Centauri Dreams
Imagining and Planning Interstellar Exploration
Shutting Down Chandra: Will We Lose Our Best Window into the X-ray Universe?
Our recent discussions of X-ray beaming to propel interstellar lightsails seem a good segue into Don Wilkins’ thoughts on the Chandra mission. Chandra, of course, is not a deep space probe but an observatory, and a revolutionary one at that, with the capability of working at the X-ray wavelengths that allow us to explore supernovae remnants, pulsars and black holes, as well as making observations that advance our investigation of dark matter and dark energy. This great instrument swims into focus today because it faces a funding challenge that may result in its shutdown. It’s a good time, then, to take a look at what Chandra has given us since launch, and to consider its significance as efforts to save the mission continue. We should get behind this effort. Let’s save Chandra.
by Don Wilkins
On July 23, 1999, the Chandra X-ray Observatory deployed from Space Shuttle Columbia. Chandra along with the Hubble Space Telescope, Spitzer Space Telescope (decommissioned when its liquid helium ran out) and Compton Gamma Ray Observatory (de-orbited after gyroscope failure), composed NASA’s fleet of ‘Great Observatories,’ so labeled because of their coverage from gamma rays through X-rays, visible light and into the infrared, spanning the electromagnetic spectrum. This scientific fleet expanded – and continues to expand – our understanding of the cosmos.
A two-stage Inertial Upper Stage booster put the observatory into an elliptical orbit. Apogee is approximately 139,000 kilometers (86,500 miles) — more than a third of the distance to the Moon – with perigee at 16,000 kilometers (9,942 miles) and an orbital period of 64 hours and 18 minutes. The numbers are significant: The consequence of this orbit is that about 85 percent of the time, Chandra orbits above the charged particle belts surrounding the Earth, allowing periods of uninterrupted observing time up to 55 hours.
Figure 1. The Chandra Observatory monitors X-rays, a critical and unique window into the hottest and most energetic places in the Universe. Credit: NASA.
A Huge Return on Investment
Chandra’s achievements include:
- The first image of the compact object, possibly a neutron star or black hole, at the center of the supernova remnant Cassiopeia A.
- The detection of jets from and a ring around the Crab Nebula.
- The detection of X-ray emitting loops, rings and filaments encircling Messier 87’s supermassive black hole.
- The discovery, by high school students, of a neutron star in the supernova remnant IC 443.
- The detection of X-ray emissions from Sagittarius A*, the black hole at the center of the Milky Way. Chandra observed a Sagittarius A* X-ray flare 400 times brighter than normal, and found a large halo of hot gas surrounding the Milky Way.
- The X-ray observation of a supernova, SN 1987A, shock wave.
- The observation of streams of high-energy particles light years in length flowing from neutron stars.
- Observations of planets within the Solar System found X-rays associated with Uranus that may not be the reflection of solar X-rays. Chandra also found that Jupiter’s X-ray emissions occur at its poles, and it detected X-ray emissions from Pluto.
- Found a dip in X-ray intensity as the planet HD 189733b transited its parent star. This is the first time a transit of an exoplanet was observed using X-rays.
- Detected the shadow of a small galaxy as it is cannibalized by a larger one.
- Found a mid-mass black hole, between stellar-sized black holes and galactic core black holes, in M82.
- Associated X-rays with the gamma ray burst GRB 91216 (Beethoven Burst). Chandra observations also associated X-rays with a gravitational source (Figure 2).
- Produced controversial data suggesting RX J1856.5-3754 and 3C58 are quark stars.
- Measured the Hubble constant as 76.9 km/s per megaparsec (a megaparsec is equal to 3.26 million light years).
- Found strong evidence dark matter exists by observing super cluster collisions. Observations placed limits on dark matter self-interaction cross-sections.
- Combined with the outputs of other telescopes yielded insights into astronomical phenomena, Figure 3.
Building Chandra was a significant technical challenge. Mirror alignment accuracy, from one end of the mirror assembly to the other, a distance of 2.7 meters or 9 feet, is accurate to 1.3 micrometers (50 millionths of an inch). X-rays are focused using complex geometrical shapes which must be precisely formed and kept ultraclean. Even today, no X-ray telescope provides the sharpness of image that Chandra does.
Figure 2 Chandra made the first X-ray detection of a gravitational wave source. The inset shows both the Chandra non-detection, or upper limit, of X-rays from GW170817 and its subsequent detection. The main panel is the Hubble image of NGC 4993. Credit: NASA.
Figure 3. The image is a combination of the outputs of three telescopes. Webb highlights infrared emission from dust; Chandra the X-ray emissions and Hubble shows stars in the field. Credit: NASA/CXC/SAO
Chandra’s Demise?
