Centauri Dreams
Imagining and Planning Interstellar Exploration
Rethinking SETI’s Targets
Have you ever given any thought to intergalactic SETI? On the face of it, the idea seems absurd — we have been doing SETI in one form or another since the days of Project Ozma and without result. If we can’t pick up radio signals from nearby stars that tell us of extraterrestrial civilizations, how could we expect to do so at distances like M31’s 2.573 million light years, not to mention even the closest galaxies beyond? Herein lies a tale, for what intergalactic SETI exposes us to is the baldness of our assumptions about the overall SETI attempt, that it is most likely to succeed using radio wavelengths, and that it may open up two-way communications with extraterrestrials. It’s the nature of these assumptions that we need to explore today.
The Visibility of a Galactic Culture
Let’s suppose, for example, that Nikolai Kardashev’s thoughts about types of civilizations are compelling enough to put to the test. A Kardashev Type III civilization is one that is able to exploit the energy resources not just of its home star but of its entire galaxy. So unimaginably beyond our present capabilities is a Kardashev Type III that we scarcely know how to describe it, but it is within the realm of reason that signs of astro-engineering on this scale might be detectable in at least nearby galaxies if such a civilization had gone to work on them. And indeed, James Annis has made such a study, concluding that neither our Milky Way nor M31 or M33, our two large, neighboring galaxies, has been transformed by the work of a Type III civilization.
Image: M33, the Triangulum Galaxy. We’ve only begun to investigate whether nearby galaxies like this one might show signs of astro-engineering on a gigantic scale. Civilizations a billion years or more older than our own might be capable of feats detectable from great distance. Credit: Adam Block/NOAO/AURA/NSF.
It should hardly be necessary to point out how preliminary such results are, and how rare such studies have been. What’s striking about Annis (and related work by Richard Carrigan and P.S. Wesson) is that these scientists are pursuing ideas that are well outside the SETI mainstream. There is a new paradigm here, one that operates without any notion of ‘contact’ and subsequent exchange of ideas between civilizations. It is a search for artifacts, for artificial structure and signs of engineering. It is all about discovery. And just as we can have no two-way conversation with Mycenaean Greece as we dig for information about the era of Agammemnon, we may with this stellar archaeology discover something just as unreachable but likewise well worth the study.
Toward a Dysonian SETI
In a recent paper, Robert Bradbury, Milan ?irkovi? (Astronomical Observatory, Belgrade) and George Dvorsky (Institute for Ethics and Emerging Technologies) consider whether intergalactic SETI may be an example of what they call a ‘Dysonian’ approach to SETI, one that is a ‘middle ground’ between the traditional radio-centric view (with contact implications) and the hostile reaction of SETI detractors who see no value in the enterprise whatsoever and think the money better spent elsewhere. The nod to Freeman Dyson is based on the latter’s conjecture that a truly developed society would surmount the limits of planetary living space and energy by building a Dyson shell, capturing most or all of the energy from the star near which it lived.
A Dyson sphere immediately changes the terms of SETI because it is in principle detectable, but unlike nearby radio signals (either from a beacon or as unintentional ‘leakage’ from a civilization’s activities), a Dyson shell might be spotted at great astronomical distances through its infrared signature. Carl Sagan was one of the first to pick up on the idea and ponder its implications. Dyson was much in favor of attacking the question in a disciplined way, using our astronomical tools, as he once wrote, “…to transpose the dreams of a frustrated engineer into a framework of respectable astronomy.” And here again, we have seen attempts, especially by the aforementioned Richard Carrigan, to study infrared data for signs of such Dyson constructs.
The new direction in SETI that the three authors of the new paper champion is one that employs a broader set of tools. Rather than limiting itself to radio dishes or dedicated optical facilities, it broadens our workspace for extraterrestrial civilizations to include astronomical data that can be gathered in tandem with other research projects, scanning a far wider and deeper field. In the authors’ view, Dysonian SETI also takes into account new developments in astrobiology and even extends into computer science and the possibility of post-biological intelligence. They advocate a Dysonian SETI drawing on four basic strategies to supplement older methods:
- The search for technological products, artifacts, and signatures of advanced technological civilizations.
