We can hope that plumes like those found emanating from the south pole of Enceladus happen on other icy worlds. There have been hints of plumes at Europa but they’ve proven elusive to pin down. However, we’re learning a great deal about the water inside Enceladus through Cassini flybys, using models based on mass spectrometry data the spacecraft has gained from the ice grains and gases in the moon’s plumes. A similar approach on other icy moons, if possible, could save us from having to drill through kilometers of ice.
What Christopher Glein (Carnegie Institution for Science) and team have done is to construct a chemical model that uses the Cassini observational data to determine the pH of the Enceladan ocean. It’s an important reading because pH tells us how acidic the water is, which gives us a look into the geochemical processes occurring inside the moon. What the new work shows is that the plume is salty, with an alkaline pH of about 11 or 12. This Carnegie Institution news release likens the pH to glass-cleaning ammonia solutions, with the same sodium chloride as Earth’s oceans.
We also find a substantial amount of sodium carbonate, which makes the Enceladus ocean similar to ‘soda lakes’ found in places like Mono Lake in California and Lake Magadi in Kenya. The paper on this work suggests that the high pH comes from serpentinization, by which rocks low in silica and high in magnesium and iron rise into the ocean floor from the upper mantle and chemically interact with surrounding water molecules. These ‘ultrabasic’ rocks are converted into new minerals — one of these is the mineral serpentine — and the water becomes alkaline.
This would be a significant finding because we’re looking for the energies needed to support life inside icy worlds like these, and serpentinization can produce molecular hydrogen (H2) to fit the bill. Says Glein:
“…molecular hydrogen can both drive the formation of organic compounds like amino acids that may lead to the origin of life, and serve as food for microbial life such as methane-producing organisms. As such, serpentinization provides a link between geological processes and biological processes. The discovery of serpentinization makes Enceladus an even more promising candidate for a separate genesis of life.”
Image: A diagram illustrating the possible interior of Saturn’s moon Enceladus, including the ocean and plumes in the south polar region, based on Cassini spacecraft observations, Credit: NASA/JPL-Caltech.
A separate genesis would not necessarily imply a continuation of life on the moon today, a point the paper is careful to make:
Aside from the origin of life, the biggest unknown that would be critical to life on Enceladus is the availability of H2. Are there “fresh” anhydrous rocks deep in Enceladus’ core, allowing H2 to be produced today (Brockwell et al., 2014); or has the ocean core system reached a state of complete chemical equilibrium…? If the latter, it is possible that life existed but died out because they ran out of food. This is an important issue to consider as we move forward in assessing the habitability of Enceladus.
The convenience of internal materials being pushed into space is hard to over-state, as it gives us the raw material for finding evidence of life through molecular analysis, and we can’t rule out the faint but real possibility of someday acquiring frozen organisms with future plume flybys.
Meanwhile, we learn more about those jets of material erupting from inside the Saturnian moon through a NASA study led by Joseph Spitale (Planetary Science Institute). The work, published in Nature, argues that rather than being discrete jets, the phenomena are ‘curtain eruptions’ that extend along the length of the prominent surface fractures on Enceladus.
Rather than intermittent geysers, we get, at least primarily, diffuse sprays whose ‘folds’ appear as separate streams with heightened brightness. “The viewing direction plays an important role in where the phantom jets appear,” Spitale said. “If you rotate your perspective around Enceladus’ south pole, such jets would seem to appear and disappear.” What we see as jets, then, would be an optical illusion, but the phenomenon of expelled materials fortunately persists.
The Glein paper is “The pH of Enceladus’ ocean,” in press at Geochimica et Cosmochimica Acta, published online 16 April 2015 (preprint). The Spitale paper is “Curtain eruptions from Enceladus’ south-polar terrain,” Nature 521 (7 May 2015), pp. 57-60 (abstract).
Not long ago I looked at the future of the Voyager spacecraft and noted a possibility once suggested by Carl Sagan. Give the Voyagers one last ’empty the tank’ burn and both could be put on a trajectory that would take them near, if not through, another star’s system (see Voyager to a Star). It would be little more than a symbolic act, for even with heroic measures to conserve power, neither Voyager will be able to communicate past the mid-2020s. With a little luck, perhaps 2030.
So we would be sending two spacecraft off to a star as a final act, turning them into markers, or monuments, that show humans are capable of producing something that will eventually reach (or come close to) another stellar system. Given their current trajectories, each Voyager passes interestingly close to another star in about 40,000 years, or roughly the amount of time since the extinction of homo neanderthalensis. The mere act of relating objects created by our species and launched in 1977 to time frames like this is itself an exercise in ‘deep time’ and the startling changes in perspective it forces.
