Centauri Dreams
Imagining and Planning Interstellar Exploration
Two Takes on Habitability
Last week’s announcement about Kepler-186f presented a world that is evidently in the outer reaches of its star’s habitable zone, with the usual caveats that we know all too little about this place to draw any conclusions about what is actually on its surface. Is it rocky, and does it have liquid water? Perhaps, but as Greg Laughlin (UC-Santa Cruz) points out on his systemic site, the widely circulated image of Kepler-186f was all but photographic in its clarity. Listen to Laughlin as he looks at the image:
I stared at it for a long time, tracing the outlines of the oceans and the continents, surface detail vivid in the mind’s eye. Yes, ice sheets hold the northern regions of Kepler-186f in an iron, frigid grip, but in the sunny equatorial archipelago, concerns of global warming are far away. Waves lap halcyon shores drenched in light like liquid gold.
He goes on to look at how the press has handled earlier stories on habitable planets, dating back to the Gliese 581c frenzy of 2007. And you can see the evolution in artist’s renderings of the various worlds under discussion, culminating in a Kepler-186f image that could indeed be misconstrued as a photograph by someone who didn’t realize the limits of our capabilities. Here’s the image again (credit: NASA Ames/SETI Institute/JPL-Caltech):
It’s beautiful work, and I ran it last Thursday along with my story on the new planet. But we have to be careful not to get too far ahead of ourselves. Yes, researchers tracking the exoplanet hunt understand that this is entirely conjectural, but upon reflection, I think we’re sending a signal to the general public that we’re more confident about what these worlds are like than is justified. In the case of Kepler-186f, the fact that a planet is close to Earth-sized does not render it Earth’s twin in any other meaningful way, especially given that the planet orbits a red dwarf and pulls in only about a quarter of the insolation that Earth receives.
The Benefits of Axial Tilt
What we do surmise about habitable zones keeps getting tweaked around the edges, and on a more theoretical plane, the work on planets with ’tilted’ orbits — this comes out of the University of Washington, Weber State and NASA — is intriguing because it pushes liquid water on the surface much further out than earlier habitable zone notions would allow. The paper, which appears in the April Astriobiology may, in fact, expand the habitable zone by ten to twenty percent, which would greatly increase the number of planets suitable for life.
We’re talking about planets whose axis has been tilted from their orbital plane thanks to gravitational interactions with other planets in the system. The contention here is that a fluctuating tilt in a planet’s orbit may enhance rather than diminish the chances for life, because glaciation becomes more difficult when polar regions melt thanks to the erratic spin. Says Rory Barnes (University of Washington), “…the rapid tilting of an exoplanet actually increases the likelihood that there might be liquid water on a planet’s surface.” From the paper:
We interpret our results to mean that planets with large and rapid obliquity oscillations are more likely to be habitable than those with negligible oscillations, such as the Earth. This perspective is at odds with the notion that the stability of the Earth’s obliquity is important to the development of life. While it still may be true that rapid oscillations can be detrimental, and certainly at some point obliquity cycles could be too large and rapid, our results clearly show that rapid obliquity evolution can be a boon for habitability. At the least, one should not rule out life on planets with rapid obliquity cycles.
Image: Tilted orbits such as those shown might make some planets wobble like a top that’s almost done spinning, an effect that could maintain liquid water on the surface, thus giving life a chance. Credit: NASA/GSFC.
One consequence is that future searches for living planets might be extended farther from the target star, given the deeper habitable zone available, a result with a bearing on how difficult it is to separate stellar and reflected planetary light. The researchers do point out in their conclusion that their simulations all began with planetary spin rates of 24 hours and obliquities of 23.5 degrees, clarifying the need for future work on a wider range of initial conditions. In particular, does a specific solar system ‘architecture’ always produce a particular obliquity cycle?
Interesting stuff, and bear in mind that it could have ramifications on another theory, that planets need a large, stabilizing moon to be suitable for life. The Earth’s axial tilt of 23.5 degrees would, in the absence of Luna, increase to the point where climate fluctuations became more extreme. This could be problematic for planets in orbits like ours, but at the outer edge of the habitable zone, in this view, those fluctuations could be precisely what is needed to prevent ice from becoming global, making the lack of a moon is a distinct plus. Large moons, then, may have a role to play in both directions, depending on the planet’s position in the habitable zone.