This list of discoveries may be ending. Although it is estimated Chandra has another decade of operation, proposed budget cuts eliminate the $74 million necessary for Chandra operation, maintenance and analysis of data. Unless the funding is restored by way of a budget line item directing NASA to spend the funds on Chandra, the agency’s X-ray observation is stopped for years until a new telescope can be brought on line. The skills of highly talented scientists and engineers working on Chandra will be dissipated.
The problem is significant. NASA’s own NuStar instrument (Nuclear Spectroscopic Telescope Array) and the joint NASA/JAXA XRISM effort (X-Ray Imaging and Spectroscopy Mission) have their virtues, as does the European Space Agency’s XMM-Newton (X-ray Multi-Mirror Mission). But Chandra’s eyes are sharper, with imagery 10 times crisper than XMM-Newton’s, 30 times sharper than NuStar, and fully 150 times sharper than XRISM.
A possible successor is in the planning stages. With 100 times Chandra’s resolution, the Lynx X-ray Observatory could study individual stars more than 16,000 light-years away, or 12.5 times the range of Chandra, using an X-ray Mirror Assembly that is the most powerful X-ray optic ever conceived. Even so, it is probable three decades or more would elapse before the new observatory would orbit and it might well involve significant cost overruns. ESA plans the Athena mission (Advanced Telescope for High ENergy Astrophysics), a large-aperture grazing-incidence X-ray telescope, but Athena would not come online until the late 2030s. In the meantime, we have a healthy Chandra.
Although we are dealing with a 25 year old spacecraft, its efficiency is both high and stable, and enough fuel remains for another decade of operations, while the cost of operations has remained stable for decades. Considered as a bridge to Lynx, Chandra is at the mercy of Congress largely through the FY25 appropriations process. The particulars of how to support this great observatory are available on the Save Chandra website, which is the voice of a grassroots effort within the astronomical community.
Saving Chandra would avert what some are describing as “an extinction-level-event for X-ray astronomy in the US.” An open community letter with 87 pages of signatures recently submitted to NASA Science Leadership strongly supports this statement.
Why X-Rays Can’t Push Interstellar Sails
Although solar sails were making their way into the aerospace journals in the late 1950s, Robert Forward was the first scientist to consider using laser beams rather than sunlight to drive a space sail. That concept, which György Marx picked up on in his 1966 paper, opened the door to interstellar mission concepts. Late in life in an unpublished memoir, Forward recalled reading about Theodore Maiman’s work on lasers at Hughes Research Laboratories, and realizing that this was a way to create a starship. His 1962 article (citation below) laid out the idea for the journal Missiles and Rockets and was later reprinted in Science Digest. Marx surely knew the Forward article and his subsequent paper in Nature probed how to achieve this goal.
Image: One of the great figures of interstellar studies, Robert Forward among many other things introduced and explored the principles of beamed propulsion. Credit: UAH Library Robert L. Forward Collection.
Marx was at that time a professor of theoretical physics at Roland Eötvös University in Budapest. He was plugged into the difficulties of interstellar flight through Les Shepherd’s work in Britain, and he cites the latter’s Realities of Space Travel (1957) in the paper as one of many sources highlighting the depth of the problem. His paper “Interstellar Vehicle Propelled by Terrestrial Laser Beam” is a mere two pages built largely around equations, reporting on the “commonly accepted view that, apart from the technical difficulties involved, the laws of conservation of energy and momentum forbid the visiting of other planetary systems in the human lifespan.”
Marx had already explored energy issues for interstellar flight in a 1960 paper for Astronautica Acta (as it was then known) and a second for the same journal in 1963 (citations below). But the idea of laser beaming offered the physicist a glimmer of hope. In the 1966 paper, he cites the advantages of beamed sailing vs conventional rocket propulsion. The paper argues that “a vehicle can be accelerated almost to the speed of light if an emitter on the Earth can accurately project light onto its mirror.”
The ideal focusing mechanism would be the laser, and it is here that he runs into trouble. For Marx worried about the size of the transmitter aperture, which determines the size and initial beam diameter that will emerge. Remember we’re in the Robert Forward era of maxed out engineering, when the idea was simply to establish what was possible even if it required building capabilities far beyond those of the present day. So here’s what Marx comes up with, the best concept he thought feasible:
…the technical conditions are extremely challenging. A range of operation of 0.1 light year would require a coherently radiating surface of the order of 1 km2 which emits hard X-rays, and the vehicle would need an X-ray mirror with an effective cross-sectional area of several km2.
