Fusion Rockets to Mars… Part 1

While there are perfectly workable concepts for sending humans to Mars by using chemical or nuclear thermal rockets (NTRs), neither option gives spectacular performance. Initial Mass in Low Earth Orbit (IMLEO) is the relevant performance metric and fortunately there’s several quasi-complete designs using both chemical propulsion and NERVA class nuclear thermal propulsion that we can refer to.

First design is Wernher von Braun’s 1969 design, using NERVA class NTRs, which sent a crew of 12 to Mars – and brought them back – in two separate vehicles, landing six on the surface in two 50 ton Mars Excursion Modules (MEMs), all for an IMLEO of 726 tons per vehicle, or 1452 tons total.

Encyclopedia Astronautica: Von Braun Mars Expedition – 1969

Von Braun 1969

The second design, using chemical rockets, was developed by NASA in 1971, with the same crew (6) and MEM (50 tons) as a single Von Braun NTR vehicle. Total IMLEO was 1900 tons.

Encyclopedia Astronautica: NASA Mars Expedition 1971

A more recent design, developed by Robert Parkinson, launched a crew of five towards Mars in three vehicles based on Shuttle-era technology – the European Space Agency’s Spacelab module (which actually flew) and the Orbital Transfer Vehicle (which was studied extensively, but never launched.) Two of the vehicles, Orbiter 1 & 2, massed just over ~210 tons each, while the unmanned Lander Assembly, carrying the MEM, massed 194 tons. Total IMLEO would be 615 tons.

Mars in 1995! (1980-1981)

[Artwork by David Hardy]

I am deliberately avoiding Robert Zubrin’s “Mars Direct” in this discussion, as it requires In Situ Resource Utilization (ISRU) to fuel up the Mars Return Vehicle, thus skewing the comparison. However incorporating ISRU is, in principle, possible for the design I wish to discuss next: the Fusion Driven Rocket.

Fusion Driven Rocket
Fusion Driven Rocket

Revised Habitable Zone… a Revisit.

Ravi Kumar Kopparapu and colleagues [Habitable Zones Around Main-Sequence Stars: New Estimates] have revised the 20-year old work of Kasting, Whitmire and Reynolds [Habitable Zones Around Main-Sequence Stars], with two seemingly minor changes with surprising effects.

Firstly, they’ve extended the range covered by the model to include stars below the 3700 K limit of the previous work, towards the much cooler 2600 K realm of very low mass Red Dwarfs. The very coldest hydrogen-fusing stars are still white-hot – Red Dwarfs drop down to just ~2300 K at the H-fusing limit of 0.08 solar masses, and 2600 K is hit by 0.09 solar mass stars. Secondly, they’ve improved the modeling of the greenhouse effect created by CO2, which has produced some startling changes in results. Earth ends up near the inner edge of the Habitable Zone – instead of 0.95 AU, the inner edge for a Solar-like star is ~0.99 AU and is even further out for redder stars. Ravi has provided a calculator of the effects on his web-site…

Hab Zone Calculator

…the default is the Sun, with an effective Temperature of 5780 K and 1 Solar Luminosity, and an Inner and Outer Edge of the Habitable Zone of 0.9928 and 1.6886 AU. Push the Calculator to 7200 K and the Edges become 0.9451 to 1.5285 AU. Drop the temperature to 2600 K and the range changes to 1.0883 to 2.1238 AU. So what’s going on? Why the changes? The peak radiation frequency of the spectrum is proportional to the effective temperature – hotter stars peak towards the ultraviolet, while cooler stars peak towards the infra-red. Planetary atmospheres are more effective scatterers of higher frequencies than lower frequencies – the cause of the blue daylight sky – and this means under a bluer spectrum, a planet is cooler, or warmer under a redder spectrum.

The Inner Edge is hit when the surface temperature runs away from our temperate ~288 K (15 C) and climbs towards 340 K (67 C), causing a wet stratosphere and eventual dessication of the planet via hydrogen loss. The Outer Edge is reached when no amount of extra carbon dioxide can further increase the surface temperature – adding more just reflects heat away. In fact at 35 bars the CO2 will condense into liquid at 273 K, but becomes opaque at lower pressures before then.

Interestingly if Venus was as far from the Sun as Mars most of its atmosphere (~90 bars CO2, 2 bars N2) would begin condensing rapidly. A turbulent atmosphere, full of convecting plumes, would carry heat away from the hot surface to space… rapidly. In a couple of decades, the place would probably find an equilibrium at something close to CO2’s critical point, about 304 K – just 31 C. An ocean of liquid carbon dioxide about 120 metres deep would form, though much of it would eventually percolate into the regolith creating a very exotic “ground-water”. Ultimately bright CO2 clouds might form and drive the temperatures lower, towards full condensation of the atmosphere…

Induced Torpor for a Trip to Mars?


New idea being funded for Phase I NIAC research, to be undertaken by aerospace firm, SpaceWorks, led by John Bradford. Phase I is for feeling out the next steps in developing the idea to a higher “Technology Readiness Level”, the standard for assessing practicality of concepts that NASA uses. The SpaceWorks NIAC abstract has the following description of the stasis-equipped Mars vehicle:

The habitat is envisioned as a very small, pressurized module that is docked around a central node/airlock permitting direct access to the Mars ascent/descent vehicle and Earth entry capsule by the crew. We believe the crew habitat mass can be reduced to only 5-7 mt (for a crew of 4-6), compared to 20-50 mt currently. The total habitat module volume would be on the order of 20 m3, compared to 200 m3 for most current designs.

The interesting thing about that mass-figure is that SpaceX’s Falcon Heavy should be able to throw a capsule of that mass, with sufficient aeroshell for braking and fuel for landing, to Mars. Such a One-Way ride, with 4-6 months of the interplanetary cruise in torpor, would be for colonists only or for cheap crew transport, if a separate return system is available. Call it the Martian “Sleeper-Car (Coach Class)”. If we got serious about colonizing Mars, then it might be a bargain basement approach used by smaller groups wanting to stake their own claim on the Red Planet. Presently the cost would be ~$200 million, factoring in the cost of developing the torpor system, but with a successful fully-reusable version of the Falcon Heavy’s lower stages (the Mars-Injection Stage would be slung into a “never-to-return” trajectory by just doing its job) the cost might drop significantly. Mass-production would allow thousands to launch to Mars, One-way, for perhaps ~$1 M/person.

Presently Mars isn’t very hospitable, but with sufficient motivation, we could build a thriving colony in one (or more) of the vast lava tubes that have been revealed via orbital cameras. If C/2013 A1 (Siding Spring) were to smash into a Martian Polar Cap, 19 October 2014, then the release of water vapour and carbon dioxide (presently enclathrated in the Cap or frozen in the soil) might induce a shift to a more clement Mars, as I have blogged previously. We might then find the motivation to fly the “Sleeper-Car” to Mars.

Starship Century: My Account

Starship Century – Right here, Right Now

The Starship is still about 100 years away, but we will begin building it this century. This was the message that Gregory Benford and his mirror-twin, James Benford, were proclaiming together, with the help of notables of both science and science fiction. And me. Just how I got involved is another story, suffice it to say, I know a lot about starships, at least about every variety that has ever been seriously proposed.

The choice of venue and the timing were serendipitous – the Arthur C Clarke Foundation and the University of California in San Diego (UCSD) had been working together on the Arthur C Clarke Center for Human Imagination, and the UCSD is the alma mater of a surprising number of modern day Science-Fiction writers. Over the month of May a variety of events were scheduled, notably a conversation between John Lethem and Kim Stanley Robinson on May 14, but the biggest would be the Starship Century. Not coincidentally there is also an associated book, though when I began working on my chapter contribution over a year ago I had no inkling of the event coming up. The details came together quickly, and are a credit to the organisers, especially Sheldon Brown and his team. The event proper was on May 21 & 22, but a UCSD function the night before set the scene, with the Chancellor and Sheldon Brown explaining what the Arthur C Clarke Center was and how it came about. UCSD is the alma mater for a surprising number of contemporary SF writers – not just the Benfords, but also David Brin, Kim Stanley Robinson, Neal Stephenson and many others – so the idea of Arthur C Clarke’s legacy finding a home there seems fitting.

I had arrived in San Diego that afternoon (May 20), after crossing the Pacific with a tail-wind for 12 hours, and had shared a Shuttle bus to the La Jolla Shores Hotel, where the invited speakers were staying, with John Cramer. I had met John briefly at the Orlando 100 Year Starship (100 YrSS), but had corresponded with him on-and-off for some time. He gave me a quick update on his retrocausality experiments (see his Alternate View column for details) and then we arrived. I grew up on Queensland’s Sunshine Coast so seeing the Pacific, but looking West, took a moment of reorientation. Once checked in I needed to stretch my legs so I walked up the nearest street to the local Cafes and shops, only to run into fellow contributor to the book, Ian Crawford – another alien in this strange land of California. We discussed exoplanets and the fate of ocean planets, whether they would dry out or remain drowned, over the aeons. Returning to the Hotel to get dressed I ran into Greg Benford – who briefly I confused with his brother Jim, as he was wearing a tie – and had an inkling I might be slightly over-dressed.

