by Paul Gilster | Jul 9, 2013 | Culture and Society |
Martin Rees’ ideas on how humans will adapt to starflight, discussed here yesterday, offer plenty of ground for speculation and good science fiction. After all, the path ahead forks in many directions, one of them being the continuing development of artificial intelligence to the point where ‘artilects’ rather than humans become the logical crew for star missions. If decades or even centuries are needed to cross to another system, then this gets around the problem of keeping people sane and cooperating across what might be generations of voyaging.
Another fork is biological, with humans being gradually engineered to make them more adaptable to environments they’ll find at their destination. Here we can imagine crews sent out in some kind of deep hibernation, their biology tweaked to allow a ready transition to the new planet. Or perhaps cyborg solutions suggest themselves, with humans augmented by digital technologies to deal with problems and interface directly with critical shipboard computer systems.
I always think of Freeman Dyson when speculating about these things because back in 1985, he lectured in Aberdeen on a one kilogram deep space probe that would be as much biological as mechanical, a genetically engineered symbiosis of animal, plant and electronics. Dyson saw the animal component as providing sensors and nerves and muscles for basic operations and navigation, while electronics dealt with communications and data return. The plant component offered a closed-cycle biochemistry fed by sunlight. Artificial intelligence would integrate all operations in a probe he described as ‘agile as a hummingbird, with a brain weighing no more than a gram.’
You can read about the concept in Dyson’s Infinite in All Directions (Harper & Row, 1988), and you might pair it with Anders Hansson’s paper “Towards Living Spacecraft,” which ran in the Journal of the British Interplanetary Society in 1996. In the same Starship Century volume as the Martin Rees essay is Dyson’s latest thinking on the matter, a lively piece called “Noah’s Ark Eggs and Viviparous Plants.” Like Rees, Dyson is captivated with the tools that molecular biology is giving us and sees them as a way to seed the universe with life, turning inhospitable venues into living worlds [video].
After all, we are now learning the language of the genome and have sequenced the genomes of several thousand species. As the speed of such sequencing increases and the costs decline, it will take no more than twenty years or so to sequence the genomes of all species now existing on our planet. It turns out that describing the entire biosphere does not take up all that much space. In fact, Dyson writes, the information content of the biosphere genome comes to something on the order of one petabyte, which is less than the amount of data held by Google. “The biosphere genome,” he writes, “could be embodied in about a microgram of DNA, or in a small room full of computer memory disks.”
The larger vision here is that we can’t talk about permanent human settlement away from Earth unless we learn how to grow complete ecosystems in remote places. Here’s the concept:
It is not enough to have hotels for humans. We must establish permanent ecological communities including microbes and plants and animals, all adapted to survive in the local environment. The populations of the various species must be balanced so as to take care of each others’ needs as well as ours. Permanent human settlement away from Earth only makes sense if it is part of a bigger enterprise, the permanent expansion of life as a whole. The best way to build human habitats is to prepare the ground by building robust local ecologies. After life has established itself with grass and trees, herbivores and carnivores, bacteria and viruses, humans can arrive and build homes in a friendly environment. There is no future for humans tramping around in clumsy spacesuits on lifeless landscapes of dust and ice.
But Dyson doesn’t stop at the kind of worlds we would consider suitable for humans. He’s talking about going well beyond this, designing biosphere populations that can survive in environments ranging from planetary moons to comets. He imagines future bioengineers designing biosphere genomes for such places (these are the Noah’s Ark Eggs of his title) and seeding them in venues like the outer Solar System, where warm-blooded plants that can collect energy from sunlight would begin the process of building an ecosystem. Many biospheres would fail but those that survived would evolve in unpredictable ways as they adapt to small, cold places without atmospheres. You can see that Dyson’s emphasis is on life at large, not just humans.
How would you engineer a warm-blooded plant? Dyson thinks even objects as distant as the Kuiper Belt could become homes for life if we can do something like this:
Two external structures make warm-blooded plants possible, a greenhouse and a mirror. The greenhouse is an insulating shell protecting the warm interior from the cold outside, with a semi-transparent window allowing sunlight or starlight to come in but preventing heat radiation from going out. The mirror is an optical reflector or system of reflectors in the cold region outside the greenhouse, concentrating sunlight or starlight from a wide area onto the window. Inside the greenhouse are the normal structures of a terrestrial plant, leaves using the energy of incoming light for photosynthesis, and roots reaching down into the icy ground to find nutrient minerals.
