Centauri Dreams
Imagining and Planning Interstellar Exploration
The Star as Starship
Moving entire stars rather than building spaceships would have certain benefits as a way of traveling through the galaxy. After all, it would mean taking your local environment with you on a millennial journey. Some have suggested it might therefore be an observable sign of highly advanced civilizations at work. But how would you move a star in the first place?
In Bowl of Heaven (Tor, 2012), Gregory Benford and Larry Niven conceive of a vast bowl — think of one-half of a Dyson sphere — wrapped around a star whose energies are directed into a propulsive plasma jet that, over aeons, moves the structure forward. Thus this snippet of dialogue, said aboard a starship by the humans who discover the alien artifact:
“…You caught how the jet bulges out near the star.”
More hand waving. “Looks to me like the magnetic fields in it are getting control, slimming it down into a slowly expanding straw…”
“A wok with a neon jet shooting out the back…and living room on the inside, more territory than you could get on the planets of a thousand solar systems. Pinned to it with centrifugal grav…”
“They don’t live on the whole bowl. Just the rim. Most of it is just mirrors. Even so, it’s more than a habitat,” said Cliff. “It’s accelerating. That jet? This whole thing is going somewhere. A ship that is a star. A ship star…”
The Benford/Niven excursion into mega-engineering came to mind over the weekend when I read Keith Cooper’s recent article for the Institute for Interstellar Studies on the ideas of Fritz Zwicky. The wildly creative astrophysicist (1898-1974) once imagined a scheme that would use the Sun as an engine that could propel us — and I do mean all of us — to Alpha Centauri. The notion was to induce ‘hot spots’ in the solar photosphere that would lead to asymmetrical flares, nudging the Sun in a new direction. Zwicky imagined that the recoil of these directed exhaust jets would make an interstellar crossing in about fifty centuries possible, pulling the Earth along with our parent star.
In a lecture in 1948, Zwicky referred to ideas like these as ‘morphological astronomy,’ which he would go on to discuss in detail in his 1969 book Discovery, Invention, Research Through the Morphological Approach, a title that ranges over everything from telescope design to aerodynamics and the concept of justice. But as Cooper notes, a number of questions are left unanswered, including details of the asymmetric thrust mechanism itself, and the always interesting question of deceleration. Just how does the Sun approach Alpha Centauri, and what effects would the move have on solar planets as well as those around the destination stars?
If a highly advanced civilization did have the ability to engineer stellar acceleration, we might spot its efforts through unusually high proper motions of particular stars. So-called ‘hypervelocity stars’ have been observed that appear to be gravitationally unbound to the Milky Way. In fact, Kelly Holley-Bockelmann and Lauren Palladino (Vanderbilt University) have identified 675 stars that were probably ejected from the galactic core, presumably because of gravitational interactions with the supermassive black hole at galactic center. Moving at velocities as high as 900 kilometers per second, stars like these would take 10 million years to travel from the core to the outer edge of the spiral. The stars in question tend to be red giants with high metallicity. Later work at Ohio State has identified a small number of hypervelocity stars with masses closer to that of the Sun.
Image: Hypervelocity stars zoom around the center of the Milky Way, where a supermassive black hole lurks. Credit: ESO/MPE.
Cooper asks whether stars with anomalous proper motion might not be worth investigating in a SETI context. From his essay:
Perhaps some advanced extraterrestrial civilisation out there has that power. Maybe we should look to stars with anomalously large proper motions. Is Barnard’s Star, which has the highest proper motion of any star in the sky at 10.3 arcseconds per year, at its distance of 5.98 light years, simply a refugee ejected from a binary star system or is it being deliberately driven? Astronomers even observed a giant flare on the star in 1998, which reached temperatures as high as 8,000 degrees Celsius, which is 2,500 degrees Celsius hotter than the Sun’s surface, or photosphere. Given that Barnard’s Star is a red dwarf with an average surface temperature typically languishing at 2,860 degrees Celsius, that’s a heck of an increase. Now, I’m not suggesting that extraterrestrials are using Barnard’s Star as a spacecraft, only that should such a feat be possible (and that’s a big if), we might expect it to look something akin to the speeding red dwarf.
