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Cosmic Engineering and the Movement of Stars

Avi Loeb’s new foray into the remote future had me thinking of the Soviet physicist Leonid Shkadov, whose 1987 paper “Possibility of Controlling Solar System Motion in the Galaxy” (citation below) discussed how an advanced civilization could get the Sun onto a new trajectory within the galaxy. Why would we want to do this? Shkadov could imagine reasons of planetary defense, a star being moved out of the way of a close encounter with another star, perhaps.

All of this may remind science fiction readers of Robert Metzger’s novel CUSP (Ace, 2005), which sees the Sun driven by a massive propulsive jet. A more recent referent is Gregory Benford and Larry Niven’s novels Bowl of Heaven (Tor, 2012) and ShipStar (2014), in which a star is partially enclosed by a Dyson sphere and used to explore the galaxy. In 1973, Stanley Schmidt would imagine Earth itself being moved to M31 as a way of avoiding an explosion in the core of the Milky Way that threatens all life (Sins of the Fathers, first published as a serial in Analog).

Loeb’s paper mentions technologies for moving stars because several recent proposals involve this kind of ‘cosmic engineering’ to change civilizational outcomes. The accelerated expansion of the universe will, after the universe has aged by a factor of 10, make all stars outside the Local Group of galaxies disappear as they recede. The process continues with continued acceleration. Wait long enough and we fall prey to cosmic winter, and as Loeb writes:

Following the lesson from Aesop’s fable “The Ants and the Grasshopper,” it would be prudent to collect as much fuel as possible before it is too late, for the purpose of keeping us warm in the frigid cosmic winter that awaits us. In addition, it would be beneficial for us to reside in the company of as many alien civilizations as possible with whom we could share technology, for the same reason that animals feel empowered by congregating in large herds.

There are places where we might do this, says Loeb, and I’ll talk about his ideas on them tomorrow. For today, note that in response to his previous papers on the oncoming ‘cosmic isolation,’ Freeman Dyson proposed to Loeb his own project that would move stars, bringing large-scale formations of stars down to a more manageable volume so that they will be bound by their own gravity. Closely spaced, the stellar collection avoids being dissipated in the accelerated expansion of the cosmos. And it turns out that Dan Hooper (University of Chicago) has also been pondering induced stellar motion, using the energy output of stars to cluster large numbers of them into an astronomically tight radius so that their energies can be harvested.

Image: A Shkadov thruster as conceived by the artist Steve Bowers.

Like Dyson and Loeb, Hooper has his eye on the acceleration of cosmic expansion and its consequences. So what kind of ‘stellar engines’ can we envision that could move objects as large as stars? Leonid Shkadov suggested using a star’s radiation pressure. The Shkadov thruster extracts energy from the star by using a vast mirror to take advantage of photon momentum. Let me turn back to an earlier post to reprint a diagram that Duncan Forgan uses in describing a Shkadov thruster. Forgan (University of Edinburgh) points out the difference between these thrusters and Dyson spheres, the latter being spherical and shaped so as to balance gravitational forces on the sphere by way of collecting a maximum amount of stellar energy. But here is the Shkadov thruster as diagrammed by Forgan:

Image: This is Figure 1 from Duncan Forgan’s paper “On the Possibility of Detecting Class A Stellar Engines Using Exoplanet Transit Curves.” Caption: Diagram of a Class A Stellar Engine, or Shkadov thruster. The star is viewed from the pole – the thruster is a spherical arc mirror (solid line), spanning a sector of total angular extent 2ψ. This produces an imbalance in the radiation pressure force produced by the star, resulting in a net thrust in the direction of the arrow. Credit: Duncan Forgan.

Thus the whole idea of the Shkadov thruster is not balance but imbalance. And Forgan goes on to say this about the idea:

In reality, the reflected radiation will alter the thermal equilibrium of the star, raising its temperature and producing the above dependence on semi-angle. Increasing ψ increases the thrust, as expected, with the maximum thrust being generated at ψ = π radians. However, if the thruster is part of a multi-component megastructure that includes concentric Dyson spheres forming a thermal engine, having a large ψ can result in the concentric spheres possessing poorer thermal efficiency.

Efficient or not, Shkadov thrusters interest Forgan as a possible SETI detection (Hooper also notes the possibility). Like Dyson spheres, their sheer scale and unusual features could make them visible in a lightcurve, perhaps with the aid of radial velocity follow-ups. Arguing against the idea, though, is the fact that the Shkadov thruster is probably not the technology our hypothetical future civilization would use to move its star (or stars).

I found this out when I wrote Avi Loeb in reaction to his new paper and mentioned the Shkadov idea. Loeb found Dan Hooper’s ideas (in “Life Versus Dark Energy: How An Advanced Civilization Could Resist the Accelerating Expansion of the Universe,” citation below) to be a better solution to the problem. Here is what Loeb told me in our email exchange yesterday:

The use of the momentum associated with the radiation emitted by the star for its propulsion, as envisioned in Shkadov’s thruster, is much less efficient than using the energy associated with same radiation. The radiation momentum equals its energy divided by the speed of light, c. However, the momentum gained by converting this energy to the kinetic energy of a massive object moving at a speed v is larger by a factor of 2(c/v). This is a huge factor of which Dan Hooper is taking advantage to argue that Sun-like stars can reach a percent of the speed light, 0.01c, in a billion years. If he would have used the momentum of the light emitted by the star as in Shkadov’s thruster, then the attainable speed would have been a hundred times smaller, only of order 30 km/s (similar to the speed of chemical rockets), and the journey being contemplated would have not been feasible. During the age of the Universe (10 billion years) one would only be able to traverse a million light years and not leave the Local Group of galaxies. This is not sufficient for gaining more fuel than available within the Local Group of galaxies.

Thus the problem of the Shkadov thruster: By using the momentum of stellar photons, Shkadov loses efficiency. Loeb adds: “One way to improve the efficiency of Shkadov’s thruster (by employing the energy and not the momentum of the star’s radiation) is to harvest the energy from the star through a Dyson sphere and then use it to ablate its surface on one side, generating a rocket effect.” This seems to be what Dan Hooper has in mind in his paper. The civilization in question would harvest the energy of stars through Dyson spheres that, quoting Hooper, “…use the collected energy to propel the captured stars, providing new and potentially distinctive signatures of an advanced civilization in this stage of expansion and stellar collection.”

Hooper has a SETI prospect in mind, while also thinking about collecting sources of energy, maximizing its amount in the form of starlight by a factor of several thousand. It is this prospect that interests Loeb. His paper makes the case that we already have massive reservoirs of fuel in the visible universe in the form of galactic clusters, each containing the equivalent of 1000 Milky Ways. Perhaps, then, we need no star-moving and collection project a la Hooper or Dyson, but rather need to think about reaching a galactic cluster for our fuel. Given the magnitude of that challenge, whether or not we take our home star along is inconsequential.