Marx talks about the absorbed energy of his sail ‘mirror’ radiating out isotropically into space, and here we run into serious problems. I took my questions to Jim Benford, CEO of Microwave Sciences and author of High Power Microwaves, a standard text which is about to go into its fourth edition at CRC Press. Jim is also a regular contributor to Centauri Dreams. And he was quick to point out that X-rays reflect from conducting surfaces in ways that defeat Marx’s purpose.
Image: György Marx in his office. Credit: REAL-I, the Image File Collection of the Academic Library.
As Jim told me, incoming X-rays reflect only at very low grazing angles. The efficiency of energy transfer is at stake here. Here’s a bit more of what he said, using one of the most important formulae in all of modern physics:
X-ray photons have far more energy than visible light or microwaves. Remember the relation E=hν, where E is the energy of the photon, h is Planck’s constant, ν the frequency. X-ray photons have energies about a thousand times that of visible light, a million times that of microwaves. If they come in normal to the surface [i.e., striking the sail head on], they ionize atoms, damaging the lattice of the material.
X-ray telescopes, as a matter of fact, work through a series of grazing incidence reflectors. In other words, we can’t direct Marx’s fantastic X-ray beam toward our sail without seriously damaging it, not unless we are willing to bring the beam to it at such a low angle that the intrinsic power of the beam is largely lost. Benford again:
There’s no way to accelerate a sail with X-rays. The cross-section of the sail must be at a slight angle to the beam, not perpendicular to it, for the X-rays to reflect. That’s hugely inefficient. Grazing incidence means that only the slight transverse component of the photon velocity vector is reversed, leaving the far larger axial component almost unchanged. Little energy is transferred to the inclined sail, and that drives it sideways to the beam, not antiparallel to it, as reflected photons do when they incident normally. So the sail is accelerated very little in the direction of the X-ray beam.
This is the coup de grâce for the X-ray sail. It’s interesting to see what Robert Forward thought of Marx’s idea. Here he is, writing in a 1984 paper called “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails,” which is one of the classics of the field:
The concept of laser-pushed interstellar lightsails was reinvented by Marx in 1966. Since Marx was unwilling to consider a laser aperture greater than 1 km2, he was forced to assume the use of hard x-rays in order to obtain the operational ranges needed for interstellar flight. The impossibility of constructing both an x-ray laser and a lightweight sail to reflect those x-rays led to Marx’s highly pessimistic conclusion about the feasibility of the concept. If Marx had been willing to consider a larger transmitter aperture, then his laser frequencies and sail requirements would have been much easier.
J. L. Redding, then at Bishop’s University in Quebec, saw Marx’s paper and responded to it in the same year, offering corrections to Marx’s equations without challenging the X-ray concept. His telling remark that “…one does not need to consider the difficulties of arranging suitable deceleration and landing facilities” refers to what he saw as the overwhelming problems in making a beamed energy propulsion system work at all. Marx had commented on the deceleration problem and Forward would go on to offer a potential solution in his 1984 paper, one so baroque that it deserves a future post of its own.
I should also mention a little referenced paper by W. E. Moeckel, “Propulsion by Impinging Laser Beams,” which ran in the Journal of Spacecraft and Rockets in 1972. Moeckel (working at what was then NASA’s Lewis Research Center in Cleveland) analyzed laser beaming to 100 ton relativistic flyby probes, each of which would require 1012 watts of X-ray energy. Making specific reference to Marx, Moeckel found X-ray beaming promising but did not know if it was feasible. His conclusion would have warmed the hearts of science fiction writers of the time:
…some future generations of mankind, with a somewhat different ordering of priorities than ours and much more available power, could conceivably explore other stars and other solar systems with highly sophisticated unmanned spacecraft capable of relaying information in elapsed times of the order of decades.
If only it worked! Fortunately, we’re not restricted to X-rays when it comes to beamed propulsion.
References
The early Forward paper is “Pluto-Gateway to the Stars,” Missiles and Rockets 10, 26 ff. (2 April 1962); reprinted in Science Digest 52, 70-75 (August 1962). Forward’s “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails” appeared in the Journal of Spacecraft and Rockets 21 (1984), pp. 187-195 (abstract).
György Marx’s paper on X-ray beaming is “Interstellar Vehicle Propelled by Terrestrial Laser Beam,” which ran in Nature on July 2, 1966 (abstract). His two other interstellar papers are “The mechanical efficiency of interstellar vehicles,” Astronautica Acta 9 (1963) 131–139, and “Über Energieprobleme der Interstellaren Raumfahrt,” Astronautica Acta 6 (1960) 366–372.