As with 100 YrSS, much of the discussion and interaction happens “off screen”. I spoke to so many people, several of whom told me amazing things, but I then promptly forgot what was on their name-badges. Familiar faces I quickly caught up with, especially Al Jackson, who has a multi-decadal career with NASA going back to the beginning and co-wrote classic papers on interstellar flight through-out the 1970s. Al astounded me by saying he only managed to see one live launch from Cape Canaveral – STS 135 – even though he had worked with many of the Apollo crews in the 1960s. A new face for me was SF writer Allen Steele, whose work I knew of, but hadn’t managed to yet read. A mutual friend, Winchell Chung, has written up much of the technical details of Allen’s novels on his “Atomic Rockets” website, and has also advised Allen on his more recent works. Other new faces, for me, were the poly-math Eric Hughes, who wrote for “Wired” in the hey-days of Cyber-Punk in the early 1990s; Mark Canter, who is a former editor of “Men’s Health” magazine and these days writes SF novels with a more anthropological basis, and John Chalmers, an astrobiologist who has worked with Stanley Miller on the chemical origins of life. The audience of Starship Century was of stellar quality, how much more so the speakers.

Day One, May 21, began with breakfast watching the breakers and discussing interstellar matters with James Benford. Instead of the UCSD Shuttle ride, I had a lift with Jim and Allen Steele as another passenger. Arriving, the puzzle was where to sit. With so many luminaries in attendance, one doesn’t just sit next to Freeman Dyson without introduction. Jim, Greg and Sheldon Brown [video] opened the day officially, and I sat back to listen to Peter Schwartz, [video] renowned Futurist and long term strategist for some very large companies, discuss the scenarios of the future that might get humanity to the stars.

Peter covered three basic scenarios, though many more can be generated. The full details can be found in the “Starship Century” anthology, but in essence three ideologies could launch us to the stars. Firstly, “God’s Galaxy”, which implies a future Earth dominated by religion, sending forth missionaries to the unconverted of the Galaxy. Secondly, “The Dying Earth”, in which we’re seeking a second home, basically the back-story of “Firefly” and countless other SF treatments. Thirdly, “Interstellar Trillionaires”, in which the ultra-rich of a fully developed interplanetary economy launch forth for adventure or curiosity’s sake. Of course, what applies to us, might also apply to other civilizations, with the logical implications for Fermi’s Paradox. Peter’s response to that, was to suggest that “They” might be too sparsely spread in space-and-time for it to yet be an issue.

Next up was Freeman Dyson [video], who has a deserved reputation as a big thinker, as well as at least one near-miss with a Nobel Prize. In interstellar matters, his seminal popular piece “Interstellar Transport” (1968) described one of the few interstellar propulsion systems we could almost build now – nuclear fusion pulse propulsion. What I hadn’t realised was that the testing ground for “Project Orion”, the USAF/NASA nuclear pulse rocket, was in San Diego – of course, the test models only used high-explosives, but the video available of those tests is quite inspiring. Since the heady days of the 1960s, Freeman has argued more for biotechnology playing a role in our interstellar plans. His lecture covered several ideas he has produced over the last 50 years, namely plant-derived habitats that we might grow on the cold bodies of the outer solar system, and the most efficient means of getting between the stars – send information. Eventually, he suggests, we might launch “biosphere seeds” to other star systems and grow new habitats for Earth-derived life as well as ourselves. Naturally this had ethical reactions from the audience as well as the rattling of chains by the Ghost of Fermi – if we can do it Out There, why hasn’t Someone done it here? After the lecture, during a break, I suggested to Ian Crawford that we might not know our biosphere’s genome well enough to tell if such a scenario hasn’t happened here.

If Freeman Dyson created controversy, the next speaker, Robert Zubrin [video], practically invited it by daring to suggest that Greenhouse warming might be preferable to billions of people living in poverty. Zubrin’s talk covered the economic Big Picture of what was needed to create an interstellar capable civilization, but also provided a chance for Robert to vent spleen about more radical environmental ideologues who are promoting what was once called “Nazism”. Naturally he has a book which covers that particular argument, so I will refer the reader to that for more details. On interstellar matters, he made a powerful case that 100 to 200 years of continued development would see humanity ready to set forth to the stars in the first generation of fusion-propelled starships. My one quibble was the “Energy at Retail Prices” fallacy being used to estimate the economic scale of interstellar flight – a 1,000 tonne spaceship moving at 0.1c and using energy at 10% efficiency would cost $125 trillion in energy bought at the retail rate of $0.1/kW.hr. The problem is that one doesn’t buy energy for a starship and just charge up the batteries. Instead a starship is more like an energy generator – using either solar energy or fusion fuels – and this requires a wholly different economic measurement. The estimates can vary significantly as a result.

Neal Stephenson’s talk [video] was something else again. Not what I expected from an SF writer at all. Instead of Big Picture discussions, he described a vast 20 kilometre Tower that he, and his Arizona State University team, have designed. His talk was thus a detailed look at an advanced theoretical engineering design study in progress. The challenge of such an immense structure, possibly hundreds of millions of tonnes of steel, working in such a changeable environment as the Earth’s atmosphere is fascinating, as is the associated novella in the anthology. But how does it relate to interstellar flight? Naturally the first thing I thought of was as an anchor for a space elevator. Greg Benford suggests another use, in the anthology, but in the current design there is a big empty volume – for future use. A space to fill, for the next generation’s imagination. A reminder, like the Pyramids became, that what one achieves in the present, will look different to the people who come after you.

Lunch at the “Starship Century” Symposium was provided by UCSD, allowing attendees to remain nearby, adding to the discussion and trading of ideas and concerns. Certainly I appreciated the chance to catch up with friends and faces from the other side of the Pacific, as well as meeting new people. Having read people’s novels, books or scientific papers for years, then meeting them on Facebook or email, I felt like I knew some of them already. Meeting authors that I had grown up with like Larry Niven, Joe Haldeman or David Brin was something I was getting used to, as I was more eager to discuss their interstellar ideas than succumb to fan-shock. I finally had my ideas about Larry Niven’s fusion-shield, from his “Known Space” stories, confirmed by the source, but didn’t quite get to talk to David Brin about the Fermi Paradox during the whole event.

The afternoon of the first day was thematically about “New Space” – what we’re doing, as a species, in the near term of a commercial nature. Of course, this was largely from the North American perspective. Patti Grace Smith [video], one of the senior Regulators of “New Space” in Washington DC, spoke about her role in helping commercial space efforts by creating a more operator friendly legal environment. Patti also gave a summary of the key-players in commercial sub-orbital and orbital commercial space efforts, the most prominent being “SpaceX”, while the most secretive has been “Blue Origin”, whom Patti has encouraged to be more open.

Once we’re in orbit the only way is Out – into the wider Solar System. Chemical propulsion isn’t up to the task, so Geoffrey Landis [video] made the argument that Nuclear Thermal Rockets (NTRs) will be the “Workhorse of the Solar System”. Geoff’s presentation was based on material presented previously, to more technical audiences, and the technical reports he referenced are also widely available. So he focussed initially on the long history of the NTR in astronautics – dating back to the late 1940s and almost brought to operational readiness by NASA’s Nuclear Engine for Rocket Vehicle Applications (NERVA) rocket program, before being shelved in the early 1970s. Since then research has focussed on newer materials and newer testing techniques of reactor designs, largely via computer simulation and hot hydrogen gas experiments to simulate the operating environment of engine components. An important point is that NERVA-style NTRs allow transport of humans and their machines in reasonable time-frames all the way to Jupiter. Inside the orbit of Jupiter there exist many sources of the chief NTR propellant – hydrogen – usually in solid form attached to oxygen as water. Conveniently water has many other uses for human beings, thus will be in demand.

That key point lead neatly into the next presenter’s talk [video], Chris Lewicki of “Planetary Resources”, who gave an intriguing overview of the next steps for one of the first “asteroid mining” companies. Chris had clearly covered the material many times before, showing a polish that only comes with practice. The Inner Solar System has abundant energy from the Sun, and convenient chunks of material orbiting in free space in the form of asteroids (and dead comets), but the first task is prospecting and finding the most convenient resources to retrieve from their distant orbits. Thus “Planetary Resources” plan of building small satellites with autonomous control, to minimise ground-control costs, and many of them, to achieve savings via mass-production. Interplanetary prospectors that are cheap enough to crash into an asteroid if that’s what the mission requires. Eventually the quest for precious high-value materials in space to return to Earth, such as the Platinum-Group Metals (PGMs), will also have the side-benefit of producing great volumes of useful in-space materials, such as high-grade iron-nickel metal and water. In time the Inner Solar System could have a viable network of resource trading, with PGMs being dropped back to Earth via “whiffle-balls” of foamed metal, and storage depots of liquid hydrogen for NERVA-style NTRs carrying people to the Moon, Mars and the asteroids.

Panel: The Future of New Space [video]

With those thoughts in mind the day ended with a special presentation and viewing of a small fraction of Arthur C Clarke’s paintings and memorabilia, now at the Geisel Library. Seeing promotional material from “2001: A Space Odyssey”, signed by the actors and similar items made me mindful of the vast legacy that Clarke’s work had inspired. In the nearly 50 years since he began working with Stanley Kubrick on 2001, we have achieved but a tiny part of what the 1960s imagined possible — a reminder of the difficulty of making dreams real.

Intense conversations ate up the hours after the scheduled activities, shadowed by my awareness that I was to be the first speaker on Day Two. My sleep was a broken few hours, an hour at a time, looking at the clock, while my sub-conscious was working on arranging what I would say. Needless to say, I have no idea how the delivery looked [video], as I covered slide-after-slide of starship concepts – most of which are covered in the anthology. One gratifying aspect was being able to point out several starship designers in the audience – Freeman Dyson nodded approvingly when I discussed his Interstellar Orion from 1968, and I covered Al Jackson’s role in the development of the Laser-Powered Ramjet. As a parting note I mentioned the “Ultimate Starship” – my personal suggestion, based on the late Robert Forward’s idea of a neutrino-rocket, to use electroweak unification physics to convert ram-scooped mass directly into a neutrino-jet. One day I will need to write the paper.

