Centauri Dreams
Imagining and Planning Interstellar Exploration
The Evolution of Antimatter Propulsion
Thinking about Eugen Sänger’s photon rocket concept inevitably calls to mind his Silbervogel design. The ‘Silverbird’ had nothing to do with antimatter but was a demonstration of the immense imaginative power of this man, who envisioned a bomber that would be launched by a rocket-powered sled into a sub-orbital trajectory. There it would skip off the upper atmosphere enroute to its target. The Silbervogel project was cancelled by the German government in 1942, but if you want to see a vividly realized alternate world where it flew, have a look at Allen Steele’s 2014 novel V-S Day, a page-turner if there ever was one.
I almost said that it was a shame we don’t have a fictionalized version of the photon rocket, but as we saw yesterday, there were powerful reasons why the design wouldn’t work, even if we could somehow ramp up antimatter production to fantastic levels (by today’s standards) and store and manipulate it efficiently. Energetic gamma rays could not be directed into an exhaust stream by the kind of ‘electron gas mirror’ that Sänger envisioned, although antimatter itself maintained its hold on generations of science fiction writers and scientists alike.
Enter the Antiproton
Sänger’s presentation at the International Astronautical Congress in 1953 came just two years ahead of the confirmation of the antiproton, first observed at the Berkeley Bevatron in 1955. Now we have something we can work with, at least theoretically. For unlike the annihilation of electrons and positrons, antiprotons and protons produce pi-mesons, or pions, when they meet. Pions don’t live long, with charged pions decaying into muons and muon neutrinos, while neutral pions decay into gamma rays. Those charged pions, however, turn out to be helpful indeed.
By the early 1980s, Robert Forward had realized that superconducting coils could be used to channel such charged pions, producing the kind of directed exhaust stream that so frustrated Sänger. Forward described the ‘magnetic nozzle’ in his book Mirror Matter (Wiley, 1988), but his first paper on the subject appeared in 1982, along with other papers on antimatter propulsion from Brice Cassenti and David Morgan, all of these in the fecund pages of the Journal of the British Interplanetary Society. Morgan (Lawrence Livermore National Laboratory) envisioned a three-meter nozzle using magnetic coils to channel charged pions, with an exhaust velocity fully 94 percent of the speed of light.
The gamma ray problem is still there, but in these designs, they appear in the exhaust well behind the rocket, even as the energy of the charged pions is used to heat a propellant like hydrogen or water which becomes the exhaust. Using methods like these, we could extract up to 50 percent of the energy unlocked by the annihilation of protons and antiprotons.
In broader terms, what Forward was proposing was to replace tons of chemical propellant with milligrams of matter. In a study he performed for the Air Force Rocket Propulsion Laboratory, he advocated the creation of facilities specifically dedicated to producing antimatter, as opposed to relying on the production of antimatter in particle accelerators as a by-product of other work. In this way, he believed, the cost could be brought down to about $10 million per milligram. Remember that a milligram of antimatter produces about the same energy as 20 tons of chemical fuel, making antimatter at this price level a better deal than chemical propulsion.
We’re not at Sänger-esque levels of specific impulse (3 X 107 seconds), but Giovanni Vulpetti was able to show later in the 1980s that a rocket working with the pions of proton/antiproton annihilation was capable of a specific impulse of 0.58c. Even so, antimatter’s numerous problems continue to bedevil us. Some of Robert Frisbee’s work in the same decade overcomes the antimatter storage issue by creating spacecraft thousands of kilometers long. These fantastic designs are rapier-thin and massive, hardly the sleek starships most science fiction has led us to expect, unless we look into SF’s most extravagant imaginings.
Image: Here’s one way of getting around those huge Frisbee rockets. This is Richard Obousy’s concept for VARIES, the Vacuum to Antimatter Rocket Interstellar Explorer System. Here the starship uses an immensely powerful laser to generate its own antimatter fuel, relying on Julian Schwinger’s work showing that electron-positron pairs can be generated out of the vacuum of space itself. Credit: Adrian Mann.
