Last week I mentioned that I wanted to get into Massimo Marengo’s new paper on KIC 8462852, the interesting star that, when studied by the Kepler instrument, revealed an intriguing light curve. I’ve written this object up numerous times now, so if you’re coming into the discussion for the first time, plug KIC 8462852 into the archive search engine to get up to speed. Marengo (Iowa State) is himself well represented in the archives. In fact, I began writing about him back in 2005, when he was working on planetary companions to Epsilon Eridani.
In the new paper, Marengo moves the ball forward in our quest to understand why the star I’ll abbreviate as KIC 8462 poses such problems. The F3-class star doesn’t give us the infrared signature we’d expect from a debris disk, yet the light curves we see suggest objects of various sizes (and shapes) transiting across its surface. What we lacked from Tabetha Boyajian’s earlier paper (and it was Boyajian, working with the Planet Hunters group, that brought KIC 8462 to our attention) was data about infrared wavelengths after the WISE mission finished its work.
That was a significant omission, because the WISE data on the star were taken in 2010, while the first events Kepler flagged at KIC 8462 occurred in March of 2011, with a long series of events beginning in February of 2013 and lasting sixty days. That gave us a small window in which something could have happened — the idea of a planetary catastrophe comes to mind, perhaps even a collision between two planets, or a planet and large asteroid. What Marengo brings to the table are observations from the Spitzer Space Telescope dating from early 2015.
Image: Astrophysicist Massimo Marengo. Credit: Iowa State University.
We learn that the Spitzer photometry from its Infrared Array Camera (IRAC) finds no strong infrared excess — no significant amount of circumstellar dust can be detected two years after the 2013 dimming event at KIC 8462. Here is what Marengo concludes from this:
The absence of strong infrared excess at the time of the IRAC observations (after the dimming events) implied by our 4.5 µm 3? limit suggests that the phenomenon observed by Kepler produced a very small amount of dust. Alternatively, if significant quantity of dust is present, it must be located at large distance from the star.
This seems to preclude catastrophic scenarios, while leaving a cometary solution intact. The paper continues:
As noted by B15 [this is the Boyajian paper], this makes the scenarios very unlikely in which the dimming events are caused by a catastrophic collision in KIC 8462852 asteroid belt, a giant impact disrupting a planet in the system, or a population of dust-enshrouded planetesimals. All these scenarios would produce very large amount of dust dispersed along the orbits of the debris, resulting in more mid-IR emission than what can be inferred from the optical depth of the dust seen passing along our line of sight to the star. Our limit (two times lower than the limit based on WISE data) further reduces the odds for these scenarios.
Image: Montage of flux time series for KIC 8462852 showing different portions of the 4-year Kepler observations with different vertical scalings. Panel ‘(c)’ is a blowup of the dip near day 793, (D800). The remaining three panels, ‘(d)’, ‘(e)’, and ‘(f)’, explore the dips which occur during the 90-day interval from day 1490 to day 1580 (D1500). Credit: Boyajian et al., 2015.
Tabetha Boyajian’s paper analyzed the natural phenomena that could account for KIC 8462’s light curve and concluded that a family of exocomets was the most promising explanation. Here the idea is that we have a family of comets in a highly elliptical orbit that has moved between us and the star, an idea that would be consistent with the lack of a strong infrared signature. Marengo has reached the same conclusion now that we are able to discount the idea of a large collision within the system. Both Boyajian and Marengo favor the comet hypothesis because it does not require a circular orbit and allows associated dust to quickly move away from the star.
In Marengo’s analysis, this fits the data, as the two-year gap between the Kepler light curves and the observations from Spitzer provide enough time for cometary debris to move several AU from the zone of tidal destruction from the star. The paper adds:
At such a distance, the thermal emission from the dust would be peaked at longer wavelengths and undetectable by IRAC. A robust detection at longer wavelengths (where the fractional brightness of the debris with respect to the star would be more favorable) will allow the determination of the distance of the cometary fragments and constrain the geometry of this scenario.
So we have a way to proceed here. Marengo notes that the measurements his paper presents cannot reveal the temperature or the luminosity of the dust that would be associated with such a family of comets, but long-term infrared monitoring would allow us to constrain both. The other day I also mentioned the small red dwarf (about 850 AU out) that could be the cause of instabilities in any Oort Cloud-like collection of comets around KIC 8462. Boyajian’s paper makes the case for measuring the motion or possible orbit (if bound) of this star as a way to tighten predictions on the timescale and repeatability of any associated comet showers.
