Centauri Dreams
Imagining and Planning Interstellar Exploration
Into Plutonian Depths
The image of Pluto on the right — an artist’s impression, to be sure (credit: NASA, ESA and G. Bacon, STScI) — suggests Ganymede to me more than Pluto, but we’ll have to wait and see what New Horizons turns up as it continues to close on its target. It’s worth thinking about how our views of this place have changed over time. The world found by Clyde Tombaugh seemed small enough when he found it, but a fraction of its light was actually coming from its yet smaller moon, which wouldn’t be discovered until USNO astronomer James Christy nailed it in 1978.
Gregory Benford depicted Pluto with a nitrogen sea in a 2006 novel called The Sunborn, one in which he explored the possibility of life at -185 degrees Celsius, the lifeforms themselves the result of an experiment by heliopause beings who drew energy from magnetic interactions far from the Sun. Even more speculative is Stephen Baxter’s story “Goose Summer” (from the Vacuum Diagrams collection of 2001), in which Plutonian life physically interacts with Charon, the latter ‘seeding’ the Plutonian surface.
But the real thing beckons. We now have images taken between May 8 and 12, downlinked last week. Here we’re looking at Pluto from a distance of 77 million kilometers using the now familiar Long-Range Reconnaissance Imager (LORRI) aboard New Horizons. The differences between this view and what we saw in April are striking. We’re now 35 million kilometers closer and have twice the pixels to work with, aided by image deconvolution techniques to tease out detail.
Here’s the May 12 imagery as contrasted with April 16 — a click on the New Horizons link will show you two other photo sets contrasting the earlier and later views.
Image: These images show Pluto in the latest series of New Horizons Long Range Reconnaissance Imager (LORRI) photos, taken May 8-12, 2015, compared to LORRI images taken one month earlier. All images have been rotated to align Pluto’s rotational axis with the vertical direction (up-down), as depicted schematically in the center panel. Between April and May, Pluto appears to get larger as the spacecraft gets closer, with Pluto’s apparent size increasing by approximately 50 percent. Pluto rotates around its axis every 6.4 Earth days, and these images show the variations in Pluto’s surface features during its rotation. All of the images are displayed using the same linear brightness scale.
The deconvolution method used to sharpen the images can, the New Horizons team reminds us, sometimes create artifacts, meaning that we’ll need to have the smaller details of these images confirmed as New Horizons gets closer. According to mission project scientist Hal Weaver (JHU/APL), as quoted in the New Horizons update linked to above, we’ll be seeing images with 5000 times better resolution when we reach closest approach during the July 14 flyby.
Are we looking at a polar cap in this imagery? Mission principal investigator Alan Stern comments:
“These new images show us that Pluto’s differing faces are each distinct; likely hinting at what may be very complex surface geology or variations in surface composition from place to place. These images also continue to support the hypothesis that Pluto has a polar cap whose extent varies with longitude; we’ll be able to make a definitive determination of the polar bright region’s iciness when we get compositional spectroscopy of that region in July.”
We’re just seven weeks away from the flyby, with Stern now moving to the east coast for the encounter operations through late July. As excitement builds, a Pluto Safari smartphone app has appeared, available for iOS as well as Android. Produced by Simulation Curriculum Corp., the free app offers interactive views of the locations of Pluto and New Horizons, along with a timeline of New Horizons mission milestones and the latest news about the spacecraft. Pluto Safari is available here in its iOS version and here for the Android iteration.
Meanwhile, looking over my collection of old science fiction magazines, I enjoyed tracking down Stanton A. Coblentz’ story “Into Plutonian Depths,” which ran in Wonder Stories Quarterly in the Spring 1931 issue. To my knowledge, this was the first story written with knowledge of Clyde Tombaugh’s discovery. Using a ‘gravity insulator,’ our protagonist and Stark, his science mentor, set out for the most distant planet known. Coblentz refers to ‘the trans-Neptunian planet’ found by Tombaugh and conjures up mystery in the name Pluto. As Stark exclaims:
“Think of it a billion miles or so beyond Neptune, a globe perhaps no larger than the earth, lost in the blackness of the outer void, its years longer than our centuries, its seasons longer than our lives! What stories it would be able to tell! Are there any living creatures there? Were any living beings ever able to endure the terror of its sunless, frozen plains? Would we find the imprint of lost races upon its shores? — races that flourished while the planet was heated from within, but that have long ago fallen in the struggle with the cold?”
