Centauri Dreams
Imagining and Planning Interstellar Exploration
Starship Musings: Warping to the Stars
by Kelvin F.Long
The executive director of the Institute for Interstellar Studies here gives us his thoughts on Star Trek and the designing of starships, with special reference to Enrico Fermi. Kelvin is also Chief Editor for the Journal of the British Interplanetary Society, whose latest conference is coming up. You’ll find a poster for the Philosophy of the Starship conference at the end of this post.
Like many, I have been inspired and thrilled by the stories of Star Trek. The creation of Gene Roddenberry was a wonderful contribution to our society and culture. I recently came across an old book in the shop window of a store and purchased it straight away. The book was titled The Making of Star Trek, The book on how to write for TV!, by Stephen E.Whitfield and Gene Roddenberry. It was published by Ballantine books in 1968 – the same year that the Stanley Kubrick and Arthur C Clarke 2001: A Space Odyssey came out. What with all this and Project Apollo happening, the late 1960s was a time to have witnessed history. Pity I wasn’t born until the early 1970s when the lunar program was winding down. I digress…
In this book, one finds the story of how Roddenberry tried to market his idea for a new type of television science fiction show. It is clear from reading it that Roddenberry was very much concerned for humankind and in the spirit of Clarke’s positive optimism, he was trying to steer us down a different path. In this book we find out many wonderful things about the origins of Star Trek, including that the U.S.S Enterprise was originally called the U.S.S Yorktown and that Captain James T.Kirk was originally Captain Robert T. April. He was described as being “mid-thirties, an unusually strong and colourful personality, the commander of the cruiser”.
The time period that Star Trek was said to be set was sometime between 1995 and 2995, close enough to our times for our continuing cast to be people like us, but far enough into the future for galaxy travel to be fully established. The Starship specifications were given as cruiser class, gross mass 190,000 tons, crew department 203 persons, propulsion drive space warp, range 18 years at light-year velocity, registry Earth United Spaceship. The nature of the mission was galactic exploration and investigation and the mission duration was around 5 years. Reading these words today, we see that what Roddenberry was doing was laying the foundations for many future visions of what starships would be like.
To Craft a Starship
What I found absolutely fascinating about reading this book however, was the process by which Roddenberry and team actually came up with the U.S.S Enterprise design. Roddenberry met with the art department and in the summer of 1964 the design of the starship was finalised. The art directors included Pato Guzman and Matt Jefferies. Roddenberry’s instructions to the team on how to design the U.S.S Enterprise were clear:
“We’re a hundred and fifty or maybe two hundred years from now. Out in deep space, on the equivalent of a cruise-size spaceship. We don’t know what the motive power is, but I don’t want to see any trails or fire. No streaks of smoke, no jet intakes, rocket exhaust, or anything like that. We’re not going to Mars, or any of that sort of limited thing. It will be like a deep-space exploration vessel, operating throughout our galaxy. We’ll be going to stars and planets that nobody has named yet”. He then got up and, as he started for the door, turned and said, “I don’t care how you do it, but make it look like it’s got power”.
According to Jefferies, the Enterprise design was arrived at by a process of elimination and the design even involved the sales department, production office and Harvey Lynn from the Rand Corporation. The various iterations produced many sheets of drawings – I wonder what happened to those treasures? The book shows some of the earlier concepts the team came up with.
Today, many in the general public take interstellar travel for granted, because Star Trek makes it look so easy with its warp drives and antimatter powered reactions. But for those of us who try to compute the problem of real starship design, we know the truth – that it is in fact extremely difficult. Whether you are sending a probe via fusion propulsion, laser driven sails or other means, the velocities, powers, energies are unreasonably high from the standpoint of today’s technology. But it is the dream of travelling to other stars through programs like Star Trek that keeps our candles burning late into the night as we calculate away at the problems. In time, I am sure we will prevail.
Fermi’s Enterprise?