Comets have plentiful sources of carbon and oxygen along with nitrogen and other key elements. Sunlight at a distance of 100 AU is reduced by a factor of ten thousand, but even the human eye can concentrate incoming light onto a spot on the retina by a factor larger than a million. So Dyson is arguing that a mirror as precise as a human eye could keep a plant warm at distances even further out than the Kuiper Belt. These self-grown mirrors would, like sunflowers, track the Sun as it moves across the sky. Plants like these would also be viviparous, with the seeds developing into viable plants before being dispersed into the frigid waste around them.
Stop being provincial, Dyson is telling us: Instead of focusing on how to find or re-create exactly Earth-like conditions wherever you go, think about helping life itself to spread into environments where it does not now exist. Give life this chance and let evolution take over to lead where it will. Seeding the universe may lead to places utterly incapable of supporting people like us, but extend the tools of molecular biology far enough and the explorers of the far future may adapt themselves to the environments they have shaped. ‘A terrible beauty is born,’ Yeats once wrote about a different kind of transformation, but in that beauty may lie habitats for new beginnings.
by Paul Gilster | Jul 8, 2013 | Culture and Society |
How we’ll go to the stars is often a question we answer with propulsion options. But of course the issue is larger than that. Will we, for example, go as biological beings or in the form of artificial intelligence? For that matter, if we start thinking about post-human intelligence, as Martin Rees does in the recently published Starship Century, are we talking about reality or a simulation? As the Swedish philosopher Nick Bostrom has speculated, a supremely advanced culture could create computer simulations capable of modeling the entire universe.
Rees recapitulates the argument in his essay “To the Ends of the Universe”: A culture that could create simulations as complex as the universe we live in might create virtual universes in the billions as a ripe domain for study or pure entertainment, allowing a kind of ‘time travel’ in which the past is reconstructed and the simulation masters can explore their history. So there we have a play on the nature of reality itself and the notion that our perceptions are constricted — as limited, Rees says, “as the perspective of Earth available to a plankton whose ‘universe’ is a spoonful of water.’
It’s a mind-blowing thought and Rees uses it to point out that our concepts of physical reality may need adjustment. What engages him about post-human evolution, though, isn’t the simulation speculation as much as the idea that genetic modifications and a simultaneous advance in machine intelligence will allow the kind of ‘directed’ evolution we’ll need if we are to expand into colonies beyond Earth. Here’s the argument compacted into a paragraph:
Darwin himself realised that ‘No living species will preserve its unaltered likeness into a distant futurity.’ We now know that ‘futurity’ extends much farther, and alterations can occur far faster than Darwin envisioned. And we know that the cosmos, through which life could spread, offers a far more extensive and varied habitat than he ever imagined. So humans are surely not the terminal branch of an evolutionary tree, but a species that emerged early in the overall roll-call of species, with special promise for diverse evolution — and perhaps of cosmic significance for jump-starting the transition to silicon-based (and potentially immortal) entities that can more readily transcend human limitations.
Humanity’s Special Moment?
Rees thinks we are at a special moment in time, a century that is the first occasion when the fate of the entire planet is in the hands of a single species. It’s also the century when we have the capability of starting the expansion of human life into the rest of the Solar System and beyond. This thought will recall Rees’ 2003 book Our Final Century (published in the US as Our Final Hour), whose subtitle says it all: ‘A Scientist’s Warning: How Terror, Error, and Environmental Disaster Threaten Humankind’s Future In This Century – On Earth and Beyond.’ There follows a study of the risk factors that hang over our culture’s head.