Jim Benford [video] covered the concept of Microwave Sail-Ships, giving a fascinating look into his experimental work in the late 1990s, with twin-brother Greg, using carbon-sails in vacuum chambers, made to do amazing things via concentrated beams of polarized microwaves. Jim, like Greg, is a physicist, an alumnus of UCSD, but an applied physicist who has literally written the book on high-power microwave systems, like the million-watt RADAR regularly used by the world’s armed forces. Thus he is well able to discuss the practicalities of propelling sails to interstellar speeds via beams of microwaves and has written several papers covering the economics of micro-wave starships. An elementary conclusion of the Benfords’ experiments is that a conical sail can very effectively ride a polarised microwave beam and be spun so it is self-stabilising. A less encouraging finding is that the cost of energy will dominate interstellar missions at high speeds. Before we can reach the stars we will need to create abundant energy supplies.

Next up was John Cramer [video], a physicist from the University of Washington, well-known to SF fans via his “Alternate View” columns in the “Analog” science fiction magazine, as well as several novels. John focussed on the use of wormholes to allow rapid transit to other star systems. Simply put, wormholes are “tunnels” between two regions in space-time, compatible with Einstein’s equations of General Relativity as one possible mathematical solution. Outside a wormhole itself, observers would see two “ends” of the one space-time structure. Whether wormholes exist or not is a matter for astronomical observation, as larger wormholes should produce distinctive gravitational lensing patterns that astronomers might be lucky enough to see. If the connection formed between the two ends of a wormhole is shorter than the distance through regular space-time, then passing through the wormhole allows apparently faster-than-light travel, though nothing ever exceeds lightspeed locally. Thanks to time-dilation — the slowing of time experienced when approaching lightspeed — a time-lag can be developed between the two ends if one end is sent to a distant star. For example, if a one end is accelerated to a time-dilation of 7,000 (0.99999999c), then only 75 minutes is required for the travelling end to appear to travel 1 light-year from the stationary end’s point-of-view. John Cramer discussed how this might allow a network of rapid-transit wormholes to be set-up throughout the Galaxy – with the caveat that the network can’t be allowed to form a “Closed Time-like Circuit,” else this might destroy the wormholes via amplifying quantum fields.

Before lunch, British astronomer Ian Crawford [video], a fellow member of “Project Icarus”, discussed what we might find amongst the nearer stars, out to 15 light-years. A planetary system probably exists around every star, something we can say with statistical confidence thanks to the work of the “Kepler” exoplanet detection mission, but discerning every planetary system will require improvements on current techniques. And we almost certainly haven’t found every small star within 15 light-years yet, as the 2013 discovery of a brown-dwarf binary at just 6.5 light-years should remind us. Ian made the forceful point that even with vast telescopes able to image those many new planets and stars, there’s only so much we can learn via telescopes. If we find a planet showing all the signs of life, we will only know more by actually going there – via robotic proxy, in Ian’s opinion.

Once we do go, will we survive? This was the after-lunch opener from Paul Davies [video], who posed the puzzling question of how terrestrial life might interact with truly alien life in another star-system. Could they co-exist, with no biochemical compatibility at all? Could they share common simple biochemicals, but foreign genetic and protein chemistry? Or could the two integrate in ways we haven’t yet imagined? Even more intriguingly, Davies suggested that we might already co-exist with “alien” biochemistries on Earth – organisms might exist in niches that otherwise exclude our kind of biology. A suggested location might be at temperatures higher than what known microbes can tolerate, or in highly alkaline fluids, such as what seeps from ocean thermal vents. Davies has suggested, in more than one book, that any life on Mars shares a common ecosystem with Earth, due to the trade in meteorites between the planets over the aeons. Mentioning this sharing of life between planets produced an out-burst from Robert Zubrin, who is an advocate of interstellar transfer of life throughout the Galaxy. A credit to Davies, his response was more interested curiosity than the reflexive dismissal Zubrin seemed to expect. His answer was that we simply don’t know enough to rule out the possibility and they should discuss it more later.

The Benfords encouraged researchers to be present in the audience, with divergent points-of-view. Despite their difference these all share a desire to bridge the space between the stars, but differ in details of how and why we’ll go to the stars. The next speaker unified the many voices by sharing his sense of wonder at the Universe, through a living work of art – Jon Lomberg [video] and his Galaxy Garden. Long-time readers of “Centauri Dreams” will know of Jon Lomberg’s artwork for Carl Sagan’s “Cosmos” in the 1970s and his Galaxy Garden in Hawaii. Having Jon share it with us, a guided-tour in slides, was inspiring and drew multiple rounds of applause from the audience. As Jon put it, we can be Citizens of the Galaxy now.

Two discussion panels concluded the Symposium. The first, chaired by Jill Tarter of the SETI Institute, featured Ian Crawford, Robert Zubrin, Geoffrey Landis, Paul Davies and myself. Our theme was “Getting to the Target Stars” [video] but with Jill as the Chair we wandered into the Search for Others who might have made the same journey. Jill gave a brief summary of false-positive detections of extraterrestrial technology, which have proven to have natural explanations. The sole exception, the distinctive spectroscopic signature of tritium, has no natural explanation – if it is ever detected. With that in mind each of the panellists made suggestions about how we might detect aliens. Robert Zubrin mentioned the distinctive radio output of a starship deploying a magnetic sail, while I suggested the Solar System be searched for dead Starships, since not everyone succeeds in their long voyages. A final task was to sum up how we thought humanity would go to the stars. A common feeling seemed to be via robotic proxies, or nano-bots. In my opinion, by the time we are ready, the distinction between “human” and “robotic” might be meaningless or arbitrary – thus my quip “Nanobots are people, too.”

The final panel was a perspective [video] by the Science Fiction writers, some involved in the “Starship Century” anthology – Joe Haldeman, David Brin, Larry Niven, Vernor Vinge – and Jon Lomberg. This was the artistic side of the event, as all these have produced visions of the starship era. The general feeling was that, given the growth in space industry that Chris Lewicki and Robert Zubrin advocated, then we would see the first star-voyagers depart in about 2200, as Freeman Dyson had extrapolated back in the late 1960s. Some envisioned the unexpected – the discovery of extraterrestrial intelligence near enough to communicate with; breakthroughs in physics that would allow rapid interstellar travel; or, as Allen Steele depicted in his award-winning “Coyote” series, the rise of a tyrant putting a nation or the world on a crash-course program of starship-building. As always, the future will surprise us, but we can prepare ourselves by listening carefully to the modern-day prophets.

Starship Century – Thinking BIG


On Day One Neal Stephenson and his team from Arizona State University presented on an awe-inspiring structure. I’m not sure I’d endorse it as a structure for space-launch, at least not yet, but a 20 km high Tower has a certain gravitas that makes it worthwhile contemplating. Certainly the exercise of designing a mega-structure (even a baby mega-structure like this Tower) will develop computational tools and experience that will help future mega-structure design, some of which will help develop and terraform the Moon, Mars and Beyond.

The mass figures were rather surprising, while the structural material choice was logical, as steel is the most abundant material we make after concrete. One estimate, before it was trimmed back, was about 2 billion tonnes. I sat next to Patti Smith, one of the chief Regulators of Commercial spaceflight, and exchanged comments with her – mostly positive, though my memory is patchy. I quickly estimated that 2 billion tonnes of steel was about 30 years of global production, but my global production figures are probably about 10 years out of date. I have a soft-spot for Colossal Mega-Structures. The automation/teleoperated machinery that building such would require could easily extrapolate to use in space, where multi-kilometre wide structures are regularly discussed. The technological spin-offs of a Space Tower would be profound – but unknowable if we never try to build it, even in detailed virtuality.

Exoplanet News & Novelty

Some interesting exoplanet news bites from around the web.

Think Outside the Box to Find Extraterrestrial Life

Sarah Seager and James Kasting are quasi-interviewed. Seager discusses the Hydrogen Greenhouse planets posited by Eric Gaidos and Raymond Pierrehumbert, while Kasting points out how difficult such worlds would be to observe. Seager also mentions her recent work on Desert Planets and how Venus missed out on becoming a still habitable planet thanks to the Runaway Greenhouse effect. Venus also doesn’t rotate fast enough to allow strong temperature gradients that might allow cooler climates near the planet’s poles.

Eyeball earths

Charles Qoi reports on Daniel Angerhausen and colleagues work on “Eyeball Earths” – planets that are tidally locked to their stars, with one face forever facing the Sun and the other forever facing the black of space. Exactly how such planets might evolve – whether their water would form a vast Ice-Cap on the night-side – is being actively researched.

Under pressure: How the density of exoplanets’ atmospheres weighs on the odds for alien life

Giovanni Vladilo and colleagues examine the effect of atmospheric pressure on a planets habitability. Thin atmospheres tend to result in oceans boiling away more easily than thick ones – thanks to the reduction in boiling point. Water being a super-star greenhouse gas is less likely to cause trouble when there’s more of other gases, like nitrogen or even carbon dioxide. Best to read the paper to get the best idea of their thesis – the URL is given with the diagram: http://wwwuser.oats.inaf.it/astrobiology/planhab/.

Overall the current trend in thinking is towards habitability – of at least part of a planet – has a wider zone around the stars than formerly believed. The inner edge might be ~0.5 AU (X4 Earth’s insolation), while the outer edge is 1.7 AU for a CO2 greenhouse, but much, much further out if there’s enough nitrogen or hydrogen to beef it up. All the way out to Saturn if the planet’s lifeforms are driven by sunlight, or all the way out if they can subsist on geothermal energy.