There is an element of developing warp drive theory however that is usually neglected and I think it is now time to raise it – the implications to the Fermi Paradox. This is the calculation performed by the Italian physicist Enrico Fermi around 1950 that given the number of stars in the galaxy, their average distance, spectral type, age and how long it takes for a civilization to grow – intelligent extraterrestrials should be here by now, yet we don’t see any. Over the years there have been many proposed solutions to the Fermi paradox. In 2002 Stephen Webb published a collection of them in his book If the Universe is Teeming with Aliens…Where is Everybody? Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life, published by Paxis.
One of the ways to address this is to ask if interstellar travel was even feasible in theory, and as discussed in my recent Centauri Dreams post on the British Interplanetary Society, Project Daedalus proved that it was. If you can design on paper a machine like Daedalus at the outset of the space age, what could you do in two or three centuries from now?
But even then travel times across the galaxy would be quite slow. The average distance between stars is around 5 light years, the Milky Way is 1,000 light years thick and 100,000 light years in diameter. Travelling at around ten percent of the speed of light the transit times for these distances would be 50 years, 10,000 years and 1 million years respectively. These are still quite long journeys and the probability of encountering another intelligent species from one of the 100-400 billion stars in our galaxy may be low. But what if you have a warp drive?
The warp drive would permit arbitrarily large multiple equivalents of the speed of light to be surpassed, so that you could reach distances in the galaxy fairly quickly. Just like Project Daedalus had to address whether interstellar travel was feasible as an attack on the Fermi Paradox problem, so the warp drive is yet another question – are arbitrarily large speeds possible, exceeding even the speed of light?
If so, then our neighbourhood should be crowded by alien equivalents of the first Vulcan mission that landed on Earth in the Star Trek universe. To my mind, if we can show in the laboratory that warp drive is feasible in theory as a proof of principle, and yet we don’t discover intelligent species outside of the Earth’s biosphere, then of the many solutions to the Fermi paradox, perhaps there are only two remaining. The first would be some variation on the Zoo hypothesis, and the second is that we are indeed alone on this pale blue dot called Earth. Take your pick what sort of a Universe you would rather exist in.
Stars for JWST
Red dwarfs or brown? The question relates to finding targets as the James Webb Space Telescope gets closer to launch. We’re going to want to have a well defined target list so that the JWST can be put to work right away, and part of that effort means finding candidate planets the telescope can probe. Yesterday’s white paper on a proposed search for brown dwarfs using the Spitzer Space Telescope lined up a number of reasons why these objects are good choices:
* for a given planetary equilibrium temperature, the orbit gets shorter with decreasing primary mass, increasing the probability of transit and providing 50+ occultations per year (and 50+ transits);
* the planet to brown dwarf size ratio means transiting rocky planets produce deep transits and permit the detection of planets down to Mars’ size in a single transit event when using Spitzer;
* the reliability of the detection is helped by the absence of known false astrophysical positives: brown dwarfs have very peculiar colors, small sizes, and being nearby, have a high proper motion allowing to check what is within their glare
All this is in addition to the fact that the fainter the star, the greater the contrast between the primary and the planet. But interest in red dwarfs remains high as well. Here again we are dealing with small stars where the habitable zone can be closer than the distance between Mercury and the Sun, making for easier transit detections than with G or K-class stars. Daniel Angerhausen (Rensselaer Polytechnic Institute) and team are thus proposing a project of their own called HABEBEE, for “Exploring the Habitability of Eyeball Exo-Earths.”
Eyeball? This online feature in Astrobiology Magazine lays out the background. Angerhausen knows that tidal lock will set in on a closely orbiting planet, with the night side likely covered in ice while the day side could offer, at the right orbital distance, clement conditions for life. The article cites the disputed candidate planet Gliese 581g as a possible ‘eyeball’ world, but there seems to be little need to single out such a controversial object. Planets meeting this description should be relatively common given that M-dwarfs make up 70-80 percent of all stars in the galaxy, so that it’s possible they are the most abundant locations of life.
“A little bit closer to the star — that is, hotter — they would completely thaw and become waterworlds,” Angerhausen tells Astrobiology Magazine‘s Charles Choi; “[A] little bit further out in the habitable zone — that is, colder — they would become total iceballs just like Europa, but with a potential for life under the ice crust. These planets — water, eyeball or snowball — will most probably be the first habitable planets we will find and be able to characterize remotely. Thats why it is so important to study them now.”
