by Paul Gilster | Sep 23, 2014 | Astrobiology and SETI |
Gatherings like the 100 Year Starship Symposium have tough organizational choices to make, and the solutions aren’t always obvious. A good part of any aerospace conference is involved in presenting papers, but do you set up a multi-track system or take a single-track approach? In Houston, the 100 Year Starship organization chose multiple tracks: We had, for example, a track on Life Sciences, one on Data Communications, another on Propulsion & Energy, and there were several others including a useful track on Interstellar Education.
The problem is that with all these tracks running at once, it was a matter of picking and choosing, and that often meant getting up after a presentation, switching rooms, and entering another track. I missed papers in Kathleen Toerpe’s Education track that I wanted to hear because I needed to hear many of the Propulsion & Energy papers, and while I caught a paper in the Becoming an Interstellar Civilization track, it was at the expense of some promising offerings in another track called Uncharted Space & Destinations.
This is how many conferences operate and it allows for a greater number of papers, but I was glad that the symposium also opened up for plenary sessions, one of which, called State of the Universe, was a discussion between Jill Tarter (SETI Institute) and Mason Peck (Cornell University). Tarter’s lengthy involvement in SETI brings particular weight to her insights, and Centauri Dreams readers already know about my admiration for Peck’s work on the miniaturized ‘satellite on a chip’ designs he and his team call ‘Sprites.’ I’m convinced that shrinking payloads through such technologies is a promising path toward interstellar missions.
Miniaturization and Cost
KickSat was a plan to take a large number of Sprites into orbit aboard a CubeSat, although the initial attempt failed when, after a launch in April of this year as part of an ISS re-supply mission, the CubeSat orbited and re-entered without deploying the nanosatellites. But the technology is fascinating. Peck refers to Sprites as ‘Sputnik on a microchip,’ and we can think of them as robotic precursor missions on a very small scale, an economic approach to exploration. A swarm of cheap Sprites could be deployed, for example, from a CubeSat solar sail mission to an outer planet, replacing telescope imagery with direct sensor readings.
But Tarter pressed Peck on astrobiology, where for our immediate purposes, Sprites aren’t going to offer the flexibility of today’s large rovers on places like Mars. How do we cope with the need to keep the costs of large missions down? The commercial sector, as well as crowdsourced funding, came up again and again. In Peck’s words:
“For Mars, the long range plan is a sample return mission. To keep the prices as low as possible, what if we buy the science? Rather than having an agency declare the boundaries of a science mission, why not offer a prize per gram of Martian soil? Let companies bid and bring economics into play. If no one rises to the challenge, no money is spent.”
Tarter was interested in what she called a ‘mega-Kickstarter from the world’ as a funding source for another kind of technology, a starshade. In reports for the NASA Institute for Advanced Concepts a few years back (still available at the NIAC site), Webster Cash (University of Colorado at Boulder) outlined the possibilities. Even the largest planets are invariably drowned out by the glare of the parent star, but a starshade approximately 20,000 kilometers away from a large space instrument like the James Webb Space Telescope uses a highly-refined ‘flower-petal’ architecture to filter out the starlight, leaving the telescope with photons from the exoplanet itself. That makes spectroscopy possible, allowing us to study the constituents of a planetary atmosphere.
Starshades are big and have to be deployed, which led Tarter to her next question: Given the kind of miniaturized technology Peck was already working with, couldn’t we make a starshade out of a swarm of smaller objects? The problems are daunting, and include keeping the surface of these structures precise enough to prevent problems with diffraction that can cloud the image. Even so, Peck noted the radical change that computation has brought to optics in the last few years, offering hope of dealing with the optics of widely distributed systems.
