by Paul Gilster | Feb 11, 2015 | Exoplanetary Science |
One of the big arguments against habitable planets around low mass stars like red dwarfs is the likelihood of tidal effects. An Earth-sized planet close enough to a red dwarf to be in its habitable zone should. the thinking goes, become tidally locked, so that it keeps one face toward its star at all times. The question then becomes, what kind of mechanisms might keep such a planet habitable at least on its day side, and could these negate the effects of a thick dark-side ice pack? Various solutions have been proposed, but the question remains open.
A new paper from Jérémy Leconte (Canadian Institute for Theoretical Astrophysics, University of Toronto) and colleagues now suggests that tidal effects may not be the game-changer we assumed them to be. In fact, by developing a three-dimensional climate model that predicts the effects of a planet’s atmosphere on the speed of its rotation, the authors now argue that the very presence of an atmosphere can overcome tidal effects to create a cycle of day and night.
The paper, titled “Asynchronous rotation of Earth-mass planets in the habitable zone of lower-mass stars,” was published in early February in Science. The authors note that the thermal inertia of the ground and atmosphere causes the atmosphere as a whole to lag behind the motion of the star. This is seen easily on Earth, when the normal changes we expect from night changing to day do not track precisely with the position of the Sun in the sky. Thus the hottest time of the day is not when the Sun is directly overhead but a few hours after this.
From the paper:
Because of this asymmetry in the atmospheric mass redistribution with respect to the subsolar point, the gravitational pull exerted by the Sun on the atmosphere has a nonzero net torque that tends to accelerate or decelerate its rotation, depending on the direction of the solar motion. Because the atmosphere and the surface are usually well coupled by friction in the atmospheric boundary layer, the angular momentum transferred from the orbit to the atmosphere is then transferred to the bulk of the planet, modifying its spin.
This effect is relatively minor on Earth thanks to our distance from the Sun, but is more pronounced on Venus, where the tug of tidal friction that tries to spin the planet down into synchronous rotation is overcome by the ‘thermal tides’ caused by this atmospheric torque. But Venus’ retrograde rotation has been attributed to its particularly massive atmosphere. The question becomes whether these atmospheric effects can drive planets in the habitable zone of low mass stars out of synchronous rotation even if their atmosphere is relatively thin.
Pressure units in a planetary atmosphere are measured in bars — the average atmospheric pressure at Earth’s surface is approximately 1 bar (contrast this with the pressure on Venus of 93 bars). The paper offers a way to assess the efficiency of thermal tides for different atmospheric masses, with results that make us look anew at tidal lock. For the atmospheric tide model that emerges shows that habitable Earth-like planets with a 1-bar atmosphere around stars more massive than ~0.5 to 0.7 solar masses could overcome the effects of tidal synchronization. It’s a powerful finding, for the effects studied here should be widespread:
Atmospheres as massive as 1 bar are a reasonable expectation value given existing models and solar system examples. This is especially true in the outer habitable zone, where planets are expected to build massive atmospheres with several bars of CO2. So, our results demonstrate that asynchronism mediated by thermal tides should affect an important fraction of planets in the habitable zone of lower-mass stars.
Here is the graph from the paper that illustrates the results:
Image: Spin state of planets in the habitable zone.The blue region depicts the habitable zone, and gray dots are detected and candidate exoplanets. Each solid black line marks the critical orbital distance (ac) separating synchronous (left, red arrow) from asynchronous planets (right, blue arrow) for ps = 1 and 10 bar (the extrapolation outside the habitable zone is shown with dotted lines). Objects in the gray area are not spun down by tides. The error bar illustrates how limits would shift when varying the dissipation inside the planet (Q ~ 100) within an order of magnitude. Credit: Jérémy Leconte et al.
The result suggests that we may find planets in the habitable zone of lower-mass stars that are more Earth-like than expected. Do away with the permanent, frozen ice pack on what had been assumed to be the ‘dark side’ and water is no longer trapped, making it free to circulate. The implications for habitability seem positive, with a day-night cycle of weeks or months distributing temperatures, but Leconte remains cautious: “Whether this new understanding of exoplanets’ climate increases the ability of these planets to develop life remains an open question.”
