (as seen by our instruments) is bathed in the star’s light, its atmosphere providing a particular ‘overlay’ to the star’s spectrum.

We’ll use the method to look for biosignatures as we refine it for smaller and smaller worlds, but we’ve already seen how successfully we can examine the atmospheres of ‘hot Jupiters’ like HD 189733b. A sufficiently capable civilization able to see our world transiting should be able to pick up the cluster of biosignatures that would identify the Earth as a living world. There have been proposals to look for atmospheric pollutants produced by industrial activity as a part of future SETI practice, and such activity could indeed become visible, though the idea of widespread pollution lasting for millennia seems a stretch. Understanding the damage it caused, surely the culture in question would either solve its pollution problems or else succumb to them.

Let’s not forget the recent flurry of interest in the unusual star KIC 8462852. Here we’re looking at what could well be a natural phenomenon in the form of clouds of comets, but could possibly be evidence of artificial megastructures throwing highly distinctive light curves. We have much work ahead on KIC 8462852, so I only bring it up to suggest that there are many ways an intelligent species might make itself visible whether its intent was to do so or not.

Turning Transits into ‘Broadcasts’

Scientists as diverse as Ronald Bracewell, Richard Carrigan, Michael Papagiannis and Robert Freitas have studied the possibilities of such detections, with Carrigan most prominently identified with the search for Dyson spheres, swarms of energy collectors that surround a star to feed its colossal energies to a growing Type II civilization. And back in 2005, Luc Arnold (now at Aix Marseille Université) suggested that a civilization wanting to be known could resort to deliberate signaling by building a particular kind of geometric megastructure, one that when viewed in a transit would all but shout, through its distinctive light curve, that it was artificial.

Kipping and Teachey are, as you would imagine, well aware of Arnold’s work, and argue that lasers offer far more practical methods. From the paper:

Whilst any number of artificial transit profiles can be created with lasers, one ideally seeks a profile which is both energy efficient and unambiguously artificial. Producing upward spikes in-transit might seem like an obvious suggestion, but star spot crossings produce these forms with complex and information rich signatures (e.g. see Beky et al. 2014). Here, we argue that cloaking the ingress/egress of a transit, but leaving the main transit undistorted, would be a highly effective strategy since no known natural phenomenon is likely to produce such an effect.


Image: Figure 6 from the paper, highlighting broadcasting rather than cloaking. Top: The power profile of a laser array designed to broadcast the Earth. An array of lasers producing a peak power of ∼ 20 MW for approximately 15 minutes nullifies the transit ingress and egress. Bottom: The resulting light curves as viewed by different broadband optical photometers. The observed impact parameter would be complex infinity, for which a normal light curve fit would be unable to explain and thus indicating the presence of artificial transit manipulation.

A monochromatic laser emission could be spotted in a transit survey (and could be searched for in Kepler data), with follow-up spectroscopy confirming the artificial nature of the transmission. Kipping and Teachey also note what James and Dominic Benford recently pointed out in their paper on SETI efforts at KIC 8462852 — a beamed signal of whatever kind could carry information, so that in addition to identifying the presence of a technological culture, the beam could attempt more detailed communication (see SETI: Power Beaming in Context).

I’ve focused in on broadcasting via transit light curve alteration because, as the paper argues, it is the most efficient use of these techniques as compared to cloaking, which I’ll describe tomorrow. I’m trying to stay out of the METI weeds here — this is not an argument in favor of identifying the Earth to ETI. Rather, it is an argument that other civilizations may use such methods, and thus this paper shows us a signature we should add to our catalog.

And it has this further implication: If a civilization did choose to broadcast its existence through these methods, it would probably choose the shortest period planet in its solar system to carry the message. The choice is obvious, as the paper notes, for this produces “a higher duty cycle of distorted events,” making the detection all the more obvious. “We therefore suggest that any survey in archival data should not be limited to rocky planets in the habitable-zone of their host star.” An excellent reminder not to succumb to easy assumptions!