And I love this idea: We have places in the Solar System like Enceladus, Europa and Triton, where cryovolcanoes are known to exist. What Tarter calls a ‘shortcut to a sample return’ is to fly a swarm of Sprites through material these cryovolcanoes are throwing into space. Peck’s response:
“I don’t see why not. Small sensors could be distributed throughout this environment. If you fly hundreds — or millions — of Sprites through a geyser, you hedge your bets for survival. Even better, you get spatially and temporally distributed sets of measurements that make a different kind of science possible. It’s a ‘village’ of satellites rather than a single big spacecraft that has to work or you lose the mission. A swarm offers a stochastic or random look at the target.”
The Assumptions of SETI
When the conversation turned to SETI, Tarter talked about current work in the radio and optical part of the spectrum and the need to move to larger apertures and greater computing power, noting that SETI would be part of the Square Kilometer Array that will go online in the 2023-2025 timeframe. In optical wavelengths, the coming generation of instruments like the European Extremely Large Telescope and the Thirty Meter Telescope might have places in the focal plane where SETI detectors could be deployed.
Peck wanted to know whether the assumption was that other civilizations are trying to contact us, or would it be possible to pick up accidental transmissions. Tarter’s response:
“We assume most of the gain is in the transmitter, so receivers don’t need to be as powerful. But in the case of other kinds of signals, those that are not deliberate — think of large astro-engineering programs, for example — any leakage is going to be extremely weak. For these we would have to build the gain into the receivers. But putting huge amounts of money into a single area doesn’t seem the right way to go. Better to do more small but different things. For now, the philosophy is to go for deliberate signals and try to be affordable.”
Driving the push into optical wavelengths has been our own experience. Peck likens the way we currently communicate and return data from spacecraft to ‘exploring the universe with a dial-up modem.’ We have low communications bandwidth and leave about 85 percent of the images we collect on Mars. Improving communications by a factor of ten through laser methods would allow far greater science return. And if we find lasers valuable, wouldn’t other civilizations come to the same conclusion? A large aperture doing individual photon counting can detect a very distant signal.
I liked the note the session ended on, the idea that working on a distant goal like a starship can have wide impact on Earth in the near future. Tarter noted that we’re going to have a large number of megacities before long, teeming with populations of 20 million or more each. Everything we need to learn about maintaining stable life support systems for a starship flows as well into how to keep such cities alive and healthy. Cities as starships. We don’t always plan the solutions that work, but experience has shown how often they emerge from pushing into the unknown.
by Paul Gilster | Sep 22, 2014 | Administrative |
A long time ago in what now seems like a different lifetime, a colleague told me that the best parts of any conference were the accidental encounters in the hallways where you ran into old friends or people whose work you knew about but hadn’t yet met. That was back when I was going to conferences about medieval literature rather than starships, but the lesson holds. There were almost too many such encounters at the 100 Year Starship 2014 Symposium in Houston to count, and it seemed that around every corner was a chance to exchange ideas and opinions.
There were also enough tracks and ongoing events that it was impossible to get everything in. Claudio Maccone and I always get together, and when I saw him crossing the lobby of the Hilton Americas hotel, I intercepted him to see if he wanted to join a group of us for dinner. But Claudio was headed for a screening of the 1951 version of The Day the Earth Stood Still, a film he had never seen, and I could hardly ask him to turn down the opportunity.
Thus the gravitational lens gave way to Gort and Klaatu and Earth’s chance to live in peace among interstellar civilizations or be burned to a cinder for our transgressions. ‘The decision rests with you,’ as Michael Rennie would say. Unlike the later version, it really was a terrific film. And Claudio and I did have the chance to catch up at a breakfast encounter filled with interstellar talk that included the lens at 550 AU and beyond. I’ll have some thoughts on using it for communications on Friday.
Image: My favorite scene in The Day the Earth Stood Still. Interstellar visitor Klaatu (Michael Rennie) adds an equation to Professor Barnhardt’s blackboard, knowing the professor will soon see it.