Excuse the digression, but Frisbee’s work calls up the memory of one of Paul Linebarger’s stranger stories. Writing as Cordwainer Smith, Linebarger created a short fable called “Golden the Ship Was – Oh! Oh! Oh!,” which ran in Amazing Stories in April of 1959. And just as Frisbee’s vast designs stretch physics to the limit by way of showing how unlikely an antimatter starship is at our current level of understanding, Linebarger’s golden ship is in most ways a chimera, although the powers threatening the Earth do not understand what they are seeing:
“That one ship is ninety million miles long, Your Highness. It shimmers like fire, but moves so fast that we cannot approach it. But it came into the center of our fleet almost touching our ships, stayed there twenty or thirty thousandths of a second. There it was, we thought. We saw the evidence of life on board: light beams waved: they examined us and then, of course, it lapsed back into nonspace. Ninety million miles, Your Highness. Old Earth has some stings yet and we do not know what the ship is doing.”
The pleasures of Cordwainer Smith are likewise vast and I won’t give anything more away about this short tale (you can find it reprinted in The Rediscovery of Man (NESFA Press, 1993). But back to proton/antiproton annihilation, which gets an interesting new wrinkle in the work of Friedwardt Winterberg. As examined in these pages by Adam Crowl (see Re-Thinking the Antimatter Rocket), Winterberg looks at how a plasma made of matter and antimatter in equal parts (an ‘ambiplasma’) can undergo extreme compression. Let me quote Crowl:
Essentially what Winterberg describes is generating a very high electron-positron current in the ambiplasma, while leaving the protons-antiprotons with a low energy. This high current generates a magnetic field that constricts rapidly, a so-called pinch discharge, but because it is a matter-antimatter mix it can collapse to a much denser state. Near nuclear densities can be achieved, assuming near-term technical advancements to currents of 170 kA and electron-positron energies of 1 GeV.
What we get is a gamma ray flux that is highly directional, forming a gamma-ray laser, a beam of gamma rays that, in conjunction with the annihilation chamber’s magnetic fields, produces thrust. The work draws on Winterberg’s thinking on deuterium fusion rockets (he was a key contributor to the original Project Daedalus starship design) and the magnetic compression of ions. Here we get a concept that would surely have delighted Eugen Sänger, as it provides for a highly directional gamma ray thrust that was the cornerstone of the photon rocket.
All these concepts assume substantial production of antimatter and major breakthroughs in storage, but in the nearer term, we will continue to explore antimatter’s possibilities in catalyzing nuclear fusion reactions, or intriguing spacecraft designs like Steven Howe’s ‘antimatter sail.’ In Howe’s work for NASA’s original Institute for Advanced Concepts, small amounts of antimatter are used to create fission as they encounter a sail impregnated with uranium. For more, see An Antimatter Driven Sail to the Kuiper Belt.
As we learn more about storage, and in particular methods involving stable antihydrogen (a positron and an antiproton), we can hope that methods of antimatter production, and even antimatter collection in the outer Solar System, will become better understood. It will take experimentation with tiny amounts of antimatter to help us understand its possible contribution to deep space exploration.
The Forward paper cited above is “Antimatter Propulsion,” JBIS 35 (1982), pp. 391-395. Brice Cassenti’s paper on antimatter is “Design Considerations for Relativistic Antimatter Rockets,” JBIS 35 (1982), pp. 396-404. David Morgan’s paper is “Concepts for the Design of an Antimatter Annihilation Rocket,” JBIS 35 (1982), pp. 405-413. Richard Obousy’s paper on VARIES is “Vacuum to Antimatter Rocket Interstellar Explorer System,” JBIS 64 (2012), pp. 378-386. Check the JBIS website for availability. The Winterberg paper is “Matter-Antimatter GeV Gamma Ray Laser Rocket Propulsion” (2011 — preprint).
Antimatter Propulsion: Birth of a Concept
I spent this past weekend poking into antimatter propulsion concepts and in particular looking back at how the idea developed. Two scientists — Les Shepherd and Eugen Sänger — immediately came to mind. I don’t know when Sänger, an Austrian rocket designer who did most of his work in Germany, conceived the idea he would refer to as a ‘photon rocket,’ but he was writing about it by the early 1950s, just as Shepherd was discussing interstellar flight in the pages of the Journal of the British Interplanetary Society. A few thoughts:
Sänger talked about antimatter propulsion at the 4th International Astronautical Congress, which took place in Zurich in 1953. I don’t have a copy of this presentation, though I know it’s available in a book called Space-Flight Problems (1953), which was published by the Swiss Astronautical Society and bills itself as a complete collection of all the lectures delivered that year in Zurich. If you like to track ideas as much as I do, you’ll possibly be interested in an English-language popularization of the idea in a 1965 book from McGraw Hill, Space Flight: Countdown for the Future, which Sänger wrote and Karl Frucht translated.