Jim Bell, who as a young grad student worked on various aspects of the Voyager encounters, brought up Sagan’s interest in an end-of-mission course change in his new book The Interstellar Age (Dutton, 2015). If we were to do something like this, what do we know about its targets? Below is an image of the star AC +79 3888, also known as Gliese 445. It’s now 17.6 light years from the Sun, an M-class dwarf in the constellation Camelopardis.
Image: Gliese 445, in an image taken by the Oschin Schmidt Telescope near San Diego, Calif., on April 22, 1998. Credit: California Institute of Technology/Palomar Observatory.
Assuming we leave the Voyagers alone, Voyager 1 will be closer to this star than to the Sun in the abovementioned 40,000 years. I often mention that if one of our Voyagers were pointed at Alpha Centauri, the journey would take roughly 75,000 years at 17.1 km/sec. But Gliese 445 is another kind of target. It’s moving in our direction at roughly 120 kilometers per second, so that at its closest approach it will be 3.485 light years out, closer than Proxima Centauri is today.
But even a trajectory-changing burn can apparently bring Voyager 1 no closer than about one light year of the star as it again moves away. Mark Biegert worked this distance out in Voyager 1 and Gliese 445, a post on his Math Encounters blog a couple of years ago, working with Voyager perfectly aimed at Gliese 445 — if anyone can tune up these numbers, I’d be interested in the results.
Voyager 2, meanwhile, should pass within 111,000 AU of HH Andromedae, otherwise known as Ross 248, in the same 40,000 years. Here again we’re dealing with an M-class dwarf with about twelve percent of the Sun’s mass and a mere 16 percent of its radius. Ross 248 is a flare star and thus far we’ve found no sign of planets around it, nor can we rule them out. The Hubble instrument was used to search for a stellar companion in the late 1990s, but none was found, and astrometric work at the Sproul Observatory found no trace of a brown dwarf companion.
Also moving in our direction, Ross 248 becomes the closest star to the Sun in the 40,000 year time frame considered here. Interestingly enough, Frederick West suggested in a 1985 paper that an unmanned craft moving at 25.4 km/sec, launched some time in the 21st Century, could reach this star by the time of its closest approach to the Solar System. West, then working as a translator of Russian scientific material at the Library of Congress, made the pitch at a session of the American Astronomical Society, envisioning a probe about the size of a large space station powered by an ion propulsion system and capable of data return after millennia. In what must go down in the annals of understatement, he told a Fredericksburg, VA newspaper that year that “We would have to build the interstellar probe exceedingly well.” Indeed.
Nudging Voyager trajectories to get them closer to these two stars — and I have no numbers on exactly how close we might come to Ross 248 in this scenario — is an opportunity we’ll have about a decade to consider as the spacecraft continue to lose effective power from their onboard plutonium. It’s the plutonium that generates electricity for the spacecraft’s computers and instruments, not to mention the critical heaters. The total power level at launch (470 watts) has now dwindled to 250 watts or so. We may get Voyager as far as 160 AU from the Sun before we lose communications. Suzy Dodd, now Project Manager for the Voyager Interstellar Mission, likes to think we could keep doing science until 2027, marking 50 years of Voyager operations.
What happens if we choose to leave the Voyager trajectories alone? Without needed power, they will fall silent but keep traveling. It will take less than 300,000 years for Voyager 2 to pass within 270,000 AU (the distance now separating us from Alpha Centauri) of Sirius, at which point it will be half as far from Sirius as we are now. In his book, Jim Bell imagines the hot blue star looming four times brighter in the sky than it does from Earth. Neither Voyager has galactic escape velocity, so 250 million year orbits around the Milky Way’s center are in store after this. It’s a prospect that evokes Bell’s deepest poetic instincts, and he does them justice:
…Voyager 1 will continue to slowly travel northward and Voyager 2 southward, relative to the sun and the surrounding stars. Over time— enormous spans of time, as the gravity of passing stars and interacting galaxies jostles them as well as the stars in our galaxy— I imagine that the Voyagers will slowly rise out of the plane of our Milky Way, rising, rising, ever higher above the surrounding disk of stars and gas and dust, as they once rose above the plane of their home solar system. If our far-distant descendants remember them, then our patience, perseverance, and persistence could be rewarded with perspective when our species— whatever it has become— does, ultimately, follow after them. The Voyagers will be long dormant when we catch them, but they will once again make our spirits soar as we gaze upon these most ancient of human artifacts, and then turn around and look back. I have no idea if they’ll still call it a selfie then, but regardless of what it’s called, the view of our home galaxy, from the outside, will be glorious to behold.