The idea of moving entire stars as a means of interstellar travel is intriguing and might fall into our toolbox of ideas on ‘interstellar archaeology,’ the search for unusual artifacts in our astronomical data. After all, moving a star simply ramps up an already existing process. We’re all on a grand tour through the Milky Way as the Sun moves at a brisk 220 kilometers per second in its orbit. An advanced civilization with clearly defined destinations in mind might find random encounters with other stars less interesting than targeted travel.
The paper on hypervelocity stars is Palladino et al., “Identifying High Metallicity M Giants at Intragroup Distances with SDSS,” The Astronomical Journal Vol. 143, No. 6 (May, 2012), p. 128 (abstract / preprint).
Leafing Through Early Interstellar Ideas
Although John Jacob Astor IV did many things in his life — as a businessman, builder, Spanish American War veteran and financier — his place in history was secured with his death on the Titanic in 1912. His was actually one of the three hundred or so bodies that were later recovered out of over 1500 who died, and he is buried in Trinity Church Cemetery in New York City. Less well known is the fact that Astor was a writer who, in 1894, produced a science fiction novel called A Journey to Other Worlds in which people travel to the outer planets.
I’ve been digging around in this curious novel and discovered an interstellar reference that was entirely new to me. Using a form of repulsive energy called ‘apergy,’ variants of which were much in vogue in the scientific romances of this era (think H. G. Wells’ The First Men in the Moon, which uses gravity-negating ‘cavorite’), Astor’s crew sets out on the Callisto to explore Jupiter and Saturn, where various adventures ensue. Along the way, his travelers see Mars’ moons Deimos and Phobos and begin to speculate:
“Either of those,” said Bearwarden, looking back at the little satellites, “would be a nice yacht for a man to explore space on. He would also, of course, need a sun to warm him, if he wished to go beyond this system, but that would not have to be a large affair–in fact, it might be smaller than the planet, and could revolve about it like a moon.”
Image: John Jacob Astor (1864-1912) in August of 1909. Credit: Wikimedia Commons.
Bearwarden’s companion Cortlandt thinks this over and decides an object as small as Deimos couldn’t maintain an atmosphere. Better, then, to create a small sun and travel along with it in a spacecraft rather than a moon or asteroid:
“It would be better, therefore, to have such a sun as you describe and accompany it in a yacht or private car like this, well stocked with oxygen and provisions. When passing through meteoric swarms or masses of solid matter, collision with which is the most serious risk we run, the car could follow behind its sun instead of revolving around it, and be kept from falling into it by partially reversing the attraction. As the gravitation of so small a sun would be slight, counteracting it for even a considerable time would take but little from the batteries.”
Conspicuously left out of the discussion is how such a small ‘sun’ would be powered, but we can forgive Astor for not anticipating Hans Bethe’s 1939 work on how fusion powers stars. Instead, he has a third character muse about creating a violent collision between two asteroids that would cause the new-made object to become luminous, which pleases an enraptured Bearwarden: “Bravo!” said Bearwarden. “There is no limit to what can be done. The idea of our present trip would have seemed more chimerical to people a hundred years ago than this new scheme appears now.”
Science fiction author and critic Richard Lupoff has speculated that Astor drew ‘apergy’ from Percy Greg’s 1880 novel Across the Zodiac, in which an anonymous narrator uses a kind of anti-gravity to travel to Mars aboard a spectacularly large spacecraft. In any case, Astor’s novel is fun to dip into in places, offering in addition to space travel a look at the world of 2000, a time in which the great project on Earth is to shift the planet’s axis, a job undertaken by the Terrestrial Axis Straightening Company to eliminate the extremes of Earth’s climate.
An Early Look at Suspended Animation
Lupoff’s book Master of Adventure is the source of his thinking on Astor. It’s also a delightful read for anyone interested in the life and works of Edgar Rice Burroughs, who came startlingly to mind after I wrote last Monday’s entry on Carl Sagan and Iosif S. Shklovskii (see Two Ways to the Stars). The duo had speculated in their book Intelligent Life in the Universe that high pressures and controlled temperatures could be the key to freezing humans for long transit times, taking advantage of ice II, which has about the same density as liquid water, as opposed to normal ice, which could disrupt human cells in the freezing and thawing process.
