Centauri Dreams

Imagining and Planning Interstellar Exploration

Remembering Dandridge Cole

I’ve been thinking all weekend about Dandridge Cole, the aerospace engineer and futurist whose death at age 44 deprived interstellar studies of one of its most insightful advocates. Cole died in 1965, just five years before a deadline he himself set (in 1953!) for a manned landing on the Moon. But then, the former paratrooper from Ohio thought a lot about the future and the need for a kind of ‘future studies’ that would look at current technological trends and project going forward just as conventional historical studies reconstruct what happened to us in centuries past.

The heart attack that struck Cole down in his office at General Electric’s Space Technology Center in Valley Forge, PA deprived us of much, but we do have the substantial legacy of a number of articles and monographs, along with three books, among which Islands in Space: The Challenge of the Planetoids, written with Donald Cox (Chilton Books, 1964) may stand out as the most influential. Andreas Hein, who is heading up the Project Hyperion worldship study for Icarus Interstellar, harks back to the inspiration of Cole in The Hollow Asteroid Starship: Dissemination of an Idea, published on the Icarus blog late last week.

Image: Dandridge Cole, who coined the term ‘macro-life’ to refer to human colonies in space and their evolution. Credit: Wikimedia Commons.

The idea is now a familiar one to science fiction fans, especially after its appropriation by George Zebrowski in his 1979 novel Macrolife, but in the mid-60s, the notion of hollowing out an asteroid to create an interstellar vehicle would hardly have been common currency. As Hein comments in his article, what Cole was doing was creating a bridge between the kind of space colonies that Gerard O’Neill would make famous and the worldships that might one day take a large human colony, a self-contained society, to a distant star.

The idea has resonance because star journeys may turn out to be multi-generational affairs that evolve naturally out of our eventually mastered skills at creating self-contained habitats in nearby space. If you can build a ship large enough and comfortable enough to re-create a planet-like environment within it, then living there might become so natural that future generations born aboard the craft would see no need for planetary living. A colony world like that might eventually disengage from the stellar system that created it and begin a voyage that would have no other aim than continuing exploration, taking ‘home’ with the crew wherever it went.

Artist and futurist Roy Scarfo provided the artwork in Cole’s 1965 book Beyond Tomorrow. On his site, Scarfo recalls going with Cole in the ambulance and being in the hospital at the time of his death. A futurist to the end, Cole had planned to have his body frozen and had made a serious study of cryogenics:

When we got to the hospital, the hospital personnel took him to a room. When they informed me that Dan was dead, and knowing that Dan wanted to be frozen, I called Ettinger, who I believe was in Chicago and who was the authority on cryogenics at the time. He knew Dan and instructed me to get in touch with a hospital and make arrangements for freezing. I believe it was the University of Pennsylvania hospital. I was racing against time as every second counted to preserve the body.

Personal and legal issues persuaded the family not to proceed with the arrangements, and Cole was buried conventionally, with Scarfo serving as one of the pallbearers.

Scarfo also wrote an appreciation of Cole on Alex Michael Bonnici’s Discovery Enterprise site in which he recalls working with Cole on Beyond Tomorrow in the evenings after work in Scarfo’s office at GE, where they would go over the chapters word by word. Says Scarfo:

Our work together gave us a handle as “the weird couple” because of the way-out material we were producing together. Today many of those concepts are as common as soap. The majority of our work together was done outside our regular responsibilities at GE, although sometimes they overlapped. We would meet almost daily for lunch at the cafeteria and afterward walk and talk during the rest of our lunch hour. This went on for years.