The paper is Leconte et al., “Asynchronous rotation of Earth-mass planets in the habitable zone of lower-mass stars,” Science Vol. 347, Issue 6222 (6 February 2015). Abstract / preprint available. Thanks to Ashley Baldwin for a pointer to and discussion of this paper.
by Paul Gilster | Feb 10, 2015 | Exoplanetary Science |
A small satellite designed to study and characterize exoplanet atmospheres is being developed by University College London (UCL) and Surrey Satellite Technology Ltd (SSTL) in the UK. Given the engaging name Twinkle, the satellite is to be launched within four years into a polar low-Earth orbit for three years of observations, with the potential for an extended mission of another five years. SSTL, based in Guildford, Surrey and an experienced hand in satellite development, is to build the spacecraft, with scientific instrumentation in the hands of UCL.
The method here is transmission spectroscopy, which can be employed when planets transit in front of their star as seen from Earth. Starlight passing through the atmosphere of the transiting world as it moves in front of and then behind the star offers a spectrum that can carry the signatures of the various molecules there, a method that has been used on a variety of worlds like the Neptune-class HAT-P-11b and the hot Jupiter HD 189733b. The goal of the Twinkle mission is to analyze at least 100 planets ranging from super-Earths to hot Jupiters, producing temperatures and even cloud maps.
Image: This artist’s impression depicts a Neptune-class world grazing the limb of its star as seen from our vantage point. Analyzing starlight as transiting worlds pass in front of and then behind their star can tell us much about the constituents of the planet’s atmosphere. Credit: NASA/JPL-Caltech.
Giovanna Tinetti (UCL), lead scientist for the mission, describes it as the first mission dedicated to analyzing exoplanet atmospheres, adding that understanding the chemical composition of an atmosphere serves as a key to the planet’s background. Distance from the parent star, notes this UCL news release, affects the chemistry and physical processes driving an atmosphere, and the atmospheres of small, terrestrial-class worlds, like that of our own Earth, can evolve substantially from their initial state through impacts with other bodies, loss of light molecules, volcanic activity or the effect of life. The atmosphere, then, can help us trace a planet’s history as we learn whether it was born in its current orbit or migrated from another part of the system..
I should mention that Tinetti was deeply involved in the discovery of water and methane in the atmosphere of HD 189733b. From Tinetti’s website at University College London:
A key observable for planets is the chemical composition and state of their atmosphere. Knowing what atmospheres are made of is essential to clarify, for instance, whether a planet was born in the orbit it is observed in or whether it has migrated a long way; it is also critical to understand the role of stellar radiation on escape processes, chemical evolution and climate. The atmospheric composition is the only indicator able to discriminate an habitable/inhabited planet from a sterile one.
Twinkle was presented several days ago at an open meeting of the Royal Astronomical Society. Interestingly, the mission is being developed with a mixture of private and public sources, at a total cost of about £50 million, including launch. That number seems strikingly low to me, with mission designers claiming that Twinkle is a factor of ten times cheaper to build and operate than other spacecraft developed through international space agency programs. The low cost is being attributed to the use of off-the-shelf components and growing expertise in small mission development. These numbers will be worth remembering if Twinkle performs as expected.
by Paul Gilster | Feb 9, 2015 | Culture and Society |
One of the reasons I do what I do is that when I was a boy, I read Poul Anderson’s The Enemy Stars. Published as a novel in 1959, the work made its original appearance the previous year in John Campbell’s Astounding Science Fiction as a two-part serial titled “We Have Fed Our Sea.” The reference is to Kipling’s poem “The Song of the Dead,” from which we read:
We have fed our sea for a thousand years
And she calls us, still unfed.