Greg Matloff has speculated that what may have drawn Sänger to antimatter is specific impulse, which reaches surreal heights if you can produce an exhaust velocity equal to the speed of light (see The Starflight Handbook for more on Matloff’s thinking). The speed of light being about 3 X 108 m/sec, Matloff worked out a specific impulse of 3 X 107 seconds. Recall that specific impulse measures engine efficiency. In other words, a higher specific impulse produces more thrust for the same amount of propellant.
Sänger must have been dazzled by this ultimate specific impulse, which he conceived possible only through the mutual annihilation of matter with antimatter. But recall that when Sänger was developing these ideas, the only form of antimatter known was the positron, or positively charged electron, which had been discovered by Carl Anderson in 1932 (he would win the Nobel for the work in 1936). When you bring positrons and electrons together, you produce gamma rays, an energetic form of electromagnetic radiation that moves at the speed of light.
Antimatter propulsion solved? Hardly. What the Sänger photon rocket had to do was to create a beam of gamma rays which could be channeled into an exhaust, somehow overcoming the problem that the gamma rays produced by the matter/antimatter annihilation emerge in random directions. They are highly energetic and would penetrate all known materials, a lethal problem for the crew and a showstopper for directed thrust unless Sänger could develop a kind of ‘electron-gas mirror’ to direct the gamma rays. Sänger never solved this problem.
The Radiator Problem
Writing in 1952, Les Shepherd went to work on antimatter equally limited by the fact that only the positron was then known — the antiproton would not be confirmed until 1955 (by Emilio Segrè and Owen Chamberlain — Nobel in 1959). Shepherd was a nuclear fission specialist who helped to found the International Academy of Astronautics and served as president of the International Astronautical Federation (see my obituary for Shepherd from 2012 for more). And his 1952 paper “Interstellar Flight” remains a landmark in the field.
Even without the antiproton, Shepherd would have known about Paul Dirac’s prediction of its existence and doubtless speculated on the possibilities it might afford. As Giovanni Vulpetti told me just after Shepherd’s death:
Dr. Shepherd realized that the matter-antimatter annihilation might have the capability to give a spaceship a high enough speed to reach nearby stars. In other words, the concept of interstellar flight (by/for human beings) may go out from pure fantasy and (slowly) come into Science, simply because the Laws of Physics would, in principle, allow it! This fundamental concept of Astronautics was accepted by investigators in the subsequent three decades, and extended/generalized just before the end of the 2nd millennium.
Vulpetti himself has been a major figure in that extension of the concept, with papers like “Maximum terminal velocity of relativistic rocket” (Acta Astronautica, Vol. 12, No. 2, 1985, pp. 81-90); and “Antimatter Propulsion for Space Exploration” (JBIS Vol. 39, 1986, pp. 391-409). Many back issues of JBIS are available for a fee on the journal’s website (http://www.jbis.org.uk/), though I haven’t yet checked for this one. But be aware that Dr. Vulpetti is also making his papers available on his website (http://www.giovannivulpetti.eu/).
Looking back at Shepherd’s “Interstellar Flight” paper is a fascinating exercise. Assuming that we could solve the Sänger problem, Shepherd saw that there were other issues that made antimatter extremely problematic. Obviously, producing antimatter in the necessary amounts would be a factor, as would the key problem of storing it safely, but Shepherd had something else in mind when he wrote “The most serious factor restricting journeys to the stars, indeed, is not likely to be the limitation on velocity but rather limitation on acceleration.”