I suddenly recalled an old Burroughs story called “The Resurrection of Jimber Jaw,” which ran in Argosy in 1937 — it was later reprinted by Lupoff himself for Canaveral Press in Tales of Three Planets. Burroughs’ characters, forced to land their plane in Siberia after mechanical trouble, find the frozen body of a caveman who is eventually returned to life. His adventures in America — he becomes a professional wrestler! — lead to an unhappy romance and he winds up re-freezing himself in a meat locker, asking not to be re-awakened. But before then, he gets off a number of comments about modern life that would not sit well with today’s sensibilities.
Various forms of freezing and suspended animation populate early science fiction going back to 18th Century romances like L. S. Mercier’s Memoirs of the Year Two Thousand Five Hundred and moving forward to H. G. Wells’ When the Sleeper Wakes (1899). Robert Heinlein would use suspended animation to great effect in one of my favorite of his novels, The Door Into Summer (1957), but magazine science fiction is packed with stories using the trope. There is Laurence Manning’s “The Man Who Awoke” (Wonder Stories 1933) and A. E. van Vogt’s classic “Far Centaurus” (Astounding, 1944), where the protagonists survive a long voyage only to learn that faster than light travel has been invented while they were enroute.
Recently Adam Crowl looked back to rocket pioneer Robert Goddard’s thoughts on suspended animation. In a short note titled “The Ultimate Migration,” written in 1918, Goddard had seen two ways for humans to reach the stars, the first being the use of atomic energy to accelerate an asteroid that had been hollowed out to serve as a spaceship. But if this didn’t work, tinkering with human cells might do the trick:
…will it be possible to reduce the protoplasm in the human body to the granular state, so that it can withstand the intense cold of interstellar space? It would probably be necessary to dessicate the body, more or less, before this state could be produced. Awakening may have to be done very slowly. It might be necessary to have people evolve, through a number of generations, for this purpose.
Goddard goes on to speculate about an immense journey in which the pilot is ‘animated’ once every 10,000 years, or at even longer intervals for longer journeys, so he can correct the spacecraft’s course. He also addresses the issue of how to build a clock that would survive such long time-frames and control the re-awakening of the pilot. The question of long-lived time-pieces is, of course, one that’s under intense investigation at the Long Now Foundation, which is engaged in the process of building a 10,000 Year Clock in the mountains of west Texas, a project conceived by Danny Hillis in which Amazon’s Jeff Bezos has already invested some $42 million.
Image: Robert Goddard (1882-1945). Credit: Wikimedia Commons.
Even 10,000 years might not seem long-term compared to some of the journeys Goddard contemplated. And what if we aren’t the only intelligence that sets about creating such missions? Adam Crowl closes his post with this intriguing thought:
…the idea of flying between the stars as mummified cryogenic life-forms has a strange allure. To travel the stars so, we would needs become like human-sized ‘tardigrades’ or ‘brine-shrimp’, both of which can undergo reversible cryptobiosis in a mostly dessicated state. Even if we can’t do so (reversibly – it’s not too difficult to make it permanent), might there not be intelligences “Out There” who have done so? What if we found one of their slow sail-ships? Would it seem like a funerary barge, filled with strange freeze-dried corpses?
There’s fodder here for more than a few science fiction stories, the creation of which is an impulse that runs through the entirety of our post-Enlightenment encounter with technology. We plug in the science of our own time in making the attempt to sketch out a future, knowing all too well that we’re only guessing at the discoveries that could change all our assumptions. That’s why I love the rich history of science fiction. It’s a genre that lets us try ideas on for size and work them through to their consequences, all the while reminding us of how much we have to learn.
Ancient Brown Dwarfs Discovered
How many brown dwarfs should we expect in the Milky Way? I can recall estimates that there could be as many brown dwarfs as main sequence stars back when people started speculating about this, but we have to go by the data, and what we have so far tells another tale. The WISE (Wide-field Infrared Survey Explorer) mission can only come up with one brown dwarf for every six stars, leading Davy Kirkpatrick (Caltech), who is part of the WISE science team, to say “Now that we’re finally seeing the solar neighborhood with keener, infrared vision, the little guys aren’t as prevalent as we once thought” (see Brown Dwarfs Sparser than Expected).