Though there’s never a wave of all her waves
But marks our English dead…
Space was, for Anderson, the new sea, one whose imperatives justify the sacrifices we make to conquer her, and “We Have Fed Our Sea” is a far better title for this work than its book version. Kipling writes:
We were dreamers, dreaming greatly, in the man-stifled town;
We yearned beyond the sky-line where the strange roads go down.
Came the Whisper, came the Vision, came the Power with the Need…
I bought The Enemy Stars at the Kroch’s and Brentano’s bookstore on S. Wabash Avenue in Chicago (this was the flagship store of the chain, and what an astonishing place it was for a book-dazzled boy like me to wander about in). I still have that paperback, and I can remember picking it up because of what was on the back cover:
They built a ship called the Southern Cross and launched her to Alpha Crucis. Centuries passed, civilisations rose and fell, the very races of mankind changed, and still the ship fell on her headlong journey toward the distant star. After ten generations the Southern Cross was the farthest thing from Earth of any human work – but she was still not halfway to her goal…
That was my first encounter with the idea that we might go to the stars in multi-generational ways. But Poul Anderson was always ahead of his time, and this is not a generation ship in the model of, say, Heinlein’s Orphans of the Sky. People do not live out their lives aboard the Southern Cross and hand on the great imperative of the mission to their descendants. Instead, crews in the Solar System are regularly teleported out to the ship.
It’s a wonderful story, and that Kipling reference will put a chill up your spine when you see what Anderson does with it. Baen Books incorporated a later novella called “The Ways Of Love” into its edition of The Enemy Stars in 1987, but I much prefer the unadorned core story. I went back to it this past weekend after writing On the Role of Humans in Starflight on Thursday, having thought for several days about the different ways we approach doing long-term things. Probably if I had read Nelson Bridwell’s just published essay To Be or Not to Be? Mankind’s Exodus to the Stars a day earlier, I would have incorporated it into the Thursday post.
But today will do just as well, for Bridwell’s themes are much to the point, reminding me of generation ships in all their manifestations, as well as generation-spanning projects. Described as a ‘senior machine vision engineer working in manufacturing automation,’ the author sees interstellar flight as a natural and necessary outcome of our space efforts, one that will take millennia and will not require violations of physical law. It is an effort, though, that should not be delayed. Writes Bridwell:
Because starships will not be ready to go for many centuries and interstellar voyages may last for thousands of years, we will want to get started sooner rather than later, initially allocating a small but steady fraction of the space budget to identify the most promising engineering approaches and performing early proof-of-concept experiments when affordable. The first application of this technology will be for unmanned interstellar probes that will conduct close-up reconnaissance of nearby solar systems.
Taken to its outer limit, a long-term perspective on Earthly life tells us that the planet will eventually meet its doom, if in no other way, through the gradual swelling of our parent star. But as Bridwell points out, we’re a long way from not just starflight but even a sustained human presence off this planet. It’s prudent, then, to do whatever we can to minimize the existential risk of something happening in the interim to destroy our species. That might involve accelerating the search for near-Earth asteroids and comets to provide plenty of time to change the trajectories of those in dangerous orbits. It also might involve a heightened awareness of and countermeasures for the kind of pandemic that could cut the population in half, if not worse.
In terms of space, bringing the long-term focus of interstellar thinking into play involves continuing the search for promising nearby solar systems where humans may eventually travel even as we develop the kind of closed-loop life support technologies that would sustain human crews over the duration of long voyages. The latter aspect receives far less attention than it should, but it is as key a driver as propulsion for figuring out how to make star journeys.
I think Bridwell has it right that an interstellar effort grows directly out of sustained development here in our own Solar System. We will learn the essentials of closed-loop life support by experimenting in places much closer to home than even the nearest star:
Because it is not at all likely that a warm, moist, green, oxygen-rich twin of Earth will be within our reach, the third goal must be learning how to live under less-ideal conditions, such as on the Moon. We should establish manned outposts on the Moon and Mars where we can develop the expertise to efficiently manufacture everything that we need from local planetary materials. Over the course of hundreds of years, as these outposts grow, they will become second homes within this solar system for humanity.