The paper is Armstrong et al., “Effects of Extreme Obliquity Variations on the Habitability of Exoplanets,” Astrobiology Vol. 14, Issue 4 (April 15, 2014), full text available. The University of Washington press release is here.
An Outward-Looking Grand Strategy
We use strategies to weigh the issues around us and maximize our chances for success. Can we create a strategy not just for a specific short-term goal but for the survival and growth of our entire species? In the essay that follows, Michael Michaud looks at the elements of such a vision, one that by necessity takes us out of our own biosphere and into the cosmos. As long-time Centauri Dreams readers know, Michaud is well suited to discussing the resolution of conflict and the attainment of goals. His lengthy career in the U.S. Foreign Service led to posts as Counselor for Science, Technology and Environment at U.S. embassies in Paris and Tokyo, and Director of the State Department’s Office of Advanced Technology. He has also been chairman of working groups at the International Academy of Astronautics on SETI issues, and is the author of the highly regarded Contact with Alien Civilizations: Our Hopes and Fears about Encountering Extraterrestrials (Springer, 2007).
By Michael A.G. Michaud
We are living amid four revolutions that draw human minds beyond the limits of the Earth.
* Astronomical exploration of the cosmos by ground-based instruments, orbiting observatories, and robotic spacecraft brings the rest of our solar system closer to us, so that we can more realistically consider living on or utilizing other worlds.
* Human spaceflight enables us to expand our presence and our field of action beyond the Earth. It changes the way we see our position in the cosmos, implying that we – and our prospects for the future — need not remain confined to our home planet.
* The search for extraterrestrial life and intelligence changes our perspective on the role of biology and sentience in the universe. Life may prove to be a widespread phenomenon, not unique to the Earth. Contact with another civilization might challenge us, or open up vast opportunities for our species.
* Proposals for extraterrestrial macroengineering , such as mining the Moon and the asteroids, building satellite solar power stations, and terraforming Mars, could enable us to expand our influence on matter and energy beyond the Earth, utilizing those resources to remove the limits to growth and open up new options for our species.
These revolutions broaden Earth-bound conceptions. They urge us to reach outward. They imply grand shared tasks for Humankind.
These revolutions also contrast us to an outside, heightening our awareness that we spring from a common origin and live in a common biosphere. They encourage us to think about the shared interests of humankind.
Synergy
Astronomy, planetary exploration, and human spaceflight are not mutually exclusive. Work in one field has stimulated new ideas – and sometimes new programs and more funding – in others.
Astronomy has been a powerful stimulus to thinking about spaceflight. It has given us a better understanding of potential destinations, and potential risks.
This can work both ways. Astronomy and planetary exploration would not have enjoyed the growth they experienced during after the beginning of the space age had it not been for the Moon landing program and the prospect of eventual human missions to Mars.
We find stimulus and response elsewhere too. The search for extraterrestrial life has been a major factor in gaining support for planetary exploration missions to Mars. The possible presence of oceans under the ice of outer planet moons is stimulating new interest. SETI, a search for evidence of alien technology, grew out of radio astronomy.
Ideas about bases on the Moon and Mars became more credible after human and robotic missions gave us geological information about lunar and planetary materials. Extraterrestrial macroengineering concepts such as mining or diverting asteroids help justify further exploration of our solar system.
Discovering planets around other stars has given new impetus to astrobiology, SETI and interstellar exploration by robotic probes.
Those who support the implementation of these grand ideas have learned to play politics, to lobby for their causes in national capitals and multinational organizations. Their efforts have concentrated on budget processes, encouraging a near term approach. Political persuasion has focused on funding specific projects.
Instead of seeing the competition for funding as a zero-sum game, we could make a more conscious effort to see connections and seek synergisms. To cite one example, ground-based astronomers have been surveying asteroids that cross or come near the orbit of the Earth. Unmanned missions to asteroids and comet nuclei might pave the way for human exploration. Those in turn could assist in developing mining operations, making those bodies part of the human resource base.
If separate advocacies worked together, the whole might be greater than the sum of its parts. What is missing is a unifying concept.