Tomorrow I’ll return to the Kipping and Teachey paper with a look at cloaking possibilities. The paper is “A Cloaking Device for Transiting Planets,” accepted by Monthly Notices of the Royal Astronomical Society (preprint).



John Ford Fishback and the Leonora Christine

by Paul Gilster on April 1, 2016

Like the Marie Celeste, the Leonora Christine is a storied vessel, at least among science fiction readers. In his 1967 story “To Outlive Eternity,” expanded into the novel Tau Zero in 1970, Poul Anderson described the starship Leonora Christine’s stunning journey as, unable to shut down its runaway engines, it moved ever closer to the speed of light. Just how a real Leonora Christine might cope with the stresses of a ramjet’s flight into the interstellar deep is the subject of Al Jackson’s latest, which draws on memories not only of Robert Bussard, who invented the interstellar ramscoop concept, but a young scientist named John Ford Fishback.

by A. A. Jackson

Project Pluto – a program to develop nuclear-powered ramjet engines – must have been on Robert Bussard’s mind one morning at breakfast at Los Alamos. Bussard was a project scientist-engineer on the nuclear thermal rocket program Rover — Bussard and his coauthor DeLauer have the two definitive monographs on nuclear propulsion [1,2]. He said many times that the idea of the hydrogen scooping fusion ramjet came to him that morning. This was sometime in 1958 or 1959 and the SLAM (Supersonic Low Altitude Missile) would have been well known to him. SLAM was an nuclear ramjet, a fearsome thing, sometimes called the Flying Crowbar. Finding a solution to the mass ratio problem for interstellar flight was also something on Bussard’s mind. Thus was born the Interstellar Ramjet, published in 1960 [3].


Image: Al Jackson delivering a plenary talk at the recent Tennessee Valley Interstellar Workshop. Credit: Joey O’Loughlin.

Most here at Centauri Dreams know that the interstellar ramjet scoops hydrogen from the interstellar medium and uses this as both a fuel and energy source by way of fusion reactor. The sun does proton fusion using gravity as the agent of confinement and compressional heating. However, doing fusion in a ‘non-gravitational’ magnetic fusion reactor makes the process very difficult [3,4]. That is, the proton and Deuterium burning is quite severe to realize on a ‘small scale’. Dan Whitmire attacked this problem by proposing the use of a carbon catalyst using the CNO cycle [4]. The CNO cycle is about 9 orders of magnitude faster than proton-proton fusion. It would still require temperatures and number densities way beyond any technology known at this time.


Bussard noted a number of problems such as losses from bremsstrahlung and synchrotron radiation. He also noted scooping with a material scoop would create a problem with erosion, hinting that magnetic fields might be used, and noting that drag would have to be accounted for.

About 8 years after Bussard’s paper, an undergraduate at MIT, John Ford Fishback, took up the problems Bussard had mentioned. He wrote this up for his Bachelor’s thesis under the supervision of Philip Morrison. The thesis was published in Astronautica Acta [5] in 1969.

Image: Physicist Robert W. Bussard.

Fishback did three remarkable things in his only journal paper: finding an expression for the ‘scoop’ magnetic field, computing the stress on the magnetic scoop sources, and working out the equations of motion of the ramjet with radiation losses. These calculations were done using a special relativistic formulation.

Fishback’s most important finding is noticing that when capturing ionized hydrogen to funnel into the fusion reactor, there is a large momentum flow of the interstellar medium which must be balanced by the scooping and confining magnetic fields. Using very general arguments, Fishback showed that sources (magnetic coils and their support) of the magnetic field determine an upper limit on how fast a ramjet can travel. The convenient measure of starship speed is the Lorentz factor


where v is the starship velocity and c the speed of light. It comes from the physical properties of the field sources, in particular the shear stress.

At the time, Fishback modeled the upper limit using diamond, because of its shear stress properties, and found that one could only accelerate until the Lorentz factor reaches about 2000 [5,6]. Tony Martin expanded on Fishback’s study [6, 7] in 1971, correcting some numbers and elaborating on Fishback’s modeling. Since that time, Graphene has been discovered and it has a shear stress that allows a limiting Lorentz factor of about 6000. This in turn implies a range of over 6000 light years when under 1 g acceleration. It does not mean the final range is 6000 light years, but one must travel at a reduced acceleration and then constant speed, which means a longer ship proper time.