Which brings me to the reason for the title of today’s post. I’m sure we’ve all had the dream where something is after you and you seem frozen into immobility, knowing you have to do something fast but are unable to act. I found myself in that position this morning. Still worn out from travel and pushed by non-aerospace obligations this afternoon, I fired up the computer intent on a first post about the symposium and an introduction to a week’s worth of musings, technical session notes and other observations about Houston. And then…
Software glitches. Operating system updates (why did I choose this morning of all mornings not to work as usual in Linux but in Windows 7?). The Mac to PC transfer of my session notes left them completely jumbled, which took time to fix. Then Internet connectivity became unpredictable, for reasons unknown. As soon as it came back, I turned to Dropbox to pull my photos from the symposium and discovered that, because I had upgraded my phone to IOS8, DropBox was now unable to download the Houston images. Multiple downloads of Dropbox updates, to no avail (DropBox: Please fix this!). Finally a Googled workaround to get the photos on the PC.
So it was a morning where time stood still. As it did in Dallas on the way to Houston. The clouds in the photo below were the remainder of a system that, the day before, delayed my Dallas-based flight for an interminable four hours. Now I seem to be running perpetually behind schedule, and am pushing up against an outside deadline. So tomorrow I’ll start digging into Houston issues, starting with a conversation between Jill Tarter and Mason Peck that evoked SETI, miniaturized spacecraft, and astrobiological signatures that might be detected by space-based telescopes.
Surely the Earth will start moving normally again and I’ll have fixed the remaining software snags by then. My son Miles said he was walking down the hall when Eric Davis called him over to join a group of colleagues, saying, “We’ve just been talking about whether we’re all living in a simulation.” That was right after a lunch with Al Jackson at a nearby Starbucks where Al explained how Roy Kerr came up with his metric for rotating black holes — Al was there in 1963 when Kerr presented the paper! What’s not to like about a place where you get invited into conversations like this? Houston gave me much to think about and I’ll start digging into it tomorrow.
by Paul Gilster | Sep 19, 2014 | Exoplanetary Science |
While I’m in Houston attending the 100 Year Starship Symposium (about which more next week), Andrew LePage has the floor. A physicist and freelance writer specializing in astronomy and the history of spaceflight, LePage will be joining us on a regular basis to provide the benefits of his considerable insight. Over the last 25 years, he has had over 100 articles published in magazines including Scientific American, Sky & Telescope and Ad Astra as well as numerous online sites. He also has a web site, www.DrewExMachina.com, where he regularly publishes a blog on various space-related topics. When not writing, LePage works as a Senior Project Scientist at Visidyne, Inc. located outside Boston, Massachusetts, where he specializes in the processing and analysis of remote sensing data.
by Andrew LePage
Like many space exploration enthusiasts and professional scientists, I was inspired as a child by science fiction in films, television and print. Even as a young adult, science fiction occasionally forced me to think outside of the confines of my mainstream training in science to consider other possibilities. One example of this was the 1983 film Star Wars: Return of the Jedi which is largely set on the forest moon of Endor. While this was hardly the first time a science fiction story was set on a habitable moon, as a college physics major increasingly interested in the science behind planetary habitability, it did get me thinking about what it would take for a moon of an extrasolar planet (or exomoon) to be habitable. And not “habitable” like Jupiter’s moon Europa potentially is with a tidally-heated ocean that could provide an abode for life buried beneath kilometers of ice, but “habitable” like the Earth with conditions that allow for the presence of liquid water on the surface for billions of years with the possibility of life and maybe a technological civilization evolving.
A dozen years later, the first extrasolar planet orbiting a normal star was discovered and a few months afterwards on January 17, 1996, famed extrasolar planet hunters Geoff Marcy and Paul Butler announced the discovery of a pair of new extrasolar giant planets (EGPs) opening the floodgate of discoveries that continues to this day. One of these new EGPs, 47 UMa b, immediately caught my attention since it orbited right at the outer edge of its sun’s habitable zone based on the newest models by James Kasting (Penn State) and his colleagues published just three years earlier. While 47 UMa b was a gas giant with a minimum mass of about 2.5 times that of Jupiter and was therefore unlikely to be habitable, what about any moons it might have? If the size of exomoons scaled with the mass of their primary, one could expect 47 UMa b to sport a family of moons with minimum masses up to a quarter of Earth’s.