Roughly twice the radius and eight times as massive as Earth, 55 Cancri e is a ‘super-Earth’ in the interesting five-planet system some 41 light years away in the constellation Cancer. No habitable conditions here, at least not for anything remotely like the kind of life we understand: 55 Cancri e orbits its G-class primary every 18 hours (55 Cancri is actually a binary, accompanied by a small red dwarf at a separation of 1000 AU). The closest super-Earth we’ve yet found, this is a tidally locked world that, helpfully for our purposes, transits its host.
What we find in a just announced study of the planet’s thermal emissions out of the University of Cambridge is an almost threefold change in temperature over a two year period. Although we’ve done it before with gas giant atmospheres, this is the first time any variability in atmosphere has been observed on a rocky planet outside our own Solar System. No other super-Earth has yet given us signs of possible surface activity, and Cambridge’s Nikku Madhusudhan, a co-author of the study, calls the changes in detected light ‘drastic.’ They imply a huge temperature swing, from 1000 degrees to 2700 degrees Celsius (?1300 – 3000 K) on the star-side of this tidally locked world. Brice-Olivier Demory is lead author of the paper on these observations:
“We saw a 300 percent change in the signal coming from this planet, which is the first time we’ve seen such a huge level of variability in an exoplanet. While we can’t be entirely sure, we think a likely explanation for this variability is large-scale surface activity, possibly volcanism, on the surface is spewing out massive volumes of gas and dust, which sometimes blanket the thermal emission from the planet so it is not seen from Earth.”
Image: Artist’s impression of super-Earth 55 Cancri e, showing a hot partially-molten surface of the planet before and after possible volcanic activity on the day side. Credit: NASA/JPL-Caltech/R. Hurt.
The 55 Cancri e work was performed with data from the space-based Spitzer instrument. To understand the results, the authors look at the entire category of ultra-short period (USP) planet candidates found by Kepler, of which there are more than 100. Most of these have radii less than twice that of Earth, and some may be undergoing periods of intense erosion. The paper notes that the planet KIC12557548b shows changes in transit depth and shape that are consistent with what it calls a ‘cometary-like environment.’ The supposition here is that KIC12557548b is sub-Mercury in size but giving off an opaque cloud of dust, perhaps driven by surface volcanism.
From such scenarios the authors derive the idea that 55 Cancri e, one of the largest known USP planets, is likely subject to volcanism, with possible magma oceans on the day side. But in comparison to KIC12557548b, this world is large enough to contain its volcanic outgassing. From the paper:
…whereas extremely small planets (nearly mercury-size) subject to intense irradiation can undergo substantial mass loss through thermal winds, super-Earths are unlikely to undergo such mass-loss due to their significantly deeper potential wells… Thus, ejecta from volcanic eruptions on even the most irradiated super-Earths such as 55 Cnc e are unlikely to escape the planet and would instead display plume behaviour characteristic to the solar system. The extent and dynamics of the plumes if large enough can cause temporal variations in the planetary sizes and brightness temperatures and hence in the transit and occultation depths.
The larger picture is that we have begun to probe atmospheric conditions on worlds as small as two-Earth radii. Theories vary as to the composition of 55 Cancri e, with some observations suggesting a carbon-rich world while others point to a silicate-rich interior with a dense atmosphere. The variability found in this study calls the earlier models into question. But as we learn more about the material surrounding 55 Cancri e, we’ll be conducting what the authors call ‘a direct probe of the planet surface composition’ that may help us understand other USP planets.
The paper is Demory et al., “Variability in the super-Earth 55 Cnc e,” submitted to Monthly Notices of the Royal Astronomical Society (preprint). A University of Cambridge news release is available.
Imagine the kind of spaceship we’ll need as we begin to expand the human presence into the nearby Solar System. We’d like something completely reusable, a vessel able to carry people in relative comfort everywhere from Mars to Venus, and perhaps as far out as the asteroid belt, where tempting Ceres awaits. Capable of refueling using in situ resources, these are ships not crafted for a single, specific mission but able to operate on demand without entering a planetary atmosphere. Brian McConnell, working with Centauri Dreams regular Alex Tolley, has been thinking about just such a ship for some time now. A software/electrical engineer, pilot and technology entrepreneur based in San Francisco, Brian here explains the concept he and Alex have come up with, one that Alex treated in a previous entry in these pages. The advantages of their ‘spacecoach’ are legion and Brian also offers a sound way to begin testing the concept. The author can be reached at bsmcconnell@gmail.com.
by Brian McConnell
“What if a spacecraft, like a cell, was made mostly of water?”