- The study of postbiological and artificially super-intelligent evolutionary trajectories, as well as other relevant fields of future studies.
- The expansion of admissible SETI target spectrum.
- The achievement of tighter interdisciplinary contact with related astrobiological subfields (studies of Galactic habitability, biogenesis, etc.) as well as related magisteria (computer science, artificial life, evolutionary biology, philosophy of mind, etc.)
The expansion of SETI into these areas would not replace ongoing SETI methods but would significantly expand the overall process in line with the great goal of learning whether other intelligent beings share the galaxy and the nearby universe with us. The paper offers more fruitful speculation than I can fit into a single entry, so we’ll be looking at these ideas over the course of the next few days. If there really is a Great Silence, to use David Brin’s phrase, these authors argue it’s one that we can only ponder usefully if we broaden our search toward the potentially observable achievements of cultures far more advanced than our own. That study has only recently begun.
The paper is Bradbury, ?irkovi? and Dvorsky, “Dysonian Approach to SETI: A Fruitful Middle Ground?” JBIS Vol. 64 (2011), pp. 156-165.
Dawn Explores Vesta’s Chemistry
The Dawn spacecraft, orbiting Vesta since July of last year, reached its lowest altitude orbit in December, now averaging 210 kilometers from the asteroid’s surface. Ceres is Dawn’s next stop, but that journey won’t begin until the close-in work at Vesta is complete, with the craft in its low altitude mapping orbit for at least ten weeks and then another period at higher altitudes before Dawn leaves Vesta in late July. The spacecraft’s Gamma Ray and Neutron Detector (GRaND) instrument is already telling us much about the giant asteroid’s surface composition.
In fact, the five weeks of mapping at low-altitude have provided the first look at global-scale variations on Vesta. GRaND measures the abundance of elements found in planetary surfaces, and while its investigations are still in the early stages of analysis, it’s clear that Vesta’s surface varies widely as opposed to the mostly uniform composition of smaller asteroids. We know that Vesta developed a core, mantle and crust, making it something closer to a planet than an asteroid. We’ll learn a great deal more as the low-altitude mapping continues and new results arrive.
Meanwhile, have a look at this close-in Dawn image, showing a crater with smaller craters on its edge, with rough textures in the crater wall (far right) that may indicate underlying bedrock. The image shows an area in the Rheasilvia basin in the south polar region, obtained with Dawn’s framing camera on December 18, 2011 — the distance to the surface here is 272 kilometers.
Image: An area in Vesta’s south polar region, centered around 58.3 south latitude and 283.7 degrees east longitude. Acquired during the LAMO (Low Altitude Mapping Orbit) phase of the mission, the image has a resolution of about 25 meters per pixel. Credit: NASA/ JPL-Caltech/ UCLA/ MPS/ DLR/ IDA.
Ion Propulsion’s Slow Push
Let’s keep an eye not just on Dawn’s science results but on the performance of its thrusters as we continue to shake out ion propulsion and tweak its capabilities with missions to the outer system. The spacecraft’s thrusters apply more than 1000 volts to accelerate charged particles (ions) that are expelled out the back at speeds up to 40 kilometers/second. Xenon is used in these thrusters because it is a relatively massive atom, offering higher thrust than other possible propellants. Even so, the acceleration is tiny, underlining the fact that ion propulsion is effective not just in its efficiency but also in its cumulative acceleration over long periods.
As the Dawn team reminds us, the force of an ion thruster on the spacecraft is comparable to the weight of a single sheet of paper. But their enormous fuel efficiency means that the thrusters can continue this tiny push not just for days but for years, allowing the effects to build. Dawn chief engineer Marc Rayman has fittingly called ion thrusting ‘acceleration with patience.’ In a 2006 report on the mission, Rayman summarized the advantages of this kind of propulsion:
All else being equal, for the same amount of propellant, a spacecraft equipped with ion propulsion can achieve 10 times the speed of a craft outfitted with normal propulsion, or a spacecraft with ion propulsion can carry far less propellant to accomplish the same job as a spacecraft using more standard propulsion. This translates into a capability for NASA to undertake extremely ambitious missions such as Dawn.