A Grand Extraplanetary Strategy
All of these fields of human endeavor are parts of an unarticulated grand strategy for our species.
At the most basic level, a strategy is simply a thoughtful way of dealing with one’s environment to improve one’s prospects for success. A grand strategy for the human species would be one designed to improve our ability to survive, to grow, to diversify, and to increase our influence on our environment and our future.
There are many elements to such a strategy, including the better management of our resources, reducing undesirable impacts on our biosphere, limiting conflict among humans, and maintaining the conviction that our future can be better than our past. Most conceptions constrain the design of such a strategy to the biosphere of our origin – a stage that many find unnecessarily narrow. The environment of a technological species is much larger than the planetary biosphere that gave it birth.
Here we may have the common purpose that underlies the four outward-looking revolutions of our time. Astronomy, planetary exploration, and SETI are reconnaissances of our larger environment. They are essential elements of any rational extraplanetary strategy for the human species; without them, we could not conduct intelligent operations beyond the Earth.
Human spaceflight is partly for reconnaissance and partly for operations, depending on the objectives of particular missions. Extraterrestrial mining and macaroengineering, including the building of large structures in space, clearly would be operations.
Whatever our differences about specific missions may be, we could share a broad vision of human activity beyond the Earth, placing astronomy, planetary spaceflight, SETI, and proposals for extraterrestrial macroengineering in a common context.
Hard times can produce new alliances. Instead of seeing other programs as rivals for funding, we could look for opportunities for each to help the others, designing missions to be synergistic wherever that is possible. For example, advocates of interstellar exploration by probes could more actively support the search for extrasolar planets and invite extrasolar planet seekers to reciprocate.
This approach will not lead to quick miracles in public funding. Governments and international organizations are unlikely to adopt a formal extraplanetary strategy, or even to agree that we should have one. But they might respond to tactical alliances among the revolutionaries.
Many differences may divide us, but we can share a unifying idea: that we are participating in the definition and implementation of a grand strategy for our species.
Armed with a shared vision, we can work quietly and persistently to see that the parts of such a grand strategy are put into place, supporting each other whenever possible. That will require patience, and an enlightened sense of self-interest.
Separately, we have worked wonders. Imagine what we could do together.
——-
This essay is based on three documents written more than thirty years ago. The author first presented a paper on this subject at the 1981 International Astronautical Congress in Rome. A more detailed discussion can be found in “Towards a Grand Strategy for the Species,” Earth-Oriented Applications of Space Technology, Vol. 2, No. 3-4 (1982), 213-219. A simpler, more popularized version entitled “Sharing the Grand Strategy” appeared in Space World, August 1984, 5-9.
Kepler-186f: Close to Earth Size, in the HZ
We have another ‘habitable zone’ planet to talk about today, one not much bigger than the Earth, but it’s probably also time to renew the caveat that using the word ‘habitable’ carries with it no guarantees. The working definition of habitable zone right now is that orbital distance within which liquid water might exist on the surface of a planet. Whether it actually does is just one of the questions. A second is whether or not we’re in fact dealing with a rocky terrestrial world.
So Centauri Dreams approaches the announcement of Kepler-186f with guarded enthusiasm for an exoplanet that looks interesting indeed. Five planets circle this star, an M-dwarf a great deal smaller and cooler than the Sun. Discovered by the Kepler space observatory, the planet presents us with transit information telling us that it is about 1.1 Earth radii, although we don’t yet know what the mass of this world is, and hence can’t make a definitive call on whether or not it is rocky. But Stephen Kane (San Francisco State), one of the researchers involved in today’s announcement, thinks we have reason to think that it is:
“What we’ve learned, just over the past few years, is that there is a definite transition which occurs around about 1.5 Earth radii,” said Kane. “What happens there is that for radii between 1.5 and 2 Earth radii, the planet becomes massive enough that it starts to accumulate a very thick hydrogen and helium atmosphere, so it starts to resemble the gas giants of our solar system rather than anything else that we see as terrestrial.”