Image: Brown dwarfs in relation to the Sun and planets. Credit: NASA/WISE mission.
This is true, at least, in the Sun’s vicinity, where WISE identifies about 200 brown dwarfs, with 33 measured within 26 light years. In the latter volume, some 211 other stars can be found. If we extrapolated this to the entire galaxy, we would get about 33 billion brown dwarfs, assuming a galactic population of 200 billion stars, but extrapolating from our own backyard may be highly unreliable. We just need to keep accumulating data to get an accurate read on these ‘failed stars,’ which are too cool to ignite hydrogen burning in their cores. Extremely low temperature Y-dwarfs are hard to spot, and it’s possible a few more will be teased out in WISE data.
I try to keep up with brown dwarf studies in the probably vain hope that we might find a Y-dwarf close enough to serve as a target for a future probe — by ‘close,’ I mean still undetected and within a few light years, though the hopes for such a find seem remote. Meanwhile, new work out of the University of Hertfordshire has uncovered two of the oldest brown dwarfs yet observed, thought to go back to the early days of the galaxy some ten billion years ago. Old brown dwarfs really up the ante for detection, for since they cannot ignite internal fusion, they fade with time. The new brown dwarfs have temperatures of 250-600 degrees Celsius. You can contrast that with the surface temperature of the Sun, about 5500 degrees Celsius.
A team working under David Pinfield found the objects in the Pisces and Hydra constellations using WISE data, with additional measurements from the Magellan, Gemini, VISTA and UKIRT instruments on the ground. WISE 0013+0634 and WISE 0833+0052 are moving at speeds of between 100 and 200 kilometers per second, a good deal faster than normal stars, and a marker for their age, which is also flagged by ancient atmospheres made up almost entirely of hydrogen.
This Royal Astronomical Society news release goes on to speculate about the implications of the new work for brown dwarf proliferation. Almost all local stars — about 97 percent — are members of the galactic ‘thin disk,’ a grouping much younger than the ‘thick disk’ in which stars move up and down in relation to galactic center at higher velocities. With only 3 percent of stars in our local volume being from the ‘thick disk’ or the ‘halo’ containing remnants of the earliest stars, it’s no surprise that these are the first brown dwarfs we’ve found from that population.
Image: A brown dwarf from the thick-disk or halo is shown. Although astronomers observe these objects as they pass near to the solar system, they spend much of their time away from the busiest part of the Galaxy. The Milky Way’s disk can be seen in the background. Credit: John Pinfield.
Given that the thick disk and halo occupy much larger volumes than the thin disk, finding a small number of brown dwarfs in the local thick disk/halo population implies a high number of brown dwarfs in the galaxy. Says Pinfield: “These two brown dwarfs may be the tip of an iceberg and are an intriguing piece of astronomical archaeology.” True enough, but we’re still working the numbers as we try to find these faint objects against a background of infrared sources ranging from distant galaxies to clouds of gas and dust. More data, and more insight, lie ahead.
The paper is Pinfield et al., “A deep WISE search for very late type objects and the discovery of two halo/thick-disk T dwarfs: WISE 0013+0634 and WISE 0833+0052,” in press at Monthly Notices of the Royal Astronomical Society (abstract / preprint).
Hacking Humanity to the Stars: The DIY Space Program
Among the numerous groups now emerging with an eye toward space exploration, SpaceGAMBIT has been the one I knew the least about. It was a pleasure, then, to hear from Alex Cureton-Griffiths, who is UK Project Lead for SpaceGAMBIT. Alex was more than happy to offer this description of SpaceGAMBIT and its plans for the future. He tells me he spends his time traveling and talking to people about our future in space and how they can get involved. In his spare time he “hunts for good coffee and hacks on steam-powered satellite thrusters for fun.” Alex, I buy coffee green and roast my own beans. If you’re a coffee guy, we need to talk, man.
by Alex Cureton-Griffiths
When I mention to people that I’m trying to help build humanity into a spacefaring species, I usually get the same reaction: ” I don’t believe it – that’s so cool!”