A puzzling question is whether such planets can be called inhabitable by oxygen-breathers like us. Desert Planets, again, have something to say about that – they withstand a Runaway Greenhouse closer in, and avoid the negative feeback of too much snow further out. But over a narrower range than planets with more exotic atmospheres.

Starship Century

Starship Century

May 21, 22 – I will be presenting on Starship Concepts, along with some amazing speakers. Check out the link. Might be hard to be there, so… buy the book!

My talk will cover “Starship Concepts” – in the book I present them in roughly chronological order, but this time I’ll mix it up a bit. Expect Bussard Ramjets, Saenger Photon Rockets, Forward Laser-Sails, Orion, Daedalus, World-ships… and more, more, more…

Terraforming Mars… Very Quickly


Comet C/2013 A1 (Siding Spring) has been discovered to be flying very close to Mars in October 2014… very close. In fact its current orbital error-ellipse has a minimum distance to Mars of 0 – i.e. IMPACT! The error ellipse is large and the current best guess is ~119,000 km from Mars, which is very close to a terrestrial planet for any identified comet in recorded history. Jupiter, of course, has suffered two impacts in the last 20 years and probably several more have been recorded in the past without observers knowing what they had seen.

So what if the comet does impact? Firstly we know the relative speed – a whopping 56 km/s, much faster than the usual asteroid impactors. This packs about 1.568E+9 J/kg in kinetic energy into the mass of the comet. So how big is it? Estimates seem to vary. At first I read the nuclear magnitude to be the nuclear size – normalised to 10.3-10.4 in the current observational figures. Read naively, that means with typical cometary albedo of 0.035, a size of over 60 km. A whopper! However, even at its initial discovery distance of 7.2 AU a comet’s nucleus can be quite active. A spherical coma much bigger than the nucleus can form. The long-time comet observers are estimating that the nucleus is about 4 km wide. Not as humungous as I first thought, but significant if it hit Mars.

So what if it did? At a density of ~1,000 kg/m3 and spherical, then the mass is 33 billion tonnes. Total energy is ~5E+22 J or 12 million Mega-tonnes TNT equivalent. If the comet wasn’t active and we really do see a 60 km monster headed for Mars, then the energy is 3,375 times higher or 42 BILLION Mega-tonnes TNT equivalent. The truth is probably somewhere in between. Assuming the former figure, that’s the equivalent of vapourising about ~20,000 cubic kilometres of ice. The Martian Polar caps total about 3.2 million cubic kilometres, so the smaller estimates won’t affect them much – though enough to be interesting – but the larger estimate would vaporise the lot and then some. The crucial question for Mars, in the long term, is amount of dry ice and liquid carbon dioxide available in the subsurface. All around the planet we see signs of significant amounts of dry-ice in the sub-surface. What if the impact liberated enough to significantly change the planet?

Firstly, how much is needed to give Mars an atmospheric pressure that humans can tolerate, given an oxygen supply, without the need for pressure suits? At the right temperature that can be as low as ~150 mb, or 15 kPa. But let’s assume 300 mb, 30 kPa, the same as the Summit of Everest. Exploring Mars would then be like exploring Everest, rather than the Moon as it is presently. Oxygen would still be needed, but in the right places, plants would be able to grow on the surface and, given time, the planet could be made more habitable. At the bottom of Hellas Planitia – a vast deep depression – the pressure would be ~500 mb, so even more hospitable and open bodies of water could form. Probably akin to the hyper-saline lakes in Antarctica, at its warmest, but on a larger scale.

The surface area of Mars is nearly 145 million km2, or 145 trillion square metres. The surface gravity is 3.711 m/s2 which means 30 kPa pressure requires a column mass of P/g = 8,084 kg per square metre. Over the whole of Mars that’s a total air mass of 1.172 quadrillion tonnes, which sounds immense, but is ~1/3 the mass of the ice-caps. Vaporising CO2 is much easier than vaporising H2O so the energy required to liberate it requires a smaller impactor… though only through directly hitting the source. If a smaller amount is liberated, say 1/10 the goal of 30 kPa, then it might tip Mars out of its current cold climatic state into something warmer. Then we’d see the sub-surface release more CO2 over time (making the surface locally unstable from gas out-bursts) and a self-accelerating warming trend kicks in.

If it’s so easy to remake Mars, then why hasn’t it happened naturally in the past? Perhaps it has. Abundant signs of climate change exist all over the planet – sudden floods of water as ice melts have formed rivers all over the planet. But then the warming ends and the planet chills. The suspicion is that temporary warmings increase trapping of carbon dioxide by dissolving it in water, thus an “Impact Summer” is followed inevitably by the return of the Endless Winter. An Impact Summer might last a few millennia, creating some new erosion, then the glaciers and dry-ice return.

Except, if it happens this time, maybe we – Humanity – can make an Impact Summer last.

Re-Enchantment of the Solar System


The Re-Enchantment of the Solar System:
A Proposed Search for Local ET’s

Dr. Gregory Lee Matloff

Adjunct Professor of Physics and Astronomy, NYU, CUNY, Pace University, The New School, SUNY

Co-author (with E. Mallove) of The Starflight Handbook, Wiley, NY (1989),
Fellow of The British Interplanetary Society
Member of the Interstellar Exploration Subcommittee, International Academy of Astronautics


It is argued using a conservative approach to interstellar travel that intelligent extraterrestrials (ET’s) may be present in our solar system, living in world ships that have colonized cometary or asteroidal objects during the last billion years. The originating star systems for these advanced beings could be solar-type stars that fortuitously approach our Sun within a light year or so at intervals of about a million years or nearby stars that have left the main sequence, prompting interstellar migration. If we are indeed within such a “Dyson Sphere” of artificial worldlets, we could detect their presence through astronomical means since a space habitat will emit more infrared radiation than a like-sized comet or asteroid. Interestingly, several Kuiper-Belt objects have recently been found to have an unexpected and substantial red excess. It is argued that, in opposition to the assumptions of current SETI searches, the very advanced occupants of this possible local Dyson Sphere may have as little interest in beaming radio signals in our direction as we do in communicating with termites. A research program is proposed whereby large and small college observatories would routinely monitor the spectral irradiances of Near Earth and Kuiper Belt objects while a concurrent theoretical effort models the spectral characteristics of various proposed space habitats. Much of the observational work, at least, could be dovetailed with projects designed to detect Near-Earth Objects (NEO’s) that might impact Earth in the future. Possible strategies and protocols for direct contact, requiring humans to be the active contactees are presented to be considered for use if such intelligent ET’s are discovered within our solar system.

(I) Introduction: Ancient and Modern Views Contrasted

To civilized people living in the late Bronze Age or early Iron Age, the nightly celestial spectacle was proof that humanity was not alone in the universe. The solar system was enchanted by the presence of those gods and goddesses of the ancient pantheon called Mercury, Venus, Mars, Jupiter, and Saturn by citizens of the Roman world.

Although recognized today as the naked-eye planets, these objects not only had the names of the divinities they actually were the divinities to many ancients. To contact a god or goddess, a person from those civilizations might visit an appropriate temple or perhaps utilize the talents of a member of that class of soothsayers that we now call astrologers.

It is fashionable today, at least among scientists, to treat these beliefs of our predecessors as somewhat quaint and superstitious. Ever since the first Greek astronomers constructed their thought models of the solar system about 2500 years ago, science has steadily pushed back the role of divinity. We understand today that the planets known to the ancients and those more recently discovered worlds circling our Sun and others are physical places more or less like the Earth, not the abode of gods and goddesses.

To actually make scientific contact with the creator(s) of our universe may be forever impossible, due to the limits on astronomy imposed by the chaotic and unprobeable nature of the Planck Epoch, the first nano-instant after the universe’s creation.

Although science may never be able to engage in direct discourse with the Creator, scientists could constructively concentrate their efforts on contacting those godlike (although mortal) minds belonging to citizens of galactic civilizations eons in advance of our own. Even though the ultimate nature of our existence and that of the universe might elude us, such a contact would tell us a good deal about our place in the universe and the cosmic function of technological civilization.

This is the purpose of the astronomical discipline known as SETI, the Search for Extraterrestrial Intelligence, through the medium of radio transmissions. After several decades of effort and many SETI searches, the results have been disappointing (1). Either there are no advanced intelligences within a hundred light years or so of the Sun, or the basic assumptions of SETI are imperfect.

But recent astronomical observation and analysis has revealed that solar systems are not uncommon in the nearby universe, early assumptions about the bounds of the ecosphere , the “life zone” about stars, may have been very conservative (2), and extinct or extant life within our own solar system may be present below the surface of Mars or within the oceans of Europa (3, 4). If life is indeed prevalent in the cosmos, where are our talkative neighbors?

Perhaps they are all around us, with some migrants even within our own solar system. Perhaps we have not found them because directed-radio signals in the hydrogen and hydroxyl bands is not the best medium of interstellar communication. Perhaps very advanced extraterrestrials are not really interested in talking with us primitives.

To establish contact then, we may have to be the active party. First humanity must unambiguously detect an alien presence, then we must be the ones to clamor for attention. After all, entomologists devote little time to communication with termites. But if a termite colony signaled us in binary or Morse codes, we would immediately take notice.

Conventional SETI is dissatisfying from a human point of view. Even if we detect beamed radio signals from a cosmic civilization located a mere 30 light years from the Sun, a simple exchange of greetings would require a human lifetime. This is a long way from the promise of a Chaldean astrologer or Roman-era priest of Jupiter.

