Centauri Dreams
Imagining and Planning Interstellar Exploration
Proposed Europa/Io Sample Return Mission
I love a long journey by car or rail, but not by airplane. Back in my flight instructing days, I used to love to take a Cessna 182 on a long jaunt, but these days the flying I do means sitting in the cheap seats in the back of a gigantic jet and suffering the various indignities of security checks, long lines and tightly packed quarters. Hence my 1000 mile rule: If the trip is less than that distance, I’ll drive it or look for a rail connection. My recent trip back to the Midwest reminded me how much I enjoy seeing the scenery at my own pace and having plenty of time to think.
One of the things I thought about was how to extract maximum value from spacecraft. A decade or so ago, JPL’s James Lesh explained to me how the signal from a distant probe passing behind a planet would be affected by that planet’s atmosphere. An elementary way to do atmospheric science! I’ve mused ever since about how to do complicated things with existing resources and how to put technology in the right place for bonus information returns. All that led to thoughts about our prime astrobiology targets in the outer system: Europa and Enceladus.
Earlier this month I wrote about Lee Billings’ Aeon Magazine essay Onward to Europa, in which he speculated about the the possibility of exploring what is beneath the Europan crust. A mission like this could be done without actually descending to the surface and penetrating the ice. Billings noted that the Hubble Space Telescope has detected water vapor, estimated at about 7000 kg of water per second, being blown into space from the surface. Europa’s high-radiation environment is challenging even for a robotic lander, but maybe we could fly through the Europan plume to sample the moon’s chemistry and possibly even detect signs of biological activity. The essay Ship of Dreams in Astrobiology Magazine speculates on the proposed Europa Clipper mission flying through the plumes, but budget issues could make the $2.1 billion Clipper too expensive.
Image: A prime target for astrobiology, Europa (as imaged here by the Galileo spacecraft) is the subject of multiple mission concepts. Credit: NASA/JPL/Ted Stryk.
Similar ideas have surfaced about Enceladus, with an eye toward flying a complicated mission on a tight budget indeed. Consider the Life Investigation for Enceladus mission concept, championed by Peter Tsou (Sample Exploration Systems). I read more about this one in an essay by Andrew LePage on his Drew ex machina website. LePage, a physicist and writer who serves as senior project scientist for Visidyne Inc. in Boston, notes that the LIFE mission would use an aerogel collector like the one NASA used in the Stardust sample return mission to return cometary dust in 2006. Some concepts also call for sample return from Saturn’s E-ring, thought to be made up of particles originally from Enceladus’ geysers.
All this came into the public eye last summer at the Low-Cost Planetary Missions Conference (LCPM-10) at Caltech, where Tsou laid out a 15-year mission that would launch in the early 2020s, reaching Saturn in May of 2030 after a series of gravity assists past Venus and the Earth. LIFE would use close passes by Titan to alter its orbit, allowing multiple low-speed approaches through the Enceladus geyser region above the moon’s south pole. At speeds slower than Stardust’s encounter with Comet Wild 2, the Enceladan material should be better preserved when captured. LIFE would then use Titan for further gravity assists followed by a return to Earth in 2036.
I love the concept as much as I love extracting atmospheric science from communications signals. The cost excluding launch might be kept as low as $425 million. The potential gain is high. LePage likes it, too, and goes on to suggest not just improvements to the Enceladus idea, but a different sample return mission that would bring back materials from both Europa and Io. The mission could launch as early as 2021, with rendezvous with Jupiter in October of 2025. LePage lays out the basics: An elongated orbit to avoid the worst of Jupiter’s radiation belts, gravity assists from Europa, Ganymede and Callisto, multiple low-velocity encounters with the Europan polar plumes and gathering of plume materials with the aerogel collector.
Then figure several months of observation to scope out a target on Io, then a close pass by the moon to sample one of its volcanic plumes, a scary swing through the most intense regions of Jupiter’s radiation belts, but one of only two passes through the worst of them (the other being at insertion into orbit around Jupiter). Re-entry to Earth’s atmosphere would occur in 2030. It’s a mission concept with intriguing resonance with the LIFE mission and builds on the same technologies. Says LePage:
While a lot more work is required to flesh out the details of a Europa-Io sample return mission (especially more information on the nature of Europa’s purported plumes), at first blush it does appear to be feasible using the same hardware proposed for the LIFE mission to Enceladus employing readily available launch vehicles. This proposed mission also nicely complements the investigations of the LIFE mission by returning samples from yet another set of plumes on a potentially life-bearing moon with the added bonus of sampling volcanic material from a second target of keen interest to planetary scientists – Io, the solar system’s most volcanically active world.
The nine-year mission LePage envisions is substantially shorter than the 15-year LIFE mission, and could be completed about the time that LIFE arrived at Saturn. This would have been a great concept to mull over on my trip, and I wish I could have read about it before I left. I’d love to see follow-up work, particularly on that white-knuckle pass by Io. The essay continues:
For minimal additional costs (i.e. a second spacecraft and launch vehicle along with the incremental cost increase of running two missions in parallel), this scientifically interesting mission could be flown in parallel with LIFE and greatly increase its total science return. And it could probably do so within the Administration’s proposed billion dollar price cap for a Europa mission.