Marengo dismisses SETI study of KIC 8462, with specific reference to Jason Wright’s recent paper on the matter, as “wild speculations,” an unfortunate phrase because Wright’s shrewd and analytical discussion of these matters has been anything but ‘wild.’
The Marengo paper is Marengo, Hulsebus and Willis, “KIC 8462852 – The Infrared Flux,” Astrophysical Journal Letters, Vol. 814, No. 1 (abstract / preprint). The Boyajian paper is Boyajian et al., “Planet Hunters X. KIC 8462852 – Where’s the flux?” submitted to Monthly Notices of the Royal Astronomical Society (preprint). The Jason Wright paper that examines KIC 8462 in the context of SETI signatures is Wright et al., “The ? Search for Extraterrestrial Civilizations with Large Energy Supplies. IV. The Signatures and Information Content of Transiting Megastructures,” submitted to The Astrophysical Journal (preprint).
In the heady days of Apollo, Mars by 2000 looked entirely feasible. Now we’re talking about the 2030s for manned exploration, and even that target seems to keep receding. In the review that follows, Michael Michaud looks at Louis Friedman’s new book on human spaceflight, which advocates Mars landings but cedes more distant targets to robotics. So how do we reconcile ambitions for human expansion beyond Mars with political and economic constraints? A career diplomat whose service included postings 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, Michael is also the author of Contact with Alien Civilizations (Copernicus, 2007). Here he places the debate over manned missions vs. robotics in context, and suggests a remedy for pessimism about an expansive future for Humankind.
by Michael A.G. Michaud
Many people in the space and astronomy communities will know of Louis Friedman, a tireless campaigner for planetary exploration and solar sailing. He was one of the co-founders of the Planetary Society in 1980, with Carl Sagan and Bruce Murray.
In his new book, entitled Human Spaceflight: From Mars to the Stars, Friedman states his argument up front: Humans will become a multi-planet species by going to Mars, but will never travel beyond that planet. Future humans will explore the rest of the universe vicariously through machines and virtual reality.
Friedman acknowledges that public interest in space exploration is still dominated by “human interest.” No one, he writes, is going to discontinue human spaceflight. Yet there is a conundrum. While giving up on manned missions to Mars is politically unacceptable, getting such a program approved and funded is not an achievable political step at this time. If another decade goes by without humans going farther in space, Friedman writes, public interest will likely decline and robotic and virtual exploration technologies will pass us by.
Friedman claims that going beyond Mars with humans is impossible not just physically for the foreseeable future but culturally forever. The long-range future of humankind, he declares, is to extend its presence in the universe virtually with robotic emissaries and artificial intelligence. This argument puts a permanent cap on human expansion, as if travel beyond Mars never will be possible.
Friedman sees having another world as a prudent step to prevent humankind being wiped out by a catastrophe. He argues that the danger of not sending humans to Mars is that we will become complacent. If that complacency overcomes making humankind a multi-planet species, we are doomed.
Friedman dismisses big ideas about exploiting planetary resources throughout the solar system and living everywhere to build civilizations and colonies on other worlds. He can’t see why or how we would do this, nor can he see waiting to do so. This illustrates an old split in the space interest community between those advocating space exploration and those supporting space utilization and eventual human expansion.
In his chapter entitled Stepping Stones to Mars, Friedman lists potential human spaceflight achievements with dates. An appendix presents a plan for a manned Mars mission in the 2040s. That first landing is to be followed later by missions establishing an infrastructure for human habitation, an effort that will take many decades.
This book’s subtitle is From Mars to the Stars. Yet Friedman dismisses interstellar travel by human beings as a subject of science fiction. People are too impatient, he writes, to wait for the necessary life-support developments. This contrasts with Carl Sagan’s 1966 comment that efficient interstellar spaceflight to the farthest reaches of our galaxy is a feasible objective for humanity.
Friedman argues that we have only one technology that might someday take our machines to the stars – light sailing. It may be another century before we have large enough laser power sources to drive small unmanned spacecraft over interstellar distances. The barrier of bigness will be overcome by the enablement of smallness.
