Sea Salt in Europa’s Dark Materials?

by Paul Gilster on May 14, 2015

‘Europa in a can’ may be the clue to what’s happening on Jupiter’s most intriguing moon. Created by JPL’s Kevin Hand and Robert Carlson, ‘Europa in a can’ is the nickname for a laboratory setup that mimics conditions on the surface of Europa. It’s a micro-environment of extremes, as you would imagine. The temperature in the vacuum chamber is minus 173 degrees Celsius. Moreover, materials within are bombarded with an electron beam that simulates the effects of Jupiter’s magnetic field. Ions and electrons strike Europa in a constant bath of radiation.

What Hand and Carlson are trying to understand is the nature of the dark material that coats Europa’s long fractures and much of the other terrain that is thought to be geologically young. The association with younger terrain would implicate materials that have welled up from within the moon, providing an interesting glimpse of what is assumed to be Europa’s ocean. Previous studies have suggested that these discolorations could be attributed to sulfur and magnesium compounds, but Hand and Carlson have produced a new candidate: Sea salt.


Image: The Galileo spacecraft gave us our best views thus far of Europa, with the discolorations along linear fractures rendered strikingly clear in this reprocessed color view. Credit: NASA/JPL.

Intense radiation peppers Europa’s surface with particle accelerator intensity. It becomes part of the story, causing the discoloration evident in the terrain. Hand and Carlson tested a variety of candidate substances, collecting the spectra of each to compare them with what our spacecraft and telescopes have found. Sodium chloride and various salt and water mixtures proved the most potent substance. When bombarded with the electron beam, they turned from white to the same reddish brown hues found on Europa in a timeframe of tens of hours, which corresponds to about a century on Europa. Spectral measurements showed a strong resemblance to the color within Europa’s fractures as seen by the Galileo spacecraft.


Image: A closer look at Europa. This is a colorized image pulled from clear-filter grayscale data from one orbit of the Galileo spacecraft combined with lower resolution data taken on a different orbit. The blue-white terrain indicates relatively pure water ice. The new work indicates that although some of the colors of Europa come from radiation-processed sulfur, irradiated salts may explain the color of the youngest regions. Highly intriguing is the possibility that these surface features may have communicated with a global subsurface ocean. Credit: NASA/JPL.

Finding sea salt on Europa’s surface would imply interactions between the ocean and the rocky seafloor, according to this JPL news release, with astrobiological implications. In any case, “This work tells us the chemical signature of radiation-baked sodium chloride is a compelling match to spacecraft data for Europa’s mystery material,” says Hand, who speculates that because the samples grew darker with increasing radiation exposure, we might be able to use color variation to determine the age of features on the moon’s surface.

The paper is Hand and Carlson, “Europa’s surface color suggests an ocean rich with sodium chloride,” accepted at Geophysical Research Letters for publication online (abstract).



SETI and Stellar Drift

by Paul Gilster on May 13, 2015

It was natural enough that Richard Carrigan would come up with the model for what he called ‘Fermi bubbles,’ which I invoked in Monday’s post. A long-time researcher of the infrared sky, Carrigan (Fermi National Accelerator Laboratory, now retired) had mined data from the Infrared Astronomical Satellite (IRAS) in 2009 to mount a search for interesting sources that could be Dyson spheres, entire stars enclosed by a swarm of power stations, or conceivably wrapped entirely by a sphere of material presumably mined from the planetary population of the system.

Carrigan’s work on infrared sources goes back well over a decade, involving not only data mining but theorizing about the nature of truly advanced civilizations. If we were to find a civilization transforming a galaxy by gradually building Dyson spheres to exploit all the energies of its stars, we would be witnessing the transformation from Kardashev Type II (a culture that uses all the power of its star) to Type III (a culture that exploits its entire galaxy’s energies. Carrigan reasoned that areas of such a galaxy would gradually grow dark in visible light, the signature of the civilization’s activities becoming traceable only in the infrared.

Both Carrigan and the researchers in the Glimpsing Heat from Alien Technologies (G-HAT) project at Penn State point out that there are natural phenomena that could mimic the Fermi bubble. In a recent paper, the G-HAT team led by Jason Wright mentions a kind of galaxy known as a flocculent spiral as a case in point. Unlike the classic spiral with well-defined structure, these are galaxies with discontinuous spiral arms. What might be perceived as a ‘bubble’ structure here would almost certainly be a natural feature.


Image: NGC 4414, a flocculent spiral galaxy in an image taken by the Hubble Space Telescope. It would be tricky business to find the signature of a Fermi bubble here given the lack of definition in the spiral arms. A bright foreground star from our Milky Way Galaxy shines in the foreground of the image. Credit: Olivier Vallejo (Observatoire de Bordeaux), HST, ESA, NASA.

Galaxy in Motion

But I think the G-HAT critique of the Fermi bubble idea truly gains strength when we consider the motion of stars in the galaxy vs. the times needed for galactic colonization to occur. For we have to remember that when we’re dealing with a galaxy of stars over billions of years, we have to set the galaxy in motion. In a 2014 paper cited on Monday, Wright and company note this:

The static model of stars, in which a supercivilization can be said to occupy a compact and contiguous region of space, is a reasonable approximation for short times (≲ 105 years) and in the case of fast ships (with velocities in significant excess of the typical thermal or orbital velocities of the stars, so ∼ 10−2 c). In such cases, the stars essentially sit still while the ships move at a significant fraction of c and populate a small region of the galaxy in some small multiple of the region’s light-crossing time.

Remember that the shorter the period for colonization, the briefer the ‘window’ for finding a Fermi bubble. But would such bubbles be apparent even assuming the slowest possible expansion?

The G-HAT team’s work makes a compelling case that they would not. For longer times, and assuming slower ships, the static model fails and fails badly. Stars do not stay in one place, and galactic rotation muddles the works. The G-HAT paper considers what it calls ‘conservative timescales’ for a ‘slow’ colonization of the Milky Way. We can use this work to consider a maximum galaxy colonization time to give us a sense of how apparent galactic colonization would be. It also has ramifications, and significant ones, for Michael Hart’s view that we are alone in the galaxy, but I’m not going to stray from the Fermi bubble question in this post.

Imagine that a single spacefaring civilization emerges that uses colony ships traveling at 10-4 c, a speed not so different from our own interplanetary probes. Also assume a very slow launch rate, so that a ship is launched every 104 years, with a maximum range of 10 parsecs. This is slow travel indeed: The travel time to the nearest stars in this scenario is roughly 105 years, a time during which 10 more ships will be launched. The paper explains that this travel speed is comparable to the velocity dispersion of stars in the galactic midplane, a fact that brings new stars into range of the colony ships.

This is an expansion without pause because as the stars mix locally, a stellar system can continue to populate the ten nearest stars every 105 years:

To first order, the stellar system can thus continue to populate the 10 nearest stars every 105 years, without immediately saturating its neighborhood with colonies or the need to launch faster or longer-lived colony ships to continue its expansion. Further, arrival of the colony ships at the nearby stars should not be modeled as a pause in the expansion of the civilization. Rather, the colonies themselves will continue to travel at these speeds with respect to the home stellar system, and themselves encounter fresh stars for colonization every 105 years, during which time they can also launch 10 colony ships.

Using halo stars, which have high relative velocities in relation to the disk, for gravitational assists can provide a boost in cruise speed that allows higher speeds. We wind up with the capability of crossing the galaxy on a galactic rotational timescale. Here is a model of slow expansion that is anything but the uniform growth imagined in a static field of stars:

The slow expansion of an ETI should thus be modeled not as an expanding circle or sphere, subject to saturation and “fronts” of slower-expanding components of the supercivilization. A better model is as the mixing of a gas, as every colonizing world populates the stars that come near it, and those stars disperse into the galaxy in random directions, further “contaminating” every star they come near. If halo stars are themselves colonizable, then those that counter-rotate and remain near the plane will provide even faster means of colonization, since they will encounter ∼ 10 times as many stars per unit time as disk stars.

Here again we note the key fact that this stellar motion obscures any well-defined Fermi bubble:

Non-circular orbits also provide significant radial mixing, and Galactic shear provides an additional source of mixing that is comparable to that of the velocity dispersion of the disk stars once the colonies have spread to vrotv ∼ 1/10 of galaxy’s size, or ∼ 1 kpc from the home stellar system.