The paper then moves to examine what happens as we unleash the power of matter/antimatter annihilation. Have a look at this:
We see that a photon rocket accelerating at 1 g would require to dissipate power in the exhaust beam at the fantastic rate of 3 million Megawatts/tonne. If we suppose that the photons take the form of black-body radiation and that there is 1 sq metre of radiating surface available per tonne of vehicle mass then we can obtain the necessary surface temperature from the Stefan-Boltzmann law…
The result is an emitting surface that would reach temperatures of about 100,000 K. We need, in other words, to dispose of waste heat in the form of thermal radiation. Even assuming a way of channeling the gamma rays of positron/electron annihilation (or looking ahead to other forms of antimatter and their uses), Shepherd could see that accelerations high enough to shorten interstellar flight times drastically would have to solve the thermal dissipation problem.
The real difficulty, always assuming that we can find suitable energy sources for the job, lies in the unfavourable ratio of power dissipation to acceleration as soon as we become involved with high relative velocities. The problem is fundamental to any form of propulsion which involves non-conservative forces (e.g., the thrust of a rocket jet) to produce the necessary acceleration. The only method of acceleration which one can conceive that would not be subject to this difficulty, would be that caused by an external field of force.
So can we produce radiators that can handle temperatures of 100,000 K? Perhaps there are ways, but Shepherd could only note that the matter was so far beyond existing technologies as to make the speculation pointless. Sänger’s photon rocket — or any vehicle somehow creating an exhaust velocity near the speed of light, has to reckon with the radiator problem.
Remarkably ahead of their time, both Les Shepherd and Eugen Sänger helped define the problems of antimatter propulsion even before we had found the antiproton, a form of antimatter that offers new possibilities that would be explored by Robert Forward and many others. But more on that tomorrow.
The Sänger references are given above. Les Shepherd’s ground-breaking paper on interstellar propulsion is “Interstellar Flight,” JBIS, Vol. 11, 149-167, July 1952. For more background on this issue, see Adam Crowl’s Re-thinking the Antimatter Rocket, published here in 2012.
An Unusual Pulsating Binary
A large part of the fascination of astronomy is the discovery of objects that don’t fit our standard definitions. KIC 8462852 — ‘Tabby’s Star’ — is deeply mysterious and high on my watchlist. But yesterday we also looked at CX330, a so-called FUor of the kind that brightens enormously over years of observation. Today we have another strange one, a system called AR Scorpii, where a white dwarf star in a binary system is releasing a blast of radiation onto a nearby red dwarf. The entire system brightens and fades every 1.97 minutes, a phenomenon that has only recently been properly understood.
“AR Scorpii was discovered over 40 years ago, but its true nature was not suspected until we started observing it in June 2015,” says Tom Marsh (University of Warwick), lead author of the paper on this work. “We realised we were seeing something extraordinary the more we progressed with our observations.”
Those observations proceeded with data from the European Southern Observatory’s Very Large Telescope (Chile) and the Isaac Newton Group of telescopes at La Palma (Canary Islands), along with other ground-based resources and the Hubble and Swift instruments in space. European amateur astronomers also played a key role in studying the star’s behavior.
AR Scorpii is about 380 light years from Earth. Its white dwarf component is roughly Earth-sized though containing 200,000 times the mass, and the associated red dwarf is about one-third the mass of the Sun. This is a tight system, with the two objects orbiting one another every 3.6 hours. What researchers have found is that the white dwarf is accelerating electrons almost to the speed of light in a beam that sweeps across the face of the M-dwarf. The brightening is dramatic, but the emissions range all the way from X-rays to radio wavelengths.
Image: This artist’s impression shows the strange object AR Scorpii. In this unique double star a rapidly spinning white dwarf star (right) powers electrons up to almost the speed of light. These high energy particles release blasts of radiation that lash the companion red dwarf star (left) and cause the entire system to pulse dramatically every 1.97 minutes with radiation ranging from the ultraviolet to radio. Credit: M. Garlick/University of Warwick, ESA/Hubble.
AR Scorpii was classified in the 1970s as a Delta Scuti variable, a kind of star (also known as a dwarf cepheid) that shows luminosity variations due to pulsations on the star’s surface. Such variables are useful as standard candles that help astronomers calculate stellar distances. But Marsh and team discovered that AR Scorpii’s pulsations were not the result of a single variable star but a binary system in which intense brightness variations were occurring. The pulses are strong enough that the star’s optical flux can increase by a factor of four within 30 seconds.