And so on. It’s a lively tale, a bit mesmerizing in its day (though it goes on far too long), and it mimics the approach of New Horizons as it describes the travelers’ view of “silvery white plains and its broken and enormous mountain ranges, whose snowy summits were offset by sheer black escarpments and ravines as hideous to contemplate as the craters of the moon…” The inhabitants of the ninth planet turn out to be nothing like Benford or Baxter’s creations, and the tale turns into something closer to Rider Haggard than modern SF as it winds its way to a conclusion.
But still, what fun to think about Pluto as it was first envisioned in fiction while we have a spacecraft moving in on the Pluto/Charon system at 1.2 million kilometers per day. Each new set of New Horizons images is going to sharpen our view of a world. If only Stanton Coblentz, and for that matter Clyde Tombaugh himself, were able to watch this encounter unfold.
LightSail Glitch: Hoping for a Reboot
The Planetary Society’s LightSail won’t stay in orbit long once its sail deploys, a victim of inexorable atmospheric drag. But we’re all lucky that in un-deployed form — as a CubeSat — LightSail can maintain its orbit for about six months. Some of that extended period may be necessary given the problem the spacecraft has encountered: After returning a healthy stream of data packets over its first two days of operations, the solar sail mission has fallen silent.
Jason Davis continues his reporting on LightSail, with the latest update on the communications problem now online. We learn that the suspected culprit for LightSail’s silence is a simple software glitch. Everything else looked good when communications ceased, with power and temperature readings stable. Davis explains that during normal operations, LightSail transmits a telemetry beacon every 15 seconds. The Linux-based flight software writes data on each transmission to a .csv file, a spreadsheet-like record of ongoing procedures.
This file continues to grow, and when it reaches a certain size, trouble can happen:
As more beacons are transmitted, the file grows in size. When it reaches 32 megabytes—roughly the size of ten compressed music files—it can crash the flight system. The manufacturer of the avionics board corrected this glitch in later software revisions. But alas, LightSail’s software version doesn’t include the update.
Late Friday, the team received a heads-up warning them of the vulnerability. A fix was quickly devised to prevent the spacecraft from crashing, and it was scheduled to be uploaded during the next ground station pass. But before that happened, LightSail fell silent. The last data packet received from the spacecraft was May 22 at 21:31 UTC (5:31 p.m. EDT).
Let’s hope we’ll still see a deployed LightSail, as in the image above. But anyone who has stared at a PC frozen into immobility knows the feeling that LightSail’s ground controllers must have experienced. The machine is not responding, which means it’s time for a reboot. A manual reboot being out of the question, a reboot command from the ground has to be used, and more than one has been sent. In fact, Cal Poly has been transmitting a new reboot command every few ground station passes. So far, no luck.
A fix may still be in the works from a natural source, but first, the situation led to a bit of humor, in the form of an email Davis received, as recorded in this tweet:
Davis also suggests a LightSail successor to be called BourbonSat, a flight spare that sits in each team member’s kitchen to offer quick stress relief. The humor is edgy but that’s because we may now be reliant on a hands-off fix: Charged particles striking an electronic component in just the right way to cause a reboot. If that sounds extreme, be aware that the phenomenon is not unusual in CubeSats. In fact, Cal Poly’s experience says that most reboot within the first three weeks of operations. You can place this in the context of the 28-day sail deployment timeline and see we might come out just fine.