The speculative buzz I get from this is science fictional indeed. The nearest cluster is Virgo, whose center is some 50 million light years away, with the Coma cluster six times farther still. Thinking in terms of a civilization that could cross such gulfs takes us into Olaf Stapledon territory, a region of spacetime with which I share Loeb’s obvious fascination. I’m running out of time today, but want to look deeper into this with the help of Loeb’s paper tomorrow. I’ll also have more thoughts on ‘stellar engines’ and their origins.

Avi Loeb’s new paper is “Securing Fuel for Our Frigid Cosmic Future” (preprint). Leonid Shkadov’s paper on the Shkadov thruster is “Possibility of controlling solar system motion in the galaxy,” 38th Congress of IAF,” October 10-17, 1987, Brighton, UK, paper IAA-87-613. The Forgan paper is “On the Possibility of Detecting Class A Stellar Engines Using Exoplanet Transit Curves,” accepted at the Journal of the British Interplanetary Society (preprint). The Hooper paper is “Life Versus Dark Energy: How An Advanced Civilization Could Resist the Accelerating Expansion of the Universe” (2018). Preprint.



On Potentially Habitable Moons

Looking through a recent Astrophysical Journal paper on gas giants in the habitable zone of their stars, I found myself being diverted by the distinction between a conservative habitable zone (CHZ) and a somewhat more optimistic one (OHZ). Let’s pause briefly on this, because these are terms that appear frequently enough in the literature to need some attention.

The division works like this (and I’ll send you to the paper for references on the background work that has developed both concepts): The OHZ in our Solar System is considered to be roughly 0.71 to 1.8 AU, which sees Venus as the inner cutoff (a world evidently barren for at least a billion years) and Mars as the outer edge, given that it appears to have been habitable in the early days of the system, perhaps some 3.8 billion years ago. ‘Habitable’ in both HZ categories is defined as the region around a star where water can exist in a liquid state on a planet with sufficient atmospheric pressure (James Kasting has a classic 1993 paper on all this).

The CHZ’s inner edge is considered to be at the ‘runaway greenhouse limit,’ where the breakdown of water molecules by solar radiation allows free hydrogen atoms to escape, drying out the planet at approximately 0.99 AU in our own system. Its outer edge, says the paper:

…consists of the maximum greenhouse effect, at 1.7 AU in our solar system, where the temperature on the planet drops to a point where CO2 will condense permanently, which will in turn increase the planet’s albedo, thus cooling the planet’s surface to a point where all water is frozen (Kaltenegger & Sasselov 2011).

It goes without saying that boundaries like these are going to vary from one planetary system to another, and it’s likewise clear that most of our thinking about habitable zone planets has gone in the direction of small rocky worlds as we mount the search for Earth analogues. What Stephen Kane (University of Southern Queensland), working with an undergraduate student at the university named Michelle Hill as well as colleagues at the University of California, Riverside has done is to identify 121 giant planets in Kepler data that could host habitable moons.

To be sure, the gas giants themselves aren’t considered candidates for life as we know it (though obviously we can’t rule out exotic species adapted to extreme conditions, like Edwin Salpeter’s ‘gasbags,’ free-floating lifeforms that might populate dense atmospheres — see Edwin Salpeter and the Gasbags of Jupiter for more). But the real focus is on those rocky moons that occur in such abundance in our own system.

“There are currently 175 known moons orbiting the eight planets in our solar system. While most of these moons orbit Saturn and Jupiter, which are outside the Sun’s habitable zone, that may not be the case in other solar systems,” says Kane. “Including rocky exomoons in our search for life in space will greatly expand the places we can look.”

Image: This is an artist’s illustration of a potentially habitable exomoon orbiting a giant planet in a distant solar system. Credit: NASA GSFC: Jay Friedlander and Britt Griswold.

As we consider the different dimensions of habitable zones around other stars, we should also keep in mind the fact that the moons that may emerge in these systems can be as various as our own. Earth’s Moon, for example, seems to be the result of a giant impact early in the system’s formation. Most moons are thought to have formed by accretion within the dust disks around planets, but others can be captured by a planet’s gravitational pull — Triton seems to be an example of this. Thus we could find moons of considerably different composition than their host planet. Considering how many moons we see orbiting our gas giants, the assumption that moons exist around other such worlds in exoplanetary systems seems reasonable.

We still have no exomoon detections, but the search continues, and I always scan the latest papers from the Hunt for Exomoons with Kepler project that David Kipping runs with anticipation, along with those of exomoon theorist René Heller. Having a database of the giant planets we’ve identified thus far as being in the habitable zone of their star may help us target future observations to refine the expected properties of their moons, assuming these exist. Such moons would receive energy from the primary star, of course, but would also receive reflected radiation from the planet they orbit. René Heller has proposed that exomoons in a habitable zone could provide a better environment for life than Earth itself. Let me quote the Hill paper:

Exomoons have the potential to be what [Heller] calls ”super-habitable” because they offer a diversity of energy sources to a potential biosphere, not just a reliance on the energy delivered by a star, like earth. The biosphere of a super-habitable exomoon could receive energy from the reflected light and emitted heat of its nearby giant planet or even from the giant planet’s gravitational field through tidal forces. Thus exomoons should then expect to have a more stable, longer period in which the energy received could maintain a livable temperate surface condition for life to form and thrive in.

Discussing the difficulties of exomoon detection, such as the fact that multiple moons around a single planet may eliminate a useful transit timing signal (this is Jean Schneider’s work) and the problems of direct imaging, it’s interesting to see that microlensing remains a candidate. It’s also intriguing to ponder the fate of exomoons, as this paper does, in terms of migrating gas giants and the likelihood that their moons will be lost. We still have much to learn about the movement of giant planets and the effect of their migration upon their own moons as well as other planets.

Once we have a firm exomoon detection, we can begin to characterize the possibilities. As we await improvements in our technology, deepening our knowledge of potential exomoon host planets is the best we can do, and that would begin, as this paper suggests, with radial velocity follow-up observations on gas giant habitable zone candidates like the ones compiled by the authors.

The paper is Hill et al., “Exploring Kepler Giant Planets in the Habitable Zone,” The Astrophysical Journal, 2018; 860 (1): 67. Abstract / preprint. The Kasting paper mentioned above is “Habitable Zones around Main Sequence Stars,” Icarus Vol. 101, Issue 1 (1993), pp. 108-128 (abstract). For René Heller’s work on ‘superhabitable’ moons, see Heller & Armstrong, “Superhabitable Worlds,” Astrobiology January 2014 (preprint). Jean Schneider’s paper on exomoon detection problems is Schneider & Sartoretti, “On the detection of satellites of extrasolar planets with the method of transits,” Astronomy & Astrophysics. Suppl. Ser. Vol. 134, No. 3 (1 February), pp. 553-560 (abstract).