Given that we’re now dealing with budget proposals that confine a NASA Europa mission to under a billion dollars, the sample return mission to Europa even without the Io component offers a profoundly interesting science return, and I like the synergies with the more fully developed LIFE concept. In any case, we have two highly intriguing astrobiology targets that are conveniently venting material into nearby space, making landing on the surface — much less trying to penetrate fissures or drill through thick ice — unnecessary at this stage of our investigations. What we learn from such missions could well determine how we press ahead with later, more complex missions that would demand operations on or below the ice.
New Horizons: The KBO Hunt Continues
Of the many interesting questions Nick Nielsen raised in last Friday’s post, the one that may be most familiar to the interstellar community is the question of potential breakthroughs. What happens if an unexpected discovery in propulsion makes all the intervening stages — building up a Solar System-wide infrastructure step by step — unnecessary? If we had the kind of disruptive breakthrough that enabled starflight tomorrow, wouldn’t the society that grew out of that capability be fundamentally different than one in which starflight took centuries to achieve?
I was mulling this over yesterday when I read Pluto-bound Probe Faces Crisis, a short article in Nature that several readers had passed along. With the New Horizons probe pressing on for a close-pass of Pluto/Charon next year, the assumption all along has been that it would make a course correction after the encounter to set up a flyby of a Kuiper Belt Object (KBO). The trick there is that the New Horizons team is running out of time to find the right KBO. The sense of urgency is revealed in the fact that mission scientists have asked for 160 orbits of observing time on the Hubble instrument, which the article calls a ‘rare request’ for an already operational mission.
Alexandra Witze sums up the reason for the delay in identifying a target in the Nature story:
In theory, project scientists should have identified a suitable KBO long ago. But they postponed their main search until 2011, waiting for all the possible KBO targets to begin converging on a narrow cone of space that New Horizons should be able to reach after its Pluto encounter. Starting to look for them before 2011 would have been impossible, says [mission co-investigator Will] Grundy, because they would have been spread over too much of the sky.
The Voyagers, Galileo, New Horizons and their ilk represent a familiar evolutionary model of our expansion into the outer Solar System as opposed to the kind of disruptive breakthrough Nick was speculating about. In this model, we learn from mission to mission, making each more capable, adding technologies that can get instruments to their destinations at a faster clip. We can’t predict disruptive technologies, but we can see a rational line of development of current tech as we tune up our deep space craft, one in which the ongoing New Horizons issues play a major role.
Image: Artist’s impression of the New Horizons spacecraft encountering a Kuiper Belt object. The Sun, more than 4.1 billion miles (6.7 billion kilometers) away, shines as a bright star embedded in the glow of the zodiacal dust cloud. Jupiter and Neptune are visible as orange and blue “stars” to the right of the Sun. Although you would not actually see the myriad other objects that make up the Kuiper Belt because they are so far apart, they are shown here to give the impression of an extensive disk of icy worlds beyond Neptune. Credit: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (JHUAPL/SwRI)
The next year is going to be filled with New Horizons news and, let’s hope, a resolution of the KBO issue. Fifty new KBOs have thus far been identified in the hunt, which has used the resources of the 8.2-metre Subaru Telescope in Hawaii and the 6.5-metre Magellan Telescopes in Chile. None, as it turns out, is close enough to New Horizons’ trajectory to make it feasible given the constraints on the spacecraft’s ability to maneuver. And as I’ve mentioned in these pages before, the search field is tricky, looking directly out along the plane of the galaxy, which means the faint signature of a KBO is readily lost in the starfield. The good news is that by adding Hubble into the mix — and a decision on this won’t be reached until June 13 — the chances of a detection soar over what they would be using ground-based telescopes alone.
Make no mistake, even a long-distance observation of a KBO from New Horizons’ 21-centimeter telescope would trump what we can see from Earth orbit, but obviously a much closer look at a primordial survivor from the Solar System’s early history would be preferable. We wait and hope for the best. Meanwhile, we in the interstellar community should be tracking this mission with great interest. New Horizons is pushing into terra incognita with instruments designed for the job, and represents, as Michael Michaud recently commented to me, a more relevant transition to our deep space future than the Voyager spacecraft. It should energize our designs for future craft that will push further into the Kuiper Belt and beyond. This incremental model works, and if along the way a disruptive breakthrough occurs, then so much the better.
How We Get There Matters
Nick Nielsen’s new essay follows up his speculations on interstellar infrastructure with a look at the kind of starships we might one day build. The consequences are profound. What if we master interstellar technologies without needing the Solar System-wide infrastructure many of us assume will precede them? A civilization’s interstellar ‘footprint’ would be radically altered if this is the case, and evidence of mega-engineering among the stars sharply constrained. Then too, how we view what is possible could be transformed by breakthroughs in biology and longevity, all part of the mix as we look at what Nick calls ‘undetermined nodes in future history.”
by J. N. Nielsen
In my previous Centauri Dreams post, The Infrastructure Problem, I sought to make a distinction between fundamentally different forms that a spacefaring civilization might take, one tending toward primarily planetary-based infrastructure, and another tending toward primarily space-based infrastructure. I am always pleased by the insightful comments I receive from Centauri Dreams readers, which never fail to spur me on to further (hopefully improved) formulations. [1] This last post was no exception. I was particularly interested in a comment by William Blight:
“A lot of ifs in this author’s presentation. Large scale industrialization of the moon for power and materials using automation and robotics for rapid bootstrapping is probably the best method for developing a powerful space infrastructure. Colonizing Mars will accelerate the development of propulsion systems. I don’t see how speculation in regard [to] Alcubierre drives has real connection to the development of near-term, space-based industry.”