Friedman suggests three interstellar precursor missions: the first launched in 2018 to the Kuiper Belt and onward to the heliopause; the second launched in 2025 to the solar gravity lens focus and on to 1,000 astronomical units; the third launched in 2040 to the Oort Cloud.
Friedman oversells virtual reality just as some others have oversold manned spaceflight. He acknowledges that we have yet to reach full cultural acceptance and satisfaction with the virtual world. Yet he seems to assume that such acceptance by the general population is inevitable.
Calling virtual reality human exploration may confuse many readers. Will we be content to watch all future exploration through robotic eyes?
There may be an unstated reason for preferring virtual reality over human presence. If future space exploration were entirely robotic, scientists would be in charge.
Cautions about Mars
Mars is far from ideal as a future home for humankind. The thin atmosphere is mostly carbon dioxide. Temperatures are low. The surface is more exposed to radiation and meteorites than Earth. Yet Mars remains the best candidate for a second planetary home within our own solar system.
Like other schedules proposed by some space advocates, Friedman’s plan for missions to Mars may be too optimistic. Yet such optimism keeps goals alive and encourages others to get involved.
What seems wildly optimistic now may be possible over the longer term. In the 1950s, some scientists thought that sending humans to the Moon was impossible.
The failure of grand visions
Friedman is correct in stating the biggest problem of space policy: the merging of grand visions with political constraints. In 1988, President Reagan’s statement on space policy included the idea of expanding human activity beyond Earth and into the solar system, an endorsement long sought by some elements of the space interest community. President George H.W. Bush fleshed out this idea in 1989 with his Space Exploration Initiative, urging that the U.S. develop a permanent presence on the Moon and the landing of a human crew on Mars by 2019. These visions failed to win the financing that would make them feasible.
Frustration and Patience
It is understandable that long-time campaigners for further exploration and use of space get frustrated, in some cases foreseeing the end of such endeavors. We all want to see major hopeful events occur in our own lifetimes. Yet we share some responsibility to look beyond.
Writing off human expansion beyond Mars for all the humans who follow us is, despite Friedman’s claim, pessimistic. The remedy is a younger generation of advocates.
A Little History
Friedman states that the settlement of Mars is the rationale for human spaceflight. The leaders of the Planetary Society did not initially support that goal. In the organization’s early years, its chief spokespersons criticized NASA’s emphasis on human missions (particularly the Space Station), which they saw as robbing funds that should have gone into further robotic exploration.
Sagan and others later realized that the planetary exploration budget rose and fell with the rise and fall of manned spaceflight programs. When NASA funding was rising, space science prospered; when NASA funding declined, space science funding declined with it. After the cancellation of further Apollo missions, planetary science was hit hardest by budget cuts . This revived a debate as old as the space program, between advocates of manned spaceflight and those who believe that priority should be given to exploration by unmanned spacecraft.
Friedman wrote in a 1984 article in Aerospace America about extending human civilization to space, suggesting a lunar base, a manned expedition to Mars, or a prospecting journey to some asteroids undertaken by an international team.
By the mid-1980s, the Planetary Society was advocating a joint U.S. Soviet manned mission to Mars. Senator Spark Matsunaga of Hawaii introduced legislation to support this idea and published a book in 1986 entitled The Mars Project: Journeys beyond the Cold War. Soviet leader Mikhail Gorbachev made overtures to the U.S. in 1987 and 1988 for a cooperative program eventually leading to a Mars landing.
Bruce Murray, reacting favorably to the 1989 Space Exploration Initiative, published an article in 1990 entitled Destination Mars—A Manifesto. Observing that the space frontier for the U.S. and the USSR had stagnated a few hundred miles up, Murray commented that neither the United States nor the Soviet Union is likely, by itself, to sustain the decades of effort necessary to reach Mars. Murray urged a joint U.S.-Soviet manned spaceflight program leading eventually to Mars.
This reviewer argued at the 1987 Case for Mars conference that relying on the Soviet Union during the Cold War made such a mission subject to political volatility. This turned out to be true. As Friedman reports, a brief flurry of interest by President Reagan and Gorbachev in a cooperative human mission to Mars disappeared quickly in the face of large global events such as the dissolution of the Soviet Union.
More recently, when the U.S. sought to punish Russia for invading Ukraine, Russian officials made public statements threatening the continuation of Russian transport of Americans to the International Space Station, even though the U.S. was paying for those flights.