Remember, these are extremely conservative assumptions, and they still show that when a civilization begins to colonize its nearest stars, it will populate the entire galaxy in no more than 108 to 109 years. The maximum timescale for galactic colonization is found to be on the order of a galactic rotation (108 years) even for present-day probe speeds. This has implications for the detectability of Fermi bubbles, for on a rotational timescale, such bubbles will be subject to rotational shear and thermal motions that disperse and ‘mix’ them. Or as Centauri Dreams regular Eniac put it in a comment yesterday, “Such bubbles would be sheared into streamers in relatively short order. The spread of civilization would look more like milk stirred into coffee than a clearly delineated expanding bubble.”

The upshot here is that it will be only during a fairly brief period of a galaxy’s history that a spacefaring civilization will have populated only a single contiguous part of that galaxy. The length of that time depends upon how fast the civilization is capable of expanding — the faster the expansion, the shorter the time to observe the interim period between Kardashev Levels II and III. The transition between this era and the galaxy-spanning civilization to follow is, by galactic standards, relatively brief. And if we assume the slowest possible expansion, our Fermi bubbles would be quickly obscured by natural stellar motion within the galaxy. Fermi bubbles, if they do exist, are going to be exceedingly hard to find.

The paper is Wright et al., “The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. I. Background and Justification,” The Astrophysical Journal Vol. 792, No. 1 (2014), p. 26 (abstract / preprint). I consider the SETI work that Jason Wright and his colleagues Matthew Povich and Steinn Sigurðsson are doing with the Glimpsing Heat from Alien Technologies project to be ground-breaking, and plan to check in with it often.



SETI: Are ‘Fermi Bubbles’ Detectable?

by Paul Gilster on May 11, 2015

I’m enough of a perfectionist that when I get something wrong, I can’t rest easy until I figure out how and why I missed the story. Such a case occurred in an article I wrote for Aeon Magazine called Distant Ruins. The article covered the rise of so-called ‘Dysonian SETI,’ which is adding an entirely new dimension to current radio and optical methods by looking into observational evidence for advanced civilizations in our abundant astronomical data.

In the story, I homed in at one point on the work that Jason Wright and his colleagues Matthew Povich and Steinn Sigurðsson are doing with the Glimpsing Heat from Alien Technologies (G-HAT) project at Penn State. Keith Cooper went over the basics of this effort on Friday, putting his own spin on the group’s recent search of 100,000 galaxies. For more background, see Jason Wright’s Glimpsing Heat from Alien Technologies essay.

I noted in the Aeon article that the G-HAT team was examining infrared data from the Wide-field Infrared Survey Explorer (WISE) and the Spitzer Space Telescope in search of the signs of an advanced civilization. What I had wrong in my description was the statement that “Wright’s group is also looking for ‘Fermi bubbles’, patches of a galaxy that show higher infrared emissions than the rest, which could be a sign that a civilisation is gradually transforming a galaxy as it works its way across it.” I know I drew the idea of Fermi bubbles from Richard Carrigan’s work, and generalized from there, but generalizing was a mistake, because it turns out that the G-HAT team doesn’t believe Fermi bubbles are something we could detect.

Below is the ‘Whirlpool’ galaxy, M51, a beautiful image and a useful object for study because we are looking at a spiral galaxy in many ways like the Milky Way from an angle that lets us see it face-on. Could we see Fermi bubbles here?


Image: The Whirlpool Galaxy is a classic spiral galaxy. At only 30 million light years distant and fully 60 thousand light years across, M51, also known as NGC 5194, is one of the brightest and most picturesque galaxies in the sky. The above image is a digital combination of a ground-based image from the 0.9-meter telescope at Kitt Peak National Observatory and a space-based image from the Hubble Space Telescope. Credit: N. Scoville (Caltech), T. Rector (U. Alaska, NOAO) et al., Hubble Heritage Team, NASA.

Richard Carrigan has studied this galaxy closely, looking for such Fermi bubbles, which he described in a 2010 paper. Here’s my description in Toward an Interstellar Archaeology, written for these pages in the same year:

Suppose a civilization somewhere in the cosmos is approaching Kardashev type III status. In other words, it is already capable of using all the power resources of its star (4*1026 W for a star like the Sun) and is on the way to exploiting the power of its galaxy (4*1037 W). Imagine it expanding out of its galactic niche, turning stars in its stellar neighborhood into a series of Dyson spheres. If we were to observe such activity in a distant galaxy, we would presumably detect a growing void in visible light from the area of the galaxy where this activity was happening, and an upturn in the infrared. Call it a ‘Fermi bubble.’

Carrigan (Fermi National Accelerator Laboratory) studied M51 and concluded that there were no unexplained ‘bubbles’ at the level of 5 percent of the galactic area. The Whirlpool galaxy seems like an ideal place to mount such a search given its orientation towards us. A Fermi bubble, if such things exist, might manifest itself as a void in the visible light we see in the image.

Carrigan talked about an expanding front of colonization as an advanced civilization moved through its galaxy, engulfing the galaxy on a time scale comparable to the galaxy’s rotation period or even less. But M51 produced no ‘bubbles,’ and James Annis would suggest that elliptical, rather than spiral, galaxies might be a better place to look for Fermi bubbles because ellipticals exhibit little structure, so that a potential void would stand out.

Here’s Carrigan in the 2010 paper (citation below) on how a civilization on its way to Kardashev Type III status might proceed:

If it was busily turning stars into Dyson spheres the civilization could create a “Fermi bubble” or void in the visible light from a patch of the galaxy with a corresponding upturn in the emission of infrared light. This bubble would grow following the lines of a suggestion attributed to Fermi that patient space travelers moving at 1/1000 to 1/100 of the speed of light could span a galaxy in one to ten million years. Here “Fermi bubble” is used rather than “Fermi void”, in part because the latter is also a term in solid state physics and also because such a region would only be a visible light void, not a matter void.

Wright and the G-HAT team are not persuaded by Carrigan’s Fermi bubbles. For one thing, as Carrigan has noted himself, bubble-like structures are not unusual in extragalactic astronomy, and spiral galaxies include areas that might mimic a void that would be hard to regard as anything but natural. In one of their recent papers, the G-HAT researchers add that with galactic arm widths on the order of ~ kpc, it is difficult to identify structures below this size scale.

The Annis idea, therefore, seems more useful, but for now let’s home in on that word ‘void.’ In the Aeon story, I referred to the galaxy VIRGOHI21 as a galaxy that contains a ‘void.’ But that’s a mistake, for as Jason Wright explained in a recent email, Virgo HI21 has no emissions at any wavelength except 21cm. It may, in fact, be a starless or ‘dark’ galaxy, a galaxy composed of dark matter, although the nature of the object is still controversial. The G-HAT team, according to Wright, has studied Virgo HI21 and found no infrared emission.

In any case, as Wright explained, the word ‘void’ isn’t appropriate, for galaxies do not actually contain them. Areas where there has been no star formation for the past 10 million years or so may manifest themselves as darker lanes between the spiral arms, and dust lanes may also appear dark, but Wright does not believe the shape of these darker lanes is consistent with the spread of a civilization. In any case, these are not voids. They contain just as many stars as other regions in the galaxy. So detecting Fermi bubbles gets to be more and more problematic.

Fermi bubbles would be hard to detect for other reasons as well, as explained by the G-HAT team and presented in their recent work. This is intriguing stuff, having to do with the time scales involved in the spread of a civilization and the motions of stars in that period — these ‘bubbles’ would not be static! I want to look at this issue next but probably won’t be able to get the piece written and published before Wednesday due to an intersection of competing duties elsewhere.

The Carrigan paper is “Starry Messages: Searching for Signatures of Interstellar Archaeology,” JBIS Vol. 63 (2010), p. 90 (preprint). The G-HAT paper I am discussing today and on Wednesday is Wright et al., “The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. I. Background and Justification,” The Astrophysical Journal Vol. 792, No. 1 (2014), p. 26 (abstract / preprint).