The Redding paper in response to Marx has the same title, “Interstellar Vehicle Propelled by Terrestrial Laser Beam,” Nature February 11, 1967 (abstract). W. E. Moeckel’s paper “Propulsion by Impinging Laser Beams” appeared in the Journal of Spacecraft and Rockets Vol. 9 (1972), 942-944 (abstract).
My thanks to Jim Benford, Greg Matloff and Al Jackson for invaluable references and commentary.
Going Interstellar via Budapest
Studying the rich history of interstellar concepts, I realized that I knew almost nothing about a figure who is always cited in the early days of beamed sail papers. Whereas Robert Forward is considered the source of so many sail concepts, the earliest follow-up to his 1962 paper on beamed sails for interstellar purposes is by one G. Marx. The paper is “Interstellar Vehicle Propelled by Terrestrial Laser Beam,” which ran in Nature on July 2, 1966. Who is this G. Marx?
My ever reliable sources quickly came through when I asked if any of them had known the man. None had, though all were familiar with the paper, but Al Jackson sent me a copy of it along with another by J. L. Redding (Bishop’s University, Canada), who published a correction to the Nature paper on February 11, 1967. It didn’t reduce my confusion that Redding’s short contribution bears the exact same title as Marx’s. My other contacts on Marx had no personal experience with him either but were curious to learn more.
Image: Astrophysicist and science historian György Marx. Credit: FOTO:Fortepan — ID 56238:Adományozó/Donor: Rádió és Televízió Újság. – This file has been extracted from another file, CC BY-SA 3.0.
This is worth digging into because it illustrates some useful facts about beamed propulsion. But first: György Marx (1927-2002) was Hungarian, an astrophysicist and historian of science who is evidently best known for his work on leptons, that class of subatomic particles that are not affected by the strong force and can carry electrical charge or be neutral (the charged leptons are the electrons, muons, and taus). As a science historian, Marx was clearly possessed of a sense of humor, authoring a study of Hungarian scientists in 2000 called The Voice of the Martians.
That title is itself worth unpacking. The reference is to that great influx of supremely gifted scientists and thinkers – among them Peter K. Goldmark, Nicholas Kaldor, Arthur Koestler, Nicholas Kurti, John von Neumann, Egon Orowan, Michael Polanyi, Leo Szilárd, Edward Teller, and Eugene P. Wigner – who were born between 1890 and 1910 and greatly influenced the growth of technology through their work in the U.S. Their gifts struck some as all but other-worldly. The Voice of the Martians has just been re-published in a new edition by Pallas Athéné Books. There’s a helpful review in The Budapest Times, available on Mary Murphy’s fine Unpacking My Bottom Drawer blog.
My friend Tibor Pacher and I joked about the extraterrestrial origin of Hungarians when we met up at a conference in Italy. Tibor is himself Hungarian, and he pointed to the old joke about the influx of Hungarian scientists being of off-planet origin. A Princeton professor upon learning that John Kemeny was Hungarian is said to have exclaimed about the mathematical prodigy “Not another one!” Leo Szilárd, asked about aliens, once quipped “They walk among us, but we call them Hungarians.” The fact that the Hungarian language is non-Indo European makes the joke even better.
Teller, by the way, when told of Szilárd’s statement, took on a worried mien and said, “Von Kármán must have been talking.” And Marx would push the notion even further, noting that Hungarians must have an extraterrestrial origin because the names of so many of them, like Szilárd, von Neumann, and Theodore von Kármán, show up on no maps of Budapest, but there is a Von Kármán crater on Mars, and another on the Moon, along with craters named for von Neumann and Szilárd. Case closed.
And then there is nuclear physicist Friedrich “Fritz” Houtermans, himself born near Danzig but knowledgeable about the Hungarian element, who opined:
“The galaxy of scientific minds, that worked on the liberation of nuclear power, were really visitors from Mars. They found it difficult to speak English without an alien accent, which would give them away, and therefore they chose to pretend to be Hungarian, whose inability to speak any language but Hungarian without a foreign accent is well known. It would be hard to check the above statement, because Hungary is so far away.”