Which do you think would be the most likely cause of our demise? The possibilities are legion, ranging from environmental collapse to biological terrorism or the inadvertent results of using new technology, perhaps in the release of nanotechnology that goes out of control. The argument in the essay is similar to the book, that we have the ability to surmount these problems if we are wise enough to expand the frontiers of science and of exploration. The outcome is by no means certain but I think the picture may look darker than it really is, for while we can see the problems Rees outlines emerging, we cannot know what new knowledge we will gain as we confront them that will make solutions possible. Put me, then, on the more optimistic side of Rees’ speculations, a space the Astronomer Royal dwells on in this essay.
Let’s assume, then, that the outcome is indeed human survival and movement into space. That calls for near-term solutions to re-energizing the global space effort. How do we avoid the blunders of the past, the failure to follow up the Apollo landings with a credible and sustained program to continue manned exploration? The answer may well lie in private funding:
Unless motivated by pure prestige and bankrolled by superpowers, manned missions beyond the moon will need perforce to be cut-price ventures, accepting high risks — perhaps even ‘one-way tickets.’ These missions will be privately funded; no Western governmental agency would expose civilians to such hazards.
Here, of course, we think of Inspiration Mars, the plan to send a crew of two on a Mars flyby with launch in 2018, or the Dutch Mars One program, in which settlers would go to Mars knowing they would not be returning. Rees continues:
There would, despite the risks, be many volunteers — driven by the same motives as early explorers, mountaineers, and the like. Private companies already offer orbital flights. Maybe within a decade adventurers will be able to sign up for a week-long trip round the far side of the Moon — voyaging farther from Earth than anyone has been before (but avoiding the greater challenge of a Moon landing and blast-off). And by mid-century the most intrepid (and wealthy) will be going farther.
On Risk as Necessity
That scenario winds up with human outposts scattered through the Solar System, beginning with Mars or, perhaps, the asteroids. The post-human era may begin as we bring genetic engineering to bear on making ourselves more adaptable to such radically alien environments. A parallel development in artificial intelligence will make it an open question whether our forays beyond the Solar System are conducted by human or machine or a mixture of both. Given the time-scales involved in interstellar journeys, any human crews would be heavily modified, but it’s more likely that they will be non-biological entities altogether, adapted for long and solitary exploration.
Is interstellar travel possible? The answer may well be yes, but for whom? Rees’ point is that the options will grow as we gain experience with the new environments we explore. SETI may help us find extraterrestrial intelligence, but maybe not. Either way, evolution into the post-human era will be driven by the imperative to protect — and extend — the species. Whether or not such extension leads in the course of millennia to a Kardashev Type III civilization, actively shaping the fate of its entire galaxy, will depend at least in part upon our ability to transcend the kind of existential risks Rees discusses in Our Final Century.
Where I disagree with Rees is in the implication that, the risks of our century being solved, we will have transcended them. Risk survival is a continual process activated by the very breakthroughs in technology and exploration the astrophysicist here explores. We can only imagine what kind of dangers a rapidly growing starfaring culture would expose itself to purely in terms of its own engineering, not to mention its possible encounters with other civilizations.
Our encounter with risk has always been a precondition for our encounter with life, making the 21st Century a bit less unique than Rees would have it, but no less significant for the opportunity it offers to re-evaluate and once again engage the inevitable risks of growing starward.
by Paul Gilster | Jul 5, 2013 | Exoplanetary Science |
How close can a planet be to its star and still be habitable? If by ‘habitability’ we mean liquid water on the surface, with whatever consequences that may bring on a particular world, then it’s clear that the answer is partially dependent on clouds. We’ve developed one-dimensional models that can study the effect of clouds in various exoplanet environments, but they’re unable to predict cloud coverage, location or altitude. A new paper now describes a three-dimensional model that can make such calculations about atmospheric circulation, with interesting results.
Focusing on planets around M-class dwarf stars, Jun Yang and Dorian Abbot (both of the University of Chicago) and Nicholas Cowan (Northwestern University) are quick to note that red dwarfs like these constitute perhaps 75 percent of all main sequence stars. Current data (based on the work of Courtney Dressing and David Charbonneau) suggest that there is an abundance of Earth-size planets in the habitable zone — one per star — around M-dwarfs. The close orbits and deep transit depth of planets in the habitable zone here make them relatively easy to detect.