Marc Millis on Mach Effect Thruster, EmDrive Tests

Marc Millis spent the summer of 2017 at the Technische Universität Dresden, where he taught a class called Introduction to Interstellar Flight and Propulsion Physics, a course he would also teach at Purdue University last November. The former head of NASA’s Breakthrough Propulsion Physics project and founding architect of the Tau Zero Foundation, Marc participated in the SpaceDrive project run by Martin Tajmar in Dresden, an effort that has been in the news with its laboratory testing of two controversial propulsion concepts: The Mach Effect Thruster and the EmDrive. Marc’s review comments on modeling for the former were almost as long as Tajmar’s draft paper. Described below, the SpaceDrive project is a wider effort that includes more than these two areas — neither the EmD or MET thruster had reached active test phase during the summer he was there — but the ongoing work on both occupies Millis in the essay that follows.

by Marc Millis

You may have noticed a renewed burst of articles about the EmDrive. What prompted this round of coverage was an interim report, part of the progress on Martin Tajmar’s ‘SpaceDrive’ project to carefully test such claims. Tajmar’s conference paper [citation below] is one of the early steps to check for false-positives. I expect more papers to follow, each progressing to other possibilities. It might take a year or so more before irrefutable results are in. Until then, treat the press stories about certain conclusions as highly suspect.

On Tajmar’s work, this quote from his conference paper:

Within the SpaceDrive project [6], we are currently assessing the two most prominent thruster candidates that promise propellantless propulsion much better than photon rockets: The so-called EMDrive and the Mach-Effect thruster. In addition, we are performing complementary experiments that can provide additional insights into the thrusters under investigation or open up new concepts. In order to properly test the thruster candidates, we are constantly improving our thrust balance facility as well as checking for thruster-environment interactions that can lead to false thrust measurements.

The Mach Effect Thruster is a different approach to the goal of a non-rocket spacedrive, but one that is rooted in unsolved questions in physics where there is a chance for new discoveries. Its theory led to a testable prediction that then evolved into an idea for a propulsive effect.

The unsolved physics question is: “What is the origin of inertial frames?” One attempt to answer that is called “Mach’s Principle” (term coined by Einstein to describe Ernst Mach’s perspective), which is roughly: “inertia here, because of matter out there.” The idea is that the phenomenon of inertia is an interaction between that mass and all the surrounding mass in the universe (presumed gravitational in nature). Jim Woodward picked up on a version of this from Dennis Sciama, and noticed that the inertial mass of an object can fluctuate if its energy fluctuates (think energy in a capacitor). That led to an idea for a propulsive effect by varying the distance between two fluctuating inertias. Unlike the EmDrive, this idea has been in the peer-reviewed literature from the beginning, with some of the more relevant papers being:

Woodward, J. F. (1990). A New Experimental Approach to Mach’s Principle and Relativistic Gravitation, in Foundations of Physics Letters, 3(5): 497-506.

Woodward, J. F. (1991). Measurements of a Machian Transient Mass Fluctuation, in Foundations of Physics Letters, 4(5): 407-423.

Woodward, J (1994), “Method for Transiently Altering the Mass of an Object to Facilitate Their Transport or Change their Stationary Apparent Weights,” US Patent # 5,280,864.

Woodward, J. (2012). Making Starships and Stargates, Springer.

Fearn, H. & Wanser, K. (2014). Experimental Tests of the Mach Effect Thruster. Journal of Space Exploration, 3: 197-205.

Martin Tajmar’s laboratory results can be summarized this way: False positive thrusts were observed under conditions where there should be no thrusting or only minor thrusting. More systematic checks have to be made prior to testing the thrusters at their nominal and maximum operating parameters. The mismatch was more pronounced for the EmDrive than for the Mach Effect Thruster. In both cases it is premature to reach definitive conclusions since this is a work in progress. And if any thrusters do pass all those tests, then more tests will commence to figure out how the thrusters operate (varying conditions to see which affect the thrust levels).

In the case of the EmDrive, only 2 W of the more normal 60 W of power was made available to the thruster. Even at that low power level, thrusts of about 4 µN were observed, which is more than the 2.6 µN expected from the claims from Sonny White’s tests. The more revealing observations were that thrusts were observed when the EmDrive was not supposed to be thrusting. When the EmDrive was pointed to a non-thrusting direction, thrusts were still observed. When the power to the thruster was sent to an attenuator to further reduce the power to the thruster by a factor of 10,000, thrusting at the prior level was still observed.

These observations do not bode well for the EmDrive’s claims of real thrust, but it is too early to firmly dismiss the possibilities. One suspect for the false positive is the interaction with the current to the device and the Earth’s magnetic field, where a current of 2-amps in a few cm of wires can produce a thrust in the µN range. Further tests are planned after adding more magnetic shielding and operating over different power levels.

In the case of the Mach Effect Thruster – which by the way, none of the press articles mentioned – the findings were less pessimistic. Again there were thrusts measured in excess of what was expected for the low power levels (0.6 versus 0.02 µN). Unlike the EmDrive’s mismatch, no thrust was observed when the Mach Effect Thruster was pointed to a non thrusting direction. There was, however, a case where the thrust direction did not change when the thruster direction was flipped. The suspected causes to be further investigated include both magnetic and thermal (expansion) effects.

A word of advice: if you plan to look at Tajmar’s paper. When I tried my usual “rush read” through the paper by reading the abstract and scanning the figures, I misled myself. Read the full text that accompanies the figures to know what you are really looking at. It’s a short article.

Regarding some representative press articles, here is a quick assessment

(1) David Hambling, New Study Casts Doubt on the “Impossible” EmDrive, But this weird propulsion idea isn’t dead yet

This one goes into more detail than the other articles about what was actually done and not done and does link to its information sources. It does not mention the Mach Effect Thruster.

(2) Mike Wall, ‘Impossible’ EmDrive Space Thruster May Really Be Impossible

This one mentions the doubt, but leaves the door open just a bit. Although it does not mention the Mach Effect Thruster also under test, it does at least give a link to the core article and mentions where it came from.

(3) Ethan Siegel, The EmDrive, NASA’s ‘Impossible’ Space Engine, Really Is Impossible: Many tests have reported an ‘anomalous thrust’ where there should be none. A researcher has finally shown where everyone else has messed up

This article talks more about the old claims and expectations than what was really in the new paper. It does not mention the Mach Effect Thruster.

(4) Mike Wehner, NASA’s ‘impossible’ fuel-free engine is actually impossible

More short-hand opinion, and again, no mention of the Mach Effect Thruster.

The takeaway: Science does not proceed by proclamation. Despite what headlines may say, laboratory work is a matter of refining techniques and bringing precision to bear on prior claims. At the moment, evaluation of the EmDrive and Mach Effect thruster continues, with no guarantee that either of these effects may prove genuine, but let’s let the process play out.

The Tajmar paper is Tajmar et al., “The SpaceDrive Project – First Results on EMDrive and Mach-Effect Thrusters,” presented at the Space Propulsion 2018 conference in Seville, Spain (full text).



Enter the ‘Clarke Exobelt’

It’s interesting to consider, as Hector Socas-Navarro does in a new paper, the various markers a technological civilization might leave. Searching for biosignatures is one thing — we’re developing the tools to examine the atmospheres of planets around nearby stars for evidence of life — but how do we go about looking for astronomical evidence of a technological society, one found not by detection of a directed radio or laser beacon but by observation of the stars around us?