I was hardly the first to consider this possibility since it was frequently mentioned at this time by astronomers whenever new EGPs were found anywhere near the habitable zone. But this realization did get me seriously researching the scientific issues surrounding the potential habitability of exomoons and I started preparing an article on the subject for the short-lived SETI and bioastronomy magazine SETIQuest, whose editorial staff I had recently joined. While working on this article, I started corresponding with then-grad student Darren Williams (Penn State) who, it would turn out, was already preparing a paper on habitable moons with Dr. Kasting and Richard Wade (Penn State). Published in Nature on January 16, 1997, their paper titled “Habitable Moons Around Extrasolar Giant Planets” was the first peer-reviewed scientific paper on the topic. They showed that a moon with a mass greater than 0.12 times that of Earth would be large enough to hold onto an atmosphere and shield it from the erosive effect of an EGP’s radiation environment. In addition, tidal heating could potentially provide an important additional source of internal heat to drive the geologic activity needed for the carbonate-silicate cycle (which acts as a planetary thermostat) for much longer periods than would otherwise be possible for such a small body in isolation.
I published my fully-referenced article on habitable moons in the spring of 1997. In addition to incorporating the results from Williams et al. and related work by other researchers, I went so far as to make the first tentative estimate of the number of habitable moons orbiting EGPs and brown dwarfs in our galaxy based on the earliest results of extrasolar planet searches: 47 million compared to the best estimate of the time of about ten billion habitable planets in the galaxy (estimates that are in desperate need of revision after almost two decades of progress). Since my research showed it was likely that habitable moons would tend to come in groups of two or more, I further speculated about the possibilities of life originating on one of these moons being transplanted to a neighbor via lithospermia. And since I did not have to contend with scientific peer-review for this article, I even speculated about the effects multiple habitable moons would have on a spacefaring civilization in such a system with so many easy-to-reach targets for exploration and exploitation.
Image: An artist’s conception of a habitable exomoon (credit: David A. Aguilar, CfA).
After SETIQuest stopped publication and I published a popular-level article on habitable moons in the December 1998 issue of Sky & Telescope, my scientific and writing interests lead me in other directions for the next decade and a half. But in the meantime, scientific work on exploring the issues surrounding habitable bodies in general and habitable moons in particular has continued. The current state of knowledge has been thoroughly reviewed in the recent cover story of the September 2014 issue of the scientific journal Astrobiology, titled “Formation, Habitability, and Detection of Extrasolar Moons” by a dozen scientists active in the field including one of the authors of the first paper on habitable exomoons, Dr. Darren Williams.
Even after 17 years of new theoretical work and observations, the possibility of habitable exomoons still remains strong. The authors show that exomoons with masses between 0.1 and 0.5 times that of the Earth can be habitable. A review of the available literature shows that exomoons of this size could form around EGPs or could be captured much as Triton is believed to have been captured by Neptune in our own solar system. Calculations also show that such exomoons, habitable or otherwise, are detectable using techniques that are available today, especially direct detection by photometric means like that employed by Kepler and by more subtle techniques such as transit timing variations (TTV) and transit duration variations (TDV) of EGPs with exomoons. As the authors state in the closing sentence of their paper:
In view of the unanticipated discoveries of planets around pulsars, Jupiter-mass planets in orbits extremely close to their stars, planets orbiting binary stars, and small-scale planetary systems that resemble the satellite system of Jupiter, the discovery of the first exomoon beckons, and promises yet another revolution in our understanding of the universe.