Louis Friedman, Human Spaceflight: From Mars to the Stars, University of Arizona Press, 2015.
Louis D. Friedman, “New Era of Global Security: Reach for the Stars,” Aerospace America, August 1984, 4.
Michael A.G. Michaud, “Choosing partners for a manned mission to Mars,” Space Policy, February 1988, 12-18.
Chapter entitled “Scientists, Citizens, and Space” in Michael A.G. Michaud, Reaching for the High Frontier: The American Pro-Space Movement, 1972-1984, Praeger, 1986, 187-213.
Bruce Murray, “Destination Mars: A Manifesto,” Nature 345 (17 May 1990), 199-200.
Iosif Shklovskii and Carl Sagan, Intelligent Life in the Universe, Dell, 1966, 449.
KIC 8462852 is back in the news. And despite a new paper dealing with the unusual star, I suspect it will be in the news for some time to come, for we’re a long way from finding out what is causing the unusual light curves the Planet Hunters group found in Kepler data. KIC 8462, you’ll recall, clearly showed something moving between us and the star, with options explored by Tabetha Boyajian, a Yale University postdoc, in a paper we examined here in October (see KIC 8462852: Cometary Origin of an Unusual Light Curve? and a series of follow-up articles).
To recap, we’re seeing a light curve around this F3-class star that doesn’t look anything like a planetary transit, but is much more suggestive of debris. Finding a debris disk around a star is not in itself unusual, since we’ve found many such around young stars, but KIC 8462 doesn’t appear to be a young star when looked at kinematically. In other words, it’s not moving the way we would expect from a star that has recently formed. Moreover, the star shows us none of the emissions at mid-infrared wavelengths we would expect from a young, dusty disk.
Jason Wright and the team at the Glimpsing Heat from Alien Technologies project at Penn State have taken a hard look at KIC 8462 and discussed it briefly in a recent paper (citations for both the Boyajian and Wright papers are at the end of this entry). It seems entirely reasonable to do what Wright did in referencing the fact that the light curve we see around the star is what we would expect to see if an advanced civilization were building something. That ‘something’ might be a project along the lines of a ‘Dyson swarm,’ in which huge collectors gather solar energy, or it could be a kind of structure beyond our current thinking.
We all know that the media reaction was swift, and we saw some outlets acting as if Wright had declared KIC 8462 an alien outpost. He had done no such thing, nor has he or the Penn State team ever suggested anything more than continuing investigation of this strange star. What seems to bother others, who have scoffed at the idea of extraterrestrial engineering, is that Wright and company have not explicitly ruled it out as a matter of course. The assumption there is that no other civilizations exist, and therefore we could not possibly be seeing one.
I come down on the side of keeping our options open and studying the data in front of us. We have a lot of work ahead to figure out what is causing a light curve so unusual that at least one of the objects briefly occulting this star caused a 22 percent dip in its flux. That implies a huge object, evidently transiting in company with many smaller ones. There seems to be no evidence that the objects are spherically symmetric. What’s going on around KIC 8462?
A new paper from Massimo Marengo (Iowa State) and colleagues looks at what Tabetha Boyajian identified as the most likely natural cause of the KIC 8462 light curves. All I have at this point is the JPL news release and a release from Iowa State — the paper has not yet appeared online — describing evidence for a swarm of comets as the culprit. The study, which has been accepted at Astrophysical Journal Letters relies on Spitzer data dating from 2015, five years later than the WISE data that found no signs of an infrared excess.
Image: This illustration shows a star behind a shattered comet. Is this the explanation for the unusual light curves found at KIC 8462852? Image credit: NASA/JPL-Caltech.
If there had been a collision between planets or asteroids in this system, it was possible that the WISE (Wide-field Infrared Survey Explorer) data, taken in 2010, reflected conditions just before the collision occurred. Now, however, we can rule that out, because Spitzer, like WISE, finds no excess of infrared light from warm dust around KIC 8462. So the idea of planet or asteroid collisions seems even less likely. Marengo, according to the JPL document, falls back on the idea of a family of comets on an eccentric orbit. He’s also aware of just how odd KIC 8462 is:
“This is a very strange star,” [Marengo] said. “It reminds me of when we first discovered pulsars. They were emitting odd signals nobody had ever seen before, and the first one discovered was named LGM-1 after ‘Little Green Men… We may not know yet what’s going on around this star, but that’s what makes it so interesting.”