Various candidates have been suggested, the most famous being the Dyson sphere, in which an advanced civilization might choose to trap the energy output of its entire star, and we’re in the era of searches for such objects, as witness the Glimpsing Heat from Alien Technologies effort at Penn State. But there are many other suggestions, ranging from detecting antimatter used for power or propulsion, analyzing Fast Radio Bursts for evidence of manipulation as a propulsion system, and looking at depletion of metals in a stellar disk (asteroid mining).

Image: Astronomer Hector Socas-Navarro (Instituto de Astrofísica de Canarias). Credit: IAC.

What Socas-Navarro has in mind is a technology we have already begun to deploy and will presumably see in accelerated use. Delightfully, he has named his idea ‘the Clarke Exobelt’ (CEB) in a nod to the father of the communications satellite, an apt choice given that he defines the idea as the collection of objects, including non-functional ones, in geostationary and geosynchronous orbits around a planet. An astronomer at the Instituto de Astrofísica de Canarias, Socas-Navarro believes that because there is no natural ‘preference’ for this orbit, the detection of a population of objects within it would be highly suggestive of an artificial origin.

Published in The Astrophysical Journal, the paper has caught the eye of enough Centauri Dreams readers that I have five copies of it in my inbox. Seeing the Clarke name associated with it is enough to pique my interest — would that we had Sir Arthur’s own thoughts on the matter! Socas-Navarro points out that there is a certain economy in searching for Clarke Exobelt objects, in that current techniques to detect exoplanets and exomoons should also be able to detect a large enough cluster of technologies in geosynchronous orbits.

The paper, then, is a suggestion that we begin looking for this kind of technosignature amongst the other possibilities. The challenge is in the extrapolation, for to be detectable with our current and near-future technologies, such an Exobelt would have to be densely populated. Our own Clarke belt is relatively sparse, with two-thirds of existing satellites in low orbits. Socas-Navarro believes, however, that the realm of geostationary and geosynchronous satellites is destined to grow, and his simulations show that if it does, it will at some point become detectable:

The geostationary orbit, often named after Clarke, who explored its practical usefulness for communication purposes (Clarke 1945), is specially interesting because satellites placed there will remain static as seen from the ground reference frame. However, the available space in that orbit is limited. A moderately advanced civilization might eventually populate it with a relatively high density of objects, making it advisable (cheaper in a supply-and-demand sense) to use geosynchronous orbits when possible for those satellites whose requirements are less strict and allow for some degree of movement along the North-South direction on the sky. Over time, one might expect that societal needs would eventually drive an increase of object density in a band around the geostationary orbit, forming a CEB.

Using Earth’s current satellite population as a reference, the author creates a Clarke Exobelt model using as parameters radius, width, face-on opacity and inclination of the equatorial plane with respect to the plane of the sky (a CEB viewed edge-on would have an inclination of 90 degrees). The model excludes eccentric orbits and assumes all objects at the same orbital altitude. Using it, Socas-Navarro explores a Clarke Exobelt as it appears in the light curve of a star.

Our current Clarke belt would be orders of magnitude below the detection threshold for observers around other stars if they were using technologies similar to ours. Socas-Navarro argues that the Clarke belt around Earth is showing exponential growth, such that extrapolating it into the future would make it visible to other-world observers by the year 2200. The author considers this a reasonable extrapolation, though one highly dependent on future technology choices including, for example, space elevator systems, which could dramatically change access to these orbits and accelerate the emergence of a detectable signature.

As I read the Socas-Navarro paper, I was struck by its reliance on finding a civilization in a state of development close to our own. He is quite clear on this, saying his intention is “to explore the consequences of a direct extrapolation of our current trends,” acknowledging that even in our own near-future, we may take a different route in the population of our own Clarke belt.

Thus far, searches for technosignatures have assumed advanced technologies of the sort needed to dismantle planets and build Dyson spheres, although there is some discussion of atmospheric change through pollution or other civilizational activities. Despite the odds against finding a civilization just at the stage when it relied heavily on a Clarke Exobelt to maintain its essential services, the author thinks it prudent to keep our eyes open for this technosignature because of the deep uncertainties of forecasting what far more advanced cultures would do.

And we are improving the methods that might help us find such a Clarke Exobelt, even though they are not fine-tuned for such. Noting the difficulty of distinguishing between a CEB and a natural planetary ring system, the paper adds this:

While the similarity between a CEB and a ring system poses an initial difficulty, it also opens new opportunities. Existing interest in the physics of exorings and exomoons means that large efforts will be devoted in future photometric missions to examine rocky planet transits for evidence of such objects. This paper shows how future positive detections of orbital material may be further scrutinized for evidence of CEBs, making the search for moderately advanced technologies “piggyback” on such missions.

Thus we have a technosignature to add to our roster. One thing finding a CEB would imply is a still-functioning civilization — active maintenance would be required to keep objects in a crowded CEB within their proper orbits over long time-periods — which could not necessarily be said of a detected Dyson sphere, conceivably a relic of a long-dead culture. CEB detection is, as the author acknowledges, a ‘long shot.’ But having the widest range of technosignatures examined in the literature is only prudent. After all, who knows what we’ll find next?

The paper is Socas-Navarro, “Possible Photometric Signatures of Moderately Advanced Civilizations: The Clarke Exobelt.” The Astrophysical Journal Vol. 855, No. 2 (13 March 2018). Abstract / Preprint.



On Those Ceres Organics

I set off an interesting conversation with a neighbor when organic material was detected on Ceres, as announced last year by scientists using data from the ongoing Dawn mission. To many people, ‘organics’ is a word synonymous with ‘life,’ which isn’t the case, and straightening that matter out involved explaining that organics are carbon-based compounds that life can build on. But organic molecules can also emerge from completely non-biological processes.

So with that caveat in mind about this word, it’s still interesting that organics appear on Ceres, especially since water ice is common there, and we know of water’s key role in living systems. A new paper looks again at data from Dawn, whose detections were made with infrared spectroscopy using its Visible and Infrared (VIR) Spectrometer. The instrument, examining which wavelengths are reflected off Ceres’ surface and which are absorbed, detected organic molecules in the region dominated by Ernutet Crater on Ceres’ northern hemisphere.

Image: Last year, the Dawn spacecraft spied organic material on the dwarf planet Ceres, largest denizen of the asteroid belt. A new analysis suggests those organics could be more plentiful than originally thought. Credit: NASA / Rendering by Hannah Kaplan.

The paper’s lead author is Hannah Kaplan, now a postdoc at the Southwest Research Institute. What Kaplan and team did was to contrast the Dawn data with laboratory spectra from both terrestrial and extraterrestrial organic materials, the latter derived from meteorites. Comparing these materials with known composition, the researchers looked anew at the Ceres spectra to gain a better picture of their composition and abundance. This analysis could help us make the call on the origin of these organics, whether natural to Ceres or delivered by an impactor.