Project Dragonfly Design Competition Funded

by Paul Gilster on May 11, 2015

Andreas Hein recently wrote up the Project Dragonfly design competition, which has been running as a Kickstarter project. Leveraging advances in miniaturization and focusing on laser-beamed lightsail technologies, Project Dragonfly aims to study the smallest possible spacecraft. From the Kickstarter announcement:

Project Dragonfly builds upon the recent trend of miniaturization of space systems. Just a few decades ago, thousands of people were involved in developing the first satellite Sputnik. Today, a handful of university students are able to build a satellite with the same capability as Sputnik, which is much cheaper and weighs hundreds of times less than the first satellite. We simply think further. What could we do with the technologies in about 20-30 years from now? Would it be possible to build spacecraft that can go to the stars but are as small as today’s picosatellites or even smaller?


You can read about the competition in Andreas’ post Project Dragonfly: Design Competitions and Crowdfunding. He tells me that the Kickstar campaign has been fully funded since last Friday. But those interested in supporting the effort further can still do so for another three days. You can access the campaign at



SETI: The Black Hole Alternative

by Paul Gilster on May 8, 2015

Our speculations about advanced civilizations invariably invoke Nikolai Kardashev’s scale, on which a Type III civilization is the most advanced, using the energy output of its entire galaxy. Given the age of our universe, a Type III has seemingly had time to emerge somewhere, yet a recent extensive survey shows no signs of them. All of this leads Keith Cooper to consider possible reasons for the lack, including societies that use their energies in ways other than we are imagining and cultures whose greatest interest is less in stars than in their galaxy’s black holes. Keith is an old friend of Centauri Dreams, with whom I’ve conducted published dialogues on interstellar issues in the past (look for these to begin again). A freelance science journalist and contributing editor to Astronomy Now, Keith’s ideas in the essay below help to illuminate the new forms of SETI now emerging as we try to puzzle out the enigma of Kardashev Type III.

By Keith Cooper


It’s not often that SETI turns up with a result that can be considered far-reaching, but the initial results from the Glimpsing Heat from Alien Technologies (G-HAT, or ‘Ĝ’ for short) survey, which Paul wrote about in April (see G-HAT: Searching for Kardashev Type III), fit the bill. Using publicly-available data from NASA’s Wide-field Infrared Survey Explorer (WISE), astronomers have searched 100,000 galaxies for anomalous infrared emission that could be an indication of heat emitted from vast energy collectors and their consumers encircling myriad stars.

The idea is that as a civilisation grows more technologically advanced, its hunger for energy increases. Civilisations could build Dyson spheres to capture all the energy from their star; as they spread to other stars, they may build Dyson spheres around them too. After perhaps a few million years, they spread amongst all the stars in their galaxy, building Dyson spheres around every one of them. The Dyson spheres grow hot and re-radiate some of that thermal energy away as mid-infrared radiation. Consequently, a galaxy that has been completely filled with intelligent, technological life should completely alter the light coming from that galaxy, pushing it more towards the infrared.

Yet the search of 100,000 galaxies has not turned up even one single galaxy that has the signature of a civilisation harvesting the energy of an entire galaxy of stars. This would be analogous to a Kardashev Type III civilisation, referring to the scale developed by Soviet astrophysicist Nikolai Kardashev to measure a civilisation’s energy usage. He based his scale on the Milky Way, so a Type III civilisation resident in our own Galaxy would have a total output of 1036 watts; an analogous civilisation in another galaxy may have a higher or lesser energy output as a consequence of the differences in the number of stars between galaxies, but for the purpose of this article we’ll describe them as Type III too.

Going down the scale, there are Type II civilisations, which harness the energy of a single star, which in the case of the Sun would be 1026 watts; again, for other stars, this will vary. Meanwhile a Type I civilisation is able to collect all the energy available to it on its home planet, which for the case of Earth is about 1016 watts. Carl Sagan further developed the scale, adding graduations between the types. Human civilisation comes in at just 0.7 on the Kardashev–Sagan scale.


Image: A Kardashev Type III civilization would be able to exploit the energy of all the stars in its galaxy.

The point of all this is that the G-HAT result throws a spanner in the works, by finding no Type III civilisations anywhere. It demands that we look again at the Kardashev scale and the assumptions that it makes.

Indeed, at first glance it may seem like bad news for SETI. After all, the Universe is very old, as are the galaxies that inhabit it. There should have been plenty of time for a civilisation, or more than one civilisation, to colonise and collect the energy from every star they come across in their galaxy, so why haven’t they?

There are a couple of reasons why the apparent absence of Type III civilisations might not be bad news for SETI. First, although there may be no Type III civilisations out there, Jason Wright of Penn State University, who founded the G-HAT project, says we shouldn’t yet discount civilisations below that level.

“This search would have only found the most extreme case of advanced civilisation, one that had spread throughout its entire galaxy and was capturing and harnessing one hundred percent of the starlight for its own purpose,” he told me when asked about G-HAT’s findings. “Kardashev 3.0 is the most extreme possible case, but there could still be a Kardashev 2.9, where only ten percent of the starlight is being used, or 2.8 where only one percent of the starlight is being used. So we’ve ruled out 3.0, but we’ve not even gotten down to 2.9 percent yet, much less something smaller like 2.5, that could be very hard [to detect].”

So far, the G-HAT analysis has found no galaxies with an infrared emission signature suggesting more than 85 percent of the starlight is being converted into thermal radiation. Fifty galaxies in the survey did stand out as having greater than 50 percent of the starlight being transformed into infrared emission, and follow up work on these is the next step, but to confuse matters there are also natural phenomena that can mimic this infrared emission, chiefly interstellar dust. Starburst galaxies, which are experiencing a severe bout of star formation, produce substantial amounts of dust. This dust absorbs starlight, heats up, and re-emits at mid-infrared wavelengths. The fifty galaxies with high infrared emission are quite possibly starburst galaxies (one of them, Arp 220, certainly is).


Image: Messier 82 (top of image), seen here with the spiral Messier 81, is a starburst galaxy, meaning it is currently forming stars at an exceptionally high rate. This huge burst of activity was caused by its close encounter with Messier 81, whose gravitational influence caused gas near the center of Messier 82 to rapidly compress. This compression triggered an explosion of star formation, concentrated near the core. The intense radiation from all of the newly formed massive stars creates a galactic “superwind” that is blowing massive amounts of gas and dust out perpendicular to the plane of the galaxy. This ejected material (seen as the orange/yellow areas extending up and down) is made mostly of polycyclic aromatic hydrocarbons, which are common products of combustion here on Earth. It can literally be thought of as the smoke from the cigar. Credit: NASA/JPL-Caltech/UCLA.

However, the analysis has not yet looked at galaxy type. “That would be an excellent next step, to separate out the galaxies that have a lot of dust and which we would expect to be giving out a lot of heat, from the ones that have hardly any dust and shouldn’t be giving out any mid-infrared radiation at all,” says Wright. He is referring here in particular to dust-free elliptical galaxies; if one was found to have infrared emission that might be relatively low compared to a starburst galaxy, but was high for an elliptical galaxy, it might signal something unusual.

It would seem then that there could still be life in these galaxies, life that could be technological, star-faring and energy consuming – we’ve barely scratched the surface. And yet, one pertinent question still remains unanswered: where are all the Type III civilisations?

The G-HAT results tell us that Type III civilisations do not exist (or, at best, have a frequency of less than one Type III civilisation per 100,000 galaxies). This is why I suggested at the top of this article that this result is far-reaching – we now know something that we didn’t know before, namely that civilisations do not seem to reach Type III status. This, though, is the second reason why the result is not necessarily bad for SETI. Think of it this way: the Kardashev scale has become part of the SETI furniture since it was first proposed in 1964. The G-HAT result forces us to question our assumptions about the Kardashev scale and broaden our thinking about extraterrestrial civilisations to encompass other ideas.

Of course, any model has assumptions inherent in it. So let’s assume that technological extraterrestrial civilisations do exist in the Universe and that they are far older than we are (dictated by the fact that the Universe is very old, and there has been plenty of time for civilisations to have gotten well ahead of us before there was even life on Earth); these seem fairly safe assumptions for this kind of discussion. Somewhere along the line they are falling off the Kardashev trajectory. Why?