This comment has helped me to understand the limitations of my exposition. There were a lot of “ifs” in my presentation. Of course what I wrote was highly speculative, as all contemporary writing on interstellar travel must be, but it was speculation with a purpose, and I am concerned that my purpose was not sufficiently clear.
We cannot see the future in detail, but we can distinguish broad patterns of development, just as we can see broad patterns of development in the past, if we look to the past for its overall lessons and not for the ideographic detail that fascinates biographers. Every “if” represents an undetermined node in future history, where under conditions of constraint we may be forced to choose between mutually exclusive alternatives, while given an open future somewhat less subject to constraint (e.g., a future in space where energy and materials are cheaply available, if only we can keep ourselves alive in space long enough to exploit them), an undetermined node represents a point of bifurcation where different communities will take different directions. These are the patterns I am trying to explicate.
Interstellar travel represents an undetermined node in future history, and we do not yet know all the constraints that will bear upon starships once we build them. It would be a mistake to think of interstellar travel in all-or-nothing terms, i.e., either we have the technological capacity or we don’t, because this technological capacity will be developed little-by-little, step-by-step. When an interstellar voyage comes at great personal cost (in time, money, opportunity cost, inconvenience, and discomfort), only a trickle of individuals will possess both the resources and the overwhelming desire to go. As the journey declines in the personal costs it demands, it will appeal to greater numbers of individuals, until the trickle eventually becomes a flood. The relative ease or lack thereof in interstellar travel will be a function of the technologies employed, so that the technologies we will eventually use to travel to the stars will shape the historical structure of that travel, and of the spacefaring civilization that undertakes interstellar travel. In other words, how we get there matters.
There is no more compelling argument for the fact that how we get there matters than the present dependency of the transportation network, and indeed of the whole of industrial-technological civilization, on fossil fuels. We all know that the geopolitics of fossil fuels has decisively shaped the world we live in, and that if some other technology, a non-fossil fuel technology, were the basis of global energy markets, the world today would be a different place.
Future technologies of interstellar flight will shape spacefaring civilization as profoundly as fossil fuels shape our world. Until particular technologies are developed and put into practice, we cannot know which will prove practicable and which will be mere curiosities of little utility, yet by reducing the possibilities for starships down to a few broadly-defined classes, we can sharpen the focus of how we think about the potential niches for spacefaring civilization. Consider this division of potential interstellar transit technologies into four classes:
- Class 0: Very long term interstellar travel, beyond the practicability of generational starships. Another way to think of this would be in terms of interstellar travel on geological time scales.
- Class 1: Generational starships, i.e., starships that would require from one to several generations (measured in ordinary human life spans) in order to reach their destination.
- Class 2: Interstellar transit within the life-span of an individual, measured in months, years, or decades.
- Class 3: Rapid interstellar transit on the order of global transportation today, measurable in hours or days.
This is a very rough and provisional division, and the reader should place no emphasis on the particular divisions I have made or the particular technologies that I cite, but only on the idea that we can divide potential interstellar transit technologies into broadly distinct classes. (The possibilities for interstellar drives are parallel to the possibility of some other industrial-technological civilization in the galaxy, not identical to us, but differing in terms of countless contingencies. The important point is not the identity of a particular technology or civilization, but the capacity it has to serve a particular role.) In each of these classes we can identify a series of technological developments that could shorten the voyage, but the voyage on the whole would remain roughly within the parameters of the classes sketched above, so that the upper edge of class 0 touches the lower edge of class 1, and so forth.
Image: Categories of starships. Credit: Nick Nielsen.
The other variables that enter into the equation of interstellar travel—longevity and destination choice among them—also admit of many possible solutions. Human life might be extended by many different technological means (incremental improvements in the life sciences, regenerative medicine, suspended animation, etc.), or even someday by the simplest of biological means. [2] And once having met the minimum interstellar threshold for a destination, interstellar travelers will have a wide choice of destinations that will affect the length of the trip. A Class 1 starship that would be a generational ship for human beings with an average life-span of today could be considered a Class 2 starship if life spans were considerably lengthened or if suspended animation technology proved to be practicable. The point here is that there is more than one way to approach the problem, and how we solve the problem matters to the kind of spacefaring civilization we eventually build.
The gravitational slingshot technology employed to send the Voyager spacecraft on interstellar trajectories could be further extrapolated with gravitational slingshots around other star systems, which might raise the velocity of a spacecraft to one percent of the speed of light. [3] This could be much faster than the Voyager spacecraft are traveling at present, but still clearly constituting class 0 interstellar transit. Were we to develop biological reconstitution technology that could remain functional for thousands of years, and we launched this on a class 0 starship (like Voyager, i.e., something that we could build with known technologies), we would then begin the era of human interstellar travel.
Image: The Daedalus starship. Credit: Adrian Mann.