When they contrasted the VIR data from Ceres with the laboratory reflectance spectra of organic materials formed on Earth, the scientists found that between 6 and 10 percent of the spectral signature on Ceres could be explained by organic material. But folding in comparisons with organic material from carbonaceous chondrite meteorites, the team found a spectral reflectance that differed from the terrestrial.

“What we find is that if we model the Ceres data using extraterrestrial organics, which may be a more appropriate analog than those found on Earth, then we need a lot more organic matter on Ceres to explain the strength of the spectral absorption that we see there,” Kaplan said. “We estimate that as much as 40 to 50 percent of the spectral signal we see on Ceres is explained by organics. That’s a huge difference compared to the six to 10 percent previously reported based on terrestrial organic compounds.”

As to the question of origins, the impact theory would seem to favor a cometary solution, comets being known to display higher abundances of organics than asteroids. The accompanying problem here is that a cometary impact would produce enough heat to destroy such organics. On the other hand, formation on Ceres itself is problematic, because other than the small patches in the northern hemisphere region already noted, organics do not appear.

“If the organics are made on Ceres, then you likely still need a mechanism to concentrate it in these specific locations or at least to preserve it in these spots,” said Ralph Milliken, an associate professor in Brown University’s Department of Earth, Environmental and Planetary Sciences and a study co-author. “It’s not clear what that mechanism might be. Ceres is clearly a fascinating object, and understanding the story and origin of organics in these spots and elsewhere on Ceres will likely require future missions that can analyze or return samples.”

Thus a major lesson: The results depend on what kind of organic material you use to make sense of the Ceres data. The comparison with extraterrestrial organics seems sensible, and it’s one we’ll doubtless invoke again as we move toward upcoming asteroid encounters. It’s worth noting in that regard that Kaplan has recently joined the teaming operating OSIRIS-REx. The spacecraft will arrive at asteroid Bennu in August of this year, while the Japanese Hayabusa 2 is expected to reach asteroid Ryugu in a matter of weeks.

The paper is Kaplan et al., “New Constraints on the Abundance and Composition of Organic Matter on Ceres,” Geophysical Research Letters 21 May 2018 (abstract).



Protoplanets: The Next Detection Frontier

Some 4 million years old, the star HD 163296 is about 330 light years out in the direction of the constellation Sagittarius. When dealing with stars this young, astronomers have had success with data from the Atacama Large Millimeter/submillimeter Array (ALMA), teasing out features in protoplanetary disks filled with gas and dust, the breeding ground of new planets. As seen below, the ALMA imagery can be striking, a closeup look at a stellar system in formation.

Image: ALMA image of the protoplanetary disk surrounding the young star HD 163296 as seen in dust. Credit: ALMA (ESO/NAOJ/NRAO); A. Isella; B. Saxton (NRAO/AUI/NSF).

Tantalizingly, ALMA can show us rings in such disks, and the gaps that imply an emerging planet. But how do we know we’re actually looking at planets, rather than other phenomena we’re only now learning how to detect in such disks? New work from Richard Teague (University of Michigan) as well as a second effort by Christophe Pinte and team (Monash University, Australia) points us strongly toward the protoplanet interpretation. Both papers are in process at Astrophysical Journal Letters and available as preprints (see below).

In each case, the focus is not on the dust that is so visible in the image above but the distribution of carbon monoxide (CO) gas throughout the HD 163296 disk structure. ALMA is able to detect the millimeter-wavelength light that molecules of CO emit, while wavelength changes owing to the Doppler effect make it possible to discern the movement of the gas within the disk. Calling the precision involved in these studies ‘mind boggling,’ Teague coauthor Til Birnstiel (University Observatory of Munich) notes that in a system where gas rotates at about 5 kilometers per second, ALMA detected velocity changes as small as a few meters per second.

“Although dust plays an important role in planet formation and provides invaluable information, gas accounts for 99 percent of a protoplanetary disks’ mass,” says Teague coauthor Jaehan Bae of the Carnegie Institute for Science. “It is therefore crucial to study kinematics of the gas.”

Teague and team found disruption in the Keplerian rotation that gas would be expected to show, the orderly motion of objects around a central star. The emergence of localized disturbances within the gas would provide evidence for a planet in the making. And indeed, the researchers found two distinct patterns, one at roughly 80 AU, the other at 140 AU. Meanwhile, the team led by Christophe Pinte — likewise looking at anomalies in the flow of gas through detection of CO emissions rather than dust — detected a third planet-like pattern, this one at 260 AU. All three worlds, the scientists calculate, would be approximately similar in mass to Jupiter.

Image: Artist impression of protoplanets forming around a young star. Credit: NRAO/AUI/NSF; S. Dagnello.

We’re out on the edge here, just as exoplanet hunting itself was in the early 1990s as we approached the first detections. These days we can use radial velocity, transits and gravitational microlensing to spot planets, with the number of confirmed worlds rising steadily. Protoplanets are another story altogether, although the evidence for them continues to mount. The Teague paper explains the significance of the CO studies:

We have presented a new method which enables the direct measurement of the gas pressure profile. This allows for significantly tighter, and more accurate, constraints on the gas surface density profile than traditional methods. Furthermore, as this method is sensitive to the gap profile, it provides essential information about the gap width in the gas which is typically poorly constrained from brightness profiles.

And indeed, what the work gives us is a way to measure changes in the gas velocity and density that correlate to the observed perturbations in the protoplanetary disk. Pinte’s team, meanwhile, working with measurements of CO velocity in the disk, found a 15 percent deviation from expected Keplerian flow. The possible protoplanet they detect at approximately 260 AU could conceivably be detected via direct imaging. If it is, the question of its formation is interesting:

Can massive planets form at a distance of 250 au from the star? The location of giant planets in the outer regions of discs would be broadly consistent with gravitational instability. On the other hand, the timescale for core accretion may also be reasonable given that HD 163296 is a relatively old disc (≈ 5 Myr). The planet may also have undergone outward migration, depending upon the initial profile of the disc. It is beyond the scope of this Letter to speculate further.

Measuring the velocity of carbon monoxide in a protoplanetary disk is an indication of how fine-grained the ALMA data on HD 163296 are. The comparison of these observations with computer models show a fit with the patterns that would be expected for young planets. The evidence is not yet conclusive, but it’s clear that the developing science of protoplanet detection is gaining traction. Applying these methods to other well-defined disks should tell us more.

The papers are Teague et al., “A Kinematic Detection of Two Unseen Jupiter Mass Embedded Protoplanets” (preprint) and Pinte et al., “Kinematic evidence for an embedded protoplanet in a circumstellar disc” (preprint), both accepted at Astrophysical Journal Letters.



New Horizons from Within

Chasing New Horizons, by Alan Stern and David Grinspoon. Picador (2018), 320 pp.

Early on in Alan Stern and David Grinspoon’s Chasing New Horizons, a basic tension within the space community reveals itself. It’s one that would haunt the prospect of a mission to Pluto throughout its lengthy gestation, repeatedly slowing and sometimes stopping the mission in its tracks. The authors call it a ‘basic disconnect’ between how NASA makes decisions on exploration and how the public tends to see the result.