What happens next depends upon when — and if — that reboot occurs, assuming the continued reboot commands from the ground are not effective. Various software fixes are being tested to see which could be inserted after contact is restored, so that the troublesome .csv file doesn’t cause further problems. Davis also says that when LightSail comes back online, the team will probably begin a manual sail deployment as soon as possible. Let’s make sure, in other words, that when we have a communicating spacecraft, we do what we sent it out there to do.
Exoplanet Exploration Organization Proposed
We’ve recently looked at the role of small spacecraft, inspired in part by The Planetary Society’s LightSail, a CubeSat-based sail mission that launched last week. It’s interesting in that regard to consider small missions in the exoplanet realm. ExoplanetSat, for example, is a 3-unit CubeSat designed at MIT as a mission to discover Earth-sized exoplanets around nearby stars. Here the beauty of the CubeSat is obvious: The platform is low-cost, the development time is relatively short, and there are frequent launch opportunities. Up to 100 ExoplanetSats are planned.
Pulling big benefits from small packages is not new, as the example of the Canadian MOST mission (Microvariability and Oscillations of STars) reminds us. MOST was the first mission dedicated to asteroseismology, to be followed by CoRoT (COnvection ROtation and planetary Transits) and then Kepler. Now we have a proposal for what is being called the United Quest for Exoplanets (UniQuE), which grows out of work performed by an interdisciplinary team hosted by the International Space University. Meeting in Montreal last summer, the group produced a report giving its recommendations to enhance the entire field of exoplanet research.
Michael Michaud passed along an article on this work called “Going Global with Exoplanets” that ran in Space Times, which is the newsletter of the American Astronautical Society. I want to dig into it a bit because one of the larger questions here is how to generate international support for planet-hunting, and as Michaud reminds me, this fits in with what could be a useful collaboration between the exoplanet community and advocates of interstellar flight.
The Montreal team, which was sponsored by NASA and Lockheed Martin, consisted of 28 participants from twelve countries. Its report proposes the creation of the Exoplanet eXploration Organization (EXO), drawing on the fact that while other organizations include exoplanets as part of their mission, most have wider agendas, or are focused solely on a specific group of people. EXO would from the outset take an international perspective that crosses numerous areas of expertise. One part of its charter, which I’ll return to in a moment, is the above-mentioned UniQuE, the creation of small satellites to conduct exoplanet work, just the kind of thing MIT has in mind for ExoplanetSat.
Image: Kepler-16b envisioned in a JPL ‘travel poster’ showing a sky with two suns from the planet that orbits both. Part of a modest but effective public outreach effort from JPL. The EXO proposal similarly looks at ways to heighten public interest in exoplanets and deep space.
But characterizing habitable planets is a multidisciplinary endeavor whose practitioners are located around the globe. If we are to move to characterizing exoplanet atmospheres and pushing into increasingly detailed observations of these worlds, we’re probably going to be in a cost range where funding by a single nation or agency is problematic. One way to encourage international cooperation is through better information exchange and the development of better databases.
Data availability is, the article argues, an area that needs improvement. We do have rich databases like the Exoplanet Data Explorer (California Planet Survey), the Extrasolar Planets Encyclopedia (L’Observatoire de Paris), MIT’s Open Exoplanet Catalogue and NASA’s Exoplanet Archive, but the Montreal team finds the presentation of data between these groups to be inconsistent both in the data provided and the metadata used to search the material. It argues that EXO could be of service:
One EXO initiative would be to support developing and promoting a common data and metadata format for publically available data. The Extended Extrasolar Planets Encyclopedia (EEPE) aims at making new data-fields, links to other databases, and raw data easier to add to the database. The rationale behind proposing EEPE and a structured format is to maximize the use of existing infrastructure, and make it easier to access and update data.