The rate at which xenon is flowed through the thruster is very low. At the highest throttle level, the system uses only about 3.25 milligrams/second, so 24 hours of continuous thrusting would expend only 10 ounces of xenon. Because the xenon is used so frugally, the corresponding thrust is very gentle. The main engine on some interplanetary spacecraft may provide about 10,000 times greater thrust but, of course, such systems are so fuel-hungry that their ultimate speed is more limited.
Dawn uses a solar array to power up the ion propulsion system. At maximum throttle, Rayman notes, the acceleration is equivalent to about 7 meters/second/day, so a full day of thrusting can change the spacecraft’s speed by something like 24 kilometers per hour. By mission”s end, the spacecraft will have accumulated over five years of total thrust time, with an effective change in speed of 11 kilometers per second. Slow but steady gets you to your target, and Dawn’s mission planners have noted that using conventional chemical propulsion, the extra mass would have put the mission well beyond budget, yet another plus for frugal ion technologies.
A Comet Consumed by the Sun
Imagine what we could do if we could attain speeds of 640 kilometers per second. That’s the velocity of a comet recently tracked just before passing across the face of the Sun and apparently disintegrating in the low solar corona. I’m just musing here, but it’s always fun to muse about such things. 640 kilometers per second drops the Alpha Centauri trip from 74,600 years-plus (Voyager-class speeds) to less than 2000 years. A long journey, to be sure, but moving in the right direction, and in any case, these are speeds that would allow exploration deep into the outer system. We’re a long way from such capabilities, but they’re a rational goal.
But back to the comet. The object was discovered on July 4, 2011 and designated C/2011 N3 (SOHO), the latter a reference to the ESA/NASA Solar and Heliospheric Observatory, whose Large Angle and Spectrometric Coronagraph made the catch. On July 6, NASA’s Solar Dynamics Observatory was able to pick up the comet some 0.2 solar radii off the Sun, where it was tracked for a mere 20 minutes before evaporating some 100,000 kilometers above the solar surface. The Extreme-Ultraviolet Imager aboard one of NASA’s Solar-Terrestrial Relations Observatories (STEREO) was also able to observe the comet, allowing researchers to get a good read on its makeup, as the lead author of the paper on this work, Karel Schrijver (Lockheed Martin Solar and Astrophysics Laboratory), points out:
“This unprecedented passage of a comet through the solar atmosphere in view of our AIA cameras [the Atmospheric Imaging Assembly aboard the SDO] presented us with a remarkable opportunity. As we witnessed this comet evaporate as it traversed a known amount of space over a specific period of time, we were able to work backward to estimate its mass just before it reached the Sun. We’ve been able to bracket its size as between 150 and 300 feet long, with a greater likelihood that it lies at the upper end of that range. And it most likely weighed in at as much as 70,000 tons, giving it about the weight of an aircraft carrier, when it first became visible to AIA.”
Image: Although it’s not C/2011 N3, this Sungrazer was also destroyed, its fiery fate recorded by the SOHO spacecraft’s Large Angle Spectrometric Coronagraph (LASCO) on December 23, 1996. LASCO uses an occulting disk, partially visible at the lower right, to block out the otherwise overwhelming solar disk allowing it to image the inner 5 million miles of the relatively faint corona. The comet is seen as its coma enters the bright equatorial solar wind region (oriented vertically). Spots and blemishes on the image are background stars and camera streaks caused by charged particles. Credit: LASCO, SOHO Consortium, NRL, ESA, NASA.
In addition to the image above, images of the ‘string of pearls’ comet Shoemaker-Levy 9 breaking apart before impact with Jupiter in 1994 inevitably come to mind, especially when we learn that C/2011 N3 likewise broke into a series of large chunks ranging up to 45 meters in size as it began to disintegrate. The comet’s coma — the envelope of ice, dust and gas surrounding its nucleus — is thought to have been some 1300 kilometers across, with a cometary tail approximately 16,000 kilometers long. The breakup of the comet was further flagged by pulsations in the brightness of the tail, a phenomenon Schrijver’s team was able to put to good use:
“I think the light pulses in the tail were one of the most interesting things we witnessed,” said Schrijver. “The comet’s tail gets brighter by as much as four times every minute or two. The comet seems first to put a lot of material into that tail, then less, and then the pattern repeats. Only because of these pulses can we measure how fast the tail falls behind the comet as its gases collide with those in the Sun’s atmosphere. And that, in turn, helps us measure the comet’s weight.”