I want to flag up three possibilities. They may not be the only possibilities. We’ll discount for now the notion that civilisations could destroy themselves – once they become interstellar the task of destroying themselves becomes inordinately more difficult, so for our purposes we’ll assume they at least reach the stage of interstellar flight. On what alternate trajectories away from the Type III destination could their evolution take them?

1. They fail to colonise all the stars

This hypothesis would to an extent fit with the G-HAT observations – extraterrestrial civilisations haven’t built Dyson spheres around 100 percent of the stars in any of 100,000 galaxies, but the result leaves room for them to have done so around a smaller percentage of stars. Perhaps the best reasoning as to why an advanced civilisation possessing the ability for interstellar travel would fail to colonise an entire galaxy is Geoffrey Landis’ percolation theory.

Landis makes the assumption that interstellar travel is short haul only. We might be able to make direct flights to alpha Centauri or epsilon Eridani, but anything much beyond that, moving at just a small fraction of the speed of light – let’s say between 5 and 10 percent – is going to take far too long. So instead, civilisations will hop across the cosmos via the stepping stones of the colonies they set up along the way. For example, imagine three worldships leaving the Solar System for pastures new: let’s say alpha Centauri, epsilon Eridani and Barnard’s Star, all of which are relatively nearby. They set up colonies there, begin building Dyson spheres and perhaps, after a few centuries, those colonies are ready to send out their own pilgrims to new stars further afield, which then found new colonies and, after a few centuries, they too head out on voyages of colonisation, and so on. Over the millennia, humankind’s reach gradually telescopes outwards.

What Landis realised was that not all colonies will seed daughter colonies. The drive to go further will not exist in every colony; cut-off from their mother-world, Earth, by time and space, they build their own cultures, their own histories, and face their own, perhaps unique, challenges. Some will be content to not explore further. Others may destroy themselves, or exhaust their resources before they can build a Dyson sphere. In some cases, there may be no worlds in nearby systems suitable for colonisation. The consequence of any of these possibilities is that some colonies will become dead ends and will fail to colonise further.

To model this, Landis assigns a probability of being colonised to a given planetary system. If that probability is above a critical threshold, then it will be colonised. If it is below the threshold, colonisation of that system will not take place. Eventually, all colonies may result in dead ends, ultimately limiting the extent to which that species colonises the galaxy it exists in. Even if there is one line of colonisation that does continue for a time, there will be voids all around it, left empty by the dead end colonies. A civilisation would struggle to reach Type III status in this fashion.

Landis’ percolation theory is not without its critics. Robin Hanson of the University of California, Berkeley, points to economics and argues that the only way to survive would be to keep up with a colonising wave because the wave would consume all the resources, leaving little of value behind it, a kind of ‘burning of the cosmic commons’ as Hanson describes it. Jason Wright is also critical, arguing that the proper motion of stars would eventually allow active colonies to spread to other stars. For what it’s worth, Landis agrees that the percolation model is not without its problems.

Landis counters that the motion of stars is slow, at least compared to the lifetimes of civilisations in human history, although Wright points out that all a colony then has to do is wait for one of its neighbours to die off before moving in. Landis is unperturbed by the critics, however.

“A lot of people have commented saying they don’t think it is a sophisticated enough model and that they think it needs more work, and that’s fair,” he told me during an interview in 2013. “I just worry that a model that has too much sophistication into which you are putting data that has no validation is hard to really justify.”

Perhaps percolation theory as it stands isn’t therefore the best solution, but instead maybe it’s a good starting point for considering alternatives to how civilisations could migrate through a galaxy.

2. Their energy requirements are low

Another alternative may be that they never really begin to climb the Kardashev ladder at all, which could lead to two outcomes.

Serbian astrophysicist Milan Ćircović has described civilisations that are driven by optimisation, rather than expansion. The optimisation is focused primarily on computation (Jason Wright suspects that Type II and Type III civilisations would use large amounts of their energy for computing, which produces heat). An optimised society would not need to colonise other stars and capture their energy because they would lack the population or computing power that would otherwise soak up vast amounts of energy.

“An optimised society is intrinsically less likely to be observed because most of the things that we tend to associate with advanced technology and advanced societies actually consist of waste energy and the waste of resources,” Ćircović, referring to the Kardashev scale, told me in an interview around five years ago.

An optimised society need not be limited to one planetary system – they may still wish to explore, sending out probes to all corners of their galaxy, but colonising star systems to harvest their energy and resources is not on their list of ‘to do’ things. Rather than building galactic empires, optimised civilisations could be like the ancient Greek city states, which would send out scouts just to explore, says Ćircović.

Jason Wright acknowledges that a galaxy-spanning civilisation need not be a Type III civilisation; it could still be possible to colonise a galaxy without having to build Dyson spheres around every star. Such galaxy-spanning civilisations could be very hard to detect. However, if an advanced civilisation has had time to colonise a galaxy, why would they not build all those Dyson spheres? The distances involved would mean that colonies, or clusters of close colonies, would develop their own societies relatively independently of the others. Some may chose to become optimised, others may be expansionist and energy-hungry, but the result would be a galaxy-spanning civilisation that does not use all the energy of that galaxy.

3. Black holes are more interesting

I confess, I’m rather taken with this idea. It could still be wrong, but it strikes me as being more purposeful than percolating slowly and somewhat randomly through a galaxy, and more ambitious than an optimised city state.

Suppose Kardashev is right, and Milan Ćircović is wrong, and that civilisations actively seek energy. So let’s imagine that a civilisation reaches Type II status, after which it heads for the stars, perhaps even building Dyson spheres around some of them. Estimates suggest that there could be as many as 100 million stellar mass black holes in our Galaxy. Some of them remain dark, while a few are lit up in X-ray binary systems, feeding off a companion star. Sooner or later a star-faring civilisation is going to bump into a black hole. What then?


Image: Simulated view of a black hole in front of the Large Magellanic Cloud. The ratio between the black hole Schwarzschild radius and the observer distance to it is 1:9. Of note is the gravitational lensing effect known as an Einstein ring, which produces a set of two fairly bright and large but highly distorted images of the Cloud as compared to its actual angular size. Credit: Alain r (Own work) [CC BY-SA 2.5], via Wikimedia Commons.

Black holes seem to hold a special fascination for physicists: they create the most extreme gravitational conditions in the Universe, making them a great place for thought experiments. Numerous physicists including John Wheeler, Roger Penrose, George Unruh and Princeton’s Adam Brown have all speculated on methods by which, in principle, it might be possible to draw energy from a black hole. And my, so much energy! Paul Davies in his book The Eerie Silence suggests that a spinning black hole could power our present human levels of energy consumption for at least a trillion trillion years, long after the stars have gone out.

There are numerous options for deriving energy from black holes. Hawking radiation is not the best option, because it leaks out at a trickle, is very low temperature and is difficult to bottle. Small black holes that evaporate relatively quickly would be more efficient for this, but they would not last long. Hawking radiation would make the perfect waste disposal system though – drop your rubbish into the black hole, wait a little while and get energy from Hawking radiation back out.

Then there is the energy radiated by the hot plasma in an accretion disc around a black hole, which is often funneled away in a magnetically collimated jet. This could be created artificially – perhaps by sending a steady stream of asteroids and comets, perhaps even planets and stars themselves using Shkadov thrusters (giant mirrors larger than a star, which act as immense solar sails, the mirror’s huge gravity pulling the star along with it) to nudge the star towards the black hole. Alternatively, there are instances in nature whereby a star naturally exists next to a black hole – the aforementioned X-ray binaries (though in many X-ray binaries the black hole is substituted for a neutron star). Jason Wright suggests that the energy efficiency of such a system would be 10 percent, making it the most efficient sustainable method of converting mass to energy.

Then there is the rotational energy of a spinning black hole. To illustrate the concept, in their book Gravitation, Charles Misner, Kip Thorne and John Wheeler imagined some form of cosmic dump truck swooping down through a black hole’s ergosphere – a region just outside a rotating black hole where an observer is forced to rotate with the black hole, but at the same time can also extract energy from the black hole. The dump trucks, each packing a million tonnes of rubbish, take a particular trajectory through the ergosphere and are able to tip out their industrial waste into the black hole. The dump trucks recoil from the ejection of the rubbish and are catapulted back the way they came, stealing away some of the black hole’s rotational energy in the process. Because the mass of the black hole has increased by the mass of the garbage dumped into it, the mass-energy of the black hole is higher than before the dump truck entered it, allowing the truck to leave with more energy than it started with. To put this in terms of the amount of energy available, up to 29 percent of the mass of the black hole is expressed in terms of its rotational energy, according to Paul Davies – this is leagues above the one percent of a star’s mass that is radiated away over a stellar lifetime.