Image: This artist’s impression shows a sunset seen from the super-Earth Gliese 667 Cc. The brightest star in the sky is the red dwarf Gliese 667 C, which is part of a triple star system. The other two more distant stars, Gliese 667 A and B appear in the sky also to the right. Finding planets in the habitable zones of red dwarfs and characterizing their atmospheres will be a major component in our search for life in the universe. Credit: ESO/L. Calçada.
There’s plenty to work with, especially given the flare situation on younger M-dwarfs, which can cause ultraviolet radiation spikes of up to 10,000 times normal levels. We have copious information about M-dwarf flares that has been gathered by observers over the years, while new observations of likely JWST candidate stars should help us characterize those more likely to host habitable planets. Radiation experiments involving the Brazilian National Synchrotron Light Source at Campinas will help the team understand the effects of radiation on ice.
The plan is to put together various models for red dwarf planets in the habitable zone that will help astronomers predict how well existing and future telescope surveys can find them. The team also hopes to travel to Antarctica to gather microbes in places that are transition zones between ice and water. They’ll use a planetary simulation chamber that was originally designed at the Brazilian Astrobiology Laboratory to mimic conditions on Mars. There the Antarctic microbes can be tested under various conditions of radiation and atmosphere to simulate M-dwarf possibilities.
So many unknowns, no matter what kind of star we home in on. I suspect that Amaury H.M.J. Triaud (Kavli Institute for Astrophysics & Space Research), who heads up the brown dwarf team, and Angerhausen himself would agree that we can’t be doctrinaire about where we look for life. Their proposals focus on brown or red dwarfs respectively not because they think these are the only possibilities for life, but because a case can be made that finding rocky worlds in the habitable zones of such stars will be quicker and the planets more easily characterized than the alternatives. The more we learn now, the better we’ll be able to use our future instruments.
Hunting for Brown Dwarf Planets
Brown dwarfs fascinate me because they’re the newest addition to the celestial menagerie, exotic objects about which we know all too little. The evidence suggests that brown dwarfs can form planets, but so far we’ve found only a few. Two gravitational microlensing detections on low mass stars have been reported, one of which is a 3.2 Earth-mass object orbiting a primary with mass of 0.084 that of the Sun, putting it into the territory between brown dwarfs and stars. The MEarth project has uncovered a planet 6.6 times the mass of the Earth orbiting a 0.16 solar mass star.
Now a new proposal to use the Spitzer Space Telescope to hunt for brown dwarfs planets is available on the Net, one that digs into what we’ve found so far, with reference to the discoveries I just mentioned:
Accounting for their low probabilities, such detections indicate the presence of a large, mostly untapped, population of low mass planets around very low mass stars (see also Dressing & Charbonneau (2013)). Arguably the most compelling discovery is that of the Kepler Object of Interest 961, a 0.13 [solar mass] star, orbited by a 0.7, a 0.8 and a 0.6 Rearth on periods shorter than two days (Muirhead et al. 2012). The KOI-961 system, remarkably, appears like a scaled-up version of the Jovian satellite system. This is precisely what we are looking for.
The plan is to use the Spitzer instrument to discover rocky planets orbiting nearby brown dwarfs, the idea being that the upcoming mission of the James Webb Space Telescope will need a suitable target list, and soon, for it to be put to work on probing the atmospheres of exoplanets. A 5400 hour campaign is the objective, the goal being to detect a small number of planetary systems with planets as small as Mars. Interestingly, the team is advocating a rapid release of all survey data to up the pace of exoplanet research and compile a database for further brown dwarf studies.
Image: The stellar menagerie: Sun to Jupiter, via brown dwarfs. Credit: Space Telescope Science Institute.
Brown dwarfs turn out to be excellent targets as we try to learn more about rocky planets around other stars. Studying the photons emitted by an atmosphere during an occultation requires relatively close targets, and as the paper on this work points out, the fainter the primary, the better the contrast between the central object and the planet. And around brown dwarfs we can expect deep transits that allow us to detect objects down to Mars size with Spitzer’s equipment. The paper also notes that brown dwarfs older than half a billion years show a near constant radius over their mass range, making it easier to estimate the size of detected planets.