Other possibilities include providing consulting services for exoplanet projects and pooling fundraising expertise, as well as helping researchers promote exoplanet projects to government agencies. Like The Planetary Society, EXO is envisioned as an advocate for its areas of interest, one that also serves as a bridge between the public and the scientific community. Thus educational projects to reach non-specialized audiences are a factor, and so are crowdsourcing efforts to consolidate, sort and analyze the vast amount of incoming data:
While much of it can be handled by computer algorithms, there are various phenomena that need human investigation. At present, raw data is gathered much faster than it is studied. One example is the website planethunters.com, which provides a crowdsourcing platform. After visitors complete a brief tutorial, participants analyze Kepler data by marking potential candidates while discarding uninteresting light curves. Another example is NASA-sponsored OSCAAR (Open Source differential photometry Code for Accelerating Amateur Research), which allows amateur astronomers to contribute their own light curves from amateur observations. These participative initiatives provide people with the ability to engage in the research community. Crowdsourcing, however, is only as powerful as the participants, which emphasizes the need for effective international outreach programs.
What is being proposed is cross-pollination between the exoplanet community and numerous other areas of public interest in space. I like the idea, for example, of offering massive open online courses (MOOCs) as a way not only of reaching the public but also of developing open source tools like lesson plans for teachers and software for amateur astronomers, who would use the EXO platform to connect with each other’s work across borders and disciplines.
But back to small satellites. The low cost and broad involvement, from space agencies to universities and small laboratories, that CubeSats offer can be leveraged here. Think of CubeSats as a route into space for countries that lack the resources for more costly missions. The UniQuE mission proposed here has much in common with ExoplanetSat in that it involves the creation not of single satellites but a constellation of low-cost small spacecraft whose mission would be characterization of the atmospheres of previously detected nearby planets:
The UniQuE team envisioned a standardized design based on a 15 kg 12 unit CubeSat layout carrying a space-proven near-IR mini-spectrometer covering the aforementioned waveband with a sensitivity range compatible with this mission. EXO would provide an overall baseline design and participating entities would have the freedom to customize and size all relevant subsystems so long as overall mission requirements are met, thereby allowing freedom for the inclusion of innovative concepts. The ideal constellation is composed of three to six pairs of satellites on a dawn-dusk sun synchronous orbit, which are launched as piggyback or secondary payloads.
What the Montreal team envisions is that all the UniQuE satellites would be required to observe the transit of an approaching planetary transit of a nearby star, while during the remaining time, the owners of the individual satellites could use them for their own scientific purposes. Results would be disclosed across the range of participating countries to provide a testbed for collaborative research and the development of ever more sophisticated spacecraft.
To read more about the thinking behind EXO, you can access the final report here. It could be argued that we have many of the tools the report recommends already in place, and that researchers currently interact through a variety of networked venues. But I think the development of an exoplanet organization with an international focus and a determined public outreach could consolidate some of these gains while providing useful collaborative tools. Moreover, the public engagement built into this kind of organization could benefit the spread of deep space ideas as we ponder future programs of exploration.
A Mass-Radius Relationship for ‘Sub-Neptunes’
The cascading numbers of exoplanet discoveries raise questions about how to interpret our data. In particular, what do we do about all those transit finds where we can work out a planet’s radius and need to determine its mass? Andrew LePage returns to Centauri Dreams with a look at a new attempt to derive the relationship between mass and radius. Getting this right will be useful as we analyze statistical data to understand how planets form and evolve. LePage is the author of an excellent blog on exoplanetary science called Drew ex Machina, and a senior project scientist at Visidyne, Inc. specializing in the processing and analysis of remote sensing data.
By Andrew LePage
As anyone with even a passing interest in planetary studies can tell you, we are witnessing an age of planetary discovery unrivaled in the long history of astronomy. Over the last two decades, thousands of extrasolar planets have been discovered using a variety of techniques. The most successful of these to date in terms of sheer number of finds is the transit method – the use of precision photometric measurements to spot the tiny decrease in a star’s brightness as an orbiting planet passes directly between us and the star. The change in the star’s brightness during the transit allows astronomers to estimate the size of the planet relative to the star while the time between successive transits allows the orbital period of the planet to be determined. Combined with information about the properties of the star being observed, other characteristics can be calculated such as the actual size of the planet and its orbit. The most successful campaign to date to search for planets using the transit method has been performed using NASA’s Kepler spacecraft, launched in 2009.