That’s what Alexander Tolley and I asked when we were working on our paper for the Journal of the British Interplanetary Society, “A Reference Design For A Simple, Durable and Refuelable Interplanetary Spacecraft” [1]. The paper explored the idea of a crewed spacecraft that used water as propellant in combination with solar electric propulsion. We dubbed them spacecoaches, as a nod to the stagecoaches of the Old West. Alex also gave the concept an excellent fictional treatment in Spaceward Ho!, also published here on Centauri Dreams. We are currently finishing a book about spacecoaches, to be published by Springer this fall. Subscribe to spacecoach.org for updates about the book and spacecoaches in general.
The idea of crewed solar electric spacecraft is hardly new. In 1954, Ernst Stuhlinger proposed a “sun-ship” powered by solar steam turbines and cesium ion drives [2,3]. Since then solar electric propulsion has been used in a wide variety of uncrewed craft. Meanwhile, the convergence of several technologies will make crewed solar electric vehicles feasible in the near future.
The core idea behind the spacecoach architecture is the use of water, and potentially waste streams, as propellant in electric engines. Water, life support and consumables are critical elements in a long duration mission, and in a conventional ship, are dead weight that must be pushed around by propellant that cannot be used for other purposes. Water in a spacecoach, on the other hand, can be used for many things before it is reclaimed and sent to the engines, and it can be treated as working mass. This, combined with the increased propellant efficiency of electric engines, leads to a virtuous cycle that results in dramatic cost reductions compared to conventional ships while increasing mission capabilities. Cost reductions of one or two orders of magnitude, which would make travel to destinations throughout the inner solar system routine, are possible with this approach.
Water is, for example, an excellent radiation shielding material, comparable to lead on a per kilogram basis, except you can’t drink lead. It is an excellent thermal battery, and can simply be circulated in reservoirs wrapped around the ship to balance hot and cold zones (this same reservoir doubles as the radiation shield). When frozen into fibrous material to form pykrete, it forms a material as tough as concrete, which can potentially be used for debris shielding or for momentum wheels, and if positioned correctly, can double as a supplemental radiation shield. If mixed with dilute hydrogen peroxide, which is safely stored at low concentrations, oxygen can be generated by passing it through a catalyst, similar to a contact lens cleaner. Dilute H2O2 is also a potent disinfectant, and can also be used to process human waste, as is done in terrestrial wastewater treatment plants. Anything the crew eats or drinks can be counted as propellant, as the water can be reclaimed and used for propulsion. This greatly simplifies planning for long missions because the longer the mission is, the more propellant you have in the form of consumables. This will also provide excellent safety margins and enable crews to survive an Apollo 13 scenario in deep space.
A spaceship that is mostly water will be more like a cell than a conventional rocket plus capsule architecture. Space agriculture, or even aquaculture, becomes practical when water is abundant. Creature comforts that would be unthinkable in a conventional ship (hot baths anyone?) will be feasible in a spacecoach. Meanwhile, inflatable structures will eventually enable the construction of large, complex habitats that will be more like miniature O’Neill colonies than a conventional spaceship [4].
In the book, Alex and I present a reference design that combines inflatable structures and thin film PV arrays to form a kite-like structure that both has a large PV array area, and can be rotated to provide artificial gravity in the outer areas [5]. The ability to generate artificial gravity while providing ample radiation protection solves two of the thorniest problems in long duration spaceflight. Alex wrote an excellent fictional treatment of the concept for Centauri Dreams called Spaceward Ho! This is intended as a straw man design to kickstart design competitions. We envision a series of design competitions for water compatible electric propulsion technologies, large scale solar arrays, and overall ship designs. Much of the reference design can be validated in ground based competitions and experiments, followed by uncrewed test vehicles (similar to what Bigelow Aerospace did by flying its Genesis I and II habitats in low earth orbit).
Spacecoaches are possible not because of any one insight or breakthrough, but because of the convergence of improvements in component technologies, specifically thin film photovoltaics, electric propulsion, and inflatable structures. The combination of the three, particularly when you add water for propulsion, leads to one or two order of magnitude improvements in mission economics.