Spitzer is the only facility that can survey a sufficient number of brown dwarfs, long enough, with the precision and the stability required to credibly be able to detect rocky planets down to the size of Mars, in time for JWST. We estimate that about 8 months of observations would be needed to complete the survey. Once candidates are detected, large ground-based facilities will confirm the transits, find the period (if only one event was captured by Spitzer) and check for the presence of additional companions. This program will rapidly advance the search for potentially habitable planets in the solar neighborhood and transmit to JWST a handful of characterizable rocky planet atmospheres.
This is a survey that not only probes a fascinating kind of object but one that should offer what the paper calls “the fastest and most convenient route to the detection and to the study of the atmospheres of terrestrial extrasolar planets.” It goes public at a time when 76 new brown dwarfs have been discovered by the UKIRT Infrared Deep Sky Survey, including two potentially useful ‘benchmark’ systems. The authors of the Spitzer proposal argue that observing the atmospheres of Earth-sized transiting worlds around M-Dwarfs with JWST will be much more challenging than equivalent work using brown dwarfs, assuming we get to work identifying the best targets.
The white paper is Triaud et al., “A search for rocky planets transiting brown dwarfs,” available online. The UKIRT Infrared Deep Sky Survey paper is Burningham et al., “Seventy six T dwarfs from the UKIDSS LAS: benchmarks, kinematics and an updated space density,” accepted at Monthly Notices of the Royal Astronomical Society (abstract).
Starship Congress Registration Opens
Our friends at Icarus Interstellar continue working on this summer’s conference. Just in from my son Miles is news about the opening of registration for the Dallas event.
Registration for the 2013 Starship Congress, hosted by Icarus Interstellar, is now open. The registration fee is $100; however, the first 25 paid registrations receive a $25 discount. This discount is also available to individuals who sign up by May 2nd, 2013. Students can register for a reduced rate of $50. Students must present a valid student I.D. at the Starship Congress to take advantage of the student rate. The $25 discount does not apply to student registrations. Group rates are also available. An optional lunch is offered for August 15, 16 and 17 for $25.
The Starship Congress will be held August 15-18 at the Hilton Anatole in Dallas, Texas. A discounted rate for Starship Congress attendees is available at the Hilton Anatole from August 12-20. To book a room at the special rate, click here.
Richard Obousy, President and Senior Scientist for Icarus Interstellar, provided Centauri Dreams readers with a preview of the Starship Congress, which you can read here. For any questions, contact registration@icarusinterstellar.org.
Robotic Replicators
Centauri Dreams regular Keith Cooper gives us a look at self-replication and the consequences of autonomous probes for intelligent cultures spreading into the universe. Is the Fermi paradox explained by the lack of such civilizations in the galaxy, or is there a far more subtle reason? Keith has been thinking about these matters for some time as editor of both Astronomy Now and Principium, which has just published its fourth issue in its role as the newsletter of the Institute for Interstellar Studies. Intelligent robotic probes, as it turns out, may be achievable sooner than we have thought.
by Keith Cooper
There’s a folk tale that you’ll sometimes hear told around the SETI or physics communities. Back in the 1940s and 50s, at the Los Alamos National Labs, where the first nuclear weapons were built, many physicists of Hungarian extraction worked. These included such luminaries in the field as Leó Szilárd, Eugene Wigner, Edward Teller and John Von Neumann. When in 1951 their colleague, the Italian physicist Enrico Fermi, proposed his famous rhetorical paradox – if intelligent extraterrestrial life exists, why do we not see any evidence for them? – the Hungarian contingent responded by standing up and saying, “We are right here, and we call ourselves Hungarians!”