The nature of the AR Scorpii pulses is what intrigues the researchers. From the paper:
Isolated white dwarfs emit most of their power from ultraviolet to near-infrared wavelengths, but when in close orbits with less dense stars, white dwarfs can strip material from their companions, and the resulting mass transfer can generate atomic line and X-ray emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic. However, even in binaries, white dwarfs are rarely detected at far-infrared or radio frequencies.
The team’s calculations show that the 1.97 minute brightness pulsations reflect the spin of a magnetic white dwarf, one that is slowing down on a timescale of 107 years.
Although the pulsations are driven by the white dwarf’s spin, they originate in large part from the cool star. AR Sco’s broad-band spectrum is characteristic of synchrotron radiation, requiring relativistic electrons. These must either originate from near the white dwarf or be generated in situ at the M star through direct interaction with the white dwarf’s magnetosphere.
Synchrotron radiation involves the acceleration of charged particles in a magnetic field, but as the quote above shows, what the researchers don’t yet know is the source of the electrons. The kind of pulsations observed here have been seen before in neutron stars but AR Scorpii is the first white dwarf system to show similar behavior. The paper notes that white dwarfs and neutron stars are the only two types of object that can support a misaligned magnetic dipole and spin fast enough to match the observed pulsations. The paper goes to some length to demonstrate that the AR Scorpii pulsations are consistent only with a white dwarf and not a neutron star.
The paper is Marsh et al., “A radio pulsing white dwarf binary star,” published online in Nature 27 July 2016 (abstract).
CX330: Distant (and Isolated) Star Formation
Given that we have fewer than a dozen examples, highly variable stars like the recently discovered CX330 have much to teach us. These stars have been nicknamed FUors, after FU Orionis, a pre-main sequence star that has shown huge variations in magnitude over the past century. Eruptions like these may be common, as Alan Boss argued last year (see A Disruptive Pathway for Planet Formation), but as we learn more about them, we have to account for dramatic changes, as when the star V1057 Cyg increased in brightness by 5.5 magnitudes over the course of a few years. What does this do to an associated circumstellar disk?
As we ponder these questions, we also have to account for CX330, which comes into the news this week because of its odd isolation. Star-forming clouds packed with young stars are rich in gas and dust, and it is in these that we find all other examples in the FU Orionis category. But CX330 is a thousand light years away from the closest region of star formation, and because it is thought to be no more than a million years old, it’s hard to explain how it could have migrated from a star-forming region without losing much of the disk it now seems to be consuming.
From a new paper on CX330:
In the YSO [Young Stellar Object] interpretation, the most interesting characteristic of CX330 is its position on the sky. It is 2? above the Galactic Plane, and well outside of any known star forming region. All known large magnitude outbursting YSOs such as FUors are located in star forming regions, likely in part because of selection biases but the actual distribution is also concentrated in these regions… Additionally, CX330 must have formed in situ, since its measured proper motion with the VVV survey has an upper limit of 3.3 mas [milliarcseconds] yr?1. The phase of stellar evolution in which disk instability events occur is generally thought to be within 106 years after the formation of the disk…so that it can have drifted less than a degree since the time of formation.
Image: This artist’s concept shows an unusual celestial object called CX330 that was first detected as a source of X-ray light in 2009 by NASA’s Chandra X-Ray Observatory while it was surveying the bulge in the central region of the Milky Way. A 2016 study in Monthly Notices of the Royal Astronomical Society found that CX330 is the most isolated young star that has been discovered. Researchers compared NASA’s Wide-field Infrared Survey Explorer (WISE) data from 2010 with NASA’s Spitzer Space Telescope data from 2007 to come to this conclusion. Credit: NASA/JPL-Caltech.
What we can say is that CX330, isolated as it is, is in the midst of an outbursting period that allowed its detection at X-ray wavelengths as well as visible light, with surveys like the above mentioned VVV (VISTA Variables in The Via Lactea) helping to measure its output. Infrared studies with data from the Wide-Field Infrared Survey Explorer (WISE) made it clear to Chris Britt (Texas Tech) that the object was associated with a large amount of warm dust that would have been heated in the outburst. From 2007 to 2010, according to WISE and Spitzer data, the star increased in brightness by several hundred times. CX330 appears more massive than known FU Orionis objects, with strong interactions with the gas and dust around it as material falls onto the star.