‘To boldly go where no one has gone before’ is an ideal, but it runs up against scientific reality:

…the committees that assess and rank robotic-mission priorities within NASA’s limited available funding are not chartered with seeking the coolest missions to find uncharted places. Rather, they want to know exactly what science is going to be done, what specific high-priority scientific questions are going to be answered, and the gritty details of how each possible mission can advance the field. So, even if the scientific community knows they really do want to go somewhere for the sheer joy and wonder of exploration, the challenge is to define a scientific rationale so compelling that it passes scientific muster.

Thus Alan Stern’s job as he began thinking about putting a probe past Pluto: Get the scientific community to see why Pluto/Charon was a significant priority for the advancement of science. And as this hard-driving narrative makes clear, advancing those priorities would not prove easy. But a few things helped, including the spectacular coincidence that Charon was discovered (by Jim Christy in 1978) just before it was about to begin a period of eclipses with Pluto, a period that would not recur for more than a century. When the eclipses began in 1985, Pluto was suddenly highly visible at planetary science conferences and we were learning a lot.

Chasing New Horizons follows Alan Stern’s efforts to use ensuing discoveries like the different composition of surface ices on Pluto and Charon and the observations of Pluto’s atmosphere to draw attention to mission possibilities. Beginning with a technical session at a American Geophysical Union meeting in 1989, Stern began arguing for what would become New Horizons, brainstorming with key Pluto scientists who would become known as the Pluto Underground on a mission the authors describe as “a subversive and unlikely idea, cooked up by a rebel alliance that seemed ill-equipped to take on an empire.”

It would prove to be quite a battle. A letter-writing campaign would develop, leading to an official NASA study of a possible Pluto mission, one led by Stern and fellow Plutophile Fran Bagenal, working with NASA engineer Robert Farquhar (who would die just months after the actual Pluto flyby). From here on it was a matter of keeping the mission visible, from pieces in Planetary Society publications to continuing talks at major conferences, where attendance was growing.

I won’t go into the intricacies of such entities as the Solar System Exploration Subcommittee, which would analyze the Farquhar report, or the personnel changes within NASA that affected the work — for that you’ll need to read the book, where the action becomes something of a pot-boiler given all the roadblocks that kept emerging, including mission cancellations — but as the New Horizons mission took early form, Pluto was likewise on the mind of engineers at JPL, who began concurrent work on a mission concept. NASA’s turn toward Rob Staehle’s Pluto Fast Flyby design was just one in a series of course changes for Stern and team. A Pluto Kuiper Express concept followed, then the formation of a NASA Science Definition Team.

Here’s a sample of how frustrating the on-again, off-again nature of New Horizons’ birth appeared to its proponents. Budget considerations had caught NASA’s eye and in the fall of 2000, a ‘stop-work order’ went out on all Pluto efforts:

Those of us who’d been working on it felt like we had been through a decade of hell running errands, with endless study variations from NASA Headquarters [says Stern]. How many iterations of this, how many committees had we been in front of, how many different planetary directors had we had at NASA, how many different everythings had we put up with? Big missions, little missions, micro-missions, Russian missions, German missions, nonnuclear missions, Pluto-only missions, Pluto-plus-Kuiper-Belt missions, and more…

New Horizons, as it would do repeatedly, came back to life, and we all know the result, but the first half of Chasing New Horizons is a fascinating and cautionary tale about how difficult mission design can be in a charged environment of tight money and competing proposals. Science surely had the last word again, because the need for a mission was now pressing, given that Pluto was moving further from the Sun in its orbit, its atmosphere could conceivably freeze out before a mission got there, and visibility considerations involved Pluto’s sharply tilted spin axis (122 degrees) and its effect on lighting across the globe.

As to the actual approach and flyby, you’ll find yourself back in those heady days, when the earliest images from New Horizons gradually gave way to more and more detail, and the stakes continued to rise even as the unexpected threatened to stymie the close approach. Exhaustive hazard searches helped Stern’s team scout the system, but the critical Core load — the lengthy command script that would get the spacecraft through its scientific observations — had to be uplinked. New Horizons received the Core load and then suddenly went silent.

Quick diagnosis made it likely that the spacecraft would restart using its backup computer, which did occur within a short time, but with the flyby near, timing was critical:

As more telemetry came back from the bird, they learned that all of the command files for the flyby that had been uploaded to the main computer had been erased when the spacecraft rebooted to the backup computer. This meant that the Core flyby sequence sent that morning would have to be reloaded. But worse, numerous supporting files needed to run the Core sequence, some of which had been loaded as far back as December, would also need to be sent again. Alice [Bowman] recalls, “We had never recovered from this kind of anomaly before. The question was, could we do it in time to start the flyby sequence…?”

With three days to do the job, the equivalent of weeks of work had to be done in three days. The task was completed with just three hours to spare. Exciting? Believe it. David Grinspoon’s method in Chasing New Horizons was to synthesize the thoughts of Alan Stern and others on the mission within a narrative that captures the drama of the event. Grinspoon is a fine stylist — search the archives here for my thoughts on his exceptional Earth in Human Hands (2016), and I’ve also written about his earlier book Lonely Planets (2003). Here he goes for clarity and narrative punch, presenting Stern’s insights inside an almost novelistic frame. This is a book you’ll want to read as we approach MU69.



New Horizons: The Beauty of Hibernation

I’ve always had a great interest in Iceland, stemming from my studies of Old Norse in graduate school, when we homed in on the sagas and immersed ourselves in a language that has changed surprisingly little for a thousand years. There’s much modern vocabulary, of course, but the Icelandic of 1000 AD is much closer to the modern variant than Shakespeare’s English is to our own. Syntactically and morphologically, Icelandic is a survivor, and a fascinating one.

New Horizons’ journey to Kuiper Belt Object MU69 occasions this reverie because the mission team has named the object Ultima Thule, following an online campaign seeking input from the public that produced 34,000 suggestions. The word ‘thule’ seems to derive from Greek, makes it into Latin, and appears in classical documents in association with the most distant northern areas then known. In the medieval era, Ultima Thule is occasionally mentioned in reference to Iceland, and sometimes to Greenland, and may have been applied even to the Shetlands, the Orkneys and, probably, the nearby Faroes. Northern and on the edge, that’s Ultima Thule.

The new Ultima Thule is a natural coinage, as New Horizons’ principal investigator Alan Stern (SwRI) has noted:

“MU69 is humanity’s next Ultima Thule. Our spacecraft is heading beyond the limits of the known worlds, to what will be this mission’s next achievement. Since this will be the farthest exploration of any object in space in history, I like to call our flyby target Ultima, for short, symbolizing this ultimate exploration by NASA and our team.”

Hence the beauty of space exploration. On Earth we eventually reach our Ultima Thule, whichever place we want to assign the name, whereas in space there’s always the next one. And indeed, New Horizons may get the chance to go after another Kuiper Belt Object after MU69. Future explorations will always find more distant targets in the cosmos.

Image: Artist’s impression of NASA’s New Horizons spacecraft encountering 2014 MU69, a Kuiper Belt object that orbits 1.6 billion kilometers beyond Pluto, on Jan. 1, 2019. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Steve Gribben.