This is bad news for the Leonora Christine of Poul Anderson’s Tau Zero [8].The range can probably be pushed to 10,000 light years, but accelerating at 1 g for 50 years would bust the Lenora Christine’s coils! That is, unless some magic material is found to take the stress loading at a Lorentz factor 1019, there is no way to circumnavigate the universe. And with the new accelerating universe, the story of Tau Zero becomes still more complicated.


Image: The interstellar ramscoop as envisioned by artist Adrian Mann.

What became of John Ford Fishback? I went to a lecture in California at Stanford in 1979 by Phillip Morrison. After the lecture I asked Morrison what had happened to Fishback. Morrison sadly told me that Fishback had gone to the University of California at Berkeley to work on his doctorate, but had committed suicide.

1. Bussard, R. W.; DeLauer, R.D. (1958). Nuclear Rocket Propulsion. McGraw-Hill.

2. Bussard, R.W.; DeLauer, R. D. (1965). Fundamentals of Nuclear Flight. McGraw-Hill.

3. Bussard, R.W., “Galactic Matter and Interstellar Flight”, Acta Astronatica, VI, pp. 179-195, 1960.

4. Whitmire, Daniel P. , “Relativistic Spaceflight and the Catalytic Nuclear Ramjet”, Acta Astronautica, 2 (5-6): 497–509, 1975.

5. Fishback, J. F., “Relativistic interstellar spaceflight,” Astronautica Acta, 15 25–35, 1969.

6. Anthony R. Martin; “Structural limitations on interstellar spaceflight,” Astronautica Acta, 16, 353-357 , 1971.

7. Anthony R. Martin, “Magnetic intake limitations on interstellar ramjets,” Astronautica Acta, 18, 1-10 , 1973

8. Anderson, Poul (2006), Tau Zero, Gollancz. ISBN 1407239139.

Table 1. Cut-Offs and Range for Ramjet accelerating at 1g. Interstellar medium 1/cm-3 using the p-p fusion reaction.

Structural Material𝛔/𝛒
dyn cm-2/gcm-3
Stainless Steel.26136.27.5

Addendum: While he was working on this article and corresponding with me, Al shared the story with Greg Benford, who had further thoughts on John Ford Fishback, as below:


Good article. I can add a touch: I was interested in this, after Edward Teller pointed out to me Fishback’s 1969 paper in Astronaut. Acta. I discussed it with Teller and did some calculations (just exploratory, never published). The idea seemed extreme but enlivened my discussions of the paper with Poul Anderson, who lived in Orinda near my Walnut Creek home and whom I saw often.

Someone told me Fishback was at Berkeley and I called him, agreed to meet. I had a one-day-per-week agreement with the Livermore Lab, where I was just turning from being a postdoc for Teller into a staff physicist — I spent Wednesdays at the Lab in Berkeley. So I met him at an Indian restaurant–a rail-thin smoker, nervous, ascerbic. “I wanted to show that we could reach the stars, really do it, with the right engineering,” he said, approximately. His anxiety was clear, but not its cause.

I found him an odd duck but was shocked when a bit later I heard he had killed himself.

When I mentioned it to Poul, he found it contrasting that a man who wanted the stars would cut off his own personal hopes. We often discussed Tau Zero, Poul once remarking that he wished he had taken more time to polish and expand the novel, since it already looked as though it might be the most remembered of his works–and indeed, seems so. He said he had written it in a few months and needed the money–its 1967 serialization in Galaxy helped, but it was tough going as a full-time pro writer then. Plus he had a word limit on the hardcover.

Poul used his Nordic background in the novel, as he liked to do. From Wikipedia:

“Incidental to the main themes is the political situation on the Earth from which the protagonists set out: a future where the nations of the world entrusted Sweden with overseeing disarmament and found themselves living under the rule of the Swedish Empire. This sub-theme reflects the great interest which Anderson, an American of Danish origin, took in Scandinavian history and culture. In later parts of the book, characters compare their desperate situation to that of semi-mythical characters of Scandinavian legend, with the relevant poetry occasionally quoted.”