A light sail might be at the upper edge of class 0 or the lower edge of class 1 interstellar travel, while a light sail further propelled by a laser might approach the upper edge of class 1. The Daedalus starship design should be considered a class 1 starship, though incremental improvements in fusion technology might boost it to the lower edge of class 2 starships. More exotic drives such as matter-antimatter reaction might qualify as class 2 starships, perhaps attaining the status of a 1G starship (such as I discussed in Stepping Stones Across the Cosmos), which would allow travel throughout our galaxy within an ordinary human lifespan, though relativistic effects would mean that accelerated communities would be temporally disjointed from non-accelerated communities. Even more exotic propulsion systems – whether the Alcubierre drive, the technology to manufacture wormholes at will, or other possibilities not imagined today – would qualify as class 3 starships that would convey us between stars as readily as jet aircraft convey us between continents today.
Image: The Bussard ramjet design. Credit: Adrian Mann.
The technological developments that could shorten the voyage of a particular class of interstellar travel represent technological succession, just as does the sequence of classes itself (which constitutes technological succession on a greater order of magnitude). In many historical cases of technological succession we see the gradual development of improved technologies, as with automobiles or integrated circuits. When technological succession happens in this way it is largely predictable, but technological succession is sometimes disruptive rather than a smooth progression. In the middle of the twentieth century many assumed that human spaceflight would be attained by the gradual improvement in supersonic flight. However, hypersonic flight has proved to be a difficult engineering challenge, and we have not yet mastered it, but chemical rocket technology leapfrogged supersonic flight and put human beings in orbit and on the moon before the gradual technological succession of improving supersonic to hypersonic to escape velocity technology could catch up. It still hasn’t caught up.
Image: Conceptualizing the Alcubierre drive. Credit: Anderson Institute.
Gradual technological succession would take place within classes of starships; disruptive technological succession would occur when one class of starship supersedes another. If we launched a class 0 starship with reconstitution technology on board, and a hundred years later (or even a thousand years later) developed class 2 starship technology, the class 2 starships would overtake the class 0 starship in a way not unlike how jet aircraft overtook propeller-driven aircraft, and chemical rockets overtook jet aircraft. If class 2 starship technology disruptively precedes practicable class 0 or class 1 starship technology, the entire era of generational starships, class 0 and class 1, will be bypassed.
We are not in a position to judge the relative success of technologies only now imagined, but once we have in place a way to differentiate between entirely different classes of starships, we can speak in terms of the kind of spacefaring civilization emergent from any technology capable of building a class x starship. What the particular technology will be is indifferent to our problem; any class x starship will do. With these considerations in mind, I can return to the point of my previous post, The Infrastructure Problem.
To restate the infrastructure problem, any sufficiently advanced class 2 starship, or any class 3 starship, that can be constructed exclusively with terrestrial infrastructure would yield a spacefaring civilization that possessed only a minimal space-based infrastructure. A spacefaring civilization with minimal space-based infrastructure would be unlikely to engage in megastructure engineering and would thus have a much more modest “footprint” in the cosmos than a Kardashevian supercivilization.
If contemporary terrestrial industrial-technological civilization continues in its present development (i.e., if it does not stagnate), and if it is not destroyed, our sophistication in science and technology will likely improve to the point at which we can build at least an advanced class 2 starship (if not a class 3 starship) and fly directly from the surface of Earth to other worlds – an SSTS spacecraft (single-stage to stellar), if you will.
Such a trajectory of development creates its own great filter, as the ongoing existential viability of a terrestrial-based industrial-technological civilization is contingent upon passing through an extended window of vulnerability when we have the technological capacity to destroy ourselves (intentionally through warfare or unintentionally through the toxic byproducts of industrialism) without bothering to exploit the technology we also possess to establish a rudimentary spacefaring civilization with multiple independent centers of civilization tolerant of local extinction, where “local” means “terrestrial.”
Ever since the advent of the Space Age in the middle of the twentieth century there have been ambitious plans to rapidly expand the human presence in space, from the “Collier’s” space program (Man Will Conquer Space Soon!) to O’Neill colonies. To date, none of these ambitious plans have come to fruition, although our technology is considerably more advanced than when humanity first entered space. Only superpower competition has proved to be a sufficient spur to a major space effort. It does not appear, then, that humanity is an “early adopter” of existential risk mitigation by way of space settlement; we are not moving in the direction of creating a spacefaring civilization predicated upon a robust space-based infrastructure.
The trajectory of development that humanity has not taken represents a possibility, a niche for spacefaring civilization, that some other intelligent species might have taken, or might yet take, and the result of taking this space-based infrastructure path of development would be a spacefaring civilization of a structure disjoint from that characterizing spacefaring civilization of a primarily Earth-based infrastructure. [4]
If none of the technologies that would make possible advanced class 2 or class 3 starships could be made sufficiently compact that they could be built on Earth and boosted into space, then a civilization would be forced into a choice between remaining stranded within its solar system or eventually building a space-based infrastructure in order to build a starship (this is an instance of “conditions of constraint” resulting in mutually exclusive alternatives mentioned above). For example, a class 1 starship like Daedalus could not be constructed without space-based infrastructure.