Yang, Abbot and Cowan put their model to work looking at the effect of water clouds on the inner edge of the habitable zone, with results showing that clouds can have a significant effect on cooling the planet, allowing liquid water to exist much closer to the star than was previously believed. Assuming M-dwarf planets close enough to be in the habitable zone will be tidally locked — with one side always facing the star — the team shows that the side of the planet exposed to the star would develop highly reflective clouds at the ‘sub-stellar’ region directly below the sun’s position in the sky.
Image: This illustration shows simulated cloud coverage (white) on a tidally locked planet (blue) that would be orbiting a red dwarf star. Credit: Jun Yang.
‘High noon’ on an M-dwarf world, in other words, should produce clouds that have a significant cooling effect. Just how significant an effect is revealed in the paper’s conclusions:
We have performed the ?rst 3D global calculations of the e?ect of water clouds on the inner edge of the HZ and predict that tidally locked Earth-like planets have clement surface conditions at twice the stellar ?ux calculated by 1D models. This brings already detected planets, such as HD 85512 b and GJ 163 c, into the HZ, and dramatically increases estimates of the frequency of habitable planets. Adopting the planetary demographics from Figure 19 of Dressing & Charbonneau (2013), our revised inner edge of the HZ increases the frequency of habitable Earth-size planets by at least 50–100%. Crucially, we have also shown how this stabilizing cloud feedback can be tested in the near future with thermal phase curves from JWST.
The behavior of clouds on planets that are not tidally locked is going to be markedly different. The researchers believe that such worlds will have an albedo (reflectivity) similar to Earth’s because only part of the tropics and the mid-latitudes will be covered with clouds and the water content of the clouds will be small. But tidal lock produces clouds with high water content covering between 60 and 80 percent of the dayside, according to these models. The thickest clouds occur where light from the star is the most intense, thereby significantly increasing the albedo of the planet.
Such clouds, under conditions described in the paper, account for 73 K of cooling, thus extending the habitable zone despite the planet’s proximity to the star. What I like about this work is the possibility of testing it in the near-term using the James Webb Space Telescope, with which we can measure the temperature of M-dwarf planets at different points in their orbit. A planet without cloud cover will show its highest temperatures when the dayside is facing the telescope (with the planet on the far side of the star). Temperatures would be lowest when the planet comes around in its orbit to show its dark side to the telescope.
But a planet with highly reflective clouds on the dayside will be different, says Jun Yang:
“…you would measure the coldest temperatures when the planet is on the opposite side, and you would measure the warmest temperatures when you are looking at the night side, because there you are actually looking at the surface rather than these high clouds.”
This University of Chicago news release points out that we can see the same effect on Earth when looking at places like Brazil or Indonesia from space. An infrared instrument will find unusually cold conditions when it observes the cloud deck, which is at high altitude. Temperature variations of this kind from an exoplanet around an M-dwarf would indicate clouds and confirm the presence of liquid water on the surface. Thus three-dimensional simulations of the way air and moisture move through the atmosphere extend the potential habitable zone in the star’s direction.
The paper is Yang, Cowan and Abbot, “Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets,” Astrophysical Journal Letters, Vol. 771, No. 2, July 10, 2013 (abstract / preprint).
by Paul Gilster | Jul 3, 2013 | Sail Concepts |
The British geneticist and biologist J.B.S. Haldane has left us with one of the more memorable lines about scientific inquiry, one that draws on the richest of all of Shakespeare’s plays for its punch. Hamlet tells Horatio that there are more things in heaven and Earth than are dreamt of in his philosophy (Act 1, Sc. 5), a thought Haldane adapts in the service of intellectual surprise. In his collection Possible Worlds and Other Essays (1927), he writes:
I have no doubt that in reality the future will be vastly more surprising than anything I can imagine. Now my own suspicion is that the universe is not only queerer than we suppose, but queerer than we can suppose … I suspect that there are more things in heaven and earth than are dreamed of, or can be dreamed of, in any philosophy.