It would take a very large comet indeed to account for the drop in flux we’ve already seen, but a swarm of comets and fragments can’t be ruled out because we just don’t have enough data to make the call. I assume Marengo also gets into the fact that a nearby M-dwarf (less than 900 AU from KIC 8462, is a possible influence in disrupting the system. The comet explanation would be striking if confirmed because we have no other instances of transiting events like these, and we would have found these comets by just happening to see them at the right time in their presumably long and eccentric orbit around the star.
Image: Left: a deep, isolated, asymmetric event in the Kepler data for KIC 8462. The deepest portion of the event is a couple of days long, but the long “tails” extend for over 10 days. Right: a complex series of events. The deepest event extends below 0.8, off the bottom of the figure. After Figure 1 of Boyajian et al. (2015). Credit: Wright et al.
So, despite PR headlines like Strange Star Likely Swarmed by Comets, I think we have to take a more cautious view. We’re dealing with a curious star whose changes in flux we don’t yet understand, and we have candidate theories to explain them. We’re no more ready to declare comets the cause of KIC 8462’s anomalies than we are to confirm alien megastructures. At this point we should leave both natural and artificial causes in the mix and recognize how long it’s going to take to work out a viable solution through careful, unbiased analysis.
The Marengo paper is Marengo, Hulsebus and Willis, ”KIC 8462852: The Infrared Flux,” Astrophysical Journal Letters, Vol. 814, No. 1 (abstract). I write about it this morning only because it is getting so much media attention — more later when I can go through the actual paper. The Boyajian paper is Boyajian et al., “Planet Hunters X. KIC 8462852 – Where’s the flux?” submitted to Monthly Notices of the Royal Astronomical Society (preprint). The Wright paper is Wright et al., “The ? Search for Extraterrestrial Civilizations with Large Energy Supplies. IV. The Signatures and Information Content of Transiting Megastructures,” submitted to The Astrophysical Journal (preprint).
Just a few days ago we looked at evidence that Kepler-438b, thought in some circles to be a possibly habitable world, is likely kept out of that category by flare activity and coronal mass ejections from the parent star. These may well have stripped the planet’s atmosphere entirely (see A Kepler-438b Caveat – and a Digression). Now we have another important study, this one out of the Harvard-Smithsonian Center for Astrophysics, taking a deep look at the red dwarf TVLM 513–46546 and finding flare activity far stronger than anything our Sun produces.
Led by the CfA’s Peter Williams, the team behind this work used data from the Atacama Large Millimeter/submillimeter Array (ALMA), examining the star at a frequency of 95 GHz. Flares have never before been detected from a red dwarf at frequencies as high as this. Moreover, although TVLM 513 is just one-tenth as massive as Sol, the detected emissions are fully 10,000 times brighter than what our star produces. The four-hour observation window was short, which may be an indication that we’re looking at a star that is frequently active.
Now considered an M9 dwarf, TVLM 513 is about 35 light years away in the constellation Boötes. It is believed to be on the borderline between red and brown dwarfs, with a radius 0.11 that of the Sun, a temperature of 2500 K, and a rotation rate of a scant two hours (the Sun takes almost a month for a complete rotation). For a habitable planet to exist here — one with temperatures allowing liquid water on the surface — it would need to orbit at about 0.02 AU. That’s obviously a problem, as Williams explains in this CfA news release:
“It’s like living in Tornado Alley in the U.S. Your location puts you at greater risk of severe storms. A planet in the habitable zone of a star like this would be buffeted by storms much stronger than those generated by the Sun.”
Image: Artist’s impression of red dwarf star TVLM 513-46546. ALMA observations suggest that it has an amazingly powerful magnetic field (shown by the blue lines), potentially associated with a flurry of solar-flare-like eruptions. Credit: NRAO/AUI/NSF; Dana Berry / SkyWorks.
Another unusual aspect of TVLM 513 is its magnetic field. Data from the Very Large Array in New Mexico had previously shown a magnetic field several hundred times stronger than the Sun’s. The paper argues that the emissions observed in the ALMA data are the result of synchrotron emission — radiation generated by the acceleration of high-velocity charged particles through magnetic fields — associated with the small star’s magnetic activity.