Now 6 billion kilometers from Earth, New Horizons has exited hibernation as of 0212 EDT (0612 UTC) on June 5, with all systems in normal operation. We’re now in the process of uploading commands to the computers aboard the spacecraft to begin preparations for the Ultima Thule flyby, including science retrieval and subsystem and science instrument checkouts. Things are heating up — we’re not that far from August, when New Horizons will begin making observations of its target, imagery that will provide information about any needed trajectory adjustments.

But back to that hibernation, which this time around lasted 165 days. New Horizons is now fully ‘awake’ and will remain so until late 2020, when all data from the Ultima Thule encounter should have been sent back to Earth. Hibernation itself was an ingenious innovation that would maximize efficiency by reducing the cost of mission control staffing. After all, a sleeping bird requires only a skeleton crew to maintain basic communications during this period.

The sheer ingenuity of the New Horizons design comes across here. No other NASA mission has attempted hibernation, but the experience of missions like Voyager demonstrated how useful it could be. Voyager required about 450 people to run flight operations, according to David Grinspoon and Alan Stern in Chasing New Horizons. Contrast that with a New Horizons flight staff of fewer than 50 people.

The numbers are striking when you look at how the project team changed after launch as well. In the four years before New Horizons’ 2006 departure, more than 2500 people were involved in building, testing and launching the spacecraft. They included those working on the Radioisotope Thermoelectric Generator (RTG) that converts radioactive decay into electricity, the ground systems necessary to monitor the mission, and of course the rocket that would launch it.

Within a month after launch, all that had changed. “The big city that was New Horizons was reduced to a small town,” write Grinspoon and Stern. As the book memorably states:

During the long years of flight to Pluto, only a skeleton crew of flight controllers and planners, a handful of engineering ‘systems leads,’ the two dozen members of the science team, their instrument engineering staffs, and a small management gaggle was needed. Alan [Stern] recalls, “Just weeks after launch nearly everyone went their own way, and the project was reduced to a little crowd of about fifty belly buttons. All of a sudden I looked around and it hit me: there are just a few of us — a tiny team — and we’re the entire crew that’s going to fly this thing for a decade and 3 billion miles and plan the flyby of a new planet.”

Image: Flight controllers Graeme Keleher and Anisha Hosadurga, of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, monitor New Horizons shortly after confirming the NASA spacecraft had exited hibernation on June 5, 2018. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Mike Buckley.

When New Horizons reached Pluto, 9.5 years had passed since launch, but because of hibernation, most of the craft’s primary systems only had 3.5 years of operational time clocked against them, which means the spacecraft was, for all intents and purposes, years younger than it would otherwise have been. Early hibernation periods tested out the concept not long after launch, easing into a process that soon increased hibernation periods to months at a time.

As New Horizons left its last hibernation period before the Pluto/Charon flyby, Alan Stern chose a ‘wake-up song’ for the occasion, a tradition dating back to Gemini 6 when flight controllers played ‘Hello Dolly’ to wake up astronauts Wally Schirra and Thomas Stafford. Stern chose ‘Faith of the Heart,’ a theme from the TV series Star Trek: Enterprise, with its lyric “It’s been a long road, getting from there to here.” Little did the team know at the time that the ‘heart’ of the title would be echoed by a famous feature on the surface of Pluto itself.

If you haven’t read Chasing New Horizons (Picador, 2018), I can’t recommend it strongly enough. This is the best inside account of a space mission I’ve yet read. Tomorrow I want to dig a little deeper into the book and talk about the New Horizons mission in context as we now begin the exciting process of preparing the craft for yet another encounter.



Lightning in the Jovian Clouds

The longer we can keep a mission going in an exotic place, the better. Sometimes longevity is its own reward, as Curiosity has just reminded us on Mars. After all, it was only because the rover has been in place for six years that it was able to observe what scientists now think are seasonal variations in the methane in Mars’ atmosphere. Thus the news that Juno will remain active in Jupiter space is heartening, and in this case necessary. The mission is now to operate until July of 2021, an additional 41 months in orbit having been approved. More time on station allows Juno to complete a primary science mission that had appeared in jeopardy.

The reason: Problems with helium valves in the spacecraft’s fuel system resulted in the decision to remain in the present 53-day orbit instead of the 14-day ‘science orbit’ originally planned, and that has extended the time needed for data collection. Thus the lengthening of operations there not only allows further time for discovery but essentially enables the spacecraft to achieve its original science objectives. NASA has now funded Juno through FY 2022, allowing for the end of prime operations in 2021 and data collection and mission close-out carrying into 2022.

“This is great news for planetary exploration as well as for the Juno team,” said Scott Bolton, principal investigator of Juno, from the Southwest Research Institute in San Antonio. “These updated plans for Juno will allow it to complete its primary science goals. As a bonus, the larger orbits allow us to further explore the far reaches of the Jovian magnetosphere — the region of space dominated by Jupiter’s magnetic field — including the far magnetotail, the southern magnetosphere, and the magnetospheric boundary region called the magnetopause. We have also found Jupiter’s radiation environment in this orbit to be less extreme than expected, which has been beneficial to not only our spacecraft, but our instruments and the continued quality of science data collected.”

In its present 53-day polar orbit, Juno moves as close as 5,000 kilometers from the Jovian cloud tops and backs out as far as 8 million kilometers. It’s an orbit that minimizes exposure to Jupiter’s radiation belts even as it allows the craft to study the planet’s entire surface over the course of its time there. The latest work on data collected during these orbits comes in two papers, one in Nature, the other in Nature Astronomy, that look at Jovian lightning and how it is produced.

The first analysis draws on data from Juno’s Microwave Radiometer Instrument (MWR), which can record emissions at a wide range of frequencies. Because lightning discharges emit radio waves, Juno can keep an eye on lightning activity on the gas giant. Jovian lightning has also been detected by optical cameras aboard spacecraft as localized flashes of light. Shannon Brown (JPL), lead author of the paper on this work, points out that until Juno, the radio signals spacecraft have detected all came from the Galileo probe, Cassini and the two Voyager flybys, but these were all found in the kilohertz range of the radio spectrum, despite attempts to find signals in the megahertz range. The reason for the discrepancy has been a mystery.

After all, terrestrial lightning emits a broad signal over the radio spectrum up to gigahertz frequencies. Juno is helping to resolve the discrepancy, detecting Jovian lightning ‘sferics’ (broadband electromagnetic impulses) at 600 MHz. That implies that the planet’s lightning discharges are not fundamentally distinct from the lightning we experience on Earth. During Juno’s first eight orbits of Jupiter, the spacecraft detected 377 sferics, finding them prevalent in the polar regions and absent near the equator, with the most frequent occurring in the northern hemisphere at latitudes higher than 40 degrees north.

”We think the reason we are the only ones who can see it is because Juno is flying closer to the lighting than ever before,” says Brown, ”and we are searching at a radio frequency that passes easily through Jupiter’s ionosphere.”