We’re surely due for a renewed look at Cole’s contribution and his ideas, especially as attention now turns to mining and the other possibilities the asteroids represent. Alex Michael Bonnici wrote his own tribute to Cole in 2007, one that encapsulates the asteroid-as-habitat idea:

In 1963, Cole wrote Exploring the Secrets of Space: Astronautics for the Layman with I. M. Levitt. In this book they suggested hollowing out an ellipsoidal asteroid about 30 km long, and rotating it about its major axis to simulate gravity. By reflecting sunlight inside with mirrors, and creating, on its inner surface, a pastoral setting an asteroid could be transformed into a permanent space colony. Cole and Cox also envisioned that asteroids would provide the raw materials to form the basis of a spacefaring civilization. And, that asteroidal materials would also serve terrestrial needs. In their view these materials could be transported using mass drivers or linear motors. Cole’s work largely presages that of Gerard K. O’Neill by more than a decade.

Extend the notion to an interstellar journey and you get what Cole would call a ‘nomadic pseudo-Earth’ that would be the seeding ground for so-called ‘macro-life.’ Cole’s view was that future human evolution inside such habitats, which includes synchrony between humans, their environment, and their technology, creates a ‘new large-scale life form.’ It was one he felt we must become, for in the years not long before his death, he had become extremely worried about our species not only in terms of population pressure but also weapons proliferation. Moving into space would be the chance to give humanity a progressing series of new and better starts.

Image: The populated asteroid from without and within. Credit: Roy Scarfo.

Here’s TIME‘s take on Cole’s macro-life views in a January 27, 1961 article:

Cole proposes the development of giant spaceships, each of which would contain at least 10,000 individual humans who would function rather like the cells of a multicelled animal; collectively, they would constitute what Cole calls a unit of “macrolife.” Stowed along with the humans in the vast body of the macroorganism would be domestic animals, plants, raw materials, machines and computers, as well as microfilms of all the books in the Library of Congress. A fully developed unit of macrolife would have rocket propulsion to enable it to move at will around the solar system. It would be able to live independently almost anywhere in space, but its normal habitat would be the asteroid belt between Mars and Jupiter where it could feed upon the mineral riches of the asteroids.

Macrolife in space would be self-adjusting, spinning off new units aboard new asteroids as necessary, but Cole freely acknowledged the difficulties in creating self-sustaining biospheres, urging that underwater bases or other sealed environments would need to become experimental testbeds for his ideas. A spacefaring species aboard a hollowed-out world, spun up for artificial gravity and provided with many of the amenities of planetary life, could well take to the stars one day. But in any case, Cole’s legacy of insightful probings of the human future will endure, the work of a man whose all too short life yielded much and has inspired interstellar theorists ever since.

For more on Cole, see Joseph Friedlander’s In Praise of Large Payloads for Space.

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Pushing Beyond Pluto

What would you do if you had a spacecraft pushing toward the edge of the Solar System with nothing much to do? The answer is to assign it an extended mission, as we found out with the two Voyagers and their continuing data return that is helping us understand the boundaries of the heliosphere. In the case of New Horizons, NASA’s probe to Pluto/Charon, two extended missions may be involved after Pluto, the first being a flyby of one or more Kuiper Belt targets, the second being a further look at what is actually going on where the solar wind meets the interstellar medium.

Alan Stern, principal investigator for New Horizons, comments on the possibility in his latest report on the mission, noting that a second extended mission isn’t out of the question, and adding that New Horizons won’t make it as far as the Voyagers before it runs out of power. But 90 to 100 AU seems a possibility, which would provide a useful supplement to Voyager data. Remember that New Horizons carries two instruments ideal for this part of the system. The first is the Solar Wind Around Pluto (SWAP) plasma instrument, the second the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI). All this is in addition to what the spacecraft’s dust counter, its two imagers and its ultraviolet spectrometer may tell us.

Stern’s report on New Horizons comes at the same time we have word, from a new paper in Science, that the assumed ‘bow shock’ at the outer edge of the heliosphere may not actually exist. The bow shock is at the boundary between the solar wind pushing out from the Sun and the interstellar medium, an area of presumed turbulence that has been observed around other stars. The new paper from David McComas (SwRI) and colleagues presents findings from the Interstellar Boundary Explorer (IBEX) that show the Sun is moving more slowly in relation to the interstellar medium than previously thought, slow enough to prevent a bow shock from forming.