Thin film solar photovoltaics, which enable the construction of large area PV sails, will enable ships to generate hundreds of kilowatts to several megawatts of electrical power (thin film PV material coincidentally is much more resistant to radiation than conventional silicon PV material) [6]. While thin film solar is not as efficient as silicon in terms of power per unit area, from a power density (watts/kilogram) standpoint, it offers multiple order of magnitude improvements, and will continue to improve for decades due to dematerialization in manufacturing processes.
SEP (solar electric propulsion) is a well understood, flight ready technology. Engines that function with water or gasified waste will be well suited to the spacecoach architecture. We simply need to test existing SEP technologies with water and waste streams to pin down performance and efficiency numbers, which can be done via an X-Prize style engineering competition. Scaling them to propel a large (40 tonne) ship will be done by clustering them in arrays, so there will be no need to build a single high power engine when an array of many 10-20 kilowatt units will do just fine, while also adding redundancy. One interesting discovery we made while doing our analysis is that ultra high specific impulse engines, such as VASIMR, are neither necessary nor desirable. Engines that operate at the low end of the electric propulsion envelope still yield excellent economics due to the synergies created by using water as propellant, while also being able to operate with less electrical power per unit of thrust, which reduces PV array size and mass.
Inflatable/expandable structures are just now beginning to be recognized as a flight ready technology, with Bigelow Aerospace’s BEAM unit due to fly on the ISS later this year. Bigelow already has two uncrewed inflatable habitats in low earth orbit. The basic idea with inflatable structures is to replace a rigid metal hull with a flexible high strength Kevlar type material and utilize pressurization to inflate and deploy the structure. This also enables a large habitable space to be compacted into a standard cargo fairing, thus requiring a minimal number of surface launches for initial delivery to orbit. We expect this technology to improve, both in terms of mass per unit of habitable space (currently about 60 kg per m3), and in terms of the types of shapes that can be created. [7]
Spacecoaches will not be mission specific ships. Even the first generation ships will be able to travel to many destinations within the inner solar system. They will be fully reusable, travelling from a high earth orbit or a Lagrange point to and from their destinations, without ever entering a planetary atmosphere. Spacecoaches will be able to travel to cislunar space, Mars, Venus, NEOs and maybe even Ceres and the Asteroid Belt. They can also be dispatched for asteroid interception and deflection missions on short notice. This is a huge departure from conventional spacecraft which are purpose built for a specific mission, usually Mars, that is planned decades in advance. Mars is certainly an interesting destination, but Ceres, with its abundant water resources and shallow gravity well, may turn out to be an even more interesting destination for human exploration and settlement.
The amount of water required for propellant on any given route will vary depending on the delta-v needed, and also the specific impulse of the engines on board, but water is easy to handle and store. Need to add an extra two kilometers per second to your delta-v budget? Just add water! (or replace the electric engines with slightly more efficient models). Because water is so easy to handle compared to conventional propellants, this will also simplify the construction and operation of orbiting fuel depots, which will be little more than orbiting water tanks.
Simplicity and upgradability is another key design element of the spacecoach. We assume that component technologies will continue to improve for decades. So instead of designing spacecoaches to fly only with today’s technology, they will be designed more like personal computers were in the 1980s. The original PCs were built around a common electrical and communication bus, the ISA bus, which allowed memory, CPUs and peripherals from many manufacturers to be combined. If you wanted to, you could buy the component parts from catalogs and build your own PC from scratch.
We envision something similar for the spacecoach, for the electrical system and engines in particular, which will have standard electrical and fluid interconnects, and uniform form factor requirements. The engines will also be mounted in a sealable compartment that can be pressurized so the crew can replace or upgrade engines without doing an EVA. This will not only make spacecoaches field upgradable, but will also reduce the need to design engines for extreme reliability. If a few units fail, crews would replace them in an operation not much different than replacing a rack mounted server. Upgrading engines will be the best way to improve performance and reduce costs, as a small increase in specific impulse can yield significant mass and cost reductions, especially for high delta-v routes like Ceres and the Asteroid Belt.
And what about cost?
Mention crewed missions to Mars, much less anywhere else, and people automatically assume you’re talking tens of billions of dollars as a starting point. We modeled approximate round-trip mission costs to destinations throughout the inner solar system, using a 40,000 kilogram (40 tonne) dry hull and SpaceX’s published launch costs to get materials, including water, into low earth orbit ($1,700/kg via Falcon 9 Heavy [8]), with electric propulsion (Isp between 1,500 to 3,000s) from there (electrode-less Lorentz force thrusters using water operate in this range). Among the missions we modeled were EML-2 (Earth Moon Lagrange point 2) to/from cislunar space, Martian moons, NEO interception, Venus orbit and Ceres. Even with engines operating at the low end of the electric propulsion performance envelope, our models predicted per mission costs in the hundreds of millions of dollars, a one or two order of magnitude reduction compared to conventional missions, some of which, such as a crewed mission to Ceres, simply are not possible via chemical propulsion.