But the solar system is an immense place, both in time and space. Our exploration of this realm has barely begun. As Papagiannis has noted (5), extraterrestrial colony ships may have crossed the interstellar gulf within the distant past and may be within our solar system, in world ships that masquerade as asteroids or comets.

If we can detect these objects and communicate with their occupants, the re-enchantment of the solar system will have been established. Direct communication with these godlike elders will become possible by masers, lasers, or spacecraft. And the answers to our queries will arrive rapidly. Then, astronomers will begin to fill the roles of the ancient temple priests and priestesses. Terrestrial civilization will enter a new golden age as we learn the wisdom of the “elders” and begin to fulfil our galactic role.

Before beginning a search for a local extraterrestrial presence, we need to develop a multi-step program. First, we must consider an evolutionary model for technological civilizations, that allows for slow interstellar expansion but not necessarily signaling. Then, we must conservatively consider those modes of interstellar travel that now appear to be feasible for a technologically advanced galactic civilization. The next step is to examine the nearby stellar environment to try to ascertain the probability that our solar system has been colonized. Finally, an observational /modeling approach is proposed to “search the skies” for signs of a local alien presence. Because this sky search can be incorporated into ongoing or proposed searches for asteroidal/cometary objects that might threaten the Earth, project-funding concerns might be eased.

And if we do detect a nearby alien presence, what then? Humanity would be faced with the dilemma of whether or not to signal, whether to bury its collective head in the sand or to strike out in anger and fear. A direct-contact protocol (6) would then be necessary as would widespread discussion of who speaks for Earth and where a meeting between humans and representatives of the alien species should occur.

(II) A Tale of Two civilizations

It can be argued that our assumptions about advanced galactic civilizations and their motives have more to do with human preconceptions than with ET’s activities. Since it is impossible to predict the course of our own civilization even a decade or two in advance, all of our preconceptions about the motives of advanced galactics must be taken with a large chunk of salt.

(A) The Communicatus

The standard SETI model for communicating extraterrestrial civilizations is based in part upon the model of Nikolai Kardashev (7), who was a leading Cold War era Soviet radio astronomer. Consider the hypothetical “Communicatus”, a race of technologically advanced extraterrestrials who evolve according to Kardashev’s model.

The Communicatus follow the basic human pattern of civilization up to the Cold War era. Evolved from carnivores, they experience in succession a long Stone Age which culminates in an agricultural revolution, then the metallurgical revolution of a Bronze and Iron Age. After discovering science, their technology rapidly develops in the direction of aircraft, spacecraft, radio, computers, and automobiles.

A long competition develops between two contending political powers upon their home world, one socialist and one capitalist. During that era, there are alternate periods of uneasy peace and limited hostilities. But instead of the capitulation of one or the other, the intense scientific competition between the two powers continues in an increasingly more ritualized fashion. Ultimately, the two social-political systems merge into a defacto world government.

In the golden age that follows the merger, the collective social responsibilities of the socialist competitor are aligned with the individual ingenuity of the capitalists. Poverty, hunger, and disease are all vanquished and the global society soon learns to productively tap all of the solar radiation reaching the home planet. All citizens of the fortunate world soon are leading long and productive lives and are contributing meaningfully to civilization’s advancement.

Expansion into the solar system begins, with the construction of freeflying space habitats from asteroidal and cometary material (8). Believing that their social structure is the most stable and successful for the needs of an expanding technological civilization on both the home world and off planet, the government of the home world elects to begin experimenting with radio signals beamed in the direction of suitable nearby sites of alien intelligence.

A binary code is developed so that any intelligent receiver will recognize the signals as artificial and readily interpret them. As the development of their home solar system continues, more energy for the transmissions becomes available through application of off-planet solar-power relay stations. The number of transmitting radio telescopes, the transmission range and the information-content of the message all increase with time.

In a sense, the Communicatus become a society of secular proselytizers. Perhaps by fortuitous accident, they have solved society’s ills and evolved as successfully on the material plane as possible. The reason for the continued funding of the interstellar transmissions is quasi-religious.

Most citizens believe that it is their responsibility to communicate the success of their social-political order to less fortunate galactic civilizations. Eventually, they hope, a galaxy-wide network of like-minded civilizations will develop. They anticipate that in the farther future, this network will be able to command galactic-scale resources and expand the interstellar transmissions to the intergalactic realm. Someday, all the universe will be fully awakened and communicative. Then consciousness will have fulfilled its purpose.

(B) The Mutatis

Up to a certain point, the cultural evolution of the “Mutatis” mirrors that of the Communicatus. They also evolve through cultural levels corresponding to the terrestrial Stone, Bronze, and Iron Ages. They also discover science, which leads to a rapid increase in the rate of technological advancement.

But their equivalent of the terrestrial Cold War ends in a manner analogous to the conclusion of that terrestrial conflict. The socialist-type Mutatis state capitulates to the capitalists.

Without any major international competition, the Mutatis capitalists construct a New World order that fails to satisfy some of the basic requirements of most world citizens. As Mutatis society develops, the gap between haves and have-nots increases. Educated citizens of the more advanced Mutatis nation states, a small but influential minority of the total world population, become increasingly disillusioned about the long-term possibilities for their civilization’s survival.

But salvation comes from an unexpected source. An Information Age had developed from the widespread application of computer technology initially developed for war. A world-wide network of personal computers had developed, an equivalent to our “World Wide Web.”

Originally developed as a means of sharing technical information during their Cold War, the Mutatis Web has now evolved into a tool for the more jaded and physically lazier Mutatis consumer. New forms of pornography, only available on the Web, can titillate all senses of the subscriber. Affluent web users can conduct all business and do their shopping from their console, without leaving the confines of their homes.

Driven by market pressures, the size and cost of the microprocessor chips that are the basis of the Web constantly decreases, information content of the chips consequently increases. In an experimental effort to emulate the complexity of the Mutatis brain on the Web, brain neurons are kept alive on computer chips. Eventually, these neurons are interfaced with the ever-complexifying network of the Web. The complexity of the evolving network soon greatly exceeds that of an individual Mutatis brain.

Neuronal interfaces are constructed so that the brains of Web subscribers can directly interact with the planet-wide network. Many citizens soon prefer the virtual reality of the Web to direct interaction with their fellows.

And then, entirely by accident, a user of the Web produces a social revolution of enormous importance. While hooked into the Web and participating in an especially graphic form of virtual pornography, an elderly and politically powerful citizen suffers the Mutatis equivalent of a fatal heart attack.

But, although his physical body has died, Web users over the planet soon notice the intruding presence of the powerful individual in their virtual worlds. The Holographic Theory of Consciousness (9) has been proven by the survival of the unfortunate individual’s mind.

Soon, the whole planet clamors for the cybernetic immortality attained by the deceased individual. Vast Web Emporia spring up all over the Mutatis home world, in which citizens can hook up to systems that preserve their bodies as long as possible while the minds of the occupants virtually surf the Web (10).

Social unrest disappears overnight as Web-connection replaces thoughts of revolution. Hunger disappears since the caloric cost of maintaining the slumbering bodies of the Web-users is much less than the cost of supporting a more physically active population. Fear of death also vanishes, since death just means the inability to return to one’s physical body after voyaging through the Web.

Soon, the individual conscious “subroutines” begin to link together. New conscious abilities emerge as complexity increases. The communal mind proposed by Olaf Stapledon (11) comes into existence not through telepathy and centralized planning, but through selfishness and greed.

Ultimately, Mutatis society grows very conservative with so many ancient minds permanently resident on the Web. The maintenance of the cybernetic network and the system of virtual immortality becomes the prime directive of all citizens of Mutatis both for those who are corporeal and those who are virtual.

The Mutatis Collective Mind begins to spread into space, not because of a desire to explore and not out of altruism towards distant galactics. The main motivation is continued immortality and the home solar system should be completely rebuilt to prevent dangerous impacts on the home world by asteroidal and cometary objects.

In the fullness of time, the Mutatis home star leaves the main sequence and begins to swell towards the subgiant and giant phases of its evolution. As the oceans of the home world evaporate, the collective mind, now housed among the assorted terminals of myriad Mutatis space habitats, begins to turn its attention to the nearer stars. In the interest of collective immortality, a long migration is planned.

(C) Which Type of ET Civilization are we Most Likely to Contact?

Although most scientists will be more sympathetic to the society and motivations of the Communicatus, the scenario of the Mutatis does not unduly strain credibility. It is impossible to predict who our closest galactic neighbors are ? talkative, altruistic explorers or quiet, inward-focussed immortals. (There are, of course, many additional possible scenarios for the evolution of a galactic civilization.)

We should continue “conventional” SETI radio searches in the hopes of hearing from Communicatus. But we should also investigate the possibility that Mutatis might exist, right here in the solar system. We next investigate the feasible forms of interstellar travel, to demonstrate that migration at the end of an advanced civilization’s home star’s main sequence life is not impossible.

(III) Feasible Interstellar Travel for Migrants

Starting from the pre-Space Age, many modes of interstellar travel have been suggested. Most can be rejected as unfeasible or impractical, at least for application to large-scale migrations. This essay introduces some of the most popular contending interstellar propulsion systems. Details of most of the proposed methods of interstellar travel have been reviewed in The Starflight Handbook and Prospects for Interstellar Travel (12, 13).

(A) Propulsion Systems that are not Presently Feasible

Probably the most popular method of interstellar travel among the general public is the “Hyperdrive”. Using magnetic fields or more esoteric means, the space/time warping characteristics of a collapsed star are simulated on a small scale and for long enough duration for a starship to take a dimensional shortcut to another star. A multi-light year voyage might take minutes. Unfortunately, although research in the construction of such artificial singularities is continuing (14), the field strengths required are so gargantuan that the hyperdrive will be found only among the pages of science fiction novels for the foreseeable future (15).