Image: IBEX has caught the interstellar wind that surrounds and compresses our heliosphere and has found that it travels more slowly and in a different direction than previously thought. This new understanding has important implications for the size and shape of the heliosphere and may inform the history and future of the solar system. Credit: SwRI.

Maybe New Horizons can help us clarify the situation with studies of the outer heliosphere, but we still lack a mission that could get us out as far as the bow shock region itself (the Voyagers have entered the heliosheath, but their signals will surely be lost before getting to the needed 200 AU or more from the Sun). All of this ties in with recent Cassini results suggesting that the heliosphere is more spherical than comet-shaped, so perhaps the interactions at system’s edge aren’t quite as fierce as has been thought. We need Innovative Interstellar Explorer to learn more, or a comparable mission specifically designed to penetrate into true interstellar space.

New Horizons in the Kuiper Belt

Meanwhile, our Pluto/Charon mission is, says Stern, doing just fine, having exited a period of hibernation on April 30 to begin a series of extensive systems checkouts. The spacecraft’s Pluto encounter occurs in the summer of 2015, but it should take a year to get all the encounter data back to Earth due to the slow data transmission rates at that distance. It’s after that that the first extended mission, subject to approval by NASA, would study objects in the Kuiper Belt. The spacecraft should have about 40 percent of its fuel still available, so a choice of KBOs should be possible, assuming the ongoing hunt for likely candidates turns up workable targets.

The New Horizons team has been using Earth-based telescopes to hunt for KBOs, but so far none has been identified that would be within range of New Horizons. It’s a tricky search, and one Stern assumes will succeed, but his recent report explains some of the problems:

First, the only KBOs within our reach are likely to be small, roughly 50 kilometers in diameter. Because they are small and far away, they will be faint as seen from Earth. In fact, calculations show that the KBOs we need to find are going to be about 25,000 times fainter than Pluto, which is itself about 10,000 times fainter than the eye can see. This means we have to search for objects with the largest telescopes and most sensitive astronomical cameras on Earth.

The second factor making the search tough is that our trajectory is pointed at the heart of the Milky Way’s densest star fields — those of the galactic center in the constellation Sagittarius. So our search is kind of a “needle in a haystack” hunt for very faint objects slowly moving against regions of the sky thick with stars!

All I could think of when I read that was an image of Clyde Tombaugh working the blink comparator at Lowell Observatory in 1930. I wonder what he would have thought then of the chances for tracking a target 25,000 times fainter than the dim planet (well, dwarf planet) he eventually found. The New Horizons effort should have at least one target defined by 2015, at which time an engine burn in the fall of that year would change the spacecraft’s trajectory to reach the first KBO, a journey that — if the team’s calculations are accurate — should last three or four years and perhaps longer. That could place the first KBO flyby as early as 2018 or as late as 2021.

What’s exciting about Stern’s report this time around is his statement that any target KBO will be approached at distances perhaps as close as the Pluto/Charon flyby, which means we should get images from the KBOs that are as detailed as those from Pluto. The down side: With no other Kuiper Belt mission in the works, we’ll need every bit of New Horizons’ observations on KBO surface composition and features, temperatures, moon or rings system and anything else the brief encounter can deliver, for as Stern puts it, “New Horizons is very likely to be the only spacecraft that will explore KBOs in the lifetime of most people alive today.”

Or maybe not. Let’s assume that first extended mission into the Kuiper Belt will be approved, which will yield not only close-up KBO images but also more distant observations of KBO satellites as well as dust particle distribution data. And while we get behind missions like Innovative Interstellar Explorer and push to see them implemented, let’s also cross our fingers for that second extended mission that would keep New Horizons active until it goes silent somewhere out around 100 AU. As the latest IBEX findings remind us, we know all too little about the boundary between our Solar System and the interstellar gulf beyond.

The new IBEX findings are in McComas et al., “The Heliosphere’s Interstellar Interaction: No Bow Shock,” published online in Science 10 May 2012 (abstract).