I see this in terms of O’Neill-class colonies that gradually grow in size and sophistication, as well as bases on planetary surfaces. The immediate need is to develop our skills at living in nearby space so that if something does happen to our planet — nuclear war, perhaps, or biological catastrophe — enough colonies will exist to perpetuate the species. Over the course of the next few centuries, creating such self-sustaining populations in space should be within our technological powers, and these will inevitably spread outward as we explore our system’s resources. I find this prospect encouraging because it assumes nothing the laws of physics do not allow.
Bridwell wonders whether our reluctance to think beyond the current moment is not itself a passing phenomenon, perhaps ‘a madness left over from the Cold War.’ I do think it is a cultural phenomenon, though I have no opinions about its origins in 20th Century geopolitics. We make our own values by choosing what to build, what to believe in, and what goals to pursue. A positive perspective is one that protects the home world first while ensuring that unexpected catastrophe cannot destroy our species. It then begins the long process of exploration and settlement that may spread as far as the outer planets, or perhaps the Oort Cloud, or if we have the determination to make it happen, the distant stars Poul Anderson saw as our destiny.
by Paul Gilster | Feb 6, 2015 | Outer Solar System |
Watching Ceres gradually take on focus and definition is going to be one of the great pleasures of February. The latest imagery comes from February 4, with the spacecraft having closed to about 145,000 kilometers. Here we’re looking at a resolution of 14 kilometers per pixel, the best to date, but only a foretaste of what’s to come. For perspective, keep in mind that while Ceres is the largest object in the main asteroid belt, its diameter is a scant 950 kilometers. Is there an ocean under this surface?
Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI.
Meanwhile, a good deal further out in the system, a small vial of Clyde Tombaugh’s ashes continues its remarkable trek, with new imagery from New Horizons, the spacecraft carrying it, being released on the same day the Ceres images were taken, February 4, which happens to be Tombaugh’s birthday. Born in 1906, Tombaugh’s long life ended in 1997, and he has stayed very much in the thoughts of New Horizons principal investigator Alan Stern, who commented:
“This is our birthday tribute to Professor Tombaugh and the Tombaugh family, in honor of his discovery and life achievements — which truly became a harbinger of 21st century planetary astronomy. These images of Pluto, clearly brighter and closer than those New Horizons took last July from twice as far away, represent our first steps at turning the pinpoint of light Clyde saw in the telescopes at Lowell Observatory 85 years ago, into a planet before the eyes of the world this summer.”
As with Ceres, we’ll be watching a distant speck turn into an actual place with features we’ll need to name as this year moves forward. The image at left, taken at a distance of over 200 million kilometers, come from New Horizons’ telescopic Long-Range Reconnaissance Imager (LORRI), an animation assembled from separate images taken on January 25 and January 27, which makes them the first acquired during the approach phase of the mission. The spacecraft’s flyby of Pluto/Charon takes place on July 14.
Image: A long distance look from LORRI: Pluto and Charon, the largest of Pluto’s five known moons, seen Jan. 25 and 27, 2015, through the telescopic Long-Range Reconnaissance Imager (LORRI) on NASA’s New Horizons spacecraft. New Horizons was about 203 million kilometers from Pluto when the frames to make the first image were taken; about 2.5 million kilometers closer for the second set. These images are the first acquired during the spacecraft’s 2015 approach to the Pluto system, which culminates with a close flyby of Pluto and its moons on July 14. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
So now New Horizons is giving us Pluto at the 2-pixel level, with Charon subtending 1 pixel as seen by LORRI, which JHU/APL describes as ” essentially a digital camera with a large telephoto telescope.” The exposure time here is one-tenth of a second, too short to make any of Pluto’s other moons, much smaller than Charon, visible. We’ll have the satisfaction of watching this system grow in our field of view over the coming months. Along the way, New Horizons will take many images of Pluto against background stars to refine distance estimates and plan course corrections needed for the flyby.