What a fascinating bunch. Marx saw this bulge in the demographic of prodigies and scientific wizards known in Hungarian as a marslakók, the term for ‘Martians,’ as a cultural phenomenon. He interviews and writes about them with insight and affection. Here he is in The Voice of the Martians in conversation with Teller as the latter reminisces about Leo Szilárd. Marx as historian was clearly able to extract great anecdotes from very deep thinkers:
“[…] fortunately, there was a Hungarian in America, Leo Szilárd, who was a versatile person. He was even capable of explaining the concept of a nuclear chain reaction to the Americans! Yet there was one thing that even Szilárd could not do: drive a car. In the summer of 1939, I was working at Columbia University in New York, just like Szilárd. One day, he came up to me and said, ‘Mr. Teller, I am asking you to drive out with me to Einstein.’ […] So, out we drove on August 2. The only problem remaining was that Szilárd again did not know where Einstein was staying for the holidays. We started asking around but nobody knew. We asked an eight year–old girl—she had a nice ponytail—where Einstein was living. She did not know either. Finally, Szilárd said, ‘You know he is that old man with long, flowing white hair.’ Then the girl gave us the direction, ‘He’s staying in the second house!’ We entered; Einstein was cordial, offered tea to Szilárd, and—being democratic—he invited in the chauffeur as well. Szilárd pulled a letter from his pocket addressed to President Roosevelt […]”
Leo Szilárd’s own sense of humor was a kind of magic dust that affected other scientists working on nuclear topics. He once announced his intention to write down everything he remembered about working on nuclear fission, “not for anyone to read, just for God.” To which Hans Bethe replied, “Don’t you think God knows the facts?” And Szilárd replied, “Maybe he does, but he does not know my version of the facts.”
All this is fine stuff, and one reason why I wander off down sideroads when I start asking questions about scientists. But it’s time to get back to interstellar concepts, because György Marx had ideas about reaching other stars that helped to focus the attention of other scientists on what Forward had been saying for some time, that interstellar flight was possible at the extreme end of engineering, and that it behooved scientists to be studying the best ways to accomplish it. But Marx’s specific choices for beamed propulsion would turn out to be ill-advised, as we’ll see next time.
A Shifting, Seething Solar Wind
In search of ever-higher velocities leaving the Solar System, we need to keep in mind the options offered by the solar wind. This stream of charged plasma particles flowing outward from the Sun carves out the protective bubble of the heliosphere, and in doing so can generate ‘winds’ of more than 500 kilometers per second. Not bad if we’re thinking in terms of harnessing the effect, perhaps by a magnetic sail that can create the field needed to interact with the wind, or an electric sail whose myriad tethers, held taut by rotation, create an electric field that repels protons and produces thrust.
But like the winds that drove the great age of sail on Earth, the solar version is treacherous, as likely to becalm the ship as to cause its sails to billow. It’s a gusty, turbulent medium, one where those velocities of 500 kilometers and more per second can as likely fall well below that figure. Exactly how it produces squalls in the form of coronal mass ejections or calmer flows is a topic under active study, which is where missions like Solar Orbiter come into play. Studying the solar surface to pin down the origin of the wind and the mechanism that drives it is at the heart of the mission.
Launched in 2020, Solar Orbiter carries a panoply of instruments, ten in all, for the analysis, including an Electron Analyzer System (EAS), a Proton-Alpha Sensor (PAS) for measuring the speed of the wind, and a Heavy Ion Sensor (HIS) designed to measure the heavy ion flow. Critical to the analysis of this paper is the Spectral Imaging of the Coronal Environment (SPICE) instrument, as we’ll see below. Steph Yardley (Northumbria University) is lead author of the paper on this work, which has just appeared on Nature Astronomy:
“The variability of solar wind streams measured in situ at a spacecraft close to the Sun provide us with a lot of information on their sources, and although past studies have traced the origins of the solar wind, this was done much closer to Earth, by which time this variability is lost. Because Solar Orbiter travels so close to the Sun, we can capture the complex nature of the solar wind to get a much clearer picture of its origins and how this complexity is driven by the changes in different source regions.”
What the work is analyzing is a theory that the process of magnetic field lines breaking and reconnecting is critical to producing the slower solar wind. Different areas of the Sun’s corona are implicated in the origin of both the fast and slow winds, with the ‘open corona’ being those regions where magnetic field lines extend from the Sun into space, tethered to it at one end only and creating the pathway for solar material to flow out in the form of the fast solar wind. Closed coronal regions, on the other hand, are those where the magnetic field lines connect to the surface at both ends, forming loops.
As you would imagine, the process is wildly turbulent and marked by the frequent breakage of these closed magnetic loops and their subsequent reconnection. The researchers have probed the theory that the slow solar wind originates in the closed corona during these periods of breakage and reconnection by studying the composition of solar wind streams, for the heavy ions emitted vary depending on their origins in either the closed or open corona. Solar Orbiter’s Heavy Ion Sensor (HIS) is able to take the needed measurements to relate the effects of this activity on the surrounding plasma.
The image below is from the Solar Dynamics Observatory spacecraft rather than Solar Orbiter, reminding us of the different views we are gaining by our various missions to our star. The comparison of key datasets tells the story.