One of the other important bulk properties of a planet that is of interest to scientists is its mass. Unfortunately, the transit method is typically unable to supply us with this information except in special circumstances where planets in a system strongly interact with each other to produce measurable variations in the timing or duration of their transits. The transit timing variation (TTV) or transit duration variation (TDV) methods can be used to estimate the masses of the planets of a system including non-transiting planets that might be present. Based on an analysis of Kepler results to date, however, this method can be used in only about 6% of planetary systems that produce transits.
A more widely applicable method to determine the mass of an extrasolar planet is through the precision measurement of a star’s radial velocity to detect the reflex motion caused by the orbiting planet. Combined with information from transit observations as well as the star’s properties, it is possible to calculate the actual mass of a planet and further refine its orbital properties. Unfortunately, NASA’s Kepler mission has discovered thousands of planets and making precision radial velocity measurements takes a lot of time on a limited number of busy telescopes that are equipped to make the required observations. In addition, many of the stars observed by Kepler are too dim or their planets too small for the current generation of instruments to detect radial velocity variations above the noise. This is especially a problem for sub-Neptune size planets including Earth-size terrestrial planets. Taken as a whole, only a small minority of all of Kepler’s finds currently have had their masses measured.
Puzzling Out a Planetary Mass
While astronomers continue to struggle to measure the masses of thousands of individual extrasolar planets found by Kepler, there have been efforts to derive a mass-radius relationship so that the mass of a planet with a known radius can at least be estimated. In addition to being useful for evaluating the level of accuracy required for detection using radial velocity measurements or other methods, such mass estimates are also valuable for scientists wishing to use Kepler radius and orbit data in statistical studies of planetary properties, dynamics, formation and evolution. Over the past few years, there have been various investigators who have attempted to derive a planetary mass-radius relationship as information on the mass and radius of known planets has expanded. These relationships have taken a mathematical form known as a power law such as M = CRγ where M is the mass of the planet (in terms of Earth mass or ME), R is its radius (in terms of Earth radii or RE) and C and γ are constants determined by analysis.
The latest work to derive a mass-radius relationship for sub-Neptune size planets (i.e. planets whose radii are less than 4RE) is a paper by Angie Wolfgang (University of California – Santa Cruz), Leslie A. Rogers (California Institute of Technology), and Eric B. Ford (Pennsylvania State University), which they recently submitted for publication in The Astrophysical Journal. These sub-Neptune size worlds are of particular interest to the scientific community since they span the size range between the Earth and Neptune where no Solar System analogs exist to provide guidance for deriving a mass-radius relationship.
Earlier work over the last few years on the planetary mass-radius relationship relied on least squares regression analysis of a set of planetary radius and mass measurements – a fairly straightforward mathematical method used to determine the constants of an equation that provides the best fit to a set of data points. Unfortunately, this classic method has some drawbacks. It does not properly take into account the uncertainty in the independent variable (i.e. the planet radius, in this case) or instances where the planet has not been detected using precision radial velocity measurements and only an upper limit of the mass can be derived. Another issue is that the least squares regression method assumes a deterministic relationship where a particular planetary radius value corresponds to a unique mass value. In reality, planets with a given radius can have a range of different mass values, in part reflecting the variation in planetary composition running from massive rocky planets with large iron-nickel cores to less massive, volatile-rich planets with deep atmospheres. These variations are expected to be especially important in sub-Neptune-class worlds.