I’m with them all the way on the second part of that reaction – it really is amazingly cool. It’s the first part that gets me – the “I don’t believe it” part. For so many people, getting to the stars is literally unbelievable. Because it’s assumed to be too difficult, too far off, or too big for individuals to make a difference. People watch movies like Star Trek and imagine we need to have shiny starships to get there, without thinking about the greasemonkeys who build the technology to get us to starships in the first place.
My organisation, SpaceGAMBIT, is a group of those greasemonkeys, and we aim to prove the doubters wrong by getting humanity hacking on hands-on, practical projects that will help us one day live amongst the stars. We’re not alone – we’ve got half a million dollars in funding from the US government to do this, and we’re working with many organisations to Make Things Happen.
How We Make a Difference
Now, it’s no good hammering away on a starship chassis if we don’t even have warp drives yet. Or for that matter, if we don’t even know that warp drives are a possible or optimal technology. Since we’re all about hands-on, just putting together plans and models for interstellar settlement doesn’t really float our boat. Instead, we focus our projects on:
1. Building near-term technologies that will get people thinking about space and settling the stars. Things like a partial space suit and 3D printed planetary rover components
2. Educating and inspiring the public about the need for getting to space and how they can get involved now. These include a range of space badges for Hacker Scouts and workshops for building Cubesats
3. Off-world habitat technologies which also have applications here on earth in disaster relief, developing countries and sustainability. Includes bioreactors and a prototype closed-system underwater habitat
Why focus on the here and now? If the public can’t see a near-term use for space technologies, they go back into “Star Trek” mode. The thing that gets people’s blood pumping is seeing it’s a “real thing” and that they can make a difference.
All of our projects are brought to life by the maker movement through hackerspaces, makerspaces and other community spaces. Basically, an ad hoc worldwide network of passionate, hands-on tinkerers with the tools, community and mindset to Get Stuff Done. Makers aren’t limited by the norms of the engineering industry, so they’re more likely to get something bodged together quickly and rapidly iterate to make it work. And since many don’t have a formal engineering background there’s a lot of out-of-the-box thinking, resulting in off-the-wall designs that might just work anyway.
There are now thousands of such community spaces around the world, including in formerly-wartorn countries like Iraq and Afghanistan, and even NASA is getting in on the act, opening their own makerspace at Ames. And if there’s no space near you, there’s absolutely nothing to stop you getting together with a group of friends and building your own!
Another key point is that all of our projects are open-source, open-hardware, open-documentation, open-everything. Anyone is free to take the projects and replicate, re-use, build on or modify them as they wish. We believe that if we’re to make it as a spacefaring civilization we’ve got to share what we know, not lock it up.
Get Involved
The fantastic thing about the maker community is that anyone can get involved. Hackerspaces and makerspaces are open for anyone to join for a small fee, and no qualifications or background are needed. Whether you’re an engineer, an artist, a kid, or whoever else, you’re free to roll up and start hacking together your own projects. As UK Project Lead for SpaceGAMBIT, I’m a walking, talking case-in-point. I have no formal engineering or space background. In fact, I flunked high school maths and majored in Chinese at university. And here I am, running a space program. If I can get involved, anyone can.
Don’t worry about coding a killer app, knowing how to wire a plug or building a 3D printer from scratch – these spaces are where you can learn from each other, and learning means failing now and again (and celebrating that failure). I was completely clueless on my first day, but over time (and many wonderful failures) I’ve learnt from the other members and developed my skills. Here’s how:
1. Find your local hackerspace. Many big cities have several.
2. Most hackerspaces have an open night or social night. Try to attend one of these to see what it’s really like. Each one has its own vibe, and words can struggle to do them justice!
3. Get involved with a cool project, or just start your own. Instructables has plenty of space projects to get you started off, and we promote many more projects in development through SpaceGAMBIT.org. When it comes to starting your own, you don’t need to ask. Just bring stuff and get working!