Image: This is part of Figure 1 from the paper. The caption reads: SDO/AIA [Solar Dynamics Observatory data using its Atmospheric Imaging Assembly] 193 Å image showing the source region from the perspective of an Earth observer. Open magnetic field lines that are constructed from the coronal potential field model are overplotted, coloured by their associated expansion factor F. The large equatorial CH [Coronal Hole] and AR [Active Region] complex are labeled in white. The FOVs [fields of view] of SO EUI/HRI and PHI/HRT [references to instruments aboard Solar Orbiter] are shown in cyan and pink, respectively. The back-projected trajectory of SO [Solar Orbiter] from 1 March 2022 until 9 March 2022 is shown by the olive dotted line (from right to left).
So because we have Solar Orbiter, we can now combine observations of the Sun from various sources including other space missions, like the Solar Dynamics Observatory, with the measurements of the solar wind actually flowing past the spacecraft. Susan Lepri (University of Michigan) is deputy principal investigator on the HIS system:
“Each region of the Sun can have a unique combination of heavy ions, which determines the chemical composition of a stream of solar wind. Because the chemical composition of the solar wind remains constant as it travels out into the solar system, we can use these ions as a fingerprint to determine the origin of a specific stream of the solar wind in the lower part of the Sun’s atmosphere.”
The results have been productive. The analysis gives us a precise breakdown of just what Solar Orbiter has encountered during the period studied. This is a thorny quotation but it includes a key finding. From the paper:
Combining the SO [Solar Orbiter] trajectory, coronal field model, magnetic connectivity tool, the SPICE composition analysis of the AR [Active Region] complex, and the in situ plasma and magnetic field parameters, we suggest that SO was immersed in three fast wind streams… originating from the three linked sections of the large equatorial CH [Coronal Hole]… These were followed by two slower streams associated with the negative polarities of the AR complex… The decrease of the solar wind speed can be explained by the expansion of the open magnetic field associated with the CH-AR complex, as the connectivity of SO transitioned across these regions. Credit: Yardley et al.
The findings described here are significant. We learn from this work just how complex the solar wind flow is, in this case involving three fast streams and two slower ones, all involving changes in magnetic field connectivity. Matching the composition of the solar wind streams to different areas on the corona gives us new insights into the turbulent mix found where the open and closed corona meet. The slow solar wind’s ‘breakout’ from closed magnetic field lines is demonstrated. The phenomenon of magnetic reconnection proves critical to the wind’s variability.
Demonstrating these linkages means that we can now use the findings to probe further into the origins of the solar wind. But this is a variability that is in no way predictable, making the prospect of riding the solar wind via electric or magnetic sail a daunting one. We’ll continue to learn more, though, as we bring in data from missions like the Parker Solar Probe. It will be fascinating to see one day how we use the solar wind to test out possible spacecraft designs in search of a faster route to the outer Solar System.
Addendum: In an earlier draft, I mistakenly criticized the authors for not initially clarifying some of the acronyms in this paper. I’ve removed that comment because a later reading showed I was mistaken about the two examples I cited.
The paper is Yardley et al., “Multi-source connectivity as the driver of solar wind variability in the heliosphere,” Nature Astronomy 28 May 2024 (full text).
And Then There Were Four (or Maybe Not)
I’m delighted to see the high level of interest in Dysonian SETI shown not only by reader comments here but in the scientific community at large. I wouldn’t normally return to the topic this quickly but for the need to add a quick addendum to our discussions of Project Hephaistos, the effort (based at Uppsala University, Sweden) to do a deep dive into data from different observatories looking for evidence of Dyson spheres in the form of quirks in the infrared data suggesting strong waste heat.
Swiftly after the latest Hephaistos paper comes a significant re-examination of the seven Dyson sphere candidates that made it through that project’s filters. You’ll recall that all seven were M-dwarfs, which struck me at the time as unusual. Only seven candidates emerged from over five million stars sampled, interesting especially because the possibility of a warm debris disk seemed to be ruled out. But Tongtian Ren (Jodrell Bank Centre for Astrophysics), working with Michael Garrett and Andrew Siemion, who share an affiliation with the same institution, has other ideas.
The researchers brought in new data from the Very Large Array Sky Survey, the NRAO VLA Sky Survey and two other sources that would allow a cross-matching of the seven Hephaistos candidates with radio sources. Hephaistos had been working with Gaia data release 3 along with the findings of the Two Micron All-Sky Survey (2MASS) and results from the Wide-field Infrared Survey Explorer, which now operates as NEOWISE. The search for radio counterparts to its Dyson candidates drew hits in three cases.