Image: Artist”s impression of a black hole and a normal star separated by a few million kilometres. That’s less than 10 percent of the distance between Mercury and our Sun. Because the two objects are so close to each other, a stream of matter spills from the normal star toward the black hole and forms a disc of hot gas around it. As matter collides in this so-called accretion disc, it heats up to millions of degrees. Near the black hole, intense magnetic fields in the disc accelerate some of this hot gas into tight jets that flow in opposite directions away from the black hole. Credit: ESO/L. Calçada.

The difference between collecting energy from stars and latching onto black holes is that you can do more with black holes than simply generating power, and it is these extra factors that could make them more attractive than colonising the stars. For one, black holes could potentially make the most powerful computers in the Universe. A computer’s computational power is a function of both its computational efficiency and its mass. Black holes have great mass, but computational efficiency? That would take a bit of organising. The trick is to use Hawking radiation, which is formed of pairs of virtual particles that appear close to the black hole’s event horizon.

For each pair, one particle heads inwards towards the black hole’s singularity, while the other quantum tunnels its way through the event horizon and escapes. However, both particles are forever connected via quantum entanglement. Now, send matter into the black hole – perhaps the waste on the dump trucks – in a specific fashion to ‘program’ the black hole, and it will interact with the infalling Hawking radiation particles. This interaction, specifically fine tuned, will then change the state of the outgoing Hawking radiation particle via entanglement, hence producing an ‘output’. Of course, all the Hawking radiation would have to be gathered, sorted through for the relevant bits of data and processed using knowledge of quantum gravity, a theory that remains stubbornly beyond our limits for the time being.

Then there is the possibility proposed by Sir Roger Penrose that black holes are the birth-sites of new universes; an advanced civilisation may choose to somehow enter one of these universes in a black hole, therefore disappearing from our Universe.

Pressing black holes into service could possibly be within reach of an advanced civilisation; black holes provide astoundingly attractive destinations for intelligence. Clément Vidal, in his book The Beginning and the End, points out that there is a surprising over-abundance of X-ray binaries within three or four light years of the galactic centre – maybe advanced civilisations around their stellar-mass black holes migrating towards the supermassive black hole at the centre of our Milky Way galaxy?

Perhaps. The stars still have their attraction, but as the G-HAT result shows, we need to start looking for alternatives to Type III civilisations. These are just three ideas – your own ideas may well be better!



A New Look Inside Enceladus

by Paul Gilster on May 7, 2015

We can hope that plumes like those found emanating from the south pole of Enceladus happen on other icy worlds. There have been hints of plumes at Europa but they’ve proven elusive to pin down. However, we’re learning a great deal about the water inside Enceladus through Cassini flybys, using models based on mass spectrometry data the spacecraft has gained from the ice grains and gases in the moon’s plumes. A similar approach on other icy moons, if possible, could save us from having to drill through kilometers of ice.

What Christopher Glein (Carnegie Institution for Science) and team have done is to construct a chemical model that uses the Cassini observational data to determine the pH of the Enceladan ocean. It’s an important reading because pH tells us how acidic the water is, which gives us a look into the geochemical processes occurring inside the moon. What the new work shows is that the plume is salty, with an alkaline pH of about 11 or 12. This Carnegie Institution news release likens the pH to glass-cleaning ammonia solutions, with the same sodium chloride as Earth’s oceans.

We also find a substantial amount of sodium carbonate, which makes the Enceladus ocean similar to ‘soda lakes’ found in places like Mono Lake in California and Lake Magadi in Kenya. The paper on this work suggests that the high pH comes from serpentinization, by which rocks low in silica and high in magnesium and iron rise into the ocean floor from the upper mantle and chemically interact with surrounding water molecules. These ‘ultrabasic’ rocks are converted into new minerals — one of these is the mineral serpentine — and the water becomes alkaline.

This would be a significant finding because we’re looking for the energies needed to support life inside icy worlds like these, and serpentinization can produce molecular hydrogen (H2) to fit the bill. Says Glein:

“…molecular hydrogen can both drive the formation of organic compounds like amino acids that may lead to the origin of life, and serve as food for microbial life such as methane-producing organisms. As such, serpentinization provides a link between geological processes and biological processes. The discovery of serpentinization makes Enceladus an even more promising candidate for a separate genesis of life.”


Image: A diagram illustrating the possible interior of Saturn’s moon Enceladus, including the ocean and plumes in the south polar region, based on Cassini spacecraft observations, Credit: NASA/JPL-Caltech.

A separate genesis would not necessarily imply a continuation of life on the moon today, a point the paper is careful to make:

Aside from the origin of life, the biggest unknown that would be critical to life on Enceladus is the availability of H2. Are there “fresh” anhydrous rocks deep in Enceladus’ core, allowing H2 to be produced today (Brockwell et al., 2014); or has the ocean core system reached a state of complete chemical equilibrium…? If the latter, it is possible that life existed but died out because they ran out of food. This is an important issue to consider as we move forward in assessing the habitability of Enceladus.

The convenience of internal materials being pushed into space is hard to over-state, as it gives us the raw material for finding evidence of life through molecular analysis, and we can’t rule out the faint but real possibility of someday acquiring frozen organisms with future plume flybys.

Meanwhile, we learn more about those jets of material erupting from inside the Saturnian moon through a NASA study led by Joseph Spitale (Planetary Science Institute). The work, published in Nature, argues that rather than being discrete jets, the phenomena are ‘curtain eruptions’ that extend along the length of the prominent surface fractures on Enceladus.

Rather than intermittent geysers, we get, at least primarily, diffuse sprays whose ‘folds’ appear as separate streams with heightened brightness. “The viewing direction plays an important role in where the phantom jets appear,” Spitale said. “If you rotate your perspective around Enceladus’ south pole, such jets would seem to appear and disappear.” What we see as jets, then, would be an optical illusion, but the phenomenon of expelled materials fortunately persists.

The Glein paper is “The pH of Enceladus’ ocean,” in press at Geochimica et Cosmochimica Acta, published online 16 April 2015 (preprint). The Spitale paper is “Curtain eruptions from Enceladus’ south-polar terrain,” Nature 521 (7 May 2015), pp. 57-60 (abstract).



Thoughts on Voyager’s Closest Stars

by Paul Gilster on May 6, 2015

Not long ago I looked at the future of the Voyager spacecraft and noted a possibility once suggested by Carl Sagan. Give the Voyagers one last ‘empty the tank’ burn and both could be put on a trajectory that would take them near, if not through, another star’s system (see Voyager to a Star). It would be little more than a symbolic act, for even with heroic measures to conserve power, neither Voyager will be able to communicate past the mid-2020s. With a little luck, perhaps 2030.

So we would be sending two spacecraft off to a star as a final act, turning them into markers, or monuments, that show humans are capable of producing something that will eventually reach (or come close to) another stellar system. Given their current trajectories, each Voyager passes interestingly close to another star in about 40,000 years, or roughly the amount of time since the extinction of homo neanderthalensis. The mere act of relating objects created by our species and launched in 1977 to time frames like this is itself an exercise in ‘deep time’ and the startling changes in perspective it forces.

Jim Bell, who as a young grad student worked on various aspects of the Voyager encounters, brought up Sagan’s interest in an end-of-mission course change in his new book The Interstellar Age (Dutton, 2015). If we were to do something like this, what do we know about its targets? Below is an image of the star AC +79 3888, also known as Gliese 445. It’s now 17.6 light years from the Sun, an M-class dwarf in the constellation Camelopardis.


Image: Gliese 445, in an image taken by the Oschin Schmidt Telescope near San Diego, Calif., on April 22, 1998. Credit: California Institute of Technology/Palomar Observatory.