A Bayesian Approach to the Mass/Radius Problem
Instead of using the least squares regression method, Wolfgang, Rogers and Ford evaluated their data using a hierarchal Bayesian technique which allowed them not only to derive the parameters for a best fit of the available data, but also to quantify the uncertainty in those parameters as well as the distribution of actual planetary mass values. Using their approach, they have derived a probabilistic mass-radius relationship where the most likely mass and the distribution of those values are determined. The team considered a total of 90 extrasolar planets with known radii less than 4RE whose masses have been measured or constrained using radial velocity or TTV methods. Neither unconfirmed planets nor circumbinary planets were considered to keep the sample as homogeneous as possible. The team also truncated the mass distribution to physically plausible values that were no less than zero (since it is physically impossible to have a negative mass) and no greater than the mass of a planet composed of iron (since it is unlikely for a planet to have a composition dominated by any element denser than iron).
Image: This plot shows the available mass and radius data (and associated error bars) used in the latest analysis of the mass-radius relationship for sub-Neptune size planets. Various fits to these data are shown including an earlier analysis by Lauren Weiss and Geoffrey Marcy (black dashed line) as well as fits for radii <8 RE, <4RE and <1.6RE (solid colored lines). (credit: Wolfgang et al.)
The detailed analysis of the dataset by Wolfgang, Rogers and Ford found that the subset of extrasolar planets whose masses were measured using the TTV method has a definite bias towards lower density planets. This bias had been suspected since a low density planet will have a larger radius than a denser planet with the same mass. And all else being equal, a larger planet is more likely to be detected using the transit method than a smaller planet. When only considering the sample of extrasolar planets with masses determined using precision radial velocity measurements, this team found that the best fit for the data set was a power law with C = 2.7 and γ = 1.3 (i.e. M = 2.7R1.3). Based on their statistical analysis, Wolfgang, Rogers and Ford found that the data were consistent with a Gaussian or bell-curve distribution of actual planet masses with a sigma of 1.9ME at any given radius value. Just as has been suspected, planets with radii less than 4RE display a range of compositions that is reflected as a fairly broad distribution of actual mass values.
In earlier work by Rogers, it was found that there seems to be a transition in planet composition at a radius no larger than 1.6 RE, above which planets are unlikely to be dense, rocky worlds like the Earth and much more likely to be less dense, volatile-rich planets like Neptune (see The Transition from Rocky to Non-Rocky Planets in Centauri Dreams for a full discussion of this work). For the sample of planets considered here with radii less than 1.6 RE, the team found that C = 1.4 and γ = 2.3. Unfortunately, the sample considered by Wolfgang, Rogers and Ford has little good data for planets in this size range and the masses with their large uncertainties tend to span the full range of physically plausible values. As a result, this analysis can not rule out the possibility of a deterministic mass-radius relationship where there is only a very narrow range of actual planet masses for any particular radius value. Recent work by others suggests that these smaller planets tend to have a more Earth-like, rocky composition which could be characterized with a more deterministic mass-radius relationship (see The Composition of Super-Earths in Drew Ex Machina for a discussion of this work).
This new work by Wolfgang, Rogers and Ford represents the best attempt to date to determine the mass-radius relationship for planets smaller than Neptune. While more data of better quality for planets in this size range are needed, it does appear that sub-Neptunes can have a range of different compositions and therefore possess a range of mass values at any given radius. This new relation will be most useful to scientists hoping to get the maximum benefit out of the ever-growing list of Kepler planetary finds where only the radius is known. Much more data will be required to determine more accurately the mass-radius distribution of planets with radii less than 1.6 RE and more precisely characterize the transition from large, rocky Earth-like planets to larger, volatile-rich planets like Neptune.
The preprint of the paper by Wolfgang, Rogers and Ford, “Probabilistic Mass-Radius Relationship for Sub-Neptune-Sized Planet”, can be found here.
LightSail Aloft!
One of the joys of science fiction is the ability to enter into conjectured worlds at will, tweaking parameters here and there to see what happens. I remember talking a few years ago to Jay Lake, a fine writer especially of short stories who died far too young in 2014. Jay commented that while it was indeed wonderful to move between imagined worlds as a reader, it was even more wondrous to do so as a writer. I’ve mostly written non-fiction in my career, but the few times I’ve done short stories, I’ve experienced a bit of this ‘world-building’ sense of possibility.