A cometary demise is a spectacular event, but it turns out that the SOHO spacecraft has observed more than 2000 comets as they approached the Sun. The so-called Kreutz Sungrazers are comets whose orbits take them within one to two solar radii of the photosphere every 500 to 1000 years. This is the largest known group of comets, one which is thought to have resulted from the breakup of a massive progenitor body several thousand years ago. The smaller Kreutz group comets rarely survive perihelion. While the SOHO observatory now tracking a Sun-grazing comet on an average of once every three days, those that make it into the solar corona should be useful tools for extending our knowledge of cometary properties.
The paper is Schrijver et al., “Destruction of Sun-Grazing Comet C/2011 N3 (SOHO) Within the Low Solar Corona,” Science Vol. 335 no. 6066 pp. 324-328 (20 January 2012). Abstract available.
NIAC Looking for New Proposals
NASA’s Innovative Advanced Concepts program has issued a second call for proposals, following the selection of its first round of Phase I concepts in 2011. NIAC (formerly the NASA Institute for Advanced Concepts) ran from 1998 to 2007 in the capable hands of Robert Cassanova, who is now external council chair for the new organization. After a four year interregnum, the program returned in 2011 with the goal of funding “early studies of visionary, long term concepts – aerospace architectures, systems, or missions (not focused technologies).” The 2011 effort resulted in funding for 30 advanced technology proposals, each of them receiving $100,000 for one year of study.
The new call for proposals continues the NIAC theme of looking for ideas that are both innovative and visionary, while remaining at an early stage of development, considered as being ten years or more from actual use on a mission. Approximately fifteen proposals are likely to win funding in the 2012 selection, with short proposals of no more than two pages in length accepted until February 9. Authors of the concepts that make the first cut will then be notified to submit a full proposal due on April 16. According to the NASA news release, the solicitation is “open to all U.S. citizens and researchers working in the United States, including NASA civil servants.”
I remember paging through reports and presentations from the first NIAC when working on my Centauri Dreams book — those reports are still available online and make for a compelling set of ideas, from antimatter collection strategies to micro-scale laser sails for deep space exploration. The new NIAC’s Phase I studies are equally provocative, and you might want to look through NIAC head Jay Falker’s presentation at the 2011 meeting to see not only an overview of the program but a set of posters explaining each of the Phase I studies chosen.
James Gilland (Ohio Aerospace Institute), for example, looks at ambient plasma wave propulsion, noting that an environment of magnetic fields and plasmas is associated not only with many planets but the Sun itself. Because plasmas with magnetic fields can support a variety of waves that can transmit energy and/or pressure, Gilland sees an opening for propulsion. Quoting the precis:
This? concept? simply? uses ?an? on?board? power? supply and? antenna? on? a? vehicle? that? operates ?in ?the? existing? plasma.? The? spacecraft ?beams? plasma? waves? in? one? direction? with? the? antenna,? to? generate? momentum? that? could? propel? the? vehicle ?in ?the? other? direction,? without? using? any? propellant? on ?the? space?ship. ?Such? a? system? could? maneuver? in ?the? plasma? environment? for? as ?long? as ?its? power? supply ?lasts,? without? refueling.? One? particular ?wave? to? consider ?is? the? Alfven? wave,? which? propagates ?in? magnetized? plasmas? and? has? been? observed? occurring ?naturally ?in? space.
Steven Howe (Universities Space Research Association), who devised the ingenious ‘antimatter sail’ concept that was analyzed in Phase I and II studies for the first NIAC, considers production methods for Pu-238, thus keeping Radioisotope Thermoelectric Generators (RTGs) in the game at a time when Pu-238 supplies are scarce. John Slough (MSNW LLC) looks at ‘a small scale, low cost path to fusion-based propulsion’ by using propellant to compress and heat a magnetized plasma to fusion conditions. Grover Swartzlander (Rochester Institute of Technology) explores so-called ‘optical lift’ and its potential to enhance solar sail missions.