I am not an engineer. I will not be designing any starships. Others will design starships, and others will formulate the ideas that are eventually translated into technologies and designs for interstellar flight. As I see it, these technologies are variables in the equation of the large scale structure of any spacefaring civilization. If there is no solution to the equation of spacefaring civilization, given some particular value for the variable of feasible interstellar travel, then we try to solve it again using a different variable. If there are no solutions at all, then we are stuck in our own solar system and the same is true of any other spacefaring civilization that emerges on any other world. [5]
What interests me is the large scale structure of civilization of any possible spacefaring civilization. I assume if a spacefaring civilization emerges more than once in our universe, these multiple spacefaring civilizations may take multiple paths of development (cf. note [4]), or they may converge upon some particular path of development to spaceflight if the parameters of possible spacefaring technologies are quite narrow. Different solutions to the equation for spacefaring civilizations yield different large scale structures of that civilization. If there is only one solution to the problem, i.e., only one technology for practicable interstellar travel, then this will exercise a strongly convergent force on the structure of any spacefaring civilization and is an equally strong condition of constraint.
From these considerations another typology begins to emerge:
1. There is no solution to the problem of interstellar travel. (Cf. note [5])
2. There is a single solution to the problem of interstellar travel, where “single solution” means only one practicable class of starships. A single class of practicable starships still admits of the possibility of technological succession within this class, so that interstellar civilizations might admit of different stages of development in their mastery of the single practicable interstellar technology.
3. There are multiple solutions to the problem of interstellar travel, so that multiple classes of starships are technologically practicable.
In the first case, all spacefaring civilizations are confined to their star system of origin. We already know this to be false, because the Voyager spacecraft are in interstellar space at this moment. However, if one redefines interstellar travel as to exclude class 0 starships, then humanity remains confined within our solar system in this first case. In the second case, spacefaring civilizations are constrained by technology to the choice of becoming an interstellar civilization or not, but all interstellar civilizations will be constrained by the parameters of the single practicable interstellar technology. In the third case, if a spacefaring civilization achieves interstellar travel, it may do so by multiple means, and interstellar civilizations will be differently constrained according to the technology or technologies they develop (in addition to other factors). [6]
Notes
[1] All of the comments I have received are greatly appreciated, and I regret that I have not responded to each comment individually, but when the reader sees the extent to which this response to a comment runs, it may perhaps be understandable.
[2] The point I am trying to make in this present argument, how we get there matters, applies equally to the technologies of transhumanism, which will not be separate from interstellar travel but will interact with the human exploration of space. Whether human beings are able to travel to distant stars because of greatly extended life-spans, or suspended animation, or reconstitution, how we get to an extended life-span matters, because each technology interacts differently with the individual life and the socioeconomic structures within which the individual finds a place. Similarly, each interstellar propulsion technology interacts differently with the individual life, making use of such propulsion technologies and the socioeconomic structure within which the individual finds a place.
[3] In a post titled, “Galactic Grand Tours, and strengthening Fermi’s Paradox” on the Well-Bred Insolence blog, Duncan Forgan writes, “…if a probe carries out a series of slingshots as it tours the Galaxy, the probe can be accelerated to approximately 1% of the speed of light without shipping enormous amounts of fuel (bear in mind Voyager 1 is travelling at 0.003% of lightspeed).”
[4] An alien civilization might take a different technological path due to different intellectual endowments. It may be that a science and technology, which remains opaque to the kind of minds that we have, will be readily mastered by an intelligent species with a different kind of mind, and vice versa. Bertrand Russell provided an imaginative example that serves as a kind of thought experiment in this respect:
We are certainly stimulated by our experience to the creation of the concept of number – the connection of the decimal system with our ten fingers is enough to prove this. If one could imagine intelligent beings living on the sun, where everything is gaseous, they would presumably have no concept of number, any more than of “things.” They might have mathematics, but the most elementary branch would be topology. Some solar Einstein might invent arithmetic, and imagine a world to which it would be applicable, but the subject would be considered too difficult for schoolboys. (Bertrand Russell, The Philosophy of Bertrand Russell, edited by Paul Arthur Schilpp, Evanston and Chicago: Northwestern University, 1944, p. 697.)
These considerations apply both to the large-scale structure of a spacefaring civilization as well as the particular technologies any such civilization pursues in the attempt to master interstellar flight.
[5] This is the position of Peter D. Ward and Donald Brownlee (best known for their book Rare Earth: Why Complex Life is Uncommon in the Universe): “The starships of TV, movies, and novels are products of wishful thinking. Interstellar travel will likely never happen, meaning we are stranded in this solar system forever. We are also likely to be permanently stuck on Earth. It is our oasis in space, and the present is our very special place in time. Humans should enjoy and cherish their day in the Sun on a very special planet… Our experience on Earth is probably repeated endlessly in the cosmos. Life develops on planets but it is ultimately destroyed by the light of a slowly brightening star. It is a cruel fact of nature that life-giving stars always go bad.” (The Life and Death of Planet Earth: How the New Science of Astrobiology Charts the Ultimate Fate of Our World, New York: Henry Holt and Company, 2002, pp. 207-208). In this case, the possibility of a large scale spacefaring civilization does not disappear (though Ward and Brownlee explicitly exclude this possibility also), but it takes on a different form, and any communication between advanced industrial-technological civilizations would have to come about by way of SETI and METI. The impossibility of interstellar travel is entirely compatible with megascale engineering within our own solar system, which megastructures could include the building of vast EM spectrum communications antennae capable of communicating across interstellar distances.