Image: Six Months of Separation: A comparison of images of Pluto and its large moon Charon, taken in July 2014 and January 2015. Between takes, New Horizons had more than halved its distance to Pluto, from about 425 million kilometers to 203 million kilometers. Pluto and Charon are four times brighter than and twice as large as in July, and Charon clearly appears more separated from Pluto. These two images are displayed using the same intensity scales. In LORRI’s current view, Pluto and Charon subtend just 2 pixels and 1 pixel, respectively, compared to 1 pixel and 0.5 pixels last July. The images were magnified four times to make Pluto and Charon more visible. Both images were rotated to show the celestial north pole at the top. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
As to Clyde Tombaugh, whenever I think of him, I think of Michael Byers’ splendid portrait in his novel Percival’s Planet (Henry Holt, 2010), although to be sure, Byers focuses not just on Tombaugh but on the entire enterprise of planet-finding and the nature of obsession (the latter being Percival Lowell’s fixation on ‘Planet X’ as well as the private demons of his subordinates). My earlier review of Percival’s Planet is here. Tombaugh was a Kansas farmboy who could grind lenses like no one else, a self-educated master craftsman whose eye for detail would change our view of the Solar System. What a pleasure to hear the words of his daughter in this New Horizons news release:
“My dad would be thrilled with New Horizons,” said Annette Tombaugh, Clyde Tombaugh’s daughter, of Las Cruces, New Mexico. “To actually see the planet that he had discovered and find out more about it, to get to see the moons of Pluto … he would have been astounded. I’m sure it would have meant so much to him if he were still alive today.”
My favorite photograph of Tombaugh has always been the one above (from the collection at New Mexico State University). Look at the sheer determination in that face! This is a man who knows what he is about — I wish I had known him.
by Paul Gilster | Feb 5, 2015 | Culture and Society |
What does it take to imagine a human future among the stars? Donald Goldsmith asks the question in a recent op-ed for Space.com called Does Humanity’s Destiny Lie in Interstellar Space Travel, playing off the tension between successful robotic exploration that has taken us beyond the heliosphere and the human impulse for personal experience of space. Along the way he looks at options for star travel both fast (wormholes) and slow (nuclear pulse, or Orion).
A fine science writer who worked with Neil deGrasse Tyson on Origins: Fourteen Billion Years of Cosmic Evolution, Goldsmith nails several key issues. The successes of robotic exploration are obvious, and we’re in the midst of several more energizing episodes — the arrival of Dawn at Ceres and the approach of New Horizons to Pluto/Charon, as well as the recent cometary exploits of Rosetta. We have much to look forward to and, as mentioned yesterday, new impetus has arisen for the Europa Clipper mission, which would constitute a fine tandem operation with the ESA’s Jupiter Icy Moons Explorer.
Freeman Dyson, in fact, thinks the success of robotics is so marked that the real work will necessarily be done by machines, with human travel in space in the category of entertainment rather than science. But Goldsmith finds this unsatisfying, and I think he speaks for quite a few people when he says a human presence on places like Mars speaks to our deepest impulses:
Just about everyone welcomes new information about the solar system, but what many really — really — want is for humanity to plant its boots on new soil, as Earth-bound explorers have done for many centuries. Lonely humans in space speak directly to our emotions, but pioneering spacecraft far less so. (Even an apparent exception, such as the hero of the movie “WALL-E,” connects with us through its seeming humanity, a fact that won’t surprise anyone who reflects for a moment on how storytelling works.)
Machines get more powerful at a mind-numbing pace, while the evolutionary changes that help us adapt to new environments move with far slower rhythms. Hybrids of human and machine may one day be feasible, or some kind of mind-uploading (a prospect I still think unworkable, as it tries to fit a consciousness that is the result of evolution in a physical body into an alien matrix). There is also the prospect of artificial intelligence achieving human-like capabilities, as witness the poetic, deeply introspective star-probe of Greg Bear’s novel Queen of Angels.