Assuming we leave the Voyagers alone, Voyager 1 will be closer to this star than to the Sun in the abovementioned 40,000 years. I often mention that if one of our Voyagers were pointed at Alpha Centauri, the journey would take roughly 75,000 years at 17.1 km/sec. But Gliese 445 is another kind of target. It’s moving in our direction at roughly 120 kilometers per second, so that at its closest approach it will be 3.485 light years out, closer than Proxima Centauri is today.

But even a trajectory-changing burn can apparently bring Voyager 1 no closer than about one light year of the star as it again moves away. Mark Biegert worked this distance out in Voyager 1 and Gliese 445, a post on his Math Encounters blog a couple of years ago, working with Voyager perfectly aimed at Gliese 445 — if anyone can tune up these numbers, I’d be interested in the results.

Voyager 2, meanwhile, should pass within 111,000 AU of HH Andromedae, otherwise known as Ross 248, in the same 40,000 years. Here again we’re dealing with an M-class dwarf with about twelve percent of the Sun’s mass and a mere 16 percent of its radius. Ross 248 is a flare star and thus far we’ve found no sign of planets around it, nor can we rule them out. The Hubble instrument was used to search for a stellar companion in the late 1990s, but none was found, and astrometric work at the Sproul Observatory found no trace of a brown dwarf companion.

Also moving in our direction, Ross 248 becomes the closest star to the Sun in the 40,000 year time frame considered here. Interestingly enough, Frederick West suggested in a 1985 paper that an unmanned craft moving at 25.4 km/sec, launched some time in the 21st Century, could reach this star by the time of its closest approach to the Solar System. West, then working as a translator of Russian scientific material at the Library of Congress, made the pitch at a session of the American Astronomical Society, envisioning a probe about the size of a large space station powered by an ion propulsion system and capable of data return after millennia. In what must go down in the annals of understatement, he told a Fredericksburg, VA newspaper that year that “We would have to build the interstellar probe exceedingly well.” Indeed.

Nudging Voyager trajectories to get them closer to these two stars — and I have no numbers on exactly how close we might come to Ross 248 in this scenario — is an opportunity we’ll have about a decade to consider as the spacecraft continue to lose effective power from their onboard plutonium. It’s the plutonium that generates electricity for the spacecraft’s computers and instruments, not to mention the critical heaters. The total power level at launch (470 watts) has now dwindled to 250 watts or so. We may get Voyager as far as 160 AU from the Sun before we lose communications. Suzy Dodd, now Project Manager for the Voyager Interstellar Mission, likes to think we could keep doing science until 2027, marking 50 years of Voyager operations.

What happens if we choose to leave the Voyager trajectories alone? Without needed power, they will fall silent but keep traveling. It will take less than 300,000 years for Voyager 2 to pass within 270,000 AU (the distance now separating us from Alpha Centauri) of Sirius, at which point it will be half as far from Sirius as we are now. In his book, Jim Bell imagines the hot blue star looming four times brighter in the sky than it does from Earth. Neither Voyager has galactic escape velocity, so 250 million year orbits around the Milky Way’s center are in store after this. It’s a prospect that evokes Bell’s deepest poetic instincts, and he does them justice:

…Voyager 1 will continue to slowly travel northward and Voyager 2 southward, relative to the sun and the surrounding stars. Over time— enormous spans of time, as the gravity of passing stars and interacting galaxies jostles them as well as the stars in our galaxy— I imagine that the Voyagers will slowly rise out of the plane of our Milky Way, rising, rising, ever higher above the surrounding disk of stars and gas and dust, as they once rose above the plane of their home solar system. If our far-distant descendants remember them, then our patience, perseverance, and persistence could be rewarded with perspective when our species— whatever it has become— does, ultimately, follow after them. The Voyagers will be long dormant when we catch them, but they will once again make our spirits soar as we gaze upon these most ancient of human artifacts, and then turn around and look back. I have no idea if they’ll still call it a selfie then, but regardless of what it’s called, the view of our home galaxy, from the outside, will be glorious to behold.



Changing Conditions on 55 Cancri e

by Paul Gilster on May 5, 2015

Roughly twice the radius and eight times as massive as Earth, 55 Cancri e is a ‘super-Earth’ in the interesting five-planet system some 41 light years away in the constellation Cancer. No habitable conditions here, at least not for anything remotely like the kind of life we understand: 55 Cancri e orbits its G-class primary every 18 hours (55 Cancri is actually a binary, accompanied by a small red dwarf at a separation of 1000 AU). The closest super-Earth we’ve yet found, this is a tidally locked world that, helpfully for our purposes, transits its host.

What we find in a just announced study of the planet’s thermal emissions out of the University of Cambridge is an almost threefold change in temperature over a two year period. Although we’ve done it before with gas giant atmospheres, this is the first time any variability in atmosphere has been observed on a rocky planet outside our own Solar System. No other super-Earth has yet given us signs of possible surface activity, and Cambridge’s Nikku Madhusudhan, a co-author of the study, calls the changes in detected light ‘drastic.’ They imply a huge temperature swing, from 1000 degrees to 2700 degrees Celsius (∼1300 – 3000 K) on the star-side of this tidally locked world. Brice-Olivier Demory is lead author of the paper on these observations:

“We saw a 300 percent change in the signal coming from this planet, which is the first time we’ve seen such a huge level of variability in an exoplanet. While we can’t be entirely sure, we think a likely explanation for this variability is large-scale surface activity, possibly volcanism, on the surface is spewing out massive volumes of gas and dust, which sometimes blanket the thermal emission from the planet so it is not seen from Earth.”


Image: Artist’s impression of super-Earth 55 Cancri e, showing a hot partially-molten surface of the planet before and after possible volcanic activity on the day side. Credit: NASA/JPL-Caltech/R. Hurt.

The 55 Cancri e work was performed with data from the space-based Spitzer instrument. To understand the results, the authors look at the entire category of ultra-short period (USP) planet candidates found by Kepler, of which there are more than 100. Most of these have radii less than twice that of Earth, and some may be undergoing periods of intense erosion. The paper notes that the planet KIC12557548b shows changes in transit depth and shape that are consistent with what it calls a ‘cometary-like environment.’ The supposition here is that KIC12557548b is sub-Mercury in size but giving off an opaque cloud of dust, perhaps driven by surface volcanism.

From such scenarios the authors derive the idea that 55 Cancri e, one of the largest known USP planets, is likely subject to volcanism, with possible magma oceans on the day side. But in comparison to KIC12557548b, this world is large enough to contain its volcanic outgassing. From the paper:

…whereas extremely small planets (nearly mercury-size) subject to intense irradiation can undergo substantial mass loss through thermal winds, super-Earths are unlikely to undergo such mass-loss due to their significantly deeper potential wells… Thus, ejecta from volcanic eruptions on even the most irradiated super-Earths such as 55 Cnc e are unlikely to escape the planet and would instead display plume behaviour characteristic to the solar system. The extent and dynamics of the plumes if large enough can cause temporal variations in the planetary sizes and brightness temperatures and hence in the transit and occultation depths.

The larger picture is that we have begun to probe atmospheric conditions on worlds as small as two-Earth radii. Theories vary as to the composition of 55 Cancri e, with some observations suggesting a carbon-rich world while others point to a silicate-rich interior with a dense atmosphere. The variability found in this study calls the earlier models into question. But as we learn more about the material surrounding 55 Cancri e, we’ll be conducting what the authors call ‘a direct probe of the planet surface composition’ that may help us understand other USP planets.

The paper is Demory et al., “Variability in the super-Earth 55 Cnc e,” submitted to Monthly Notices of the Royal Astronomical Society (preprint). A University of Cambridge news release is available.



A Stagecoach to the Stars

by Paul Gilster on May 1, 2015

Imagine the kind of spaceship we’ll need as we begin to expand the human presence into the nearby Solar System. We’d like something completely reusable, a vessel able to carry people in relative comfort everywhere from Mars to Venus, and perhaps as far out as the asteroid belt, where tempting Ceres awaits. Capable of refueling using in situ resources, these are ships not crafted for a single, specific mission but able to operate on demand without entering a planetary atmosphere. Brian McConnell, working with Centauri Dreams regular Alex Tolley, has been thinking about just such a ship for some time now. A software/electrical engineer, pilot and technology entrepreneur based in San Francisco, Brian here explains the concept he and Alex have come up with, one that Alex treated in a previous entry in these pages. The advantages of their ‘spacecoach’ are legion and Brian also offers a sound way to begin testing the concept. The author can be reached at

by Brian McConnell


“What if a spacecraft, like a cell, was made mostly of water?”