“CX330 is both more intense and more isolated than any of these young outbursting objects that we’ve ever seen,” said Joel Green, study co-author and researcher at the Space Telescope Science Institute in Baltimore. “This could be the tip of the iceberg — these objects may be everywhere.”
Which leads me back to Alan Boss’ 2015 paper. In it, Boss (Carnegie Institution for Science) modeled interactions between protoplanetary disks and their stars, studying in particular periods of gravitational instability in the disk that can scatter larger objects (1 to 10 meters) away from the star. What we wind up with is a star that consumes about half the gaseous disk mass but allows these larger objects to grow into planetesimals. Thus we have a mechanism for planet formation despite infalling gas and violent upheavals in the star’s accretion disk.
If outbursts like these are relatively short in astronomical terms, we wouldn’t have observed many of them, and Joel Green’s comment could be right on target. In fact, we may learn that most or all stars go through periods of violent development in their youth. This would follow a hierarchical model of star development in which a surrounding gas cloud reaches a critical density and begins to collapse onto the young star. The paper argues that this may explain stars like CX330:
Hierarchical SF [star formation] posits that stars form in a fractal distribution on a smoothly varying range of scales dominated by turbulence rather than magnetic support… There is evidence of hierarchical SF as a result of supernovae shocks…and in at least one association of massive stars in a larger SF region… If star formation proceeds in a fractal distribution of scales, some few stars should be observed to form even in total isolation from the cloud complexes which have been the focus of large star formation studies.
In any case, the effect of outbursts from young stars could be a vital element in planet formation. Finding more stars in the FU Orionis class would be useful, particularly since no FU Orionis star has ever been observed closing down from the high luminosity phase. We’re dealing with a short period in a star’s evolution but one that holds clues to the growth of planetary systems.
The paper is Britt et al., “Discovery of a Long-Lived, High Amplitude Dusty Infrared Transient,” Monthly Notices of the Royal Astronomical Society, Vol. 460, Issue 3 (18 May, 2016). Abstract available. The Alan Boss paper is “Orbital Survival of Meter-size and Larger Bodies During Gravitationally Unstable Phases of Protoplanetary Disk Evolution,” Astrophysical Journal Vol. 807 (July 1, 2015), No. 1, 10 (abstract).
Jupiter’s Great Red Spot as Heat Source
Speculating about what an advanced extraterrestrial civilization might do has kept us occupied for the last two days, with gas giants like Jupiter the primary topic of conversation. We don’t know if it’s possible to ignite a gas giant to provide new sources of energy. But with Juno getting ready to measure Jupiter’s aurorae, we’re looking at naturally produced energy today, and now we have interesting work on the planet’s Great Red Spot that comes out of Earth-based observations. The enormous storm turns out to be a key factor in heating Jupiter’s atmosphere.
And what a storm it is. We knew about the Great Red Spot as early as the 17th Century because its span — three Earth diameters — qualifies this highly visible maelstrom as the largest hurricane we know of. Winds can take six days to complete one circuit of the Great Red Spot, which has varied in size and color ever since it was discovered. It is now observed to span 22,000 km by 12,000 km in longitude and latitude, respectively.
The Great Red Spot gives us a source of energy to heat Jupiter’s upper atmosphere but thus far we have lacked evidence of its effect upon temperatures. Now an analysis based upon new infrared data is changing our view of temperatures high above Jupiter’s visible disk.
Image: Acquiring Jovian spectra. Bright regions at the poles result from auroral emissions; the contrast at low- and midlatitudes has been enhanced for visibility. Great Red Spot (GRS) emissions at mid latitudes are indicated by the red arrow. Additional info: The vertical dark line in the middle of the image indicates the position of the spectrometer slit, which was aligned along the rotational axis of Jupiter. Image shown is taken from the slit (slit-jaw imaging) using the “L-filter” (3.13 – 3.53 μm). Credit: J. O’Donoghue, NASA Infrared Telescope Facility (IRTF).
The results come from James O’Donoghue (Boston University) and colleagues, who report their findings on the matter today in Nature, using 2012 data from the SpeX spectrometer on NASA’s Infrared Telescope Facility in Hawaii. The issue caught the astronomers’ attention because at mid- to low latitudes, temperatures in Jupiter’s upper atmosphere are hundreds of degrees warmer than heating from the Sun can explain. We’re looking at non-Solar energy whose sources could be studied by creating heat maps of the entire planet.