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KOI-872: Timing is Everything

It’s no surprise that the techniques we’re using to look for moons around exoplanets should start turning up new planets on their own. We’re still looking for that first exomoon, but a team of researchers working with the Hunt for Exomoons with Kepler (HEK) project has found transit variations that have revealed a second planet around a star already known to have one transiting planet. The star is the intriguing KOI-872 (KOI stands for Kepler Object of Interest), the data on which were recently released by the Kepler team and analyzed swiftly by HEK.

Kepler’s transit methods examine the change in starlight when an exoplanet passes in front of the star being observed. This lightcurve, however, can tell more than a single tale. David Kipping (Harvard-Smithsonian Center for Astrophysics) is head of HEK and second author of the paper on the new work, which was published online today in the journal Science:

“For a planet following a strictly Keplerian orbit around its host star, the spacing, timing and other properties of the observed transit light curve should be unchanging in time. Several effects, however, can produce deviations from the Keplerian case so that the spacing of the transits is not strictly periodic.”

Those effects are clear cut, changes in how long the transit takes (Transit Duration Variation, or TDV), and changes to when it actually occurs (Transit Timing Variation, or TTV). These deviations are sensitive to objects as small as 0.2 Earth masses, which shows their utility in hunting for large exomoons. In the case of the Sun-like star KOI-872, transits of the known planet show time variations of two hours. The TTVs are large and precise enough to allow scientists to calculate the orbit of the second planet. According to Kipping, both the transiting planet and the unseen planet perturbing its orbit are Saturn-class worlds. Moreover, they orbit close to a 5:3 mean motion resonance, accounting for the size of the timing variations.

Image: Using Kepler Telescope transit data of planet “b”, scientists predicted that a second planet “c” about the mass of Saturn orbits the distant star KOI-872. This research, led by Southwest Research Institute and the Harvard-Smithsonian Center for Astrophysics, is providing evidence of an orderly arrangement of planets orbiting KOI-872, not unlike our own solar system. Credit: Southwest Research Institute.

You may recall from last fall the story of Kepler-19b (discussed here in On Planets and What We Can See), a probable mini-Neptune some 650 light years away in the constellation of Lyra. Researchers at the Harvard-Smithsonian Center for Astrophysics were able to uncover a second world in the same system, Kepler-19c, by studying the transit timing variations of Kepler-19b. In this case, the timing variations were about five minutes long, but other than revealing the planet’s existence, they allowed no further deductions about its size or composition. In fact, the prospects — all consistent with the limited data available — ranged from a rocky planet on a circular five-day orbit to a gas giant on an oblong 100-day orbit.

But the new planet around KOI-872 is another matter entirely, and in Kipping’s words represents the transit timing variation model coming into full maturity, allowing us to measure the mass of the unseen planet and to determine the 5:3 orbital resonance. Moreover, we get tight constraints on the inclination, semi-major axis and eccentricity of both planets, which indicate they follow circular, co-planar orbits much like planets in our own Solar System. We now know that the unseen planet orbits its star every 57 days. Further Kepler observations will allow us to refine these details and tune up TTV and TDV methods for future planet and exomoon discoveries.

Image: Scientists analyzed Kepler Telescope data and identified KOI-872 as a stellar system where measured transits of a planet orbiting the star show large time variations (the shifting bumps in the data) indicative of a hidden companion. A team led by Southwest Research Institute and the Harvard-Smithsonian Center for Astrophysics determined that the observed variations can be best explained by an unseen planet about the mass of Saturn orbiting the host star every 57 days. Credit: Southwest Research Institute.

When the HEK team went to work on KOI-872, it was motivated in part by the fact that the transiting planet was large enough to hold on to a large moon of the size that current TTV techniques could uncover. The large TTVs yielded a planet rather than a moon in this case, but future finds of large moons around gas giants in the habitable zone will give us some idea as to whether single, rocky planets are the only likely venue for life. Are there as many habitable moons in the galaxy as habitable planets, or even more? HEK is the first step toward helping us find out.