If a starship crew is capable of traveling at near-optic velocities, general relativistic time dilation greatly shortens the voyage’s duration, from the crew’s point of view. There are two conceptual methods of achieving such speeds that do not strain the laws of physics.

The Bussard Ramjet (Figure 1) works by using a magnetic scoop (or ramscoop) to ingest interstellar protons over a large area (16). The fuel passes through a fusion reactor capable of converting hydrogen directly into helium plus energy, as does the Sun and other main sequence stars. The released energy is used to accelerate the helium nuclei exhaust out the rear of the spacecraft. Although recent research indicates that a suitable ramscoop may be feasible (17), there seems to be no way to create a proton fusion reactor. Many less capable ramjet variants have been proposed, the most feasible is the use of a solenoidal field ramscoop to reflect oncoming interstellar ions and thereby decelerate a speeding starship (18, 19).

Technology and physics do not provide substantial barriers, on the other hand, for the Antimatter Rocket (20). Nuclear fission and fusion, those nuclear reactions currently in use by humans are relatively inefficient in that only a small percentage of the reactant mass is converted to energy. A fuel mix consisting of antiprotons (which can be produced in nuclear accelerators) and protons is the most volatile substance in the universe, on the other hand, since essentially all the reactant mass is converted into energy. Antiprotons can now be produced in small quantities and stored for long periods in “Penning Traps” (21). Although antimatter rocketry (Figure 2) is certainly feasible technologically, there is one “small” problem. The cost of antimatter production must fall by many orders of magnitude before the antimatter rocket can be considered economically feasible.

For decades or century duration interstellar voyages at speeds as high as 10% of the speed of light (0.1c), a favored propulsion system is the laser light sail (22). A solar-pumped laser power station is constructed in space and maneuvered into position between the Sun and the destination star. A laser beam from the power station impinges against the light sail of the starship – a highly reflective metal sheet with a thickness of nanometers or microns (Figure 3). The pressure of the laser photons accelerates the spacecraft in the direction of the destination target star. Optimum performance seems limited because of the requirement for the power station to be “suspended” between Sun and starship very accurately for decades, but projection instead of both power station and probe on the same initial slow hyperbolic trajectory towards the target star may somewhat alleviate pointing problems (23).

Since the recent experimental confirmation of the Casimir Effect (24), starship designers must consider the possibility that a future propulsion option might be energy from stabilized vacuum quantum fluctuations or controlled modification to a spacecraft’s inertia (25). Although a NASA workshop on these and related topics has occurred (26), it must be admitted that research in the field of ZPE (Zero Point Energy) is too underdeveloped at this time to comment on its ultimate utility for interstellar travel or other applications.

(B) Currently Feasible Propulsion Options

During the 1970’s and 1980’s, the first open literature engineering study of the difficulties of interstellar travel, Project Daedalus, was conducted by the British Interplanetary Society (27). At the conclusion of Daedalus, several of the team members collaborated on a consideration of what types of interstellar missions would be possible for humans, as opposed to robots, in the foreseeable future (28). After considering and eliminating the propulsion systems reviewed above, the Daedalus team members were left only two contenders. In contrast to science fiction, the only feasible mission was the “1000-year ark” or world ship (29, 30), in which the population of a kilometer-dimension ship with a closed ecosystem travels for many generations between neighboring stellar systems.

(1) Fusion Pulse

One method of world ship propulsion, a product of the Cold War, is nuclear pulse propulsion (Figure 4). Thrust is produced by the reaction of either a thermonuclear bomb external to the ship’s reaction product, or an electron beam (or laser beam) imploded inertial fusion micropellet (31, 32).

Possible reactants include deuterium/tritium, deuterium/helium-3, lithium-proton, and boron-proton. The deuterium/tritium thermonuclear fusion reaction is relatively easy to ignite, but has the disadvantage of producing copious amounts of thermal neutrons and a consequent radiation hazard. Although the deuterium/helium-3 reaction is much cleaner and almost as easy to initiate, helium-3 is extremely rare on the earth. We might obtain it though, directly from the solar wind, by mining upper lunar regolith layers that have been exposed for eons to the solar wind, or from the atmospheres of the giant planets. Although relatively clean and utilizing rather common reactants, the last two reactions listed are much more difficult to initiate.

The exhaust velocity of a fusion pulse starship might be in the range 0.01 – 0.03c (where c is the speed of light). If deceleration is accomplished using a magnetic field to decelerate reflect oncoming interstellar ions (18, 19), a fusion-propelled interstellar ark might require 500-1000 years to reach the nearest star. But until controlled fusion is actually achieved and the reactants are optimized, it is difficult to estimate whether an entire space-faring civilization could be relocated using fusion alone.

(2) The Hyper-thin Solar Sail

The only currently feasible alternative to fusion pulse propulsion is the hyper-thin solar sail unfurled near the sun (33, 34). A space manufactured, highly reflective sheet sail with a thickness measured in nanometers is attached to the payload with diamond strength (or silicon carbide) cable. After injection into a parabolic or slightly hyperbolic solar orbit, the sail is gradually unfurled as close to the Sun as possible. The sail/cables/payload is then “blown” out of the solar system by the radiation pressure of sunlight (Figure 5).

For a small interstellar ark massing a few million kilograms, the fully unfurled sail dimensions are typically about 100 km and peak accelerations are in the neighborhood of a few g (where 1 g = 1 Earth surface gravity). One-way voyage times to the nearest stars (Proxima and Alpha Centauri at an approximate distance of 4.3 light years) are typically about 1000 years.

After the acceleration phase, the cables and sail can be wound around the habitat to provide additional cosmic ray protection. As the target star approaches, the sail can be unfurled and used in reverse for deceleration.

A number of methods have been suggested to reduce travel time or increase payload. These include use of very thin and highly reflective cables that are partially supported by solar radiation pressure, a perforated sail surface to reduce sail mass, and more complex pre-perihelion trajectories (35 – 38). But the greatest trip-time reduction is caused by increasing the solar luminosity (34), which enhances this propulsion system’s utility for those civilizations relocating from a dying star.

(C) Solar Sail Operations from Post-Main-Sequence Stars

We assume here that an advanced galactic civilization might not commence interstellar migration until its home star leaves the main sequence. At that time, the star’s luminosity and physical size both increase.

To examine the effects of this stellar luminosity change upon solar sail performance, we next define a solar sail “runway”. This is the distance between sail partial unfurlment at perihelion and the point at which the sail is refurled. For the purposes of the argument presented here, assume that the sail is refurled when the distance to the Sun’s center has increased by a factor of 2X from the perihelion distance and the radiant flux received from the Sun (in watt/area) has decreased to one-quarter of its perihelion value. As the sail is unfurled along the runway, its radiation pressure acceleration radially outward from the Sun is here assumed to be constant.

For example, if the sail is partially unfurled at 0.02 AU (Astronomical Units) from the Sun’s center, the sail is refurled at 0.04 AU and the runway length is 0.02 AU. The perihelion distance is a function of the reflective and thermal properties of the sail and other aspects of sail design (33, 34).

Assuming the same sail design characteristics as above, imagine that the Sun is replaced by a star with twice the solar luminosity. Applying the inverse square law, the perihelion distance is now 0.028 AU from the star’s center for the same perihelion radiant flux on the sail. The sail is refurled at 0.056 AU and the runway length has increased to 0.028 AU. These arguments can be generalized to demonstrate that runway length, Srw, varies with the square root of stellar luminosity, Lstar.

Applying elementary kinematics, the sail’s velocity (relative to its star) at the conclusion of sail operation, vfin, can be written:


where a = starship acceleration on “runway” and v0 = starship perihelion velocity relative to its star.

Assuming that 2aSrw >> v0, the final starship velocity varies approximately with the square root of runway length or the fourth root of stellar luminosity.

This assumption can be checked for the case of a 1000X solar luminosity star, considered in Ref. 34. According to the approximate theory presented above, such a craft should be faster than an identical ship departing the Sun by a factor of the fourth-root of 1000, or about 5.6X.

Recent work (39, 40) has revealed that an optimum sail material is beryllium and that a 20 nm thick beryllium sail closely approximates the aluminum-boron bilayer sail performance presented in Fig. 3 of Ref. 34. From Ref. 40, a 20 nm thick sail can project a 1 x 107 kg payload to Alpha Centauri (at a distance of 4.3 light years), in about 1800 years, starting from a thermally limited perihelion distance of 0.02 1 AU and an initially parabolic solar orbit.

If the Sun were replaced by a giant star with 1000X the solar luminosity, the trip time to Alpha Centauri should be reduced by a factor of about 5.6X, to about 320 years. Note from Fig. 7 of Ref. 34 that the optimization program presented in that reference yields a corresponding travel time of about 300 years, which is in good agreement with the approximate theory presented here.

Even though the approximate dependence of starship final velocity with the fourth root of stellar luminosity is apparently confirmed by the above example, it should be treated as very approximate. Perihelion velocity may not always be negligible compared to the product of acceleration and runway length. The perihelion trajectory may not always be parabolic and the sail’s attitude at perihelion may not always be normal to the Sun, as assumed in Refs. 33, 34 and 40 (37, 38). For really accurate work, it will be necessary to treat the Sun or any other star as an extended light source if the perihelion distance is close, rather than as a point source as has been done in the optimization program in Ref. 34 (41).