Even so, it’s always striking how science and technology keep moving in ways that defy our expectations. Take yesterday’s launch of The Planetary Society’s crowd-funded LightSail, which went aloft thanks to the efforts of a United Launch Alliance Atlas V from Cape Canaveral. LightSail violates expectations on a number of fronts. For one thing, the crowd-funding thing, which is a consequence of an Internet era that science fiction writers lustily engaged, but which enters homes on desktop computers that SF had trouble anticipating.
My old saying applies: It’s the business of the future to surprise us, even those of us who keep thinking about the future every day. Another LightSail surprise is its size. Many science fiction tales have covered solar sails dating back to the wondrous “The Lady Who Sailed the Soul,’ from Cordwainer Smith, and Arthur C. Clarke’s “The Wind from the Sun.” We’ve looked at a number of the early stories in these pages over the years. But imagined sails in those days were vast, just like Robert Forward’s gigantic designs, and I can’t think of anyone in those days who anticipated matching up sails with tiny satellites — CubeSats — which have brought space capabilities down from the level of government organizations to small university groups.
Image: The launch of LightSail aboard an Atlas V, as captured by remote camera on May 20. Credit: Navid Baraty / The Planetary Society.
So we have a CubeSat about the size of a loaf of bread that is about to deploy a sail measuring 32 square meters. CubeSats are cheap, and while they can’t mount missions of the complexity of a Juno or a Cassini, I can see a robust future for them. The beauty of The Planetary Society’s effort here is that while CubeSats can be readily orbited, they’ve had no real propulsion capabilities. Until now. So we’re not testing just one sail. We’re testing a broader concept.
Can we get a CubeSat to another planet? I can see no reason why not if it turns out that the solar sail strategy employed here does the job. And if we can get one CubeSat to another planet, we can surely get more. Thus the possibility of future missions designed around ‘swarms’ of CubeSat descendants, deployed on missions in which the components of a much larger spacecraft are effectively distributed among a host of carriers, all driven by solar photon momentum. Perhaps LightSail is the first step in making such a vision a reality.
Remember, too, that LightSail was launched as only one payload among many. Much media attention went into the launch of the X-37B, understandable because the small space plane has been operated with relative secrecy. But the Atlas V carrying LightSail also carried several other CubeSats into space. Contrast this with the early days of the space program, when each rocket lifted a single payload, and consider where miniaturization and improved design have begun to take us. With Mason Peck’s ‘sprites,’ we’re now exploring an even smaller realm some call ‘satellites on a chip,’ where the idea of swarm operations takes on a whole new luster.
We have about four weeks to wait before LightSail attempts deployment of its mylar sail. Even then the craft will quickly be pulled back into the Earth’s atmosphere, returning along the way images and data on spacecraft performance that will flow to the ground stations at Cal Poly San Luis Obispo and Georgia Tech (LightSail was designed by San Luis Obispo firm Stellar Exploration, Inc.) Data return has already begun. You’ll want to follow Jason Davis’ updates on The Planetary Society’s site as this story unfolds. LightSail’s first telemetry file can be downloaded — according to Jason, the early values appear to be ‘nominal or near predicted ranges.’ Here’s the one item that could be problematic:
The team’s only major concern is a line of telemetry showing the indicator switches for solar panel deployment have been triggered. (Look for line 77 in the telemetry file—the “f” is a hexidecimal value indicating all switches were released.) Under normal circumstances, the solar panels do not open until the sail deployment sequence starts, because the sails have a tendency to start billowing out of their storage cavities.
This telemetry reading, however, does not necessarily mean the panels are open. The switches were once inadvertantly triggered during vibration testing, so it’s possible they popped loose during the ride to orbit. We’ll know for sure after flight day four, when we test out the camera system. This is one time we don’t want to see a pretty picture of Earth—it would mean the panels are open.