This looks strongly like data contamination, and the Jodrell Bank scientists think they’ve found the sources of the infrared signatures for these three:
Candidates A and G are associated with radio sources offset approximately ∼ 5 arcseconds from their respective Gaia stellar positions. We suggest that these radio sources are most likely to be DOGs (dust-obscured galaxies) that contaminate the IR (WISE) Spectral-Energy Distributions (SEDs) of the two DS candidates. The offsets for candidate B are smaller, approximately ∼ 0.35 arcsecond. Since M-dwarfs very rarely present persistent radio emission (≤ 0.5% of the sample observed by Callingham et al. (2021)), we suspect that this radio source is also associated with a background DOG lying very close to the line-of-sight. We note that the radio source associated with G has a steep spectral index with a best fit of α = −0.52 ± 0.02 – this value is typical of synchrotron emission from a radio-loud AGN with extended jets.
Let’s untangle this. A dust-obscured galaxy is generally studied at infrared wavelengths, being too difficult a target for visible light observations. There is likely strong star formation going on here, and perhaps an AGN, or active galactic nucleus, emitting energy across the electromagnetic spectrum. Usefully a DOG with an AGN can also be examined at radio wavelengths, which can tease out information about the gas content of the galaxy. So here we have background objects that can contaminate our infrared observations and can be identified by using surveys at different wavelengths.
All seven of the Hephaistos candidates are implicated in possible contamination if we bring in the objects known as hot dust-obscured galaxies, which have inevitably achieved the acronym Hot DOGs. The authors propose that the Spectral-Energy Distributions (SEDs) of each of the Hephaistos objects are “significantly contaminated” by background galaxies of this category. If this is the case, then the oddity of finding seven Dyson sphere candidates around M-dwarfs is resolved, but it will take deeper observations of all seven to confirm this, an effort the authors believe is warranted.
Image: Here is an artist’s impression of the Hot DOG W2246-0526, based on the results of a 2016 paper by Díaz-Santos et al. (2016). In that work (not connected with today’s paper), the authors used ALMA observations to show that the interstellar medium in the Hot DOG is dominated by turbulence, and may be unstable against the energy being injected by the AGN here, potentially producing an isotropic outflow. The WISE mission was essential to finding this galaxy because the galaxy is covered in dust, obscuring its light from visible-wavelength telescopes. But the radio signature of such objects, detected by other methods, raises questions about the recent Hephaistos findings. Image credit: NASA/JPL.
Bear in mind that only 1 out of every 3,000 galaxies that WISE observed fits into this category, so we are dealing with comparatively unusual objects. But given that the Hephaistos survey ran five million objects through its pipeline, the possibility of contamination in the data in the seven proposed candidates seems worth pursuing. The hunt continues, but more and more it appears that if Dyson spheres are achievable by advanced civilizations (and if such civilizations actually exist), they are seldom built.
The paper is Tongtian Ren et al., “Background Contamination of the Project Hephaistos Dyson Spheres Candidates,” available as a preprint.
Tantalizing New Images of Europa
What a pleasure to see new images from JunoCam, the visible-light camera aboard the Juno spacecraft that has now imaged in its peregrinations around Jupiter the surface of its most interesting moon. Our probing of Europa’s secrets has depended heavily upon the imagery returned by the Galileo spacecraft. That mission made its last flyby in 2000, and we have another wait while ESA’s Juice mission and Europa Clipper make the journey, the former enroute, the latter scheduled for an October launch.
Juno’s 2022 flyby thus gave us a helpful visual update, one that is complemented by an informative snapshot taken by the spacecraft’s Stellar Reference Unit (SRU) star camera. While we have five high resolution images to work with, the Stellar Reference Unit’s black-and-white image has produced the most detail. The image is intriguing because of its method, for bear in mind that the SRU is designed to track stars for navigation purposes. That makes it a dim light instrument, one that must be handled carefully to avoid washing out the image. The Juno team used it on Europa’s nightside, where the ambient light was sunlight reflected off Jupiter itself and the Sun was safely hidden.
Image: From Juno’s SRU, this image shows the location of a double ridge running east-west (blue box) with possible plume stains and the chaos feature the team calls ‘the Platypus” (orange box). These features hint at current surface activity and the presence of subsurface liquid water on the icy Jovian moon. Credit: NASA/JPL-Caltech/SwRI.