Image: John Slough’s Phase I study, a ‘small scale, low cost path’ to fusion?based propulsion. It is accomplished by employing the propellant to compress and heat a magnetized plasma to fusion conditions, and thereby channel the fusion energy released into heating only the propellant. Passage of the hot propellant through a magnetic nozzle rapidly converts this thermal energy into both directed (propulsive) energy and electrical energy. Credit: John Slough/NIAC.
I won’t go through all of these Phase I ideas here, as they’re available in Falker’s excellent presentation, and you can also see some of them discussed in Enabling the future: NASA call for exploration revolution via NIAC concepts, an article on the NASASpaceflight.com site. NASA’s 2012 solicitation page for NIAC is here. Ahead for NIAC is the 2012 Spring Symposium, planned for March 27-29 at the Westin Hotel in Pasadena, CA, where current NIAC fellows will give presentations about their Phase I research. Public attendance at the meeting is encouraged.
Exoplanetary Ring Systems and Their Uses
What to say about an extrasolar ring system that has already had its four distinct rings named? Rochester, Sutherland, Campanas and Tololo are the Earth-bound sites where the unusual system was first detected and analyzed, and the international team of researchers involved thought them suitable monickers for the four rings thus far detected. The light curve of the young, Sun-like star they’ve been studying in the Scorpius-Centaurus association — a region of massive star formation — shows what appears to be a dust ring system orbiting a smaller companion occulting the star.
The data here come from SuperWASP (Wide Angle Search for Planets) and the All Sky Automated Survey (ASAS) project. The star in question is 1SWASP J140747.93-394542.6, which displays a complex eclipse event with, at some points, 95 percent of the light from the star being blocked by dust. Similar in mass to the Sun, the star is only about 16 million years old, and lies about 420 light years away from the Solar System. Eric Mamajek (University of Rochester) thinks the object at the center of the system is either a low-mass star, a brown dwarf, or a planet, but it will take follow-up studies to determine the answer. Says Mamajek:
“When I first saw the light curve, I knew we had found a very weird and unique object. After we ruled out the eclipse being due to a spherical star or a circumstellar disk passing in front of the star, I realized that the only plausible explanation was some sort of dust ring system orbiting a smaller companion—basically a ‘Saturn on steroids.’ This marks the first time astronomers have detected an extrasolar ring system transiting a Sun-like star, and the first system of discrete, thin, dust rings detected around a very low-mass object outside of our solar system.”
Image: Rings found in a young stellar system may offer clues to planet formation, including the moons around gas giants. Credit: Michael Osadciw/University of Rochester.
The discovery is unusual but it’s not likely to be our last look at such systems. A circumstellar disk can have a huge radius, and we can imagine seeing one star in a binary system with such a disk that would regularly eclipse the other star. The same is true of a giant planet in a young system that is building moons out of its own circumplanetary disk. Seeing such disks in eclipse could tell us much about the era of planet formation, and we already have long-period eclipsing systems like EE Cephei, Epsilon Aurigae and OGLE-LMC-ECL-17782 to build on. In fact, the eclipses associated with Epsilon Aurigae last almost 2 years, and we may be seeing “…rings and gaps in a forming planetary system around a lower mass secondary,” as the paper on this work notes.
But we could also be looking at moons in formation around a gas giant, a subject that calls up our recent discussion of the HEK (Hunt for Exomoons with Kepler) project and the detectability of these objects. The exomoon discussion in the paper is worthy of an extended quote:
A simple thought experiment illustrates the potential observability of moon-forming circumplanetary disks around young gas giants (and indeed this was the back of the envelope calculation that spawned our interest in the interpretation of the eclipsing star)… If one were [to] take the Galilean satellites of Jupiter and grind them up into dust grains, and spread the grains uniformly between Jupiter and Callisto’s orbit, one would have a dusty disk of optical depth O(105). The size of such a proto-moon disk in this case would be a few solar radii – i.e. large enough and optically thick enough to potentially eclipse a star’s light. Of course such a disk need not be face on – more likely the disk would have a nonzero inclination with respect to the planet-star orbital plane, so the star need not be completely geometrically eclipsed by such a circumplanetary disk.