[6] A further distinction could be made in the third case between “more than one solution to the problem of interstellar travel exists” (i.e., at least two solutions exist to the problem of interstellar travel), and, “all classes of interstellar travel are technologically practicable.”
Thoughts on a Spacecraft’s Rebirth
According to a recent NASA news release, the agency has never before signed the kind of agreement it has made with Skycorp, Inc., a Los Gatos, CA-based firm that will now attempt contact with the International Sun-Earth Explorer-3 (ISEE-3) spacecraft. You’ll recall that this is the vehicle that scientists and space activists alike have been talking about resurrecting now that, having completed its studies of the solar wind in 1981 and later comet observations, it is making its closest approach to the Earth in more than thirty years (see ISEE-3: The Challenge of the Long Duration Flight).
According to its website, Skycorp is in the business of bringing “…new technologies, new approaches, and reduced cost to the manufacture of spacecraft and space systems.” Founded in 1998, the company signed a Space Act Agreement with NASA for the use of the International Space Station in 1999, and qualified the first commercial payload used in the filming of a television commercial (for Radio Shack) in 2001. In addition to its ISEE-3 involvement, Skycorp is now working on an orbit servicing system (for Orbital Recovery Corporation) and the design of lunar surface systems with NASA.
The document NASA has now signed is a Non-Reimbursable Space Act Agreement (NRSAA) with Skycorp that involves not just contact with the ISEE-3 spacecraft but, possibly, command and control over it. ISEE-3 will near the Earth this August, and the agreement lays out the variety of what NASA describes as “technical, safety, legal and proprietary issues” that will need to be addressed before contacting and re-purposing the spacecraft can be attempted.
“The intrepid ISEE-3 spacecraft was sent away from its primary mission to study the physics of the solar wind extending its mission of discovery to study two comets.” said John Grunsfeld, astronaut and associate administrator for the Science Mission Directorate at NASA headquarters in Washington. “We have a chance to engage a new generation of citizen scientists through this creative effort to recapture the ISEE-3 spacecraft as it zips by the Earth this summer.”
It’s hard not to get excited about the prospects here. The ISEE-3 Reboot Project works with a spacecraft that, although inactive for many years, still contains fuel and probably functional instruments. Of course, ISEE-3’s reactivation will be handled remotely, but in the 1960s this would have made a great scenario for a short story in one of the science fiction magazines. In that era, ideas like in-space repair of satellites and salvage and re-use of older equipment by human crews were concepts made fresh by the sudden progress of the manned space program. After all, we were doing space walks!
I’m remembering “The Trouble with Telstar,” a 1963 story by John Berryman (the SF writer, not the poet) that brought home to readers what would be involved in maintaining a space infrastructure. In the editorial squib introducing it, John Campbell wrote: “The real trouble with communications satellites is the enormous difficulty of repairing even the simplest little trouble. You need such a loooong screwdriver.” It was a lesson we’d learn again in spades with the Hubble repairs. Berryman, a writer and engineer who died in 1988, followed up with “Stuck,” another tale of space repair that inspired the gorgeous John Schoenherr cover at the right.
Fortunately, the reactivation of ISEE-3 isn’t a hands-on repair job and we can attempt to salvage this bird from Earth. Current thinking is to insert the spacecraft into an orbit at the L1 Lagrangian point, at which time the probe would be put back into operations. In this sense, ISEE-3 is an interesting measure of our ability to build long-term hardware. Like Voyager, the diminutive spacecraft was never intended for activities over this kind of time-frame, but new operations do appear possible. Everything depends, of course, upon the satellite’s close approach this summer, for if communications cannot be established, it will simply continue its orbit of the Sun.
So we have a “citizen science” program hard at work on a novel problem, with the help of the agency that put the spacecraft into motion all those years ago. Any new data from a re-born ISEE-3 is to be broadly shared within the science community and the public, offering a useful educational tool showing how we gather data in space and disseminate the results. We’ll also learn a good deal about how spacecraft endure the space environment over a span of decades, information that will contribute to our thinking about future probes on long missions and potentially extendable observation windows.
Not bad for a satellite sent out over three decades ago to study how the solar wind can affect satellites in Earth orbit and possibly disrupt our sensitive technological infrastructure. I’m now wondering whether there are other spacecraft out there that might be brought back to life, and reminded that when we build things to last, we can discover uses that the original designers may not have dreamed of. That’s a lesson we’ll want to remember as we create mission concepts around any new space hardware.
Exomoons: A New Technique for Detection
A friend asked me the other day whether my interest in exomoons — moons around exoplanets — wasn’t just a fascination with the technology of planet hunting. After all, we’ve finally gotten to the point where we can detect and confirm planets around other stars. An exomoon represents the next step at pushing our methods, and a detection would be an affirmation of just how far new technology and ingenious analysis can take us. So was there really any scientific value in finding exomoons, or was the hunt little more than an exercise in refining our tools?
I’ve written about technology for a long time, but the case for exomoons goes well beyond what my friend describes. We’ve found not just gas giants but ‘super-Earths’ in the habitable zones of other stars, and it’s a natural suggestion that around one or both classes of planet, an exomoon might he habitable even if the parent world were not. It’s a natural assumption that moons exist around other planets elsewhere as readily as they do around the planets of our own Solar System, and given the sheer number of planets out there, the prospect of adding potential sites for life — perhaps more common sites than habitable planets themselves — is irresistible.