The next section presents a sample sail-launched interstellar ark that could be flown from the Sun or a more luminous star. Stars in the solar vicinity are next examined to attempt to determine the frequency of post-main-sequence stars from which interstellar migration may be occurring. Starship velocities for expeditions leaving these systems will be estimated. From a consideration of stellar motions, an estimate is derived for the number of migrations to our solar system that may have occurred during the last billion years.

(IV) Nearby Stellar Candidates for Migrating Civilizations

Before examining various nearby candidate stars for the sites of migrating interstellar civilizations, some design considerations for a small interstellar ark are presented. These are based upon calculations using the solar sail optimization program presented in Ref. 34, for a 20 nm thick beryllium sail, for diamond tensile-strength cables, a rip-stop mesh array in front of the sail with an areal mass thickness of 1.24 x 10-5 kg/m2, and a sail unfurlment fraction of 0.25 at perihelion (40).

Table 1 presents characteristics of a small Sun-launched interstellar ark with a payload of 10 kg and a perihelion distance of 0.021 AU. This payload mass is similar to that of Gilfillan in an early treatment of interstellar arks (42) and suggested to Apollo 11 astronaut Buzz Aldrin by this author for use in his science fiction novel with John Barnes, Encounter with Tiber (Warner, NY, 1996).

Table 1. Design Characteristics for a Small Sail-Launched Interstellar Ark

Payload: 105 kg

Sail: 20 nm beryllium 

Cables: Diamond tensile strength 

Ripstop mesh thickness: 1.24 x 10-5 kg/m2

Perihelion: 0.021 AU 

Perihelion sail unfurlment fraction: 0.25

Unfurled sail radius: 120 km 

Total mass: 2.1 x 106 kg

Peak acceleration: 6.33g 

Interstellar cruise velocity: 0.0036c 

Time to Alpha Centauri: 1193 years 


Such a large space-manufactured solar sail is a good way beyond the test sail unfurled at space station Mir in 1993 (43). It is also far beyond the Earth-launched sails under consideration today to propel an interstellar precursor probe early in the 21st century (44). Computer simulations of sail structural parameters do indicate that such a large sail craft could be engineered (45).

Some may argue that the example chosen is too small. But an interstellar colonizing expedition could consist of many ships of this type. The population of each might be around 20 (42). Also, if the goal is to create large space habitats or world ships from asteroidal and cometary material in the destination star system, not to reenter a gravity well and colonize a planetary system, the required mass budgets are of course more modest.

Attention is next turned to the stars in the solar neighborhood that might be home to a migrating interstellar civilization. Stars of interest are subgiant (luminosity class IV) and giant (luminosity class III) since civilizations around such stars have a strong motivation to emigrate.

(A) Procyon

Also called Alpha Canis Minor, the nearest candidate star (Procyon A) is classified as F5 IV-V, is in the process of leaving the main sequence and is located 11.3 light years from the Sun. It has a white dwarf companion (Procyon B) with 65% the Sun’s mass, at a mean separation of 4.55″, or 15 AU. The eccentricity of this binary star’s orbit is about 0.4 and the semimajor aids is 15.8 AU. The periastron of the orbit is about 7.63 AU, the apastron is approximately 17.35 AU. Procyon A has about 6X the solar luminosity and twice the Sun’s radius (46). From the mass-luminosity relationship, Procyon A has about 1.5X the mass of the Sun (47).

In their classic Planets for Man, Dole and Asimov estimate that a single F5 V star has a 0.0344 probability of possessing a habitable planet. This is only somewhat less than the 0.0545 probability of a Sunlike GOV star possessing a habitable planet (48).

But because F5 stars have a lower main sequence life expectancy than G and K main sequence stars and the proximity of its nearby white dwarf companion, analysts generally conclude that Procyon A is unlikely to possess life-bearing planets. Recently however, Whitmire et al. have reconsidered the likelihood of finding habitable planets in binary star systems (49).

Using Eq. (12) of Whitmire et al.’s analysis, the critical binary semi-major axis beyond which planet formation will not be inhibited at 1 AU from the primary star can be estimated as a function of secondary mass, orbital eccentricity, average planetesimal semimajor axis during planet formation and the critical planetesimal disruption velocity Uc (that value of planetesimal velocity required to disrupt the planet-formation process). Application of this equation reveals that Procyon A could possess a habitable planet if Uc is approximately 1000 m/sec. Whitmire et al. states that, in current planet-formation models, Uc >> 100 m/sec (49).

Therefore, Procyon A must be considered a marginal candidate for being the home star of a migrating interstellar civilization. If such a civilization is present there, the spacecraft considered in Table 1 departs from this 6X solar luminosity star at about 1.6X its velocity when launched from the Sun. At this velocity of 0.0056c, the craft could travel one light year in less than two centuries and reach the Sun in less than 2000 years.

(B) Beta Hydri

Dole and Asimov estimated that Beta Hydri, at 21.3 light years from the Sun, has a 0.037 probability of possessing a habitable planet (48). According to a recent study by Dravins et al. however, this single star should be classified as a G2 IV star (50).

The nearest subgiant, ? Hydri has an age of approximately 6.7 billion years and is slightly metal-poor. It is approximately 3.3X as luminous as the Sun (50).

The solar sail starship outlined in Table 1 would reach a velocity of about 0.0049c, if launched from ? Hydri. This ship could cover a distance of one light year in slightly more than 200 years, reach our solar system in about 4,400 years.

(C) Pollux

Pollux, also called Beta Geminorum, is 35 light years from the Sun and is classified as a KO III giant. Although not the nearest red giant, Pollux is the nearest giant star that may have spent its main sequence lifetime as an F-dwarf, which may have had habitable planets (51).

Currently, Pollux is about 35X as luminous as the Sun (51). If the starship considered in Table 1 departed Pollux, its interstellar cruise velocity would be about 0.0088c. This craft could traverse 1 light year in less than 115 years, but would require about 4,000 years to reach the Sun.

(D) The Effects of Stellar Motions

If an interstellar civilization is migrating from any of the three candidate stars considered above, it seems very unlikely that colony ships would reach our solar system. However, it must be emphasized that stars are not truly fixed in space; they all follow independent paths around the center of our Milky Way Galaxy.

Sometimes stars approach the Sun a lot closer than Proxima/Alpha Centauri at 4.3 light years. According to the results of a NASA JPL study of the Voyager space probes interstellar trajectories led by R. J. Cesarone, stars approach the Sun within distances of about 2 light years at intervals of approximately 100,000 years. Every million years or so, a star will approach the Sun within 1 light year (52).

Appendix 3 of The Starflight Handbook lists the nearest 74 star systems, out to about 21 light years (12). The list includes Procyon and ? Hydri, two candidate home stars for migrating interstellar civilizations. Conservatively, then one can reasonably expect that 1% of the stars in the solar neighborhood can be considered as candidate home stars for interstellar migrants.

During the past billion years, about 10,000 stars have approached the Sun within 2 light years, about 1,000 stars have approached the Sun within 1 light year. It is not unreasonable, therefore, to conclude that 10-100 candidate home stars for interstellar migrants have closely approached our solar system within the past billion years. It is therefore not impossible that space habitats constructed and occupied by interstellar migrants lurk in the depths of our solar system, waiting to be discovered.

(V) Conclusions: A New Approach to SETI

If starships have crossed to our solar system within past eons (perhaps accelerated by solar sails and decelerated by a combination of magnetic reflection of interstellar ions and solar sails), they may have created a myriad of artificial worldlets from asteroidal and cometary material. We may live within a “Dyson Sphere” (53) of millions of space habitats, each masquerading as a small comet nucleus or asteroid.

In the absence of directed radio transmissions from the extraterrestrials to the Earth, detection will be challenging, but not impossible. One method of detection might be a search for excess infrared emissions from asteroidal objects (5).

Consider the case of a 1 km radius spherical worldlet located 2 AU from the Sun. A system of mirrors is utilized to reflect sunlight into the worldlet’s interior, so that it’s internal temperature is 300 K (degrees Kelvin), a comfortable room temperature for humans. Such mirror systems have been proposed to supply solar energy for human space habitat designs (54).

To model reflected and emitted radiation from this object, the reflectivity (?) is assumed to be spectrally flat in the 0.3- 0.8 ? spectral range, where most of the Sun’s radiant flux is found. For the sake of this modeling exercise, it is assumed that ? = 0.1. If the habitat’s walls are fully opaque, the emissivity (?) is equal to 1-?, or 0.9 (55).

Located near Earth at 1 AU from the Sun, a flat plate oriented normal to the Sun receives about 1,400 watt/m2 of sunlight (47). At the 2 AU location of our hypothetical artificial worldlet, the solar radiant flux is reduced to about 350 watt/m2.

The total solar radiant power reflected (Sref) from the worldlet is equal to the product of its reflectivity, cross-sectional area, and the incident solar flux:

watts, (2)

where rad = space habitat radius, in meters. For a 1000 meter spherical habitat at 2 AU from the Sun with a reflectivity of 0.1, about 108 watts of solar radiation are reflected into space.

Applying blackbody radiation theory (55), the radiant power of infrared reradiated by the spherical space habitat (Srad) can be written:

watts, (3)

where T is the habitat’s absolute radiation temperature (here 300 K) and ? is the Stefan-Boltzmann constant (5.67 x 10-8 watt/m2K4). About 5 x 109 watts of infrared radiation will be radiated by this hypothetical space habitat, which is roughly 50X the radiant power of the reflected sunlight.

The size of the mirror necessary to reflect the required sunlight into the habitat can be estimated as follows. The amount of sunlight absorbed by the (fully opaque) habitat walls is 9X the amount reflected, or about 109 watts. To preserve the 300 K internal habitat temperature, about 4 x 109 watts of sunlight must be continuously supplied by the mirror. Since the solar radiant flux 2 AU from the Sun is about 350 watt/m2, the approximate mirror area required is about 107 square meters. If the mirror is disc shaped, its radius is about 2 km.