This is what the O’Donoghue set out to do, realizing that what his team refers to as an ‘energy crisis’ occurs not just on Jupiter but on other giant planets as well. One explanation for Jupiter has been auroral heating mechanisms that pump energy into the upper atmosphere. But the low to mid-latitudes lack this kind of heat source and yet remain 600 K warmer than can be explained by solar heating. The paper makes the case for a different kind of source:
A more likely energy source is acoustic waves that heat from below (also via viscous dissipation); this form of heating requires vertical propagation of disturbances in the low-altitude atmosphere. Acoustic waves are produced above thunderstorms, and the subsequent waves have been modelled to heat the Jovian upper atmosphere by 10K per day and on Earth have been observed to heat the thermosphere over the Andes mountains. On Jupiter, acoustic-wave heating has been modelled to potentially impart hundreds of degrees of heating to the upper atmosphere. However, to the best of our knowledge, no such coupling between the lower and upper atmosphere has been directly observed for the outer planets, so vertical coupling has not been seriously considered as a solution to the giant-planet energy crisis.
The team found that high altitude temperatures on Jupiter are greater than expected when the Great Red Spot is directly below. In fact, the atmosphere above this region is hundreds of degrees hotter than anywhere else on the planet. The temperature spike above the Great Red Spot points to coupling between lower and upper atmosphere. The authors believe the heating is caused by atmospheric turbulence that rises because of the shear between the storm and surrounding atmosphere, with propagating waves depositing their energies high above.
Image: Turbulent atmospheric flows above the storm produce both gravity waves and acoustic waves. Gravity waves are much like how a guitar string moves when plucked, while acoustic waves are compressions of the air (sound waves). Heating in the upper atmosphere 800 kilometers above the storm is thought to be caused by a combination of these two wave types ‘crashing’ like ocean waves on a beach. Credit: Art by Karen Teramura, UH IfA, James O’Donoghue
Co-author Tom Stallard (University of Leicester) puts the work into the context of ongoing missions like Juno:
“This fantastic result, showing how the upper atmosphere is heated from below, was produced directly from Leicester’s 2012 observing campaign, which was designed to try and answer why Jupiter’s upper atmosphere is so hot. Juno will be measuring the aurora and its sources, and we expected the auroral energy to flow from the pole to the equator. Instead, we find the equator appears to be heated from plumes of energy coming from Jupiter’s vast equatorial storms.”
The paper is O’Donoghue et al., “Heating of Jupiter’s upper atmosphere above the Great Red Spot,” Nature, published online 27 July 2016 (abstract).
SETI: Detecting ‘Stellified’ Objects
When Nikolai Kardashev looked into the question of where to find advanced extraterrestrial civilizations, he argued that the obvious starting point would be in the vicinity of extreme astrophysics. Active galactic nuclei (AGN) come to mind, or even the centers of comparatively quiet galaxies like our own. Clément Vidal picked up the same point in his The Beginning and the End (Springer, 2014), arguing persuasively that we should consider how black holes could be used, perhaps by manipulating the merger of such objects. And yes, this is astroengineering utterly beyond our skills, but possibly not those of an advanced ETI.
Using black holes for energy is extreme, but Roger Penrose has imagined a super-civilization extracting black hole rotational energy by the injection of matter, and there are a number of other propositions on how such advanced engineering might work. Extracting energy from a black hole’s accretion disk might be the most efficient method, but lower-grade operations could exist around neutron stars. To that idea we might add, as Milan ?irkovi? does in the paper we looked at yesterday, the exploitation of X-ray binaries or quasars like SS433.
The new ?irkovi? paper homes in on gas giants and brown dwarfs, with the possibility of making either into a star. It’s an idea with a popular pedigree, the ignition of Jupiter having gone viral with the film 2010, but the real action is beyond the Solar System entirely. Consider that the number of substellar objects in interstellar space has been estimated to be as high as 105 times greater than the number of main sequence stars (see Island-Hopping to the Stars for more on this estimate and the kind of substellar objects it references).