It turns out that the story is apocryphal, started by Philip Morrison, one of the fathers of modern SETI [1]. But there is a neat twist. You see, one of those Hungarians, John Von Neumann, developed the idea of self-replicating automata, which he presented in 1948. Twelve years later astronomer Ronald Bracewell proposed that advanced civilisations may send sophisticated probes carrying artificial intelligence to the stars in order to seek out life and contact it. Bracewell did not stipulate that these probes had to be self replicating – i.e able to build replicas of themselves from raw materials – but the two concepts were a happy marriage. A probe could fly to a star system, build versions of itself from the raw materials that it finds there, and then each daughter probe could continue on to another star, where more probes are built, and so on until the entire Galaxy has been visited for the cost of just one probe.
The combination of Von Neumann machines and Bracewell’s probes made Fermi’s Paradox all the more puzzling. There has been more than enough time throughout cosmic history for one or more civilisations to send out an army of self-replicating probes that could colonise the Galaxy in anywhere between three million and 300 million years [2] [3]. By all rights, if intelligent life elsewhere in the Universe does exist, then they should have colonised the Solar System long before humans arrived on the scene – the essence of Fermi’s Paradox. The conundrum it is about to be compounded further, because human civilisation will have its own Von Neumann probes within the next two to three decades, tops. And if we can do it, so can the aliens, so where are they?
To Build a Replicator
A self-replicator requires four fundamental components: a ‘factory’, a ‘duplicator’, a ‘controller’ and an instruction program. The latter is easy – digital blueprints that can be stored on computer and which direct the factory in how to manufacturer the replica. The duplicator facilitates the copying of the blueprint, while the controller is linked to both the factory and the duplicator, first initiating the duplicator with the program input, then the factory with the output, before finally copying the program and uploading it to the new daughter probe, so it too can produce offspring in the future.
‘Duplicator’, ‘controller’, ‘factory’; these are just words. What are they in real life? In biology, DNA permits replication by following these very steps. DNA’s factory is found in the form of ribosomes, where proteins are synthesised. The duplicators are RNA enzymes and polymerase, while the controllers are the repressor molecules that can control the conveyance of genetic information from the DNA to the ribosomes by ‘messenger RNA’ created by the RNA polymerase. The program itself is encoded into the RNA and DNA, which dictates the whole process.
That’s fine for biological cells; how on earth can a single space probe take the raw materials of an asteroid and turn it into another identical space probe? The factory itself would be machinery to do the mining and smelting, but beyond this something needs to do the job of constructing the daughter probe down to the finest detail. Previously, we had assumed that nanotechnology would do the duplicating, reassembling the asteroidal material into metal paneling, computer circuits and propulsion drives. However, nanotechnology is far from reaching the level of autonomy and maturity where it is able to do this.
Perhaps there is another way, a technology for which we are only now beginning to see its potential. Additive manufacturing or, as it is more popularly known, 3D printing, is being increasingly utilised in more and more areas of technology and construction. Additive manufacturing takes a digital design (the instruction program) and is able to build it up layer by layer, each 0.1mm thick. The factory, in this sense, is then the 3D printer as a whole. The duplicator is the part that lays down the layers while the controller is the computer. It’s not a pure replica in the Star Trek sense, but it can build practically anything, including moving parts, that can otherwise only be manufactured in a real factory.
Gathering Space Resources
3D printing is not the technology of tomorrow; it’s the technology of today. It’s not a suddenly disruptive technology either (well, not in the sense of how it has gradually evolved), having been around in its most basic form since the 1970s and in its current form since 1995. Rather, it is a transformative technology. The reason it is gaining traction in modern society now is because it is becoming affordable, with small 3D printers now costing under $2,000. Within a decade or so, we’ll all have one; they’ll be as ubiquitous as a VCR, cell phone or a microwave. This will have huge consequences for manufacturing, jobs and the economy, potentially destroying large swathes of the supply chains from manufacturing to the purchaser, but, whereas the factory production lines on Earth may dry up, in space new economic opportunities will open up.
As spaceflight transitions from the domain of national space agencies to a wider field of private corporations, economic opportunities in space are already being sought after, including the mineral riches of the asteroids. One company in particular, Deep Space Industries, has already patented a 3D printer that will work in the microgravity of space [4] and they intend to use additive manufacturing to construct communication and energy platforms, space habitats, rocket fuel stations and probes from material mined from asteroids and brought into Earth orbit. For now, they envisage factory facilities in orbit and the asteroids mined will be those that come close to Earth [5]. Nevertheless, it has already been mooted that astronauts on a mission to Mars will be able to take 3D printers with them and, as we utilise asteroids further afield, we’ll start to bundle in the 3D printers with automated probes, creating an industrial infrastructure in space, first across the inner Solar System and then expanding into the outer realms.