The fully referenced review paper is René Heller et al., “Formation, Habitability, and Detection of Extrasolar Moons”, Astrobiology, Vol. 14, No. 9, September 2014 (preprint).
by Paul Gilster | Sep 18, 2014 | Outer Solar System |
Since we don’t yet have flight-ready systems for getting to the outer Solar System much faster than New Horizons, we might as well enjoy one of the benefits of long flight times. Look at it this way: For the next ten months, we can look forward to sharper and sharper images and an ever increasing flow of data about Pluto/Charon and associated moons. It’s going to be a fascinating story that unfolds gradually, culminating in the July flyby next year, and then, of course, we can hope for further exploration of a Kuiper Belt object.
So New Horizons, launched in 2006, is going to be with us for a while, and it has already given us a brief look at asteroid 132524 APL and a shakeout of its science instruments during a gravitational assist maneuver at Jupiter. Now we’re getting down to much finer-grained imagery from Pluto. The first image distinguishing Pluto and Charon was returned in July of 2013. The latest imagery using the spacecraft’s Long Range Reconnaissance Imager (LORRI) shows Pluto’s diminutive moon Hydra, taken as part of a long-exposure strategy that controllers are using to search for other moons or debris near Pluto/Charon.
Interestingly, this report on the New Horizons site points out that the science team didn’t expect to detect Hydra until January. The spacecraft took 48 10-second images on July 18 and repeated the process on July 20, combining them to show the evidence of Hydra.
Image (click to enlarge): Hydra revealed in summer data from New Horizons. Credit: Alan Stern/JHU/APL.
To untangle the image, start with the top row. With Pluto overexposed at image center, you can see a dark streak at the right that is an artifact of the imaging process. Both dates show similar images, but at the right is a ‘difference’ image that largely pulls out the background starfield. Here you can see Hydra as an overlapping smudge of bright and dark that appears immediately above Pluto in the image. In the bottom row are the same images, showing Hydra’s expected position on these dates as marked by red and green crosshairs. It’s tough to make Hydra out, but a close look at the enlarged image will identify it.
The JHU/APL article quotes Science Team member John Spencer as saying that at this point, Hydra is several times fainter than the faintest objects New Horizons’ camera is designed to detect. That’s good news overall because it speaks to the quality of the equipment as well as its operational status. It also confirms the efficacy of the plan to look for satellites and possibly hazardous smaller objects as the spacecraft approaches the system. Keeping New Horizons healthy and collision-free is obviously job number one — we only get one shot at this.
I also want to quote New Horizons co-Investigator Randy Gladstone (Southwest Research Institute) on the question of how we’ll study Pluto’s atmosphere during the flyby. We know, of course, that it’s low in pressure, mostly made up of molecular nitrogen with small amounts of methane and carbon monoxide, but Gladstone points out that models of the atmosphere are in disagreement because of the sparse data available. New Horizons will be engaged in survey observations that make few assumptions about what may be found, including a Pluto solar occultation operation:
The Alice ultraviolet spectrograph will watch the Sun set (and then rise again) as New Horizons flies through Pluto’s shadow, about an hour after closest approach. Watching how the different colors of sunlight fade (and then return) as New Horizons enters (and leaves) the shadow will tell us nearly all we could ask for about composition (all gases have unique absorption signatures at the ultraviolet wavelengths covered by Alice) and structure (how those the absorption features vary with altitude will tell us about temperatures, escape rates and possibly about dynamics and clouds).
We’ve only known about this atmosphere since 1988, when Pluto occulted a distant star. The refracted starlight observed then was hard evidence for an atmosphere. Like the atmosphere on Neptune’s large moon Triton, Pluto’s has a surface pressure of 30 to 100 microbars — that’s 3 to 100 millionths of Earth’s surface pressure. Sublimation of ices on the surface is responsible for what little atmosphere Pluto has. As Pluto continues to move away from the Sun following its closest approach in 1989, condensation should take over, but New Horizons should be getting there before the atmosphere has condensed back onto the surface.