4. As soon as you’ve got a project going, you may need funding to continue it: SpaceGAMBIT offers funding for certain types of projects, and there’s always Kickstarter or Indiegogo, both of which have successfully funded space projects.
5. If you have kids (or if you’re a kid yourself), there are lots of great programs out there to get started. Curiosity Hacked (formerly Hacker Scouts) are based in the USA, and Code Club offer after school coding classes around the world.
What’s Next for SpaceGAMBIT?
We’re now working with NASA on the asteroid grand challenge, because we can’t become a starfaring species if we’re wiped out by death rocks from outer space. In a few months, we’ll launch a new round of project solicitation, focusing on how to spot dangerous near-earth-objects and figuring out how to divert them. We’ll also run a couple of competitions to get more people scouring the skies, with the aim of making telescopes cheaper and easier to use.
Like last year, we plan to continue funding projects focused on space and maker education, habitat technologies, and small satellite technologies. For projects we fund, we emphasise rapid iteration and getting things done on a budget. All projects have a funding limit of $20,000, should take 3-4 months to complete and be totally open-source, open hardware and open everything. For now you can check our previous project submission announcement for more information, and get in touch if you have a project you’d like to submit!
Space Weathering: The Mars Connection
I don’t usually have much to say about Mars, for this site’s focus is on deep space — the outer Solar System and beyond. But with both the Mangalyaan and MAVEN Mars missions in progress, I’ll take this opportunity to mention new work out of MIT that deals with the effect of Mars on asteroids. The topic is ‘space weathering,’ the result of impacts from high energy particles and more. Richard Binzel and colleague Francesca DeMeo have been looking at disruption to asteroid surfaces, finding that close planetary encounters can explain an unusual fact: The surfaces of most asteroids appear redder than the remnants of asteroids that have crashed as meteorites to Earth.
Back in 2010, Binzel established what he sees as the basic mechanism. Main belt asteroids, orbiting between Mars and Jupiter, are exposed to cosmic radiation that changes the chemical nature of their surfaces. But take an asteroid out of the main belt and give it a close pass by the Earth and ‘asteroid quakes’ will occur, moving surface grains about and exposing fresh surfaces underneath because of the gravitational disruption. Binzel calls these ‘refreshed’ asteroids, and argues that when asteroids of this kind get too close to Earth, they break apart and fall to the surface as meteorites.
How Mars fits into this picture is a bit more of a stretch. At one-tenth the mass of Earth and only one-third its size, Mars seems unlikely to be considered a major gravitational disruptor. But placement is important, for Mars is closely situated to the main belt, which makes asteroid encounters that much more likely to happen. To study the effect, Binzel and DeMeo have tracked asteroids in the database maintained by the International Astronomical Union’s Minor Planet Center, which currently holds data on 300,000 asteroids and their orbits. The researchers’ new paper in Icarus maps orbital intersections between asteroids, Earth and Mars.
Image: Mighty Mars? It’s a small world, but its effect on asteroids passing by it is only now being understood. Credit: NASA.
The duo chose 64 asteroids and calculated the probabilities over the past half million years of close encounters that could have stirred up the asteroid surfaces. The paper focuses on a class of asteroids called Q-type, found primarily among Near-Earth Objects and matching ordinary chondrites spectroscopically over visible to near-infrared wavelengths. Because they are so similar to meteorites, they are assumed to have gone through weathering of the surface regolith, meaning older reddish grains have been churned and replaced. From the preprint:
Ten percent of the Q-types in our sample have not experienced Earth encounter on recent timescales. Thus, the orbited distribution of Q-types suggests Earth encounter is not the only resurfacing mechanism that counteracts the effects of space weathering. These non-Earth encountering objects do cross the orbit of Mars and show low Mars-MOID [Minimum Orbit Intersection Distance] values. We conclude that Mars is likely to play an important role in refreshing NEO surfaces due to its large mass and frequent asteroid encounters.
Two other mechanisms for refreshing an asteroid surface are considered, one being collisions between asteroids in the main belt, the other growing out of the results of the YORP [Yarkovsky, O’Keefe, Radzievskii, Paddack] effect, by which asteroids can be ‘spun up’ by photons streaming outward from the Sun. Binzel and DeMeo’s work found no conclusive evidence that either of these would play a significant role in refreshing asteroid surfaces, although the paper suggests further observations of small main belt asteroids to measure their effect.