Image: A ‘fuel harvestor’ concept as developed by Deep Space Industries. Credit: DSI.
Here’s the key; these 3D printers that will sit in orbit and are designed to build habitats or communication platforms, could easily become part of a large probe and be programmed to just build more probes. All of a sudden, we’d have a population of Von Neumann probes on our hands.
Without artificial intelligence, the probes would just be programmed automatons. They’d spend their time flitting from asteroid to asteroid, following the simple programming we have given them, but one day someone is inevitably going to direct them towards the stars. This raises two vital points. One is that if we can build Von Neumann probes, then a technological alien intelligence could surely do the same and their absence is therefore troubling. And two, Von Neumann probes will soon no longer be a theoretical concept and we are going to have to start to decide what we want them to be: explorers, or scavengers.
A Future Beyond Consumption
It seems clear that self-replicating probes will first be used for resource gathering in our own Solar System. Gradually their sphere of influence will begin to edge out into the Kuiper Belt and then the Oort Cloud, halfway to the nearest stars. That may not be for some time, given the distances involved, but when we start sending them to other stars, do we really want them rampaging through another planetary system, consuming everything like a horde of locusts? How would we feel if someone else’s Von Neumann probes entered our Solar System to do the same? Once they are let loose, we need to take responsibility for their behaviour, lest we be considered bad parents for not supervising our creations. That would not be the ‘first contact’ situation we’ve been dreaming of.
On the other hand, Bracewell’s probes were designed for contact, for communication, for the storage and conveyance of information – a far more civilised task. But standards, however low, can be set early. If our Von Neumann probes are only ever used for mining, will we be wise enough to have the vision in the future to appropriate them for other means too? It seems we need to think about how we are going to operate them now, rather than later after the horse has bolted.
And perhaps there lies the answer to Fermi’s Paradox. Maybe intelligent extraterrestrials are more interested in making a good first impression than the incessant consumption of resources. Perhaps that is why the Solar System wasn’t scoured by a wave of Von Neumann probes long ago. The folly of our assumption is that we see all before us as resources to be utilised, but why should intelligent extraterrestrial life share that outlook? Maybe they are more interested in contact than consumption – a criticism that can be levelled at other ideas in SETI, such as Kardashev civilisations and Dyson spheres that have been discussed recently on Centauri Dreams. Perhaps instead there is a Bracewell probe already here, lurking in in a Lagrange point, or in the shadow of an asteroid, watching and waiting to be discovered. If that’s the case, it may be one our own Von Neumann probes that first encounters it – and we want to make sure that we make the right impression with our own probe the day that happens.
References
[1] H Paul Schuch’s edited collection of SETI essays, SETI: Past, Present and Future, published by Springer, 2011.
[2] Birkbeck College’s Ian Crawford has calculated that the time to colonise the Galaxy could be as little as 3.75 million years, as described in an article in the July 2000 issue of Scientific American.
[3] Frank Tipler’s estimate for the time to colonise the Galaxy was 300 million years, as written in his famous 1980 paper “Extraterrestrial Intelligent Beings Do Not Exist,” that appeared in the Royal Astronomical Society’s Quarterly Journal.
[4] Deep Space Industries 22 January 2013 press announcement.
[5] Private correspondence with Deep Space Industries’ CEO, David Gump.
The Alpha Centauri Angle
Apropos of yesterday’s article on the discovery of Proxima Centauri, it’s worth noting that Murray Leinster’s story “Proxima Centauri,” which ran in Astounding Stories in March of 1935, was published just seven years after H. A. Alden’s parallax findings demonstrated beyond all doubt that Proxima was the closest star to the Sun, vindicating both Robert Innes and J. G. E. G. Voûte. Leinster’s mile-wide starship makes the first interstellar crossing only to encounter a race of intelligent plants, the first science fiction story I know of to tackle the voyage to this star.