Kepler-186f is thus well below the value where we would expect it to accumulate a thick hydrogen and helium envelope, causing Kane to add “there’s a very excellent chance that it does have a rocky surface like the Earth.” If that’s the case, then we have a planet on the outer edge of its star’s habitable zone, though one that may have a somewhat thicker atmosphere than Earth’s because of its somewhat larger size. Perhaps the surface can avoid freezing. In any case, this is what lead author Elisa Quintana (NASA Ames) calls “the first definitive Earth-sized planet found in the habitable zone around another star.” The work appeared today in Science.
Image: The artist’s concept depicts Kepler-186f, the first validated Earth-size planet orbiting a distant star in the habitable zone—a range of distances from a star where liquid water might pool on the surface of an orbiting planet. The discovery of Kepler-186f confirms that Earth-size planets exist in the habitable zone of other stars and signals a significant step closer to finding a world similar to Earth. Credit: NASA Ames/SETI Institute/JPL-Caltech.
The discovery team used so-called ‘speckle imaging’ in obtaining its high resolution observations from the eight-meter Gemini North telescope on Mauna Kea as well as adaptive optics observations from the ten-meter Keck II telescope to rule out extraneous sources that could account for the Kepler data, concluding that the signal has to be that of a transiting planet. The speckle data allowed direct imaging of the system to within 400 million miles, confirming there were no other stellar-sized objects orbiting within this distance from the star. “The Keck and Gemini data are two key pieces of this puzzle,” adds Quintana. “Without these complementary observations we wouldn’t have been able to confirm this Earth-sized planet.”
The new planet orbits its star once every 130 days, receiving about a third of the heat energy that Earth does from the Sun. The four inner planets — Kepler-186b, Kepler-186c, Kepler-186d, and Kepler-186e — are all too hot for life as we know it, with periods of 3, 7, 13 and 22 days.
Image: Kepler-186 and the Solar System: The diagram compares the planets of the inner solar system to Kepler-186, a five-planet system about 500 light-years from Earth in the constellation Cygnus. The five planets of Kepler-186 orbit a star classified as a M1 dwarf, measuring half the size and mass of the sun. The Kepler-186 system is home to Kepler-186f, the first validated Earth-size planet orbiting a distant star in the habitable zone—a range of distances from a star where liquid water might pool on the surface of an orbiting planet. Credit: NASA Ames/SETI Institute/JPL-Caltech.
The objection to Kepler-186f as a home for life rests on the dangers of orbiting an M-dwarf, a class of star prone to flare activity. Move a planet close enough to the star to be in its habitable zone and the assumption is that it’s also tidally locked, presenting the same side to the star throughout its orbit, with all the complications that brings to climate models. Neither of these factors are complete show-stoppers — some climate studies show that temperature extremes can be mitigated by winds or ocean currents — and in the case of Kepler-186f, we do have a world on the habitable zone’s outer edge, perhaps far enough out not to suffer tidal lock.
So it’s an interesting place, this new world, about 490 light years away in the constellation Cygnus and thus tantalizingly out of reach for atmospheric analysis even with instrumentation planned for the near future. The James Webb Space Telescope itself won’t be able to help us with that task. But it’s pleasing to note that Kepler-186f has been studied over a frequency range of 1 to 10 GHz looking for emissions, though none has so far been found. Getting a detectable signal here from this star would require a transmitter between 10 and 20 times as powerful as the planetary radar system at Arecibo. SETI keeps coming up empty, but good for us if, in addition to our other studies, we keep our ears open for a long-shot detection.
The paper is Quintana et al., “An Earth-Sized Planet in the Habitable Zone of a Cool Star,” Science Vol 344, No. 6181 (18 April 2014), pp. 277-280 (abstract). This news release from the Gemini and Keck observatories is also helpful, as is this one from San Francisco State University.
A New Look at Sea Floor Astrobiology
How do you produce life on an early Earth bathed in ultraviolet radiation? The presumption when I was growing up was that the combination of chemicals in ancient ponds, fed energy by lightning or ultraviolet light itself, would produce everything needed to start the process. Thus Stanley Miller and Harold Urey’s experiments, beginning in 1953 at the University of Chicago, which simulated early Earth conditions to produce amino acids out of a sealed ‘atmosphere’ of water, ammonia, methane and hydrogen, with electrodes firing sparks to simulate lightning.