We can imagine Olaf Stapledon nodding as he read those lines. Haldane sketched out a human history covering the coming 40 million years in his essay “The Last Judgement,” one that Stapledon drew on in creating his own Last and First Men (1930). We can also imagine the reader surprise that probably met Haldane’s use of solar sails as a way of getting around the Solar System, all part of the same essay, in which Venusian colonists (after the demise of the Earth) contemplate crossing to a passing star on beams of the Sun’s light.
Image: John Burdon Sanderson Haldane (1892-1964), who gives us one of our earlier references to solar sails as a way of reaching the stars.
In Defense of the Sail
These days we have seen solar sailing in action through the Japanese IKAROS mission and the smaller NanoSail-D project from NASA, even as larger sails have been constructed for ground tests and we look forward to a late 2014 launch of NASA’s Sunjammer, which will deploy a sail 38 meters on a side (the sail’s name comes from an Arthur C. Clarke story that envisioned a race to the Moon using solar sails, one that ends with the abandoned ship being targeted for interstellar space). Solar sails, in other words, are not — like fusion, like antimatter — propulsion systems of the future. They’ve reached flight status for deployment and testing in space.
These sails work because light, although it has no mass, can impart momentum. If we can make the necessary upgrades in materials to enable a close pass by the Sun (perhaps with the aid of an occulter to shield the craft at perihelion), we can imagine driving a sail deep into the Solar System at speeds of several hundred kilometers per second. But as Jim Benford shows in his essay “Sailships” in Starship Century, we need to go much faster for an interstellar mission, and that involves beaming energy that will produce force at great distances.
From the essay:
I say this because sailships have a singular advantage: they leave the engine behind. So we can build a spacecraft that consists of only payload and structure — no fuel at all. The propellant is light itself, so sails reflect light waves, whether visible or microwave or laser produced, from a beam generated elsewhere. Sails can be made both light and smart, in the sense that control systems, sensors and computational ability can be embedded in the structure of the sail itself — a smart sail, with dispersed circuitry, and therefore far harder to damage by meteors or accident.
All this makes a solid case for sail technologies, and so does the fact that in the year 2000, Benford and his brother Gregory produced laboratory data showing that carbon sails could be driven by microwave beams to produce accelerations of several g’s. When I wrote about this work in Centauri Dreams (the book), I was struck by the fact that this laboratory work demonstrated beam-riding, in which the pressure of the beam and the concave shape of the sail work together to produce a sideways restoring force. The sail can also be stabilized against yaw and drift because the beam being directed at it can carry angular momentum that can be imparted to the spacecraft, thus spinning it.
Near-Term Work on Beamed Power
Benford looks at issues of stability, deployment and large-scale space construction for sails and the beam sources that drive them, all of which demand a stable infrastructure to build upon. But the fact that we already have sails capable of space deployment points toward building experience with sail engineering and in particular materials as we learn to optimize our work with carbon nanotubes and carbon micro-truss structures. It’s an intriguing thought that, sails being large structures we will learn to work with in space, their growth should also help in the development of large transmitting antennae of the kind we will eventually need to build.
A stair-step series of beamed power applications builds the groundwork for the sail infrastructure, one that grows out of engaging commercial interests:
In today’s frugal climate, it is important for technology development to be coupled to commercial applications. Several of the missions we’ve described are potentially commercial matters. Starting with orbital debris mapping, one can see an incremental commercial development leading first to satellite power recharging. Eventually, as the space market and business confidence grows and capital becomes more available, this development plan leads to the repowering of satellites in GEO and ultimately to launch services. Investment costs are minimized because the research program leads to applications, which feed capital back into research, leading to new applications.
Ultimately, we’d like to build a true beamer — the source of the laser or microwave beam we’re sending to the sail — that would be assembled in space from materials mined from the moon or from asteroids. Placing the beamer in a close solar orbit would maximize the power available. The beauty of the beamed energy concept is obvious: Early fusion designs like Daedalus could carry a payload that was less than one percent of its initial mass, while sailship payload masses can be considerably higher. After all, we’re leaving the propellant behind us in the Solar System. Moreover, a single beamer, once established, could be used for many sailship missions.