The paper is Nesvorný, Kipping et al., “The Detection and Characterization of a Nontransiting Planet by Transit Timing Variations,” published online in Science May 10 2012 (abstract).

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Space Exploration: A Closing Window?

Our expectations determine so much of what we see, which is one of the great lessons of Michael Michaud’s sweeping study of our attitudes toward extraterrestrial intelligence in Contact with Alien Civilizations (Springer, 2006). But extraterrestrials aside, I’ve also been musing over how our attitudes affect our perceptions in relation to something closer to home, the human space program. Recently I was reminded of Richard Gott’s views on the space program and the Copernican Principle, which suggest that just as our location in the universe is not likely to be special, neither is our location in time.

My expectation, for example, is that whether it takes one or many centuries, we will eventually have expanded far enough into the Solar System to make the technological transition to interstellar missions. But Gott (Princeton University) has been arguing since 2007 that there is simply no assurance of continued growth. In fact, his work indicates we are as likely to be experiencing the latter stages of the space program as its beginnings. The view is controversial and I like to return to it now and again because it so shrewdly questions all our assumptions.

Image: Apollo 17 Saturn V rocket on Pad 39-A at dusk. Will manned space exploration ever achieve the levels of funding that made Apollo possible again? Credit: NASA.

So ponder a different, much more Earth-bound future, one in which funding for human spaceflight may end permanently. Examples abound, from the pyramid-building phase of Egypt’s civilization to the return of Cheng Ho’s fleet to China — not every wave of technology is followed up. Thus Gott, in a short but intriguing discussion called A Goal for the Human Spaceflight Program:

Once lost, opportunities may not come again. The human spaceflight program is only 48 years old. The Copernican Principle tells us that our location is not likely to be special. If our location within the history of human space travel is not special, there is a 50% chance that we are in the last half now and that its future duration is less than 48 years (cf. Gott, 2007). If the human spaceflight program has a much longer future duration than this, then we would be lucky to be living in the first tiny bit of it. Bayesian statistics warn us against accepting hypotheses that imply our observations are lucky. It would be prudent to take the above Copernican estimate seriously since it assumes that we are not particularly lucky or unlucky in our location in time, and a wise policy should aim to protect us even against some bad luck. With such a short past track record of funding, it would be a mistake to count on much longer and better funding in the future.

This application of the Copernican Principle goes against my deepest presumptions, which is why I appreciate the intellectual gauntlet it hurls down. Because what Gott is sketching is a by no means impossible future, one in which the real question becomes how we can best use the technologies we have today and will have in the very near future to ensure species survival. Gott’s answer is that within the first half of this century or so, we will have the capability of planting a self-sustaining colony on Mars, making us a two-planet species and thus better protected against global disaster of whatever sort. We will have created an insurance policy for all humanity.

Let’s act, in other words, as if we don’t have the luxury of an unbroken line of gradual development, because an end to the space program some time in the 21st Century might mark the end of any chance we have to get into the Solar System, much less to the stars. Skip the return to the Moon, a hostile environment not conducive to colonization, and go for the one best chance for extending the species, a planet with water, reasonable gravity and the resources needed to get an underground base off to a survivable start. The real space race? The race to get a colony planted in the most likely spot before all funding for human spaceflight ends.

Gott is reminded of the library of Alexandria, a laudable effort to collect human knowledge but one that eventually burned, taking most (but thankfully not all) of Sophocles’ plays with it. Here he’s thinking of the surviving seven Sophoclean plays and weighing them against the 120 that the dramatist wrote, by way of making the case for off-world colonies as soon as possible:

We should be planting colonies off the Earth now as a life insurance policy against whatever unexpected catastrophes may await us on the Earth. Of course, we should still be doing everything possible to protect our environment and safeguard our prospects on the Earth. But chaos theory tells us that we may well be unable to predict the specific cause of our demise as a species. By definition, whatever causes us to go extinct will be something the likes of which we have not experienced so far. We simply may not be smart enough to know how best to spend our money on Earth to insure the greatest chance of survival here. Spending money planting colonies in space simply gives us more chances–like storing some of Sophocles’ plays away from the Alexandrian library.