Mirror size of course depends upon worldlet location within the solar system. Smaller mirrors are required for habitats within the Near-Earth-Object (NEO) population, more distant Kuiper-Belt or inner-Oort-Belt worldlets require larger mirrors. It seems unlikely that we would confirm or detect an artificial worldlet’s existence by stray sunlight reflected in the Earth’s direction by the mirror since all or most of the light striking the mirror is directed into the habitat’s interior, probably through a system of windows (54).

Comparison of reflected solar spectral irradiance and reradiated infrared spectral irradiance (both in units of watts/m2-? ) has been accomplished using a published curve of the Sun’s spectral irradiance (56), corrected for the 2 AU habitat distance and a General Electric (GEN-15-C) Radiation Calculator (55). The peak spectral irradiance of the reflected flux is concentrated in near-visible waveband, mostly between 0.3 and 0.8 ?. Most of the infrared flux is concentrated in the 5 – 15 ? waveband.

Interestingly, the peak of both spectral irradiance curves is similar, in the neighborhood of 30 – 50 watts/m2-? . At 2.2 ? in the near infrared, the reflected solar spectral irradiance is about 1000X greater than the reradiated infrared spectral irradiance curve.

The situation would be different, however, if we moved our hypothetical alien space habitat from 2 AU to a position in the Kuiper Belt. If it were located at 50 AU, for example, the infrared flux would be the same, although a mirror radius of about 50 km would be required. But the reflected flux would be decreased by a factor of almost 1000X, so the two fluxes would be about equal at 2.2 microns. Interestingly, a recent photometric study of Kuiper Belt objects has revealed that some of them have emit more energy in the red/near-infrared spectral bands than expected (57). The reason for this color effect is unknown.

(A) A New SETI Search Strategy

Here, then, is the outline of a proposed research program to search for advanced extraterrestrials within the solar system:

  1. Small College and advanced amateur telescopes in the approximate aperture range 0.5 – 1 meters should be enlisted to search for NEOs that seem unnaturally smooth. Large terrestrial observatory instruments should applied in the same manner for Kuiper Belt objects. Such smooth objects would show less rotational brightness variation than objects of asteroidal or cometary origin. Such studies of NEOs and Kuiper Belt objects could be conducted in concurrence with present and future observational programs to inventory those objects that might pose a future collision threat to Earth.
  2. Modelers and theoreticians could devote themselves to accurate calculations of reflected and reradiated radiant fluxes and spectral irradiances for the many suggested shapes of space habitats – spheres, cylinders, toroids, etc. (54) – at a variety of distances from the Sun. This will allow determination of the optimum spectral wavebands to utilize in a search for artificial alien worldlets in the solar system.
  3. High altitude or space telescopes operating in the infrared wavebands selected should be used to check the selected smooth appearing objects for the predicted infrared excess radiation.
  4. A concomitant study among diplomats and international lawyers could determine an appropriate protocol for direct contact (6). If we locate nearby extraterrestrials, should we visit them, should they visit us, should a contact be by remote means only, or should we meet at a neutral location? We also need to determine (in the case of a direct face-to-face meeting) who should speak for humankind and how to respond if the aliens remain noncommunicative or display signs of hostility. Science fiction writers could contribute to this aspect of the study by developing multiple scenarios for the evolutionary paths of technologically advanced civilizations.
  5. A final step in the research project would be to determine whether tailored radar pulses (from Arecibo perhaps) could serve the dual purpose of mapping a suspicious body and initiating contact with the object’s suspected occupants.

We see that very conservative considerations of interstellar travel techniques demonstrate that extraterrestrials circling sub-giant or giant stars could relocate to neighboring stellar systems on trips requiring only a few centuries. As many as 100 such stars may have approached the Sun within 1 – 2 light years within the last billion years.

If technologically advanced extraterrestrial beings from these star systems have migrated to our solar system, they may have chosen to remain uncommunicative and may be living in space habitats within the Kuiper Belt or regions closer to the Earth. We should not be deterred by negative results to date in the search for Dyson Spheres circling nearby stars, since observational sensitivity limits such searches to heavily populated Dyson spheres that convert about 1% of the central star’s energy output to infrared “waste heat” (58).

It is up to the present generation of astronomers to begin the survey of small solar system objects to determine if any of them seem to be artificial. True, we may have to observe many thousands of asteroids or comets to find a likely candidate. But “conventional” SETI searchers are used to laborious searches since they must investigate hundreds or thousands of stars in the hope of finding one communicative civilization. Perhaps a broadened search strategy will enlarge the likelihood of a successful detection.

The research project proposed could utilize the talents of graduate students and scholars in a wide range of disciplines. The cost need not be outrageous and the results may change humanity’s conception of the universe and the role of technologically advanced conscious life.

References and Notes

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  33. G. L. Matloff and E. F. Mallove, ‘Solar Sail Starships: Clipper Ships of the Galaxy’, JBIS, 34, 371-380 (1981).
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  36. G. L. Matloff, ‘An Approximate Heterochromatic Perforated Light-Sail Theory’, IAA-95-IAA.4.1.01.
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  58. J. Jugaku and S. Nishimura, ‘A Search for Dyson Spheres Around Late-Type Stars in the Solar Neighborhood II’, in Astronomical and Biochemical Origins and the Search for Life in the Universe, eds. C. B. Cosmovici, S. Boyer, and D. Werthimer, Editrice Compositori, Bologna, Italy, 1997, pp. 707-709.


Habitable or Earth-Like?

A useful distinction, lost in the current breathless prose of the science media, is between Habitable and Earth-like planets. Back in the early 1960s Stephen Dole defined Habitable planets as having Earth-like atmospheres, gravities and at least 10% of their surface lying in a temperature-range suitable for human habitation. Since the 1980s, in the work of Martyn Fogg and his successors, there has been a broadening of the term to mean the less restrictive “able to sustain liquid water on its surface”. This has been the “official” meaning of “habitable” ever since Kasting, Whitmire and Reynolds, Habitable Zones Around Main Sequence Stars (1993) set the parameters of the modern debate. Thus a planet with a thick, unbreathable mostly CO2 atmosphere is habitable because it can keep liquid water on its surface, but it is emphatically not Earth-like.

However, truly Earth-like planets are still being studied – planets with Earth gravity and low-CO2 atmosphere. So what happens if the relative amount of water is varied? Desert planets, or for “Dune” fans, Arrakis-like planets, with low amounts of water, actually have wider habitable zones than water-planets, like Caladan. According to work by Yutaka Abe & Kevin Zahnle, Arrakis-like planets are able to retain liquid water – in the form of lakes near the poles – to much higher insolation levels than planets covered by water. Here’s their paper: Habitable Zone Limits for Dry Planets. Venus might have gone through a Dry Planet phase as recently as 1 Gya, before the greehouse runaway finally took hold. The critical limit is an insolation of 170% of what Earth currently experiences, implying desert surfaces above boiling point. The habitable zone will be clustered around the poles, depending on the axial tilt and season variations.

Another variable in planetary conditions is the eccentricity, e, of the orbit. The point of closest approach to a star, the periastron, is at a distance of a*(1-e) where a is the mean orbital distance. As insolation increases with the inverse square of the radius, the maximum insolation experienced is 1/(1-e)2 times the average. Contrariwise, the maximum distance (apoastron) is at a*(1+e) and thus the relative insolation drops by 1/(1+e)2. Thus the ratio of maximum to minimum insolation is ((1+e)/(1-e))2, which means an e~0.5 implies the periastron sunlight is 9 times stronger than apoastron sunlight. Yet according to the climate modelling work of Darren Williams, Earth-Like Worlds on Eccentric Orbits: Excursions Beyond the Habitable Zone, habitable regions can be found on planets with eccentricities of up to 0.7 – an insolation ratio over 32!

Another variable modeled in Williams’ work, and others since, is the effect of varying the axial tilt of a planet’s spin-axis, its obliquity. Earth’s present day tilt is 23.5 degrees, around which it oscillates on a multimillennial time-scale by a couple of degrees. In principle greater obliquities are perfectly possible – the planet Uranus has a tilt of more than 90 degrees, for example – with potentially dramatic climatic effects. While seasons on such worlds might be of an intensity we don’t experience on Earth, they remain essentially habitable worlds. The imagination boggles trying to imagine how life might adapt to scorching endless Summers followed by Sygian, glacial Winters, but Life finds a way. Combining eccentricity and obliquity at un-Earthly extremes makes for an exotic climate, famously used to best advantage by Hal Clement when he designed Mesklin, in “A Mission of Gravity”, a world covered in methane seas, except during periastron when the seas moved away from the liquid ammonia level of heat in the opposite hemisphere’s summer.

From deep-diving experience we know that humans, and other mammals (pigs are regular experimental subjects), can breathe mixtures of hydrogen/helium and oxygen. If we accept this rather exotic possibility then we should mention the Hydrogen Greenhouse planet, which Raymond Pierrehumbert and Eric Gaidos have studied. In their 2011 paper, Hydrogen Greenhouse Planets Beyond the Habitable Zone, they make the case for photosynthetic life being viable out to 10 AU or so thanks to a H/He rich atmosphere keeping the planet warm. With such broad habitable zones (from 2 AU to 10 AU in the case of the Sun) then such planets might prove more abundant than traditional “Earth-like” planets.

Thus, while Earth-like might seem a restrictive description, there could be quite a diversity of Earth-like worlds throughout the Galaxy.