Then in catching up with work after my recent trip, I noticed René Heller’s work on the exomoon question, as reported in Astrobiology Magazine. Heller (McMaster University, Ontario) is proposing a new method of detection that involves the particular eclipsing effect of moons during the kind of transit studies that Kepler has provided so much data on. What is striking here is that if Heller is right, we may be able to detect not just moons several times larger than Ganymede — this is where the current state of the art seems to be — but moons much smaller still, on the scale of the moons we find in our own Solar System.
Heller’s paper, recently published in The Astrophysical Journal, picks up on further scientific advantages in studying exomoons. Such objects can offer us insights into how exoplanets formed. Consider this (internal references omitted for brevity):
The satellite systems around Jupiter and Saturn, for example, show different architectures with Jupiter hosting four massive moons and Saturn hosting only one. Intriguingly, the total mass of these major satellites is about 10-4 times their planet’s mass, which can be explained by their common formation in the circumplanetary gas and debris disk…, and by Jupiter opening up a gap in the heliocentric disk during its own formation… The formation of Earth is inextricably linked with the formation of the Moon…, and Uranus’ natural satellites indicate a successive ‘collisional tilting scenario’, thereby explaining the planet’s unusual spin-orbit misalignment.
And so on. Between astrobiology and planet formation, I’d say there is plenty of reason to be interested in exomoons, though my friend is right that I still get jazzed by the ability of skilled researchers to develop the strategies we need for their detection. We’ve followed the fortunes of the Hunt for Exomoons with Kepler (HEK) for some time in these pages, and it’s fascinating to me that what Heller is proposing would also make use of the abundant Kepler data. But while HEK works through the study of variations in transit timings and durations, Heller’s method, which he calls the ‘Orbital Sampling Effect,’ relies on a different kind of analysis.
Addendum: My mistake. A note from exomoon hunter David Kipping, who heads up the Hunt for Exomoons with Kepler, sets this straight. Let me quote it:
“This is not true, actually HEK uses dynamics and photometric effects in combination. We model both the moon transit effects (which Heller is talking about) plus the dynamical perturbations, such as transit timing and duration variations, simultaneously. The difference is that Heller stacks all of the transit data on top of itself and we do not. An important point to bear in mind is that this stacking process does not give you any more signal-to-noise than using the entire time series and fitting it globally, as HEK does. Since one does not actually gain any greater sensitivity by using the OSE method, one cannot finds moons smaller than the sensitivity limits we’ve achieved thus far in HEK and I can tell you that Galilean satellites are only detectable in very exceptional cases- generally Earth-like radius sensitivity is the norm.
“Let me say though, the OSE method is fast and computationally cheap but lacks the dynamics of our models, which ultimately allow for an exomoon confirmation. So I think this method could be useful tool for quickly scanning the Kepler data to identify interesting anomalies worthy of further analysis.”
And back to the original post:
What more can we tease out of an exoplanetary transit? Observation of a moon orbiting an exoplanet around its equatorial midline and passing in front of the planet, then behind it, should — over the course of time and numerous observations — build up a series of dots representing its position at any given moment in its orbit. With enough observations, the effect is of two ‘wings’ coming out of the sides of the planet, an effect that will appear lighter at the inner edges and darker at the outer edges. Astrobiology Magazine explains:
That’s because when the moon reaches the extent of its orbit and then starts circling back around the planet, its positions overlap more in a tighter space. As such, the “wingtips” look darker; that is, there is increased eclipsing of background starlight at the moon’s farthest apparent positions from the planet.
And of course to spot this effect requires constant observation of the star over a long period of time, allowing the moon to complete a large number of orbits “in order for its light-blocking effect to preferentially stack up at the wingtips.” And voila, what we have with the Kepler spacecraft is an observatory that gave us precisely this, observations of about 150,000 stars through its long stare of data gathering, some four years before equipment failure laid it low. Heller’s point is that we need wait for no future technology to hunt today for moons like those in our Solar System.
Image: This figure from Heller’s paper depicts the effects of a transiting exoplanet with exomoon averaged over time. We would expect a drop in starlight as the exoplanet moves in front of the host star, but Heller’s method focuses on the change to the lightcurve caused by the exomoon before and after it crosses the stellar disk, an effect which repeats on the other side. Credit: René Heller.
The ‘stacking up’ effect of these transit observations over time as the moon and planet transit the star in different configurations allows the detection. Because we need plentiful statistical data to make all this work, red dwarf stars are ideal candidates because habitable zone planets there have extremely short years and thus make many transits. According to Heller, moons down to Ganymede size should be detectable around M-dwarfs, while around warmer orange dwarf stars, exomoons about ten times Ganymede’s mass would be within range. G-class stars like the Sun are not represented well enough in the Kepler data because they lack sufficient transits.
The minute effects discernible in an exoplanet’s regular transit are what make the exomoon detection possible. Heller notes that the Orbital Sampling Effect (OSE) yields data indicative of the moons’ radii and planetary distances, while study of the planet’s transit timing variations (TTV) and transit duration variations (TDV), in conjunction with Orbital Sampling Effect, allow measurements of the moon’s mass. More complex transit signatures could, using this method, even allow the detection of multiple exomoon candidates.