But for those who insist upon human bodies aboard a starship, these options aren’t enough, which leads us to the confrontation with the reality of distance, the nearest star, Proxima Centauri, being approximately 260,000 times the distance from the Earth to the Sun. Goldsmith takes a look at the Project Orion study in which Dyson played such a major role, envisioning a spacecraft that would be driven by a series of nuclear explosions behind the craft, their energies extracted by a pusher plate and a crew-saving system of enormous shock absorbers.
Image: An early conception of Orion as an interplanetary vehicle, one that would eventually be reworked into Freeman Dyson’s interstellar design. Credit: Adrian Mann.
Dyson’s 1968 paper on the ultimate Orion, a starship capable of reaching the nearest stars in 140 years, gave us what Goldsmith calls ‘the gold standard for visions of interstellar travel,’ in that Orion used technologies not impossibly far from what was currently available. But it’s telling that Dyson still sees the key requirement for interstellar flight as a society that can think in terms of centuries and work with long-term planning and execution of generational projects. Orion ran afoul of test ban treaties and the ever-controversial issue of radiation, but in any case it’s hard to see a culture with such short time-horizons as ours building such a vessel.
I’m glad to see Goldsmith referring to Steve Kilston’s ideas on slow expansion, which throws out the false dichotomy between fast results or none at all. Kilston’s idea is best described as a worldship, one we’ve looked at before in these pages. An astronomer and something of a philosopher, Kilston believes that within 500 years we will be able to build a vast structure capable of carrying a million people on a journey at a small fraction of the speed of light. It’s a generation ship, and one that banks on serious changes in the human outlook. Says Goldsmith:
Kilston’s “Plausible Path,” like any other low-velocity journey, requires that generations upon generations of spacefarers pass their entire lives short of their goal. Today, this plan would attract few volunteers. But if human society came to feel sure of its long-term viability, so that our time horizon stretched beyond the current limits of (at most) our grandchildren’s lifetimes, the situation would become quite different. Perhaps the wisest aspect of Kilston’s plan lies in its final pre-launch phase: a 100-year cruise through the solar system to demonstrate the full feasibility of the spacecraft and the willingness of its crew to pass their lives in space.
You can read more about Kilston’s ‘plausible path’ in The Ultimate Project, a presentation the scientist made at the Jet Propulsion Laboratory back in 2006. What he is arguing is that starflight will not become a reasonable expectation unless we reach a point where at least some people think that travel times of thousands of years are acceptable given the goal to be accomplished. Here I want to quote Kilston himself, from a comment he wrote on this site in 2013, responding to a suggestion that there may be reasons for interstellar flight that are irrational:
I’m not sure there is such a thing as an “irrational reason” — explanations and motivations certainly should pay attention to emotional factors. The pursuit of long-term goals and dreams is as vital for our mental and societal health as a concern and empathy for other humans is.
Children respond with wonder and enthusiasm when they hear about a grand project like interstellar travel. It can continue to magnificently inspire them long after we initiate it. As Pierre Teilhard de Chardin wrote, “The future belongs to those who give the next generation reason for hope.”
Goldsmith himself seems to be in this camp. And I think he’s practical enough to acknowledge that the outcome is very much up to us. There is no certainty that our species will ever attain interstellar flight, but if we are to make it happen, we’ll have to learn how to live off the Earth long-term. That would in my view involve ever increasing colonization within the Solar System to master the technologies needed for starflight and the human issues of survival in deep space.
At that point, I see no reason why space habitats on the scale of what Steve Kilston has long studied could not be built, either as explicit starships or as O’Neill-style colony worlds. Would generations accustomed to living in constructed habitats like these eventually decide to take one of their vessels all the way to another star? We have trouble imagining people who would be willing to live this way, but several centuries of technological development and experience in space could make the prospect far less onerous. I agree with Kilston that it’s a plausible path, and whether it happens or not, we still have rapidly advancing artificial intelligence to fall back on. In one form or another, I think human efforts will indeed result in interstellar journeys.