But what would account for the fact that Earth’s lightning activity is highest near the equator, while Jupiter’s is most frequent in the polar regions? Brown and company suggest that Jupiter’s poles allow more warm air to rise from within because there is less upper-level warmth from sunlight. Possibly the heating from sunlight at Jupiter’s equator can stabilize the upper atmosphere to inhibit warm air rising from below as it does at the poles. If this is the case, we would expect the polar regions to experience the convective forces that lead to lightning.

From the paper’s abstract:

Because the distribution of lightning is a proxy for moist convective activity, which is thought to be an important source of outward energy transport from the interior of the planet, increased convection towards the poles could indicate an outward internal heat flux that is preferentially weighted towards the poles. The distribution of moist convection is important for understanding the composition, general circulation and energy transport on Jupiter.

Image: This artist’s concept of lightning distribution in Jupiter’s northern hemisphere incorporates a JunoCam image with artistic embellishments. Data from NASA’s Juno mission indicates that most of the lightning activity on Jupiter is near its poles. Credit: NASA/JPL-Caltech/SwRI/JunoCam.

What scientists now have to explain, as this JPL news release points out, is why the north pole is so much more active than the south. Our understanding of energy flow and circulation on Jupiter is clearly a work in progress, something the Juno data trove may help us untangle. Meanwhile, Ivana Kolmašová (Czech Academy of Sciences, Prague) and colleagues have offered what NASA is calling ‘the largest database of lightning-generated low-frequency radio emissions around Jupiter (whistlers) to date.’ The dataset includes more than 1600 signals collected by Juno’s Waves instrument, 10 times the number recorded by Voyager 1.

We’re not only further along in detection technology than in the Voyager days, with advances in microwave and plasma wave instruments to sense lightning amidst Jupiter’s emissions, but we’re also dealing with a spacecraft that has come closer to Jupiter than any other craft in history, allowing a vast increase in signal strength. The knowledge that Juno will now be able to proceed through its entire primary data collection mission is thus a cause for celebration.

The papers are Brown et al., “Prevalent lightning sferics at 600 megahertz near Jupiter’s poles,” Nature 558 (2018), 87-90 (abstract); and Kolmašová et al., “Discovery of rapid whistlers close to Jupiter implying lightning rates similar to those on Earth,” Nature Astronomy 6 June 2018 (abstract).



Scouting Alpha Centauri at X-ray Wavelengths

One of the benefits of having Alpha Centauri as our closest stellar neighbor is that this system comprises three different kinds of star. We have the familiar Centauri A, a G-class star much like our Sun, along with the smaller Centauri B, a K-class star with about 90 percent of the Sun’s mass. Proxima Centauri gives us an M-dwarf, along with the (so far) only known planet in the system, Proxima b. Questions of habitability here are numerous. Along with possible tidal locking, another major issue is radiation, since M-dwarfs are known for their flare activity.

As we learn more about the entire Alpha Centauri system, though, we’re learning that the two primary stars are much more clement. They may have issues of their own — in particular, although stable orbits can be found around both Centauri A and B, we still don’t know whether planets are likely to have formed there — but scientists studying data from the Chandra X-ray Observatory have found that levels of X-ray radiation are far lower here than around Proxima Centauri.

This is good news, because high radiation levels could prove fatal for surface life, with the additional effect of possible damage to planetary atmospheres. Chandra has been involved in a multi-year campaign targeting Centauri A and B stretching back to 2005, with observations every six months. No other X-ray observatory is capable of resolving the two primary stars during their current close orbital approach. What we wind up with is a look at radiation activity over time, covering a period analogous to our own Sun’s 11-year sunspot cycle.

Image: A new study involving long-term monitoring of Alpha Centauri by NASA’s Chandra X-ray Observatory indicates that any planets orbiting the two brightest stars are likely not being pummeled by large amounts of X-ray radiation from their host stars. This is important for the viability of life in the nearest star system outside the Solar System. Chandra data from May 2nd, 2017 are seen in the pull-out, which is shown in context of a visible-light image taken from the ground of the Alpha Centauri system and its surroundings. Credit: X-ray: NASA/CXC/University of Colorado/T.Ayres; Optical: Zdeněk Bardon/ESO.

Tom Ayres (University of Colorado Boulder) presented these results at the just concluded meeting of the American Astronomical Society in Denver. Any planets in the habitable zone of Centauri A would actually receive a lower dose of X-rays, on average, than planets around the Sun, while the X-ray dosage for a planetary companion of Centauri B is about 5 times higher than the Sun. This contrasts sharply with Proxima Centauri’s planet, which would receive an average dosage 500 times larger than the Earth, rising to 50,000 times higher during a major flare. If we find planets around either A or B, it may be that Breakthrough Starshot will want to prioritize these at the expense of the more endangered Proxima b.

In the animation below, we can see the proper motion of Centauri A and B.


Image: This movie shows Chandra observations of Alpha Centauri A and B taken about every 6 months between 2005 and 2018. Alpha Cen A is the star to the upper left. The motion of the pair from left to right is their “proper motion”, showing the movement of the pair in our galaxy with respect to the solar system. The change in relative positions of the pair shows the motion in their 80 year long orbit and the wobbles show the small apparent motion (called parallax) caused by the year long orbit of the Earth around the Sun. The Chandra images are shown in black and white. To place these semi-annual images in context, the two colored circles show the expected motion of Alpha Cen A (yellow) and Alpha Cen B (orange) when taking account of proper motion, orbital motion and parallax. The size of the circles is proportional to the X-ray brightness of the source. Credit: Thomas Ayres.

Ayres has also written up some of the results in Research Notes of the American Astronomical Society, where I learned that the central AB pair has actually been under X-ray study for almost four decades, dating back to the late 1970s and the HEAO-2 satellite (also known as the Einstein Observatory), which was the first fully imaging X-ray telescope ever put into space. Subsequent observations were conducted by ROSAT (Röntgen-Satellit), XMM-Newton and now Chandra. Here, Ayres explains why X-ray studies may help us learn about habitability in this system as well as giving us information closer to home:

The modest coronae (106 K) of α Cen AB are on par with our own Sun’s. X-ray studies of these objects can help us understand how the “Dynamo” in the stellar interior produces the episodic surface magnetic eruptions at the core of solar activity and “Space Weather.” The hard radiation and particle bombardment from flares and coronal mass ejections can affect Planet Earth, so the interest is not solely academic. Exoplanets of other sunlike stars can be exposed to analogous extreme high-energy transients from their hosts, with perhaps serious repercussions for habitability.

Image: Figure 1 from the Ayres note. Caption: X-ray light curves of a Cen AB and the Sun 1995–2018. Credit: T. R. Ayres.

I was fortunate enough to be in the audience when Ayres spoke to Breakthrough Discuss in 2016 in a presentation called “The Ups and Downs of Alpha Centauri.” Here’s Breakthrough’s video of that talk, which I highly recommend.

The research note is Ayres, “Alpha Centauri Beyond the Crossroads,” Research Notes of the AAS Vol. 2, No. 1 (22 January 2018). Full text.