As I said, this is bracing stuff (and thanks to Larry Klaes for the pointer). Gott is not the only one wondering whether there is a brief window that will allow us to move into the Solar System and then close, but he is becoming one of the more visible proponents of this view. The motto of the Tau Zero Foundation — ad astra incrementis — assumes a step-by-step process over what may be centuries to develop the technologies for travel to other stars. But Gott’s point is emphatic and much more urgent: For incremental development in space to occur, we should multiply the civilizations that can achieve it, spinning off colonies that back up what we have learned against future catastrophe.

That’s a job not for the distant future but for the next 4-5 decades. Gott reckons that if we put up into low Earth orbit as much tonnage in the next 48 years as we have in the last 48 years (in Saturn V and Shuttle launches alone) we could deliver 2,304 tons to the surface of Mars. And while he talks about heavy lift vehicles like the Ares V, we also have commercial companies like SpaceX with its Falcon Heavy concept and the continuing efforts of Robert Zubrin’s Mars Society to make something like this happen even absent massive government intervention.

Will the first interstellar mission be assembled not by an Earth team but by the scientists and engineers of a colony world we have yet to populate? There is no way to tell, but a Mars colony of the kind Gott advocates would give us at least one alternative to a future Earth with no viable space program and no prospects for energizing the species through an expansive wave of exploration. One colony can plant another, multiplying the hope not only of survival but renaissance. But all of it depends upon getting through a narrow temporal window that even now may be closing.

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Jupiter Icy Moons Explorer

Mars has always been a tempting destination because of the possibility of life. Thus the fascination of Schiaparelli’s ‘canals,’ and Percival Lowell’s fixation on chimerical lines in the sand. But look what’s happened to the question of life elsewhere in the Solar System. We’ve gone from invaders from Mars and a possibly tropical Venus — wonderful venues for early science fiction — to a vastly expanded arena where, if we don’t expect to find creatures even vaguely like ourselves, we still might encounter bacterial life in the most extreme environments.

Astrobiology will push exploration. This is not to say that objects in deep space aren’t worth studying in their own right, possible life or not, but merely to acknowledge that if we find life on another world, it deepens our view of the cosmos and fuels the exploratory imperative. A ‘second genesis’ off the Earth, once confirmed, would heighten interest in other targets where microbial life might take hold, from the cloud tops of Venus out to the icy moons of Jupiter and Saturn. We can’t completely discount even the remote Kuiper Belt in terms of dwarf planets and their possible internal oceans.

Jupiter’s Intriguing Moons

The latest mission news from the European Space Agency makes the point as well as anything. The Jupiter Icy Moons Explorer (JUICE) mission, recently approved as part of the agency’s Cosmic Vision 2015-2025 program, takes us from French Guiana aboard an Ariane 5 to Europa, Ganymede and Callisto, all three candidates for internal oceans. It’s no surprise that the major themes of Cosmic Vision at play here are the conditions for planet formation and the emergence of life.

2022 is the scheduled launch date, with arrival in Jupiter space in 2030, after which the spacecraft will spend three years studying these interesting worlds and reporting back to Earth. The Guardian quotes Leigh Fletcher (Oxford University) in this recent article:

“Scientists have had a lot of success detecting the giant planets orbiting distant stars, but the really exciting prospect may be the existence of potentially habitable ‘waterworlds’ that could be a lot like Ganymede or Europa.

“One of the main aims of the mission is to try to understand whether a ‘waterworld’ such as Ganymede might be the sort of environment that could harbour life.”

The notion of a habitable zone — habitable for human beings — gives way to the much broader ‘life zone’ where some form of life might emerge, and Jupiter offers an extremely useful environment in which to probe it. How does Ganymede’s magnetic field, for example, protect it from the hostile radiation belts spawned by the solar wind interacting with Jupiter’s huge magnetosphere and Io’s plasma? How do Europa and Callisto compare to what we’ll find on Ganymede, and which of the three is most likely to offer conditions in which life might prosper?