Image: Artist’s impression of a free-floating gas giant. Credit: NASA/JPL-Caltech.
Could a technologically manipulated gas giant or brown dwarf be a SETI observable? Here we can look at several possibilities. The luminosity of a ‘stellified’ object should be greater than its mass would lead us to expect for natural objects. And, taking the long view, its luminosity should evolve differently from natural stars. Moreover, we might find anomalies in the spectra of such objects, especially early and late in their astronomically-brief lifetimes. And bear in mind that stellified objects would be bright power sources, unlike artificial orbital habitats or other large structures which would only reflect light or become apparent through their thermal emissions.
In other words, if such objects exist, they would be useful targets for Dysonian SETI investigation. ?irkovi? notes that determining the mass of a field star is a tricky proposition, and if we’re considering possible artifacts, we can’t just try to position the object on the Hertzsprung-Russell diagram as if it were a normal main sequence star. An accurate mass determination would require a multiple star system, and even here the measurement is fraught with uncertainties. But where we can determine it, mass is worth pursuing.
From the ?irkovi? paper, referring to Martyn Fogg’s 1989 paper on stellified gas giants:
The outliers from the low-mass stellar luminosity-mass relation deserve our best observational scrutiny, especially if the anomaly is extreme. In Fogg’s model, for instance, luminosity of [a] stellified Jupiter will fairly soon after the beginning of the process be ten orders of magnitude or so above the expected luminosity of such a low-mass free-floating object. If such artefacts are numerous in the Galaxy, their considerably easier detectability could deform and leave an imprint on the substellar mass function.
The evolution of luminosity in such objects would be challenging to gauge because of the timeframes involved — Fogg estimated a 50 million year span of exponentially increasing luminosity in the case of a stellified gas giant like Jupiter. As ?irkovi? notes, we could easily have such artifacts in our stellar catalogs now, for depending on their stage of stellification, they would simply mimic a particular type of star. We seem to be best off in hunting for stellified objects at the beginning and end of their lifetimes, looking for gamma and X-ray flares, for example, during the early stages of a planet’s transformation into a star.
Even so, we still are dealing with relatively transient phenomena compared to main sequence star lifetimes, and advanced technologies about which we can only speculate. Remember that the scenario Martyn Fogg originally came up with assumes using a small black hole, merging it with Jupiter in a carefully controlled orbit that eventually brings it toward the planet’s center. 100 million years of habitability are provided to the Jovian satellite system, but several hundred million years later, runaway accretion would have to be prevented. ?irkovi? comments:
For the initial phase, the period of unstable bursting and flaring can be shortened by sufficiently gently placing the mini-black hole into the substellar objects; for the final phase, duration and properties of instabilities depend on the manner of removing the excess mass from the black hole, as well as on the existing installation/swarm surrounding it. If such anomalies are observed in a planetary system containing at least one planet in the circumstellar habitable zone, this could be further incentive to give it high priority as a SETI target.
My take on all this is that as we are at the beginning of Dysonian SETI, we’re early in the process of developing the necessary parameters. ?irkovi? speaks to this point near the end of this paper, calling for improved quantitative models for the kind of astroengineering projects we can imagine and their possible SETI signatures. The advantage of stellified gas giants is that they are larger and simpler than many of the conjectured astroengineering projects that have been proposed, but we would want to have sound models for a wide range of possibilities.
And there’s the nub of the problem: We’d like to be able to observe an anomaly in our astronomical data and relate it swiftly to a potential technology, using what we believe to be its observables. But Dysonian SETI is built around the concept of abandoning anthropocentrism and simply observing. How does a Kardashev sub-Type I culture like ours envision what a Type II might do? Our conjectures invariably grow out of our preconceptions, and the models we build can only be crude templates. Rather than abandoning the process, we have to keep all this in mind, continually adjusting our assumptions while being alert for data that fit no previous niche.
The ?irkovi? paper is “Stellified Planets and Brown Dwarfs as Novel Dysonian SETI Signals,” in press at JBIS. I also referenced a Nikolai Kardashev paper above; it’s “On the inevitability and possible forms of supercivilizations”, in The Search for Extraterrestrial Life: Recent Developments, ed. M.D. Papagiannis, IAU, Dordrecht, pp.497-504, 1985.
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