The work surrounding Proxima Centauri was intensive, but another fast-moving star called Gamma Draconis in Draco, now known to be about 154 light years from Earth thanks to the precision measurements of the Hipparcos astrometry satellite, might have superseded it. About 70 percent more massive than the Sun, Gamma Draconis has an optical companion that may be an M-dwarf at about 1000 AU from the parent. Its bid for history came from the work of an astronomer named James Bradley, who tried without success to measure its parallax. Bradley was working in the early 18th Century on the problem and found no apparent motion.
Stellar parallax turned out to be too small an effect for Bradley’s instruments to measure. Most Centauri Dreams readers will be familiar with the notion of observing the same object from first one, then the other side of the Earth’s orbit, looking to determine from the angles thus presented the distance to the object. It’s no wonder that such measurements were beyond the efforts of early astronomers and the apparent lack of parallax served as an argument against heliocentrism. A lack of parallax implied a far greater distance to the stars than was then thought possible, and what seemed to be an unreasonable void between the planets and the stars.
It would fall to the German astronomer Friedrich Wilhelm Bessel to make the first successful measurement of stellar parallax, using a device called a heliometer, which was originally designed to measure the variation of the Sun’s diameter at different times of the year. As so often happens in these matters, Bessel was working on 61 Cygni at the same time that another astronomer — his friend Thomas Henderson — was trying to come up with a parallax reading for Alpha Centauri. Henderson had been tipped off by an observer on St. Helena who was charting star positions for the British East India Company that Alpha Centauri had a large proper motion.
Henderson was at that time observing at the Cape of Good Hope, using what turned out to be slightly defective equipment that may have contributed to his delays in getting the Alpha Centauri parallax into circulation. In any event, Bessel’s heliometer method proved superior to Henderson’s mural circle and Dollond transit (see this Astronomical Society of Southern Africa page for more on these instruments), and Bessel’s findings on 61 Cygni were accepted by the Royal Astronomical Society in London in 1842, while Henderson’s own figures were questioned.
Bessel thus goes down as the first to demonstrate stellar parallax. Henderson went on to tighten up his own readings on Alpha Centauri, using measurements taken by his successor at the Royal Observatory at the Cape of Good Hope, but it took several decades for the modern value of the parallax to be established. But both astronomers were on to the essential fact that parallax was coming within the capabilities of the instruments of their time, and by the end of the 19th Century, about 60 stellar parallaxes had been obtained. The parallax of Proxima Centauri, for the record, is now known to be 0.7687 ± 0.0003 arcsec, the largest of any star yet found.
Image: A portrait of the German mathematician Friedrich Wilhelm Bessel by the Danish portrait painter Christian Albrecht Jensen. Credit: Wikimedia Commons.
While the Hipparcos satellite was able to extend the parallax method dramatically, it falls to the upcoming Gaia mission to measure parallax angles down to an accuracy of 10 microarcseconds, meaning we should be able to firm up distances to stars tens of thousands of light years from the Earth. Indeed, working with stars down to magnitude 20 (400,000 times fainter than can be seen with the naked eye), Gaia will be able to measure the distance of stars as far away as the galactic center to an accuracy of 20 percent. The Gaia mission’s planners aim to develop a catalog encompassing fully one billion stars, producing a three-dimensional star map that will not only contain newly discovered extrasolar planets but brown dwarfs and thousands of other objects useful in understanding the evolution of the Milky Way.
One can only imagine what the earliest reckoners of stellar distance would have made of all this. Archimedes followed the heliocentric astronomer Aristarchus in calculating that the distance to the stars, compared to the Sun, was proportionally as far away as the ratio of the radius of the Earth was to the distance to the Sun (thanks to Adam Crowl for this reference). Using the figures he was working with, that works out to a stellar distance of 100 million Earth radii, a figure then all but inconceivable. If we translated into our modern values for these parameters, the stars Aristarchus was charting would be 6.378 x 1011 (637,800,000,000) kilometers away. The actual distance to Alpha Centauri is now known to be roughly 40 trillion (4 x 1013) kilometers.
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