Assessing the Radiation Risk

The JUICE mission will make flybys of Callisto and Europa in search of answers, making the first measurements of the thickness of Europa’s crust. It will then enter into orbit around Ganymede in 2032 to study both the surface and internal structure of the moon, the only one in the Solar System known to generate its own magnetic field. You can find ESA’s matrix of science objectives here. Following its selection, the mission now enters a definition phase lasting 24 months. As you might guess, radiation is a major concern, with a late 2011 technical report noting that a shielding analysis should be carried out as soon as possible and a major effort put into shielding simulations to clarify the impact radiation protection will have on payload:

Since 2008 a development was conducted which re-analysed all available in situ measurement data from all missions that visited the Jupiter system (gravity assist and the mission that orbited Jupiter, Galileo), but using primarily Galileo data. The locations of these measurements were first mapped into the Jupiter magnetic field and then parameterised This so called JOREM model was just concluded and validated… at the beginning of the Reformulation Study and was therefore taken as the new baseline. The mean level prediction of the environment by JOREM is higher than the previously used model by about a factor of 2. Furthermore Europa flybys were added to the mission profile, increasing the total dose by about 25%. In comparison, the Callisto phase is only contributing about 9% to the total dose.

Image: Electrodynamic interactions play a variety of roles in the Jupiter system: generation of plasma at the Io torus, magnetosphere/satellite interactions, dynamics of a giant plasma disc coupled to Jupiter’s rotation by the auroral current system, generation of Jupiter’s intense radiation belts. Credit: ESA.

The effects of intense radiation on glasses, fibre optics and other optical and electro-optical components all come into play here, just as they do in the astrobiological questions that go beyond the issue of building the spacecraft. The interaction between the Galilean moons and Jupiter itself through gravitational and electromagnetic forces will be illuminating as we look at the question of possible life in these ‘water worlds.’ From the ‘Yellow Book’ report on JUICE, which contains the results of ESA’s assessment study of the mission:

…organic matter and other surface compounds will experience a different radiation environment at Europa than at Ganymede (due to the difference in radial distance from Jupiter) and thus may suffer different alteration processes, influencing their detection on the surface. Deep aqueous environments are protected by the icy crusts from the strong radiation that dominates the surfaces of the icy satellites. Since radiation is more intense closer to Jupiter, at Europa’s surface, radiation is a handicap for habitability, and it produces alteration of the materials once they are exposed…

That difference will be useful as we compare and contrast the three moons for potential astrobiology. And the differences affect the instruments needed to do the job:

The effect of radiation on the stability of surface organics and minerals at Europa is poorly understood. Therefore, JUICE instrumentation will target the environmental properties of the younger terrains in the active regions where materials could have preserved their original characteristics. Measurements from terrains on both Europa and Ganymede will allow a comparison of different radiation doses and terrain ages from similar materials. The positive side of radiation is the generation of oxidants that may raise the potential for habitability and astrobiology. Surface oxidants could be diffused into the interior, and provide another type of chemical energy…

I’ve focused on radiation here as simply one of the major issues that makes Jupiter such an interesting target when we’re looking at astrobiological possibilities. The Yellow Book report says the Galilean satellites “…provide a conceptual basis within which new theories for understanding habitability can be constructed.” Voyager and Galileo have given us enough of a look at these worlds to know how much we will benefit from an orbiter around Ganymede, even if a far more radiation-hardened Europa orbiter isn’t yet in the cards. But we do get the Callisto and Europa flybys with JUICE, and the path ahead is clearly defined as we try to set needed constraints on the emergence of life on icy satellites in our own Solar System and around other stars.

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Planetary Annihilation around White Dwarfs

Can we tell something about the planets around another star by examining that star’s atmosphere? A new study out of the University of Warwick makes a strong case for the method in the study of white dwarfs, following up on a landmark 2007 paper by Benjamin Zuckerman (UCLA) that looked at pollution in white dwarf photospheres. ‘Pollution’ as in metals that shouldn’t be there, which suggests an accretion disk of material feeding the star, which itself would have collapsed from a red giant stage and is perhaps now absorbing planetary material around it.