So we’re learning more about asteroids even as we discover oddities like P/2013 P5, the unusual object that sprouts six comet-like tails [see What a Strange Asteroid Can Tell Us]. How both their composition and history define their characteristics is going to be an essential study for future efforts to reach and mine asteroids. This MIT news release offers more, including this comment from Vishnu Reddy (Planetary Science Institute), who was not involved in the research:
“On each of the asteroids we have visited so far, every one of them has shown a different kind of space weathering. So it appears that not only is composition an important factor, but also the location of the asteroid with respect to the Sun.”
The paper is DeMeo, “Mars Encounters cause fresh surfaces on some near-Earth asteroids,” Icarus Vol. 227 (1 January 2014), pp. 112-122 (abstract). See also Binzel et al., “Earth encounters as the origin of fresh surfaces on near-Earth asteroids,” Nature 463 (21 January 2010), pp. 331-334 (abstract).
Two Ways to the Stars
I often cite Robert Forward’s various statements to the effect that “Travel to the stars is difficult but not impossible.” Forward’s numerous papers drove the point home by examining star travel through the lens of known physics, conceiving of ways that an advanced civilization capable of the engineering could build an interstellar infrastructure. But while Forward was early in this game, so were Iosif S. Shklovskii and Carl Sagan. A Russian astronomer, Shklovskii had written a book’s whose Russian title translates roughly as ‘Universe, Life, Intelligence’ in 1962. Four years later, Sagan would join Shklovskii as co-author and the two would tackle the original book afresh, adding new material that reflected on and expanded the 1962 version’s ideas.
The result was the volume now called Intelligent Life in the Universe. I sometimes recommend books that are essential parts of a deep space library, and this is surely one, significant not only for its historical treatment of starflight but a compelling source of ideas even today. 1966 was early times for the interstellar idea, with the pioneering 1950s papers of Les Shepherd and Eugen Sänger still fresh in the memory; the work of Robert Bussard on ramjets that could move at a high percentage of the speed of light was very much in play. Shklovskii and Sagan thought there were two ways of achieving human interstellar flight, the first of which involved slowing down the human metabolism to allow it to survive long voyages.
Slow Journey, Frozen Time
Automated interstellar probes were still a reach for most scientists in this era, but Shklovskii and Sagan thought that velocities of up to 100,000 kilometers per second — one-third the speed of light — were not beyond reach as our technology advanced. Given that, humans could be frozen for the duration and awakened upon arrival. The challenge was obvious: The density of ice is lower than the density of water, so that freezing a human being, who is composed largely of water, works serious damage to the cells during the freezing and thawing process. Delicate cell structures are disrupted in each case, and anti-freezing chemicals would kill the subject.
Image: The Russian astronomer Iosif S. Shklovskii, whose collaboration with Carl Sagan produced a classic of interstellar studies.
As so often in Intelligent Life in the Universe, Shklovskii and Sagan thought about possibilities that most in the scientific community hadn’t yet considered, particularly Sagan, whose contributions are flagged in the text to make it clear when he is speaking rather than Shklovskii. Sagan had been working with a Swedish biologist named Carl-Göran Hedén, who specialized in microbiology and biotechnology (Hedén would go on to become the founder of the first chair in biotechnology in Sweden, and the first president of the International Organisation for Biotechnology and Bioengineering). Sagan’s conversations with Hedén focused on the difference in density between water and ice, noting that at high pressures different crystal structures and different densities emerge, producing ice that may have applications for human freezing.
Thus a pressure of 3000 atmospheres and a temperature of -40 degrees Celsius turns ordinary ice into ice II, a kind of frozen water that has nearly the same density as the liquid. The conclusion seemed obvious to Sagan, who wrote:
If a human being could be safely brought to and maintained at an ambient pressure of several thousand atmospheres, and then quickly and carefully frozen to very low temperatures, it might be possible to preserve him for long periods of time. This is only one of many possible alternatives. It seems possible that by the time interstellar space vehicles with velocities of 1010 cm sec-1 are available, techniques for long-term preservation of a human crew will also be available.