We have a lot to learn about small stars, their magnetic fields and their flare processes, and even in this study, the paper offers a caveat:
… confident inferences based on the broadband radio spectrum of TVLM 513 are precluded because the ALMA observations were not obtained contemporaneously with observations at longer wavelengths, and TVLM 513’s radio luminosity, and possibly its radio spectral shape, are variable. Additional support from the Joint ALMA Observatory to allow simultaneous observations with other observatories would be highly valuable.
The authors add that while it has long been known that both stars and gas giant planets have magnetic fields, the mechanisms at work are different and it is unclear what kind of magnetic activity we should expect from objects of intermediate size. Learning more about magnetic processes in small stars should help us understand more about exoplanets and their magnetic activity. This first result at millimeter wavelengths thus points to the work ahead:
Modern radio telescopes are capable of achieving ?µJy sensitivities at high frequencies (?20 GHz), raising the possibility of probing the means by which particles are accelerated to MeV energies by objects with effective temperatures of ?2500 K.
So we’re going to learn a lot more about small red dwarfs as we study whether or not such stars can host habitable planets. The argument against red dwarfs and astrobiology used to focus on tidal lock and the problems of atmospheric circulation, but we’re now wondering whether, particularly in young red dwarfs, flare activity may not be the key factor. If TVLM 513 is representative of a category of flare-spitting stars, the smallest red dwarfs may be hostile to life.
The paper is Williams et al., “The First Millimeter Detection of a Non-Accreting Ultracool Dwarf,” in press at The Astrophysical Journal (preprint).
Justin Atchison’s name started appearing in these pages all the way back in 2007 when, in a post called Deep Space Propulsion via Magnetic Fields, I described his work at Cornell on micro-satellites the size of a single wafer of silicon. Working with Mason Peck, Justin did his graduate work on chip-scale spacecraft dynamics, solar sails and propulsion via the Lorentz force, ideas I’ve tracked ever since. He’s now an aerospace engineer at the Johns Hopkins University Applied Physics Laboratory, where he focuses on trajectory design and orbit determination for Earth and interplanetary spacecraft. As a 2015 NIAC fellow he is researching technologies that enable asteroid gravimetry during spacecraft flybys. In the entry that follows, Justin reports on his trip to Jerusalem for this fall’s International Astronautical Congress.
by Justin A. Atchison
Greetings. I’m Justin Atchison, an aerospace engineer at the Johns Hopkins University Applied Physics Laboratory. I’m proud to have previously had my graduate research included on Centauri Dreams (1,2, and 3). Now, I’m guest-writing an article about the three most futuristic talks I saw at the International Astronautical Congress in Jerusalem this past October. I was able to attend the conference thanks to a travel fellowship through the Future Space Leaders Foundation (FSLF). I’d strongly encourage any student or young-professional (under 35) to apply for this grant next year. It’s a fantastic opportunity to attend this premier conference and interact with a variety of international leaders and thinkers in the aerospace field. FSLF also hosts the Future Space Event on Capitol Hill each summer, which offers engagement with US Congress and aerospace executives on the latest and most relevant space-related subjects.
Image: Justin in the IAC-2015 exhibition hall trying on a protective harness that minimizes radiation exposure to the pelvis bone, which is particularly sensitive to radiation due to its high bone marrow production.
So with that note of thanks and recommendation, I give you “The 3 Most Futuristic Talks at IAC 2015.”
1. An Approach for Terraforming Titan
Abbishek G., D. Kothia, R.A. Kulkarni, S. Chopra, and U. Guven, “Space Settlement on Saturn’s Moon: Titan,” International Astronautical Congress, Jerusalem, Israel, IAC-15-D4.1.5, 2015.
University of Petroleum and Energy Studies, India
The authors of this paper explore options for terraforming Titan in the distant future. Specifically, this means liberating oxygen and increasing the surface temperature.
In addition to having water-ice, Titan is a candidate for human settlement for a few compelling reasons:
- Abundant Water-Ice – Water is obviously critical for life and is a source of oxygen.
- Solar Wind Shielding – Saturn’s magnetosphere “contains” Titan for 95% of its orbit period and is relatively stable.
- Earth-like Geology – Observations of Titan show a relatively young, Earth-like surface with rivers, wind-generated dunes, and tectonic-induced mountains.