What emerges is a jumble of chaotic terrain cut by ridges and laden with a reddish-brown material familiar from Galileo imagery of the moon. These dark stains have been hypothesized to be the deposits of cryovolcanic plumes. Amidst this terrain, a new feature emerges that interrupts different forms of terrain. The Juno team has christened it the Platypus. Here the ridge topography breaks down as it encounters what is clearly younger material laden with ice blocks, a disrupted area that is some 37 kilometers by 67 kilometers in size. A double ridge line north of the Platypus is also apparent, the complex terrain suggesting the kind of surface change that researchers believe may allow ocean water to come close to the surface in isolated pockets.
The mention of plumes is intriguing because of the possibility of one day collecting samples from a spacecraft during a flyby, although no plumes are evident in the Juno imagery. Both the Platypus and the double ridges suggest recent activity. On the possibility of plumes, the SRU paper notes:
Diffuse discontinuous low-albedo deposits flank double ridges ∼50 km north of the “Platypus” chaos margin, extending radially outward from the lineaments. The morphology of these deposits is similar to features observed elsewhere on Europa that have been associated with hypothesized plume activity, the discontinuous low-albedo spots flanking Rhadamanthys Linea being a prominent example (Quick & Hedman, 2020). Quick and Hedman (2020) surmise that 1–10 m thick deposits can be emplaced by sufficiently compact plumes and detected by high-resolution visible wavelength cameras. The radii of the deposits observed by the SRU are ∼2–5 km, which Quick & Hedman’s models associate with <10 km high plumes.
We can also compare the Juno imagery with that of Galileo, as the JunoCam paper does:
The number of documented craters larger than 1 km on Europa has gone from 41 to 40 craters. Careful comparisons of the JunoCam images with overlapping images from Galileo show no surface changes due to plume deposits or ongoing geologic activity over time intervals of 23–26 yr, though admittedly the images are not well matched in resolution, viewing geometry, and wavelength. No active eruptions were detected. Finally, from the Europa data set taken on 2022 February 24, we can say that the north polar cap of Europa at this image scale looks similar to lower latitudes.
It’s worth adding here that a recent search using the Atacama Large Millimeter/submillimeter Array (ALMA) collected data over four days to examine the moon’s entire surface, coming up with no evidence of plume activity. We’re clearly not dealing with a geyser phenomenon anywhere as active as what we find at Enceladus, and thus far evidence from the Hubble instrument has been the most compelling, but even the data from its 2013 observations remain at the edge of detection. Clearly the search for active plumes will continue given their exciting implications.
Meanwhile, evidence for surface activity of other kinds on Europa continues to emerge, presenting new targets for Europa Clipper as well as Juice. Juice (Jupiter Icy Moons Explorer) launched on April 14, 2023 and will arrive in July of 2031, while Europa Clipper is scheduled to reach the giant planet in April of 2030. The new imagery suggests that Europa’s outer ice shell moves freely over the ocean (“true polar wander”), capturing steep depressions up to 50 kilometers wide near the equator. These ovoid features are similar to those found in other parts of Europa. Candy Hansen, who leads JunoCam planning at the Planetary Science Institute in Tucson, AZ, notes their relevance:
“True polar wander occurs if Europa’s icy shell is decoupled from its rocky interior, resulting in high stress levels on the shell, which lead to predictable fracture patterns. This is the first time that these fracture patterns have been mapped in the southern hemisphere, suggesting that true polar wander’s effect on Europa’s surface geology is more extensive than previously identified.”
The landscape of ice blocks and troughs near Europa’s equator broken by depressions tells a tale that must be interpreted in terms of light and shadow. The feature called Gwern, for example, an apparent impact crater found in Galileo imagery, turns out under different lighting to be nothing more than an oval shadow caused by the intersection of prominent ridges. Cross-cut ridges and the dark stains that may mark the residue from ancient (or recent) plumes offer a compelling landscape. New features like the Platypus will get a particularly hard look from our incoming spacecraft.
Image: Jupiter’s moon Europa was captured by the JunoCam instrument aboard NASA’s Juno spacecraft during the mission’s close flyby on Sept. 29, 2022. The images show the fractures, ridges, and bands that crisscross the moon’s surface. Credit: Image data: NASA/JPL-Caltech/SwRI/MSSS. Image processing: Björn Jónsson (CC BY 3.0).
The SRU paper is Becker et al., “A Complex Region of Europa’s Surface With Hints of Recent Activity Revealed by Juno’s Stellar Reference Unit,” JGR Planets 22 December 2023 (full text). The paper on the JunoCam imagery is Hansen, “Juno’s JunoCam Images of Europa,” Planetary Science Journal Vol. 5, No. 3 (21 March 2024), 76. Full text. The paper on the ALMA observations is Cordiner et al., “ALMA Spectroscopy of Europa: A Search for Active Plumes,” submitted to IAU Symposium 383 conference proceedings (preprint).
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