But there are other ways of explaining life’s origins, as a new study from the Jet Propulsion Laboratory and the Icy Worlds Team at the NASA Astrobiology Institute reminds us. Hydrothermal vents on the sea floor have been under consideration since the 1980s, with some researchers pointing to the ‘black smokers’ that produce hot, acidic fluids. The new NASA work looks at much cooler vents bubbling with alkaline solutions like those in the ‘Lost City,’ a field of hydrothermal activity in the mid-Atlantic on the seafloor mountain Atlantis Massif.
Image: This image from the floor of the Atlantic Ocean shows a collection of limestone towers known as the “Lost City.” Alkaline hydrothermal vents of this type are suggested to be the birthplace of the first living organisms on the ancient Earth. Credit: JPL.
Here there is a field of about thirty large calcium carbonate chimneys — some 30 to 60 meters tall — and a number of smaller structures venting mainly hydrogen and methane into the surrounding water. The so-called ‘water world’ theory that JPL’s Michael Russell has been working on since 1989 draws on the idea that warm alkaline vents like these would have maintained a state of imbalance with ancient oceans that were acidic. Life is, in this formulation, seen as the inevitable outcome of disequilibrium, producing enough energy to drive its formation.
Thus we have a proton gradient with hydrogen ions concentrated largely on the outside of the vent’s chimneys, which the work refers to as ‘mineral membranes.’ We also have an electrical gradient between oceans rich with carbon dioxide. and hydrogen and methane from the vents as they meet at the chimney wall. The transference of electrons could have produced complex organic compounds, using processes not so different from those that occur in mitochondria.
“Within these vents, we have a geological system that already does one aspect of what life does,” said Laurie Barge, second author of the study at JPL. “Life lives off proton gradients and the transfer of electrons.”
The work represents a fundamental shift in focus over older ‘chemical soup’ models, its examination of membrane-spanning gradients pre-empting prebiotic chemistry. The paper explains:
…there is an advantage to be gained from examining the transition from geochemistry to biochemistry from the bottom up, that is, to “look under the hood” at life’s first free energy-converting nanoengines or “mechanocatalysts.” Such an approach encourages us to see life as one of the last in a vast hierarchical cascade of emergent, disequilibria-converting entropy-generating engines in the Universe. In doing so, we keep our sights on the “astro” in astrobiology.
The researchers speculate that minerals may have played the role of enzymes in the ancient ocean, interacting with local chemicals and driving reactions. A mineral called ‘green rust’ (fougèrite) could use the proton gradient to produce phosphate-laden molecules capable of storing energy. Molybdenum is also in play, a rare metal that can drive important chemical reactions. Thus basic metabolic reactions around sea floor hot springs may help to explain not only how life emerged on our own planet but also how it may emerge on worlds far beyond.
On this latter point, the paper explains how to proceed:
In considering habitability and the potential for life elsewhere in the Solar System and beyond, the physical and chemical disequilibria that obtain on wet icy rocky worlds, and the various processes that might relieve them, need to be established. If life’s origin is ultimately coupled to geophysical convection in a particular geochemical context, one should be able to make predictions about life’s likelihood on a planet or moon of interest from application of coupled chemical and fluid/geodynamical modeling, and from the availability of key feedstocks, thus accounting for other planetary energetic drivers, for example, tidal and radiogenic heating, solar wind interactions, magnetic dynamos—appropriate to the object in question.
We’d like to account, in other words, for the disequilibrium-producing factors that could play an astrobiological role on multitudes of exoplanets. The possibilities range widely, from gravitational effects to thermal and chemical gradients that can all play a role in life’s inception. Particularly close to home, of course, we focus in on places like Enceladus and Europa, where we have nearby laboratories for observing these processes in action. Until we can put the right kind of instrumentation on the scene, continuing Earth-bound lab work on these ideas is the way forward.
The paper is Russell et al., “The Drive to Life on Wet and Icy Worlds,” Astrobiology Vol. 14, Issue 4 (April 15, 2014). Available online. A JPL news release is also available.