- Native Atmosphere – Titan’s atmosphere is nitrogen rich (95%) and shows strata similar to Earth (troposphere, stratosphere, mesosphere, and thermosphere). The atmospheric pressure on the surface is only 60% higher than that on Earth.
However, Titan presents challenges for habitation, namely a lack of breathable oxygen and the presence of extremely cold surface temperatures (90K). The authors suggest that the solution to these two challenges is a nuclear fission plant that can dissociate oxygen from water and produce greenhouse gases.
Generating Oxygen – The main idea is to use beta or gamma radiation to set up a radiolysis process that converts hydrogen and oxygen atoms into usable constituents, including O2. This requires an artificial radiation source and a means of liberating the hydrogen and oxygen atoms from the native cold ice. They suggest a nuclear fission plant as the source of the radiation, and a duoplasmatron as the means of liberating and exciting the H and O atoms. The duoplasmatron would accelerate a beam of argon ions, which would be aimed at the water-ice. The collisions cause sputtering, where the argon ions literally knock O and H atoms out of the ice. These atoms are then collected and radiated to generate usable O2.
Heating Up the Atmosphere – At about 9.5 AU from the Sun, Titan receives only ~1% as much solar energy as Earth. The goal for raising the temperature on Titan is to capture and retain that limited energy. The authors consider the generation of greenhouse gases as the solution. There are two options they suggest:
- If lightning is present on Titan, then the oxygen generated by the nuclear reactor can energetically react with the already-present nitrogen to produce nitrogen oxides, namely NO, NO2, N2O, and N2O2. Once these nitrogen oxides are able to raise the surface temperature by roughly 20 K, Titan’s methane lakes will begin to boil off, releasing gaseous methane as an additional greenhouse gas, and potentially raising the surface temperature to a habitable value.
- If lightning isn’t present, or if its generation of nitrogen oxides is too inefficient, one could boil the methane lakes directly using the previously mentioned nuclear reactor. In this setup, the reactor is simply increasing the amount of vapor in the already-present methane cycle (vaporization and condensation of methane). To cause the lakes to naturally vaporize, one needs to generate sufficient vapor to affect the global climate and raise the surface temperature by 20 K.
The authors don’t estimate the total time required for terraforming, the size of the nuclear plant required to start the process, or the maximum theoretical surface temperature achievable, but they nonetheless posit a potential path forward for planetary habitation…and that’s a meaningful contribution.
2. Eternal Memory
Guzman M., A. M. Hein, and C. Welch, “Eternal Memory: Long-Duration Storage Concepts for Space” International Astronautical Congress, Jerusalem, Israel, IAC-15-D4.1.3, 2015.
International Space University and Initiative for Interstellar Studies, France
How can present day humanity leave a message for distant future civilizations (human or alien)? This question first became an option with Carl Sagan’s famous Voyager Golden Record. The authors of this review paper evaluate the requirements and near term options available to store and interpret data for millions to billions of years in space. That’s a long enough timescale that you have to start to consider the lifetime of the sun (5 billion years) and the merging of the Milky Way and Andromeda galaxies (long term dynamics may destabilize orbits in the solar system). There are a variety of current efforts, most of which are crowd-funded, including: The Helena Project, Lunar Mission Project, Time Capsule to Mars, KEO, The Rosetta Project, The Human Document Project, One Earth Message, and Moonspike.
The data storage mechanism has to survive radiation, micrometeoroids, extreme temperatures, vacuum, solar wind, and geologic processes (if landed on a planet or moon). In terms of locating the data, the authors consider just about every option: Earth orbit, the Moon, the planets, planetary moons, Lagrange points, asteroids and comets, escape trajectories, and even orbiting an M-star. The Moon appears to be a good candidate because it remains near enough to Earth for future civilizations to discover, yet distant enough to avoid too common access (it can’t be too accessible or it might be easily destroyed by malicious or careless humans). One of the proposed implementations is to bury data at the lunar north pole, where regolith can be a shield against micrometeoroid impacts.