That’s what Alexander Tolley and I asked when we were working on our paper for the Journal of the British Interplanetary Society, “A Reference Design For A Simple, Durable and Refuelable Interplanetary Spacecraft” [1]. The paper explored the idea of a crewed spacecraft that used water as propellant in combination with solar electric propulsion. We dubbed them spacecoaches, as a nod to the stagecoaches of the Old West. Alex also gave the concept an excellent fictional treatment in Spaceward Ho!, also published here on Centauri Dreams. We are currently finishing a book about spacecoaches, to be published by Springer this fall. Subscribe to for updates about the book and spacecoaches in general.

The HD 7924 Planetary System

The idea of crewed solar electric spacecraft is hardly new. In 1954, Ernst Stuhlinger proposed a “sun-ship” powered by solar steam turbines and cesium ion drives [2,3]. Since then solar electric propulsion has been used in a wide variety of uncrewed craft. Meanwhile, the convergence of several technologies will make crewed solar electric vehicles feasible in the near future.

The core idea behind the spacecoach architecture is the use of water, and potentially waste streams, as propellant in electric engines. Water, life support and consumables are critical elements in a long duration mission, and in a conventional ship, are dead weight that must be pushed around by propellant that cannot be used for other purposes. Water in a spacecoach, on the other hand, can be used for many things before it is reclaimed and sent to the engines, and it can be treated as working mass. This, combined with the increased propellant efficiency of electric engines, leads to a virtuous cycle that results in dramatic cost reductions compared to conventional ships while increasing mission capabilities. Cost reductions of one or two orders of magnitude, which would make travel to destinations throughout the inner solar system routine, are possible with this approach.

Water is, for example, an excellent radiation shielding material, comparable to lead on a per kilogram basis, except you can’t drink lead. It is an excellent thermal battery, and can simply be circulated in reservoirs wrapped around the ship to balance hot and cold zones (this same reservoir doubles as the radiation shield). When frozen into fibrous material to form pykrete, it forms a material as tough as concrete, which can potentially be used for debris shielding or for momentum wheels, and if positioned correctly, can double as a supplemental radiation shield. If mixed with dilute hydrogen peroxide, which is safely stored at low concentrations, oxygen can be generated by passing it through a catalyst, similar to a contact lens cleaner. Dilute H2O2 is also a potent disinfectant, and can also be used to process human waste, as is done in terrestrial wastewater treatment plants. Anything the crew eats or drinks can be counted as propellant, as the water can be reclaimed and used for propulsion. This greatly simplifies planning for long missions because the longer the mission is, the more propellant you have in the form of consumables. This will also provide excellent safety margins and enable crews to survive an Apollo 13 scenario in deep space.

A spaceship that is mostly water will be more like a cell than a conventional rocket plus capsule architecture. Space agriculture, or even aquaculture, becomes practical when water is abundant. Creature comforts that would be unthinkable in a conventional ship (hot baths anyone?) will be feasible in a spacecoach. Meanwhile, inflatable structures will eventually enable the construction of large, complex habitats that will be more like miniature O’Neill colonies than a conventional spaceship [4].

In the book, Alex and I present a reference design that combines inflatable structures and thin film PV arrays to form a kite-like structure that both has a large PV array area, and can be rotated to provide artificial gravity in the outer areas [5]. The ability to generate artificial gravity while providing ample radiation protection solves two of the thorniest problems in long duration spaceflight. Alex wrote an excellent fictional treatment of the concept for Centauri Dreams called Spaceward Ho! This is intended as a straw man design to kickstart design competitions. We envision a series of design competitions for water compatible electric propulsion technologies, large scale solar arrays, and overall ship designs. Much of the reference design can be validated in ground based competitions and experiments, followed by uncrewed test vehicles (similar to what Bigelow Aerospace did by flying its Genesis I and II habitats in low earth orbit).


Spacecoaches are possible not because of any one insight or breakthrough, but because of the convergence of improvements in component technologies, specifically thin film photovoltaics, electric propulsion, and inflatable structures. The combination of the three, particularly when you add water for propulsion, leads to one or two order of magnitude improvements in mission economics.

Thin film solar photovoltaics, which enable the construction of large area PV sails, will enable ships to generate hundreds of kilowatts to several megawatts of electrical power (thin film PV material coincidentally is much more resistant to radiation than conventional silicon PV material) [6]. While thin film solar is not as efficient as silicon in terms of power per unit area, from a power density (watts/kilogram) standpoint, it offers multiple order of magnitude improvements, and will continue to improve for decades due to dematerialization in manufacturing processes.

SEP (solar electric propulsion) is a well understood, flight ready technology. Engines that function with water or gasified waste will be well suited to the spacecoach architecture. We simply need to test existing SEP technologies with water and waste streams to pin down performance and efficiency numbers, which can be done via an X-Prize style engineering competition. Scaling them to propel a large (40 tonne) ship will be done by clustering them in arrays, so there will be no need to build a single high power engine when an array of many 10-20 kilowatt units will do just fine, while also adding redundancy. One interesting discovery we made while doing our analysis is that ultra high specific impulse engines, such as VASIMR, are neither necessary nor desirable. Engines that operate at the low end of the electric propulsion envelope still yield excellent economics due to the synergies created by using water as propellant, while also being able to operate with less electrical power per unit of thrust, which reduces PV array size and mass.

Inflatable/expandable structures are just now beginning to be recognized as a flight ready technology, with Bigelow Aerospace’s BEAM unit due to fly on the ISS later this year. Bigelow already has two uncrewed inflatable habitats in low earth orbit. The basic idea with inflatable structures is to replace a rigid metal hull with a flexible high strength Kevlar type material and utilize pressurization to inflate and deploy the structure. This also enables a large habitable space to be compacted into a standard cargo fairing, thus requiring a minimal number of surface launches for initial delivery to orbit. We expect this technology to improve, both in terms of mass per unit of habitable space (currently about 60 kg per m3), and in terms of the types of shapes that can be created. [7]

Spacecoaches will not be mission specific ships. Even the first generation ships will be able to travel to many destinations within the inner solar system. They will be fully reusable, travelling from a high earth orbit or a Lagrange point to and from their destinations, without ever entering a planetary atmosphere. Spacecoaches will be able to travel to cislunar space, Mars, Venus, NEOs and maybe even Ceres and the Asteroid Belt. They can also be dispatched for asteroid interception and deflection missions on short notice. This is a huge departure from conventional spacecraft which are purpose built for a specific mission, usually Mars, that is planned decades in advance. Mars is certainly an interesting destination, but Ceres, with its abundant water resources and shallow gravity well, may turn out to be an even more interesting destination for human exploration and settlement.

The amount of water required for propellant on any given route will vary depending on the delta-v needed, and also the specific impulse of the engines on board, but water is easy to handle and store. Need to add an extra two kilometers per second to your delta-v budget? Just add water! (or replace the electric engines with slightly more efficient models). Because water is so easy to handle compared to conventional propellants, this will also simplify the construction and operation of orbiting fuel depots, which will be little more than orbiting water tanks.

Simplicity and upgradability is another key design element of the spacecoach. We assume that component technologies will continue to improve for decades. So instead of designing spacecoaches to fly only with today’s technology, they will be designed more like personal computers were in the 1980s. The original PCs were built around a common electrical and communication bus, the ISA bus, which allowed memory, CPUs and peripherals from many manufacturers to be combined. If you wanted to, you could buy the component parts from catalogs and build your own PC from scratch.

We envision something similar for the spacecoach, for the electrical system and engines in particular, which will have standard electrical and fluid interconnects, and uniform form factor requirements. The engines will also be mounted in a sealable compartment that can be pressurized so the crew can replace or upgrade engines without doing an EVA. This will not only make spacecoaches field upgradable, but will also reduce the need to design engines for extreme reliability. If a few units fail, crews would replace them in an operation not much different than replacing a rack mounted server. Upgrading engines will be the best way to improve performance and reduce costs, as a small increase in specific impulse can yield significant mass and cost reductions, especially for high delta-v routes like Ceres and the Asteroid Belt.