The paper is Heller, “Detecting extrasolar moons akin to solar system satellites with an orbital sampling effect,” The Astrophysical Journal, Vol. 787, No. 1 (2014), abstract and preprint available. Thanks to Dave Moore for the pointer to this work.
A New Marker for Planet Formation
Given how many planet-hosting stars we’re finding, any markers that can tell us which are most likely to have terrestrial worlds would be welcome. New work out of Vanderbilt University is now providing us with an interesting possibility. Working with the university’s Keivan Stassun, graduate student Trey Mack has developed a model that studies the chemical composition of a given star and relates it to the amount of rocky material it has ingested during the course of its lifetime. Stars with a high amount of such material may be places where small, terrestrial worlds are rare.
What Mack has done is to look at the relative abundance of fifteen specific elements. According to this Vanderbilt news release, he was most interested in elements with high condensation temperatures like aluminum, silicon, calcium and iron, the kind of materials that become building blocks for planets like the Earth. In this context, it’s important to remember that stars are 98 percent hydrogen and helium, with all elements heavier than these being referred to as ‘metals.’
Bear in mind how stellar metallicity may be affected during planet formation, as discussed in the paper:
There are at least two planet formation processes that may alter stellar surface abundances: (1) the accretion of hydrogen-depleted rocky material (Gonzalez, 1997), which would result in the enrichment of the stellar atmosphere, and (2) H-depleted rocky material in terrestrial planets may be withheld from the star during their formation, which would result in the depletion of heavy elements relative to H in the stellar atmosphere (Melendez et al. 2009).
All of this can lead to scenarios involving planetary migration:
For the enrichment scenario, Schuler et al. (2011a) suggest that stars with close-in giant planets (?0.05 AU) may be more enriched with elements of high condensation temperature (TC). This is thought to be a result of giant planets which form in the outer planetary system migrating inward to their present close-in positions. As they migrate, they can push rocky material into the host star (e.g., Ida & Lin 2008; Raymond et al. 2011). For the depletion scenario, Melendez et al. (2009) and Ram?rez et al. (2009) propose that the depletion of refractory elements in Sun-like stars may correlate with the presence of terrestrial planets.
For the Vanderbilt work, the wide binary pair HD 20782/81 proved a useful study, chosen by Mack because both stars have planets and both evidently condensed out of the same cloud of dust and gas, thereby beginning their lives with the same chemical compositions. To my knowledge, this is the only known wide binary in which both stars host detected planets. The two stars are G-class dwarfs like the Sun, one being orbited by two Neptune-class planets, the other by a single Jupiter-size world in a highly eccentric orbit. The spectra of the stars indicates that both show an abundance of refractory materials significantly higher than the Sun.
The abundances of these metals is high enough, in fact, to indicate that each of the two stars would have had to consume an amount of rocky material equal to 10-20 Earth masses to produce the observed spectra. On that score it is significant that both stars host giant planets on eccentric orbits closing as tightly as 0.2 AU. Mack summarizes the finding:
“Imagine that the star originally formed rocky planets like Earth. Further, imagine that it also formed gas giant planets like Jupiter. The rocky planets form in the region close to the star where it is hot and the gas giants form in the outer part of the planetary system where it is cold. However, once the gas giants are fully formed, they begin to migrate inward and, as they do, their gravity begins to pull and tug on the inner rocky planets. With the right amount of pulling and tugging, a gas giant can easily force a rocky planet to plunge into the star. If enough rocky planets fall into the star, they will stamp it with a particular chemical signature that we can detect.”
Image: What if we could determine if a given star is likely to host a planetary system like our own by breaking down its light into a single high-resolution spectrum and analyzing it? A spectrum taken of the Sun is shown above. The dark bands result from specific chemical elements in the star’s outer layer, like hydrogen or iron, absorbing specific frequencies of light. By carefully measuring the width of each dark band, astronomers can determine just how much hydrogen, iron, calcium and other elements are present in a distant star. The new model suggests that a G-class star with levels of refractory elements like aluminum, silicon and iron significantly higher than those in the Sun may not have any Earth-like planets because it has swallowed them. Credit: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF.
With this in mind, the possibility that either of the binary twins hosts terrestrial planets is sharply reduced. The star orbited by two Neptune-class planets seems to have ingested more rocky material than its twin, with Mack and Stassun speculating that the two planets proved to be more efficient orbital disruptors for any terrestrial worlds that may have once been there. Even so, the other star, orbited by a Jupiter-class world, evidently pushed a large amount of rocky material into its star as well. The chemical composition is telling us that G-class stars with such high metallicities probably lack the kind of inner rocky planets many astronomers are searching for.
If these signatures are born out by subsequent study, we may have found a way to quickly determine whether a given G-class star is likely to have terrestrial planets, simply by analyzing its chemical composition. Several previous studies have linked star metallicity with planet formation, with one concluding that gas giants are found around high-metallicity stars, while terrestrial planets can be found around stars with a wide range of metal content. This work extends the use of metals as a marker in interesting new directions, playing off the link between a G-class star’s chemical composition and the kind of solar system it is likely to produce.
The paper is Mack et al., “Detailed Abundances of Planet-Hosting Wide Binaries. I. Did Planet Formation Imprint Chemical Signatures in the Atmospheres of HD 20782/81?” The Astrophysical Journal Vol. 787, No. 2 (2014), p. 98 (abstract / preprint).
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