There are a variety of near-term technologies available for this challenge, including three approaches that could likely survive the requisite millions to billions of years:
- Silica Glass Etching – “Silica is an attractive material for eternal memory concepts because it is stable against temperature, stable against chemicals, has established microfabrication methods, and has a high Young’s modulus and Knoop hardness.” In this implementation, femtosecond lasers are used to etch the glass and achieve CD-ROM like data densities. A laboratory test exposed a sample wafer to 1000°C heat for two hours with no damage.
- Tungsten Embedded in Silicon Nitride – A group in the Netherlands has developed and tested a process for patterning tungsten inside transparent, resilient silicon nitride. The resulting wafer can be read optically. The materials were selected for their high melting points, low coefficients of thermal expansion, and high fracture toughness. A sample QR code was generated and successfully tested at high temperatures, the result of which implied 106 year survivability.
- Generational Bacteria DNA – This approach uses DNA as a means of storing data (see Data Storage: The DNA Option). Although this may sound extreme, consider that bacterial DNA has already survived millions of years in Earth’s rather unstable environment. It is a demonstrated high-density, resilient means of storing data. In this implementation, we would write data into the genome of a particularly hardy strain of bacteria, and rely on its self-survival to protect our data for the future. This option presents the challenge that it requires the future civilization to have the capability to study the bacteria’s DNA and identify the human generated code.
As humans continue to send probes to unique places in the solar system, I hope that we’ll consider and incorporate these new technologies. Who knows–In a few million years, our “cave paintings” may be hanging in some intergalactic museum.
3. Carbon Nanotubes for Space Solar Power…(And Eventually Interplanetary Travel?)
Gadhadar R., P. Narayan, and I. Divyashree, “Carbon-Nanotube Based Space Solar Power (CASSP),” 4th Space Solar Power International Student and Young Professional Design Competition, Jerusalem, Israel, 2015.
NoPo Nanotechnologies Private Limited and Dhruva Space, India.
Single-Walled Carbon Nanotubes (SWCNT) have remarkable properties:
- Incredibly high strength-to-weight ratio (300x steel) [~50,000 kN m / kg]
- High electrical conductivity (higher than copper) [106-107 m/?]
- High thermal conductivity (higher than diamond) [3500 W/(m K)]
- High temperature stability (up to 2800°C in a vacuum)
- Tailorable semiconductive properties (based on nanotube diameter)
- Ability to sustain high voltage densities (1-2 V/µm)
- Ability to sustain high current densities (~109 A/cm2)
- High radiation resistance
These properties, specifically the strength-to-weight ratio, make them candidates for things like space elevators and momentum exchange tethers.
The authors of this paper posit a different application for SWCNT: space based solar power. This is the concept where an enormous array of solar cells is placed in orbit around Earth. Power is collected, and then beamed down to the surface at microwave frequencies for terrestrial use. The main idea is to collect solar power outside of Earth’s atmosphere, which attenuates something like 50-60% of the energy in solar spectra. The spacecraft has access to a higher energy density of solar light, which it then beams down to Earth at microwave frequencies, at which the atmosphere is transparent. A second advantage is that high orbiting satellites have much shorter local nights (eclipses) than someone on the Earth (there’s only up to about 75 minutes of darkness for geostationary orbit).
In this paper, the authors describe an implementation of a solar power satellite that would use semiconducting SWCNT as the solar cells. Based on the authors’ analysis, it’s feasible to mature this TRL 4 technology to achieve a peak energy of 2 W/g at 10 cents per W. This is compared to current TRL 9 options that offer roughly 0.046 W/g at 250 dollar per W. The design is entirely developed from different forms of SWCNT, which are used to make a transparent substrate, a semiconducting layer, and a conducting base. The three-layered assembly would have a density of 230 g/m2, roughly a third of current technologies.
Additionally, the authors advocate for SWCNT based microwave transmitters, which could potentially be more efficient than traditional Klystron tubes and wouldn’t require active cooling.
As an added benefit, this type of SWCNT microwave source could potentially be used in the newly discussed (and certainly controversial) CANNAE drive. In the paper’s implementation, CANNAE propulsion would only be used for station-keeping…But it’s not hard to extrapolate and conceive of a solar powered, CANNAE-driven spacecraft for interplanetary exploration.
I have to admit, I’m a bit skeptical of the economics of space-based solar power concepts. But this paper is nonetheless exciting as it highlights the potential applications for this relatively new engineered material. I can’t wait to see how SWCNT are used in the coming decades and what new exploration technologies they’ll enable.