Saturn: Commotion in the A Ring
After yesterday’s look back at the ambitious Project Orion planners and their hopes of reaching Saturn’s moons by the 1970s, let’s stay in the same vicinity today to look at what may be the emergence of an entirely new moon. As always, we have Cassini to thank for this work, which shows a disturbance at the outer edge of Saturn’s A ring. This is the outermost of the large, bright rings, with a width of approximately 14,600 kilometers. Its inner boundary is the Cassini division, a 4800 kilometer wide region between it and the B ring.
The image below shows the disturbance, an area in the shape of an arc that is about 20 percent brighter than its surroundings. The region is some 1200 kilometers long and 10 kilometers wide, and it is accompanied by breaks in the otherwise smooth profile at the edge of the ring. The current thinking is that both the arc and the protuberances are the result of gravitational effects caused by a nearby object. Are the rings, then, giving birth to a new moon?
Image: The disturbance visible at the outer edge of Saturn’s A ring in this image from NASA’s Cassini spacecraft could be caused by an object replaying the birth process of icy moons. This view looks toward the illuminated side of the rings from about 53 degrees above the plane of the rings. It was obtained from a distance of approximately 775,000 miles (1.2 kilometers) from Saturn, with a sun-Saturn-spacecraft, or phase, angle of 31 degrees. The scale is about 7 kilometers per pixel. Credit: NASA/JPL-Caltech/Space Science Institute.
Informally dubbed ‘Peggy,’ the proto-moon, assuming that is what it is, cannot yet be resolved in Cassini’s imagery, although the spacecraft will move closer to the outer edge of the A ring in late 2016, perhaps offering an opportunity to study it in greater detail. Scientists estimate it to be no more than a kilometer in diameter, but the diminutive object could give us the chance to shake out a recent proposal that all the icy moons formed originally from ring particles before moving further away from the planet, growing over time as they merged with other moons.
“The theory holds that Saturn long ago had a much more massive ring system capable of giving birth to larger moons,” said Carl Murray (Queen Mary University, London), lead author of the paper on this work. “As the moons formed near the edge, they depleted the rings and evolved, so the ones that formed earliest are the largest and the farthest out.”
Many of Saturn’s moons are composed largely of ice, and they do indeed increase in size with distance from the planet. “We may,” adds Murray, “be looking at the act of birth, where this object is just leaving the rings and heading off to be a moon in its own right.”
This news combined with the thought of using Enceladus to refuel a Project Orion vessel, as we discussed yesterday, somehow calls to mind Isaac Asimov’s story “The Martian Way,” a novella first published in Galaxy Science Fiction (1952) and later made available in The Martian Way and Other Stories (1955). Martian colonists make the trip to Saturn to bring back a cubic mile of ice that will supply the colony for 200 years. They, embed their ships in the ice block for the return even as they use its resources as reaction mass.
Martians, it turns out, are the perfect crew for deep space vehicles because — Gerald Driggers, author of the Earth-Mars Chronicles, will like this — they’ve been forced to acclimatize to cramped conditions and the rigors of space travel. Asimov’s characters look down upon the unfortunate planet-bound population of Earth and discuss what they see as an inevitable future:
“Even if they come to Mars, it will only be their children that are free. There’ll be starships someday: great huge things that can carry thousands of people and maintain [their] self-contained equilibrium for decades, maybe centuries. Mankind will spread through the whole Galaxy. But people will have to live their lives out on shipboard until new methods of interstellar travel are developed, so it will be Martians, not planet-bound Earthmen, who will colonize the Universe. That’s inevitable. It’s got to be. It’s the Martian way.”
The new work on the A-ring anomaly is Murray et al, “The discovery and dynamical evolution of an object at the outer edge of Saturn’s A ring,” published online by Icarus, 28 March 2014 (abstract).
Remembering ‘Saturn by 1970’
One day in the late summer of 1958, at a time when the Jet Propulsion Laboratory was still in the hands of the U.S. Army (the transfer to NASA wouldn’t happen until the end of that year), Freeman Dyson and Ted Taylor showed up at the facility outside Pasadena. Try to imagine the scene: At the time, JPL was busy building the Explorer 6 satellite, all 65 kilograms of it. And here came two Project Orion scientists talking about not just satellites but auxiliary vehicles, additional payload to fly aboard their proposed 4000 ton spacecraft that they hoped would explore the outer planets.