I’ll be checking in with Jason’s blog frequently during the mission as we get closer to sail deployment. Meanwhile, be aware that the second iteration of LightSail is scheduled for a 2016 flight, this one a full demonstration of solar sailing in Earth orbit, with launch aboard a SpaceX Falcon Heavy to an orbit of about 720 kilometers. The KickStarter campaign supporting the LightSail project can be accessed here. The level of support that has emerged is encouraging indeed, as success with LightSail will energize the entire community of sail researchers.
Enter the ‘Warm Titan’
Our definition of the habitable zone is water-based, focusing on planetary surfaces warm enough that liquid water can exist there. New work by Steven Benner (Foundation for Applied Molecular Evolution) and colleagues considers other kinds of habitable zones, specifically those supporting hydrocarbons, which can be liquids, solids or gases depending on the ambient temperature and pressure. Benner’s work focuses on compounds called ethers that can link together to form polyethers, offering life a chance to emerge and adapt in hydrocarbon environments.
Out of this comes the notion of ‘warm Titans,’ moons with hydrocarbon seas that are not made up of methane. We have no such worlds in our Solar System, and they needn’t be moons of gas giants to fit the bill. Think of them, as this Astrobio.net news release does, as being oily Earths drenched in hydrocarbons like propane or octane. Although they do not appear in any genetic molecules on Earth, ethers may be the key to fill the function of DNA and RNA on such worlds.
The nucleobases in the four-letter code of DNA and RNA can mutate even as the molecule’s form is retained, and out of this come the proteins that help life interact and adapt with its environment. Like DNA, ethers show repeating elements, in this case of carbon and oxygen, in their chemical backbones. But unlike DNA and RNA, they have no outward negative charge of the kind that lets them dissolve and float freely so they can interact with other biomolecules. Says Benner:
“This is the central point of the ‘polyelectrolyte theory of gene,’ which holds that any genetic biopolymer able to support Darwinian evolution operating in water must have an ever-repeating backbone charge. The repeating charges so dominate the physical behavior of the genetic molecule that any changes in the nucleobases that influence genetic information have essentially no significant impact on the molecule’s overall physical properties.”
Molecules like DNA and RNA cannot dissolve in a hydrocarbon ocean, making them unable to provide the necessary interactions on worlds like Titan. But ethers, strung together in complex polyethers, while they lack an outward charge, do have internal charge repulsions that allow small parts of the molecule to function in ways similar to DNA and RNA nucleobases.
Image: An artist’s impression of the low-lit surface of Titan under the moon’s thick, orange haze, with liquid hydrocarbons pooling and eroding the surface much like water on Earth. Credit: Steven Hobbs (Brisbane, Queensland, Australia).
Benner’s experiments with ethers show that they are not soluble when we get down to temperatures as low as Titan’s, making Saturn’s largest moon an unlikely venue for such life. But while methane has a narrow liquid range (between -184 and -173 degrees Celsius), we can still put ethers to work in warmer hydrocarbon oceans. Thus the emergence of the ‘warm Titan,’ a world perhaps covered with propane instead of methane oceans that can stay liquid over a broad range (-184 degrees Celsius to -40 degrees). Octane turns out to be even better, not freezing until it reaches -57 degrees Celsius or vaporizing until it hits a temperature of 125 degrees.
Thus hydrocarbon molecules larger than methane come to the rescue. Once again we reconsider the notion of a habitable zone. Certainly in terms of life that we are familiar with, liquid water at the surface is a prerequisite. But as we’ve seen on the icy moons of our system’s gas giants, oceans can provide subsurface environments where life could conceivably emerge. Now we have to consider a hydrocarbon habitable zone where propane or octane can exist in a liquid state. “Virtually every star,” says Benner, “has a habitable zone for every solvent.”
The paper is Christopher et al., “Solubility of Polyethers in Hydrocarbons at Low Temperatures. A Model for Potential Genetic Backbones on Warm Titans,” Astrobiology Vol. 15, Issue 3 (11 March 2015). Thanks to Ivan Vuletich for the pointer to this one.
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