Imagine, then, future sails many kilometers in diameter that are deployed by spinning up the initial, folded package and, in close proximity to the beamer, are pushed out by laser or microwave beam. The acceleration quickly increases as the beam stays fixed on the sail for hours, then days. As the sailship reaches the outer Solar System, the beam switches off and the spacecraft is launched on its interstellar journey, perhaps stowing the sail for cruise. The fact that electromagnetic waves can transfer power over long range makes this scenario possible.
Image: An interstellar sail pushed by laser or microwave. Credit: Michael Carroll.
Fusion, Benford notes, is still struggling with physics and engineering issues, so that cost estimates for continued research and development are wide open. If the idea is to solve the physics, then tackle the engineering questions and finally look at the economic feasibility, then sails have the edge. We know the basic physics and have an engineering requirement that demands large antenna and optical arrays, along with assembly of our photon sources. Usefully, we have considerable experience in the sub-systems that are a foundation of this work.
We often think of beamed sail concepts in terms of gigantic structures like the Fresnel lens that Robert Forward wrote about in the outer Solar System, thousands of kilometers in diameter and massing half a million tons, or his 75,000 ton staged laser sails destined for Epsilon Eridani. But researchers like Geoffrey Landis have gone to work on Forward’s concepts using high-temperature materials like boron and carbon that would allow better acceleration, and proposing a string of lenses that would drastically reduce the size of the Fresnel lens.
The theoretical work continues as we press on with our early sail deployments. J.B.S. Haldane had no doubt that the future would surprise him, and doubtless the fiction of Olaf Stapledon took his thoughts in directions he could never have anticipated. We will learn in the coming century whether the audacious idea of a sail being pushed between the stars is another Haldane whim that nudges our philosophy, pointing to a workable approach to crossing the interstellar gulf.
by Paul Gilster | Jul 2, 2013 | Culture and Society |
Tracking down starflight in literature is an absorbing pastime. When I was writing my Centauri Dreams book, I found that I was vaguely familiar with many of the antecedents of today’s science fictional journeys, but a book called Wunderwelten, by Friedrich Wilhelm Mader, took me by surprise. A 1911 adventure novel for young readers, Wunderwelten imagines a sphere that, in the fashion of the time’s space fiction, was moved by antigravity in a multi-year journey to Alpha Centauri. Mader’s ship, called ‘Sannah,’ was a precursor to all the Centauri-bound starships to come.
What a delight to find Sannah emerge in the form of Sannah III in Stephen Baxter’s story “Star Call,” which appears in the recently published Starship Century. But Baxter’s updated ship is a far cry from the 50-meter antigravity vessel imagined by Mader. For one thing, it’s gifted with artificial intelligence:
I am called Sannah III because I am the third of four copies who were created in the NuMind Laboratory at the NASA Ames research base. I was the one who was most keen to volunteer for this duty. One of my sisters will be kept at NASA Ames as backup and mirror, which means that if anything goes wrong with me the sentience engineers will study her to help me. The other sisters will be assigned to different tasks. I want you to know that I understand that I will not come home from this mission. I chose this path freely. I believe it is a worthy cause.
Image: Science fiction novelist Stephen Baxter, who has revived Mader’s antique starship.
Sannah’s cause, and the thinking around it, are regularly reported back to Earth as it travels, all by means of the Star Call system, which allows people to buy a share in the mission and in return get once-a-decade message exchanges with the starship. A poignancy in these communiques emerges that reminds me of Greg Bear’s sentient starship in Queen of Angels as we begin to realize the mission is not going well and Sannah may not have all the facts.
Emergence of Pellet Propulsion
Interestingly, the stardrive on Sannah is not antimatter but what Baxter dubs a ‘Singer-Nordley-Crowl’ drive after Clifford Singer, who studied pellet propulsion technologies back in the late 1970s. The Nordley reference is to Gerald Nordley, whose own pellet propulsion methods revised and significantly upgraded Singer to allow for ‘smart pellets’ with course correction. Crowl, of course, is our own Adam Crowl, who has been writing and commenting on this site almost since its inception, and whose own entry in Starship Century is a comprehensive look at how researchers have envisioned starships in our time.