SETI Looks at Red Dwarfs

by Paul Gilster on March 31, 2016

When it comes to astrobiology, what we don’t know dwarfs what we do. After all, despite all conjecture, we have yet to find proof that life exists anywhere else in the universe. SETI offers its own imponderables, adding on to the question of life’s emergence. How often does intelligence arise, and if it does, how often does it produce civilizations capable of using technology? Even more to the point, how long do such civilizations last if they do appear?

We keep asking the questions out of the conviction that one day we’ll start retrieving data, perhaps in the form of a signal from another star. It’s because of the lifetime-of-a-civilization question that I’m interested in a SETI search focused on red dwarf stars. True, M-dwarfs have a lot going against them, as Centauri Dreams readers know. A habitable planet around an M-dwarf may be tidally locked, which could be a showstopper except that some scientists believe global weather patterns may make at least part of such planets habitable.

Flare activity is always an issue on younger M-dwarfs, though it’s possible to conceive of this as an evolutionary spur, and we can’t rule out life’s ability to adapt to extreme circumstances. But despite all these unanswered issues, my interest in these stars draws primarily from two main points. First, they are the most common stars in the galaxy, comprising perhaps as much as 80 percent of the total. That gives us a huge number of candidates for life and potential civilization.

And while we can’t say how long civilizations live, not being sure if we ourselves will survive, we can take heart from the idea that if enough of them come into being, at least a few may get past whatever culture-shredding ‘filter’ they encounter to move into a serene maturity. Here red dwarfs truly stand out, because they live so much longer than any other stars. Every red dwarf that has formed in the universe is still there, and we can expect such stars to live for trillions — not billions — of years.

I like the odds, but I’m also trying to imagine what a civilization would look like a billion years after the emergence of tool-making. Or five billion. If a culture can survive for aeons, it will have mastered issues of conflict that plague us daily and much else besides. Surely a mature species long past emotional and technological infancy would want to know about its neighbors. Would such a culture reach out to others, if only to exchange notes? Or would it have moved into realms of philosophy and thought that make all this irrelevant?

We’re deep in imponderables here, but all we can do is look and listen. Thus I was pleased to see that the SETI Institute is initiating a search using the Allen Telescope Array that targets red dwarf stars. As the Institute’s news release explains, we now believe that somewhere from one-sixth to one-half of red dwarfs have planets in their habitable zones, which is a percentage that may be comparable to stars like the Sun, and for all we know at this point, may exceed it.


“Significantly, three-fourths of all stars are red dwarfs,” notes SETI Institute astronomer Seth Shostak. “That means that if you observe a finite set of them – say the nearest twenty thousand – then on average they will be at only half the distance of the nearest twenty thousand Sun-like stars.”

That, of course, means that we have a larger population of stars whose potential signal to us would be stronger. The SETI Institute is drawing on a target list of 20,000 M-dwarfs compiled by Boston University astronomer Andrew West, one that will incorporate new data as it is collected by missions like TESS, the Transiting Exoplanet Survey Satellite, slated for launch next year. Using the ATA’s 42 antennae, the red dwarf survey will take two years to complete, working in several frequency bands between 1 and 10 GHz. Says Institute scientist Gerry Harp:

“Roughly half of those bands will be at so-called ‘magic frequencies’ – places on the radio dial that are directly related to basic mathematical constants. It’s reasonable to speculate that extraterrestrials trying to attract attention might generate signals at such special frequencies.”

My assumption is that as resources become available (never an easy matter), SETI will search broadly through the various stellar types — we can’t know what we’ll find until we look. But it’s heartening to find a SETI attempt specifically turning to a category of star that has generally received little attention. It may well be that a race that is deep into philosophical maturity will have moved beyond beaming signals to other stars. It may, for all we know, have moved beyond biology! But let’s keep up the search and learn as much as we can about the small red stars that pepper the cosmos and may, if in any way habitable, hold clues about life’s emergence.