Relating this to our own system, the paper goes on:
The rings of Saturn have optical depth near ?1 even at a relatively old age (4.6 Gyr), however the vast majority of mass orbiting Saturn is locked up in satellites (Mrings ? 10?4 Msatellites). Presumably a disk of much higher optical depth and signi?cant radial substructure existed during the epoch of satellite formation. While there have been studies investigating the detectability of thin, discrete planetary rings similar to Saturn’s… there has been negligible investigation of the observability of the dense proto-satellite disks that likely existed during the ?rst ?107 years. Relaxing the assumptions about the size, mass, composition and structure of the disk in our back-of-the-envelope calculation has little impact on the feasibility of the idea that dusty disks of high optical depth may be a common feature of young gas giant planets, and such objects may be observable via deep eclipses of young stars.
Putting all this together, our knowledge of the moons of gas giants in our own Solar System tells us that when they were in formation, circumplanetary disks would have produced equally complex eclipses if seen in transit by distant astronomers, showing dense areas alternating with gaps where the young moons were in the process of formation. Viewing such events around exoplanets, then, we may be seeing a testbed for moon and planet formation theories that will become increasingly more valuable as the number of such disk observations increases.
The paper is Mamajek et al., “Planetary Construction Zones in Occultation: Discovery of an Extrasolar Ring System Transiting a Young Sun-like Star and Future Prospects for Detecting Eclipses by Circumsecondary and Circumplanetary Disks,” in press at the Astronomical Journal (preprint).
Circumbinary Planets: A New Class of Systems?
Last week’s meeting of the American Astronomical Society is still much in the news, and I want to cover several more stories from the Austin conclave this week, starting with yet another circumbinary planetary system, in which a planet orbits two stars. Not long ago we looked at Kepler-16b, a circumbinary planet orbiting two stars in this mode — as opposed to a binary system where planets orbit one or the other of the two stars. Kepler-16b was interesting but perhaps unusual given the perceived difficulties in finding stable orbits around close binaries.
But things are happening quickly on the exoplanet front. Needing more information about the prevalence of this kind of planet and the range of orbital and physical properties involved in such systems, we now get news of not one but two more, Kepler-34b and Kepler-35b. Note the nomenclature: We could as easily call these Kepler-34(AB)b and Kepler-35(AB)b. We confront the real possibility that ‘two sun’ systems are not necessarily rarities. At least, that’s the view of William Welsh (San Diego State University), who presented the findings at the AAS meeting:
“It was once believed that the environment around a pair of stars would be too chaotic for a circumbinary planet to form, but now that we have confirmed three such planets, we know that it is possible, if not probable, that there are at least millions in the Galaxy.”
Here’s what we know about these worlds. Kepler-34b orbits two Sun-like stars every 289 days, while the two stars in question orbit and eclipse each other every 28 days. At 4,900 light years from Earth, the planet is in the constellation Cygnus, as is Kepler-35b, although the latter is more distant at 5400 light years. Both are thought to be Saturn-sized gas giants. Kepler-35b orbits its two stars (80 and 89 percent of the Sun’s mass) every 131 days, with its central stars orbiting and eclipsing each other every 21 days. Between these worlds and Kepler-16b, we are building our knowledge of a new class of planets, one Kepler may supplement with still more examples.
Image: Twin suns would yield not only spectacular visual effects but climate changes that could be equally breathtaking. Credit: Lynette R. Cook.
Laurance Doyle (SETI Institute), a co-author of the paper on this work, speaks of “…the new field of comparative circumbinary planetology,” which he believes is now established by these findings. Here is yet more fodder for science fiction writers looking for unique settings, for planets in such orbits would receive continually changing amounts of sunlight. The effects on local weather patterns alone would be enough to spin an absorbing tale, a year’s worth of seasonal change packed into short and dramatically changing time frames. “The effects of these climate swings on the atmospheric dynamics, and ultimately on the evolution of life on habitable circumbinary planets,” says Welsh, “is a fascinating topic that we are just beginning to explore.”
The paper is Welsh et al., “Transiting circumbinary planets Kepler-34 b and Kepler-35 b,” published online by Nature on 11 January, 2012 (abstract).
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