“The reception there was rather cool,” Dyson would later say. “The lady at the front office decided Taylor and I were a pair of crackpots and tried to get rid of us. After about half an hour of arguing we got inside and then it all went very well.”
Image: Freeman Dyson, whose payload ideas must have confounded the team working on early Earth satellites. Credit: Courtesy of Princeton University Archives. Princeton University Library.
The entertaining tale is told in George Dyson’s Project Orion: The True Story of the Atomic Spaceship (Henry Holt, 2002), and it’s easy to see why even hardened rocket scientists would be confounded by what the duo proposed. By mid-1958, the largest payload ever lifted into orbit was Sputnik III, weighing in at 1325 kilograms. Project Orion was intended to loft 1600 tons to low-Earth orbit, or in its advanced version, 1300 tons to a landing on one of Saturn’s moons. The moon that most drew Dyson’s eye in 1958 was tiny Enceladus.
When I wrote about this in connection with the recent findings of an ocean within the distant moon, I was delighted to receive the diagram below from George Dyson, which shows the numbers as tabulated by Freeman Dyson in 1958, when details about the outer planets’ moons were sketchy at best. I want to run this as a bit of deep space history, the working figures that would later turn into Freeman Dyson’s document “Trips to Satellites of the Outer Planets, which was declassified in 1987.
Image: Thinking about deep space destinations in 1958, as the Orion team pondered their best options for a trip that might take place as early as 1970. Credit: Freeman Dyson, courtesy of George Dyson.
With reference to the figures, George Dyson comments:
“Note that the .618 density for Enceladus was not a transcription or arithmetic error, it is due to the mass and radius of the outer planet satellites being known only approximately at that time. (I believe Thomas “Tommy” Gold was brought in as a consultant on the question of selecting landing sites.) These calculations were made to determine the best destination both in terms of an optimum velocity match and highest probability of being able to obtain water ice or hydrocarbons on the surface to replenish the vehicle’s propellant mass.”
Below is the title page of the “Trips to Satellites of the Outer Planets’ report.
Image credit: Freeman Dyson, courtesy of George Dyson.
We’ve discussed Orion many times in these pages, though it’s been long enough that it may be time for a general review in the near future. Most Centauri Dreams readers will be familiar with George Dyson’s definitive book on the project’s history, and with the overall concept of detonating nuclear devices behind the craft, with a system of pusher plates and shock absorbers to cushion the crew, and the capability of launching payloads that were mind-boggling in the days of Sputnik. Interestingly, Mars was the first destination the team had in mind, though a landing on the Moon along the way would have been part of that mission. A four or five year mission seemed a possibility, one that Freeman Dyson would liken to the voyage of Darwin’s Beagle.
But the allure of the outer planets and their satellites was hard to resist, particularly when you threw in two ways to make the mission lighter and more efficient. For one thing, it was possible to use atmospheric braking (‘aerobraking’) to reduce propellant mass. I’ll quote from George Dyson’s book on the other:
The second part of the strategy is to gather propellant for the return trip at the destination, thereby reducing the average takeoff weight of the bombs. “We assume that we can use as propellant either ice, ammonia, or hydrocarbons,” wrote Freeman, explaining why Enceladus was such a good place to stop. “We suppose that each propulsion unit contains one-third of its mass in the form of the bomb and other fabricated parts, and two-thirds of its mass in the form of propellant. This means that, when propellant refueling is possible, only one-third of the mass required for the homeward trip need be carried out from Earth.” When you put these numbers together, the end results were astonishing. “With the use of atmospheric drag a round-trip to satellites of either Jupiter or Saturn could be made with a total velocity increment of the order of 40 km/sec. With refueling and braking, all the satellites become accessible with a round-trip mass-ratio less than 2.”
The Mars ship can thus become an outer planet ship that refuels along the way. And given the document shown above, I have to close with this last quote from the book:
Forty years later, Freeman and I review a two-page handwritten General Atomic calculation sheet, “Outer Planet Satellites,” dating from 1958 or 1959. It lists, for nine different satellites, ten different parameters such as orbital velocity, escape velocity, density, and gravity that determine the suitability of the satellites as places to land. Freeman smiles as he carefully studies the numbers.
“Enceladus still looks good,” he says.
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