What we would expect to find in the atmosphere of a white dwarf is little more than hydrogen and helium — heavy elements should quickly sink to the core and not be observable. But white dwarfs with metal-contamination in their atmospheres have been observed for almost a century now. Let me Boris Gänsicke and colleagues on this, from the paper on the University of Warwick work (internal references deleted for brevity):

…the rapidly growing number of white dwarfs that are accreting from circumstellar discs… unambiguously demonstrates that debris from the tidal disruption of main-belt analogue asteroids or minor planets… or Kuiper-belt like objects…, likely perturbed by unseen planets…, is the most likely origin of photospheric metals in many, if not most polluted white dwarfs.

In a study of more than 80 white dwarfs using the Cosmic Origin Spectrograph on the Hubble Space Telescope, the researchers found four that showed not only oxygen, magnesium, iron and silicon, but a small amount of carbon in their photospheres, closely matching the composition of rocky planets, including the Earth, that orbit close to our Sun. The evidence is that all four stars once had at least one rocky planet orbiting them which has now been destroyed. And because heavy elements like these would be pulled into the core in short order, the researchers believe they are observing the final phase of the destruction of these worlds, an inflow of material falling into the stars at a rate of up to 1 million kilograms every second.

Image: A white dwarf sits in the centre of the remnant of a planetary system. Asteroid sized debris is scattered inwards by interaction with the remaining planets and is tidally disrupted as it approaches the white dwarf forming a disc of dust some of which is raining down onto the star. The researchers have found that the composition of the debris that has just fallen onto the four white dwarfs matches the composition of Earth-like rocky worlds. Credit: Mark A. Garlick.

The white dwarf PG0843+516 turns out to be particularly interesting because of the amount of iron, nickel and sulphur in its atmosphere — the study refers to it as ‘extremely polluted’ — strongly suggesting the star is swallowing the core of a rocky planet that had undergone the same kind of differentiation that occurred in the Earth. Gänsicke sees this as a glimpse of the processes that will one day play out long after our Sun has left its red giant phase:

“What we are seeing today in these white dwarfs several hundred light years away could well be a snapshot of the very distant future of the Earth. As stars like our Sun reach the end of their life, they expand to become red giants when the nuclear fuel in their cores is depleted. When this happens in our own solar system, billions of years from now, the Sun will engulf the inner planets Mercury and Venus. It’s unclear whether the Earth will also be swallowed up by the Sun in its red giant phase – but even if it survives, its surface will be roasted.”

Not a pretty picture, but the rest of the Solar System will be likewise disrupted:

“During the transformation of the Sun into a white dwarf, it will lose a large amount of mass, and all the planets will move further out. This may destabilise the orbits and lead to collisions between planetary bodies as happened in the unstable early days of our solar system. This may even shatter entire terrestrial planets, forming large amounts of asteroids, some of which will have chemical compositions similar to those of the planetary core. In our solar system, Jupiter will survive the late evolution of the Sun unscathed, and scatter asteroids, new or old, towards the white dwarf. It is entirely feasible that in PG0843+516 we see the accretion of such fragments made from the core material of what was once a terrestrial exoplanet.”

All of the more than 80 white dwarfs in the study are within several hundred light years of Earth, offering us a glimpse into deep time, a reminder that our own system formed long after many nearby stars were fully mature and doubtless orbited by planets of their own. The paper is Gänsicke et al., “The chemical diversity of exo-terrestrial planetary debris around white dwarfs,” accepted for publication in the Monthly Notices of the Royal Astronomical Society (preprint). The Zuckerman paper cited above is “Externally Polluted White Dwarfs with Dust Disks,” Astrophysical Journal 663 (2007), p. 1285 (preprint). A University of Warwick news release is also available.

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Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).

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