Such large cost reductions are possible due to a combination of the fuel efficiency of electric engines, and the synergies created by using water as propellant. On one hand electric engines require far less propellant for a given delta-v. On the other, virtually everything the crew consumes or uses for life support can eventually be sent to the engines. As a result the only dead weight on the ship is the hull and whatever non-consumable materials and equipment are brought on board, which will also allow spacecoaches to carry larger crews. Reusability will also enable operators to amortize development and construction costs across many missions.
Spacecoaches are also well suited for in situ resource utilization. Should we reach low gravity destinations with accessible water (Ceres is an especially interesting location), it will eventually be possible to refuel spacecoaches at these destinations, or even ship water inbound to cislunar depots. We assume for now that spacecoaches are fully supplied from Earth, but exploring ISRU destinations and capabilities will be a high priority early on. Partially reusable launch vehicles offer another way to reduce costs. Water will be an ideal payload for a heavily re-used Falcon 9R booster. Unlike most payloads, it has essentially zero replacement cost, so the launch operator can fly the reusable boosters until they fail, and can learn about potential failure modes and fixes in the process (all while delivering more water to orbit).
If you are part of a team working on electric propulsion technology, here’s one way you can help make these a reality. Test your engine with water vapor, carbon dioxide and gasified waste (or a good analogue), and publish your results. The most important parameters ship designers will be interested in are specific impulse, efficiency (ideally the “wall plug” efficiency of the entire system so it can be modeled as a black box) and thrust/mass ratio. We already know several SEP technologies work reasonably well with water, but it will be great to examine all systems to see how well each works with water, compare performance across a variety of technologies, and identify opportunities for further improvement.
It is easy to be cynical about new spaceflight concepts, especially one that promises large cost reductions, but most of this can be validated on the ground and via uncrewed testbeds in a short time and at little expense. It is a paradigm shift, and that will take people some time to accept. The rocket + capsule design pattern served us well in the early years of spaceflight, so its hard to get away from that, but it’s time to move on to something that is more adaptable, something that’s more like a ship that can sail wherever her captain wants to go.
Spacecoaches will form the basis for a real world Starfleet, a fleet which will grow as ships are built, and which will reach new destinations as component technologies continue to improve in the coming decades. They will open the inner solar system out to the Asteroid Belt to human exploration and settlement, and with some spacecoaches operating in cislunar space, humanity will also have a rapid response capability should we be surprised by the discovery of an Earth threatening object.
Visit spacecoach.org to learn more, and to subscribe for notices about the upcoming book, which examines the spacecoach reference design and potential missions in detail. If you are interested in obtaining an advance copy of the book, acting as a technical reviewer or inviting us to speak, please get in touch.
References
[1] “Reference Design for a Simple, Durable and Refuelable Interplanetary Spacecraft”, B. S. McConnell; A. M. Tolley (2010), JBIS, 63, 108-119
[2] Image credit: Frank Tinsley/American Bosch Arma Corporation, 1954
[3] “Possibilities of Electrical Space Ship Propulsion,” E. Stuhlinger, Bericht über den V Internationalen Astronautischen Kongreß, Frederich Hecht, editor, 1955, pp. 100-119; paper presented at the Fifth International Astronautical Congress in Innsbruck, Austria, 5-7 August 1954
[4] “A Shape Grammar for Space Architecture – I. Pressurized Membranes”, Val Stavrev* Aeromedia, Sofia, Bulgaria, 40th International Conference on Environmental Systems, http://www.spacearchitect.org/pubs/AIAA-2010-6071.pdf
[5] Image credit: Rüdiger Klaehn
[6] “Super radiation tolerance of CIGS solar cells demonstrated in space by MDS-1 satellite”, Photovoltaic Energy Conversion, 2003. Proceedings of 3rd World Conference on, 18-18 May 2003, pp. 693 – 696 Vol.1
[7] Estimate based on BA330 mass per cubic meter of habitable space, per Bigelow Aerospace’s published specifications
[8] Per SpaceX published launch cost and delivery capacity for Falcon 9 Heavy, as of April 2015
In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).
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