And what about Charon? No evidence exists for an atmosphere there, but the moon is small enough that any atmosphere present would have to be considerably less dense than that on Pluto. While sublimation could produce gases that fed the atmosphere for a time, the tiny world would not be able to prevent their rapid dispersion into space.
by Paul Gilster | Sep 17, 2014 | Outer Solar System |
Hard to believe that it’s been ten years for Cassini, but it was all the way back in January of 2005 that the Huygens probe landed on Titan, an event that will be forever bright in my memory. Although the fourth space probe to visit Saturn, Cassini became in 2004 the first to orbit the ringed planet, and since then, the mission has explored Titan’s hydrocarbon lakes, probed the geyser activity on Enceladus, tracked the mammoth hurricane at Saturn’s north pole, and firmed up the possibility of subsurface oceans on both Titan and Enceladus.
I mentioned the Galileo probe last week, its work at Europa and its fiery plunge into Jupiter’s atmosphere to conclude the mission. Cassini has a similar fate in store after finishing its Northern Solstice Mission, which will explore the region between the rings and the planet. As discussed at the recent European Planetary Science Congress in Cascais, Portugal, the spacecraft’s final orbit will occur in September of 2017, taking Cassini to a mere 3000 kilometers above the planet on closest approach. A final Titan encounter will then provide the gravitational muscle to hurl the craft into Saturn’s atmosphere, where it will be vaporized.
But of course we’re not quite through with Cassini yet. New work out of the SETI Institute delves into the creation and destruction of moons on extremely short time-scales within the Saturnian ring system. Robert French, Mark Showalter and team accomplished their studies by comparing the exquisite photographs Cassini has produced with pictures made during the Voyager mission. The upshot: The F ring has taken on a completely new look.
“The F ring is a narrow, lumpy feature made entirely of water ice that lies just outside the broad, luminous rings A, B, and C,” notes French. “It has bright spots. But it has fundamentally changed its appearance since the time of Voyager. Today, there are fewer of the very bright lumps. We believe the most luminous knots occur when tiny moons, no bigger than a large mountain, collide with the densest part of the ring. These moons are small enough to coalesce and then break apart in short order.”
Short order indeed. It appears that the bright spots can appear and disappear in the course of mere days and even hours. Explaining the phenomenon is the nature of the F ring itself, which is at the critical point known as the Roche limit. This is the range within which the gravitational pull on a moon’s near side can differ enough from that on its far side to actually tear the moon apart. What we seem to be seeing is moon formation — objects no more than 5 kilometers in size — quickly followed by gravitationally induced breakup.
Image: Cassini spied just as many regular, faint clumps in Saturn’s narrow F ring (the outermost, thin ring), like those pictured here, as Voyager did. But it saw hardly any of the long, bright clumps that were common in Voyager images. Credit: NASA/JPL-Caltech/SSI
Mark Showalter compares these small moons to bumper cars that careen through moon-forming material, taking form and then fragmenting as they go. Adding to the chaos is the moon Prometheus, a 100-kilometer object that orbits just within the F ring. Alignments of this moon that occur every 17 years produce a further gravitational influence that helps to launch the formation of the tiny moonlets and propel them through their brief lives. Prometheus, then, should cause a periodic waxing and waning of the clumps of moon activity.
Further work with the Cassini data should help the researchers firm up this theory, for the Prometheus influence should cause an increase in the clumping and breakup activity within the next few years. Cassini has a sufficient lifetime to test that prediction. What adds further interest to the story is the fact that we’re seeing in miniature some of the elementary processes that, 4.6 billion years ago, led to the formation of the Solar System’s planets. Consider Saturn’s F ring, then, a laboratory for processes we’d like to learn much more about as we turn our instruments to young stars and the planets coalescing around them.
The paper is French et al., “Analysis of clumps in Saturn’s F ring from Voyager and Cassini,” published online in Icarus on July 15, 2014 (abstract). This news release from the SETI Institute is also helpful.