Adam, serious congratulations, buddy. I mean, to have a stardrive named after you…
Nordley was interested in nanotechnology and proposed that the problems of getting small particles moving at relativistic speeds to their target (where they would push against its magnetic field to drive it forward) could be handled by artificial intelligence and minute rockets. ‘Smart pellets’ wouldn’t be easy to send on their way but nanotechnology worked there as well. Says Crowl:
To power either system would require immense solar power-collection systems, which Nordley proposed to be built via self-replicating machines. Optimistically assuming a single self-replicating power-satellite that supplies one gigawatt of power that copies itself in a year, then within mere decades sufficient power would be available to propel a 1,000-ton starship to 0.86c at five-Gs, and a decade later a thousand such starships could be propelled per year.
0.86c is an interesting figure. Nordley told me in an interview years back that he thought the first human crew to reach Alpha Centauri would get there after a journey lasting about three years. That’s three years as experienced by the crew. Moving at 0.86c, he added, those aboard the starship would experience a time compression factor of two — half as much time would expire for them as would expire for the people left behind on Earth. Add in acceleration and deceleration time and you get the result, a three year passage (as perceived by those onboard) to the nearest star. It’s about the same amount of time it took Magellan to circumnavigate the Earth.
Image: Interstellar researcher Gerald Nordley, speaking at the Space Access 2010 Conference in Phoenix, Arizona.
Interactions with the Medium
Crowl’s paper runs through all the starship concepts I’ve ever encountered, among the most fascinating of which are the lesser known. Back in the 1970s, for example, as NASA studied the possibility of pushing a probe up to interstellar speeds using lasers, Philip C. Norem and Robert Forward went to work on the question of how to slow down a probe for rendezvous. One of Forward’s sail deceleration concepts was ingenious enough to merit separate treatment, and I’ll talk about it tomorrow as we discuss Jim Benford’s ideas on laser and microwave sails. But there are other ways of doing these things, and Norem and Forward found that a starship could be turned by using large charged wires to interact with the galactic magnetic field.
The key to this is the fact that a charged object moving through a magnetic field experiences a Lorentz force at right angles to its direction of motion and the magnetic field itself. If you give it enough time to work, the Forward/Norem method can actually slow a probe down and turn it so that it approaches the target star (from our perspective on Earth) from behind. At that point a laser beam from Earth could be trained on the starship’s sail to slow it for the rendezvous. The same method could be used in reverse to enable a return journey. The main problem is that the large turning circles require centuries of additional travel time to pull off the feat.
Both Forward and Norem were fascinated by the concept of ‘thrustless turning,’ written up by Norem in a 1969 paper. I’ll mention another aspect of this that may be germane here, a 2005 paper by Gregory Matloff and Les Johnson that studies how to use the interstellar medium not for turning but for generating power aboard the spacecraft. This could be done through the interactions between an electrodynamic tether and the interstellar magnetic field.
We might throw Freeman Dyson into the mix as well. Dyson studied propellantless braking after observing the magnetic interactions between the large inflatable satellites of the early 1960s and the plasma around them. A starship using these methods to decelerate would release electromagnetic energy that might be observable, thus allowing a search for extraterrestrial space vehicles. Crowl discusses what Dyson called Alfven braking in relation to magnetic sail concepts that emerged in the late 1980s. And it’s to sails, though not magnetic ones, that I want to turn tomorrow as we ponder Crowl’s many propulsion alternatives.
The Philip Norem paper is “Interstellar Travel: A Round Trip Propulsion System with Relativistic Capabilities,” AAS 69-388 (June, 1969). Robert Forward’s paper on Lorentz force turning is “Zero-Thrust Velocity Vector Control for Interstellar Probes: Lorentz Force Navigation and Circling,” AIAA Journal 2 (1964), pp. 885-889. Gregory Matloff and Les Johnson write about electrodynamic tether possibilities in “Applications of the Electrodynamic Tether to Interstellar Travel,” JBIS 58 (June, 2005), pp. 398-402. Cliff Singer’s first pellet paper is “Interstellar Propulsion Using a Pellet Stream for Momentum Transfer,” JBIS 33 (1980), pp. 107-115.