Sagan thought that with these means, journeys of up to 100,000 years would be possible. A spacecraft moving at 100,000 kilometers per second could reach a star 1000 light years away in 3000 years, with whatever adjustments would be needed for acceleration and deceleration. A trip to the galactic core would take 60,000 years. He went on to say: “If such voyages are to be feasible, the lifetime of our civilization should perhaps exceed the length of the voyage. Otherwise, there will be no one to come home to.” True enough, but it’s hard to imagine coming back to Earth after 60,000 years thinking that the place you returned to would still be ‘home.’
The Relativistic Solution
The second way to make manned interstellar missions happen was, Sagan believed, the use of relativistic spacecraft, which would, because of time dilation, act as a different kind of metabolic inhibitor. This was mind-boggling stuff back in the 1960s, though it followed as a consequence of the by then well established special theory of relativity. Continue to accelerate at 1 g and you reach the nearest stars in a few scant years of ship time. 21 years take you to the galactic center, while 28 years get you all the way to M31, the great galaxy of Andromeda. The ship nudges up ever closer to c, 300,000 kilometers per second, but never reaches it. Poul Anderson explored all this in his wonderful novel Tau Zero, which has blown minds and inspired science careers since it first appeared in Galaxy in 1967.
Shklovskii and Sagan saw such trips as a communications tool. A radio signal would take 2.5 million years to reach Andromeda, and another 2.5 million years would elapse before any possible response. At the time Intelligent Life in the Universe was being written, SETI seemed to solve the intractable propulsion problem by allowing us to ‘explore’ — i.e., to listen — with radio waves that move at the speed of light. Shklovskii and Sagan reversed the paradigm when it came to destinations well beyond the nearest stars. Now it would be actual journeys to these places that allowed a human presence to be known. As Sagan wrote:
…if relativistic interstellar spaceflight were used for such a mission, the crew would arrive at the galaxy in question after perhaps 30 years in transit, able not only to sing the songs of distant Earth, but to provide an opportunity for cosmic discourse with inhabitants of a certainly unique and possibly vanished civilization. Despite the dangers of the passage and the length of the voyage, I have no doubt that qualified crew for such missions could be mustered. Shorter, round-trip journeys to destinations within our Galaxy might prove even more attractive. Not only would the crews voyage to a distant world, but they would return in the distant future of their world, an adventure and a challenge certainly difficult to duplicate.
Surely this passage is the source of the title for Arthur Clarke’s The Songs of Distant Earth, published in 1986, which explores the effects of long-term interstellar flight. Addendum: See Adam Crowl’s comment below — the influence evidently flowed the other way. My mistake.
Given that reaching M31 within the lifetime of a human crew would require a velocity of 0.99999 c, the only solution that fit the bill was Robert Bussard’s interstellar ramjet, which feeds off the interstellar medium to gorge itself with reaction mass, burning a fusion torch aboard a vessel that becomes more efficient the faster it moves. Sagan liked the Bussard concept and thought it violated no physical principles — he even expected it to be achieved in prototype form in no less than a century — but as we’ve seen (see Catalyzed Fusion: Tuning Up the Ramjet), the problems of lighting proton-proton fusion are immense, and so are issues of drag.
Much work would go into demonstrating this in the next few decades, obviously unknown to Shklovskii and Sagan in 1966, but Bussard variants using a catalytic cycle called the CNO bi-cycle (carbon-nitrogen-oxygen) are still intriguing, and as you might imagine, we’re not through with them here on Centauri Dreams. We can take Shklovskii and Sagan as our models. Both had a taste for bold venturing and pushing the limits of possibility, a taste confirmed in their choice of epigram to introduce the book, Pindar’s Six Nemean Ode:
There is one
race of men, one race of gods; both have breath
of life from a single mother. But sundered power
holds us divided, so that the one is nothing, while for the
other the brazen sky is established
their sure citadel forever. Yet we have some likeness in great
intelligence, or strength, to the immortals,
though we know not what the day will bring, what course
after nightfall
destiny has written that we must run to the end.
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