And what about cost?

Mention crewed missions to Mars, much less anywhere else, and people automatically assume you’re talking tens of billions of dollars as a starting point. We modeled approximate round-trip mission costs to destinations throughout the inner solar system, using a 40,000 kilogram (40 tonne) dry hull and SpaceX’s published launch costs to get materials, including water, into low earth orbit ($1,700/kg via Falcon 9 Heavy [8]), with electric propulsion (Isp between 1,500 to 3,000s) from there (electrode-less Lorentz force thrusters using water operate in this range). Among the missions we modeled were EML-2 (Earth Moon Lagrange point 2) to/from cislunar space, Martian moons, NEO interception, Venus orbit and Ceres. Even with engines operating at the low end of the electric propulsion performance envelope, our models predicted per mission costs in the hundreds of millions of dollars, a one or two order of magnitude reduction compared to conventional missions, some of which, such as a crewed mission to Ceres, simply are not possible via chemical propulsion.

Such large cost reductions are possible due to a combination of the fuel efficiency of electric engines, and the synergies created by using water as propellant. On one hand electric engines require far less propellant for a given delta-v. On the other, virtually everything the crew consumes or uses for life support can eventually be sent to the engines. As a result the only dead weight on the ship is the hull and whatever non-consumable materials and equipment are brought on board, which will also allow spacecoaches to carry larger crews. Reusability will also enable operators to amortize development and construction costs across many missions.

Spacecoaches are also well suited for in situ resource utilization. Should we reach low gravity destinations with accessible water (Ceres is an especially interesting location), it will eventually be possible to refuel spacecoaches at these destinations, or even ship water inbound to cislunar depots. We assume for now that spacecoaches are fully supplied from Earth, but exploring ISRU destinations and capabilities will be a high priority early on. Partially reusable launch vehicles offer another way to reduce costs. Water will be an ideal payload for a heavily re-used Falcon 9R booster. Unlike most payloads, it has essentially zero replacement cost, so the launch operator can fly the reusable boosters until they fail, and can learn about potential failure modes and fixes in the process (all while delivering more water to orbit).

If you are part of a team working on electric propulsion technology, here’s one way you can help make these a reality. Test your engine with water vapor, carbon dioxide and gasified waste (or a good analogue), and publish your results. The most important parameters ship designers will be interested in are specific impulse, efficiency (ideally the “wall plug” efficiency of the entire system so it can be modeled as a black box) and thrust/mass ratio. We already know several SEP technologies work reasonably well with water, but it will be great to examine all systems to see how well each works with water, compare performance across a variety of technologies, and identify opportunities for further improvement.

It is easy to be cynical about new spaceflight concepts, especially one that promises large cost reductions, but most of this can be validated on the ground and via uncrewed testbeds in a short time and at little expense. It is a paradigm shift, and that will take people some time to accept. The rocket + capsule design pattern served us well in the early years of spaceflight, so its hard to get away from that, but it’s time to move on to something that is more adaptable, something that’s more like a ship that can sail wherever her captain wants to go.

Spacecoaches will form the basis for a real world Starfleet, a fleet which will grow as ships are built, and which will reach new destinations as component technologies continue to improve in the coming decades. They will open the inner solar system out to the Asteroid Belt to human exploration and settlement, and with some spacecoaches operating in cislunar space, humanity will also have a rapid response capability should we be surprised by the discovery of an Earth threatening object.

Visit to learn more, and to subscribe for notices about the upcoming book, which examines the spacecoach reference design and potential missions in detail. If you are interested in obtaining an advance copy of the book, acting as a technical reviewer or inviting us to speak, please get in touch.


[1] “Reference Design for a Simple, Durable and Refuelable Interplanetary Spacecraft”, B. S. McConnell; A. M. Tolley (2010), JBIS, 63, 108-119

[2] Image credit: Frank Tinsley/American Bosch Arma Corporation, 1954

[3] “Possibilities of Electrical Space Ship Propulsion,” E. Stuhlinger, Bericht über den V Internationalen Astronautischen Kongreß, Frederich Hecht, editor, 1955, pp. 100-119; paper presented at the Fifth International Astronautical Congress in Innsbruck, Austria, 5-7 August 1954

[4] “A Shape Grammar for Space Architecture – I. Pressurized Membranes”, Val Stavrev* Aeromedia, Sofia, Bulgaria, 40th International Conference on Environmental Systems,

[5] Image credit: Rüdiger Klaehn

[6] “Super radiation tolerance of CIGS solar cells demonstrated in space by MDS-1 satellite”, Photovoltaic Energy Conversion, 2003. Proceedings of 3rd World Conference on, 18-18 May 2003, pp. 693 – 696 Vol.1

[7] Estimate based on BA330 mass per cubic meter of habitable space, per Bigelow Aerospace’s published specifications

[8] Per SpaceX published launch cost and delivery capacity for Falcon 9 Heavy, as of April 2015



Pluto/Charon: Surface Features Emerging

by Paul Gilster on April 30, 2015

One of the more memorable moments from yesterday’s teleconference on the New Horizons mission was Alan Stern’s comment that the latest pixelated images of Pluto/Charon constituted his ‘meet Pluto moment.’ If anyone has an interest in meeting Pluto, it’s Stern (Southwest Research Institute), who serves as principal investigator and whose unflagging efforts made it possible. As for those pixelated views, well, they’re a glimpse of what is to come, but even now, they’re telling us helpful things about the target. The animation below speaks volumes, with the first showing Charon’s rotation with the center of Pluto fixed in the frame. The images were acquired with the Long Range Reconnaissance Imager (LORRI) camera.


Image: A series of LORRI images of Pluto and Charon taken at 13 different times spanning 6.5 days, from April 12 to April 18, 2015. During that time, the spacecraft’s distance from Pluto decreased from about 111 million kilometers to 104 million kilometers. Pluto and Charon rotate around a center-of-mass (also called the “barycenter”) once every 6.4 Earth days, and these LORRI images capture one complete rotation of the system. The direction of the rotation axis is shown in the figure. In one of these movies, the center of Pluto is kept fixed in the frame, while the other movie is fixed on the center of mass (accounting for the “wobble” in the system as Charon orbits Pluto). Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

The second image shows the same view with the motion around the barycenter clearly revealed, a spectacular teaching tool for those trying to explain rotation around the center of mass.


Image: The 3x-magnified view of Pluto highlights the changing brightness across the disk of Pluto as it rotates. Because Pluto is tipped on its side (like Uranus), when observing Pluto from the New Horizons spacecraft, one primarily sees one pole of Pluto, which appears to be brighter than the rest of the disk in all the images. Scientists suggest this brightening in Pluto’s polar region might be caused by a “cap” of highly reflective snow on the surface. The “snow” in this case is likely to be frozen molecular nitrogen ice. New Horizons observations in July will determine definitively whether or not this hypothesis is correct. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

The zoomed image is particularly notable because it clearly shows different brightness patterns as Pluto rotates (and notice that Charon, even though much smaller, still stays relatively the same, an indication that the surface is more uniform than Pluto, or as Stern said in the news conference, more ‘muted’). An apparent oddity here actually is not — If you look closely at the view of Pluto in the center, it seems to show an uneven surface. This is only an optical effect caused by the fact that dark areas on the dwarf planet are rotating in and out of view.

These albedo changes on the Plutonian surface are striking. Stern commented that it was unusual that we should be seeing such marked effects from such a distance. The possibility of a polar cap — probably made of highly reflective frozen molecular nitrogen ice — seems strongly enhanced by the bright area that persists in the zoomed image. JHU/APL has used a mathematical technique called deconvolution to improve the resolution of the raw LORRI images, giving us about the best resolution the camera and detector can provide at this distance.

And speaking of raw images, be aware of the raw image archive being assembled on the LORRI Images from the Pluto Encounter page. The imagery is, of course, going to keep getting better. We’ll get another sequence from an observing run scheduled for May 8 through May 14, producing another ‘movie’ in mid-May, followed by a lengthy pause to downlink the data that lasts until the 27th. The spacecraft will then begin observing Pluto/Charon daily at ever higher resolution. Fasten your seat belt.