Centauri Dreams
Imagining and Planning Interstellar Exploration
A Space Catapult with Interstellar Potential
A new propulsion method with interstellar implications recently emerged on the arXiv site, and in an intriguing video on David Kipping’s Cool Worlds channel on YouTube. Kipping (Columbia University) has built a video production process that is second to none, but beyond the imagery is his ability to translate sophisticated mathematical concepts into clear language and engaging visuals. So while we’re going to discuss his new propulsion concept using the arXiv paper, don’t miss the video, where this novel new idea is artfully rendered.
I was delighted to see the author invoking J.R.R. Tolkien in the video (though not in the paper), for he begins the Cool Worlds episode with some musings on interstellar flight and why it has come to engage so many of us. Tolkien devotees will already know the lovely term he used to explain our yearnings for something beyond ourselves: ‘sea-longing.’ It’s a kenning, to use the scholarly jargon, a metaphorical double construction that links two ideas. Anglo-Saxon poetry, about which Tolkien was a master, is rife with such turns of phrase.
Image: Columbia’s David Kipping, astrophysicist and guiding force of the Cool Worlds Lab.
Tolkien’s work on Beowulf was hugely significant to scholarship on that great poem, and The Lord of the Rings is peppered with linguistic echoes of the language. Here’s the relevant quote from The Two Towers, in which the elf Legolas invokes the things that drive his race:
And now Legolas fell silent, while the others talked, and he looked out against the sun, and as he gazed he saw white sea-birds beating up the river.
’Look!’ he cried. ‘Gulls! They are flying far inland. A wonder they are to me and a trouble to my heart. Never in all my life had I met them, until we came to Pelargir, and there I heard them crying in the air as we rode to the battle of the ships. Then I stood still, forgetting war in Middle-earth,; for their wailing voices spoke to me of the Sea. The Sea! Alas! I have not yet beheld it. But deep in the hearts of all my kindred lies the sea-longing, which it is perilous to stir. Alas! for the gulls. No peace shall I have again under beech or under elm.’
Sea-longing. If it was an innate component of Tolkien’s elvish personalities, it’s one common among all humans, I think, though clearly in greater or lesser amount depending on the person. I grew up in the American Midwest far from any ocean, but I had ‘sea-longing’ as a boy and have it still. It’s not just about oceans, of course, but about vast expanses that are partly real and partly a matter of the yearning imagination. It’s why some people have to explore.
Turning Yearning into Hardware
Kipping’s reputation is already secure as an innovator of a very high order. His work on exo-moons solidified the hunt for these objects, which surely exist but which have yet to be confirmed in the only two cases that look plausible so far. His vision of a ‘terrascope’ is reminiscent of gravitational lensing but draws on the Earth’s atmosphere to provide refractive lensing, a telescope concept that although it cannot compete with the gravity lens, nonetheless offers huge magnifications for a space-based telescope. His ‘Halo drive’ gathers energy from light boomeranging around a black hole while using no onboard fuel.
That latter idea is fully consonant with the laws of physics, but of course demands we find a way to get to a black hole to use its energies. By contrast, Torqued Accelerator using Radiation from the Sun (TARS) is a means of acceleration that could be built now. It offers no ‘warp drive’ type travel, and in fact in its most powerful iteration would weigh in at about 1000 kilometers per second. But interstellar flight pushes us to follow our leads, and we should keep in mind how huge a step 1000 km/s represents when weighed against the current defender of the velocity crown, Voyager 1 at about 17 km/s.
So let’s talk about this, because it’s a remarkable way to overcome a serious problem with solar sails, creating a way to push a payload beyond Solar System escape velocity with energy extracted from the Sun. As opposed to the Breakthrough Starshot concept, a politically impossible 100 GW laser array high in the Atacama, TARS offers us an exceedingly economical way to send not one but swarms of tiny probes. And if a journey to Proxima Centauri would take about a millennium, ask yourself what we could do with this in our own system.
The concept is blindingly simple once it’s been thought of, and like Jim Bickford’s TFINER design (see TFINER: Ramping Up Propulson via Nuclear Decay) it’s almost jarring. Why hadn’t someone thought of this before? Kipping, pondering the dilemma of interstellar propulsion, asked whether a deep space sail necessarily has to be beam-driven. True, light from the Sun diminishes rapidly with distance, so that beyond Jupiter, a solar sail is getting little propulsive effect. But maybe pushing a sail is the wrong approach.
For that matter, does it have to be shaped like a conventional solar sail? Kipping began thinking about using sail materials to harvest the energy of solar photons, storing it in what could be considered a battery, and then using that stored energy, transformed into kinetic energy, to hurl a small spacecraft outwards. We thus get the huge advantage of harvesting abundant energy from a system that can be serviced because it remains relatively close to home, not to mention system reusability.
The notion is shown in the figure below, drawn from the paper. Imagine taking two light sails attached to each other by a tether, both identical and each coated on one side with highly reflective material and non-reflective material on the back. Now we can rotate one of them 180 degrees around, so that they are facing in opposite directions. The TARS unit begins to spin because of incident solar photons, and that spin gets faster and faster until the stresses on the tether close in on its design limits. Let me quote from the paper here:
At this point, one (or both) sails are detached (or a sail section) and will head off at high speed tangential to the final rotational motion. The light sail(s) will then continue to enjoy thrust from solar radiation in what follows, but crucially the initial high speed provides sufficient momentum to escape our solar system. The concept is attractive since it only involves two light sails and a tether, and is powered by the Sun. In practice, one might consider an initial spin-up phase with directed energy (but far less than 100 GW) or micro-thrusters, since TARS is more stable once rotation is established.
Image: This is Figure 1 from the paper. Caption: A simplified version of the TARS system. Here, the system comprises one tether and two paddles, which together are orbiting around the Sun, with an instantaneous velocity vector along the Y-axis. Incident solar radiation is largely reflected by the α-surface (the reflective surface) of the paddles, but largely absorbed by the β-surface. This leads to a radiation pressure torque that gradually spins up TARS. Note that both paddles experience both reflection and emission; we only show one of each for the sake of visual clarity in the above. Credit: Kipping & Lampo.
Below is an animation showing the basic concept, with the sails depicted here in the form of panels or paddles, with the same characteristics – a reflective side, a non-reflective side, and the two panels configured in such a way that the incident solar photons spin the system up. Now imagine a small payload at the end of one of these paddles being released just when the system has reached maximum spin-up, so that the craft, possibly the size of a small computer ship, hurtles away with enough force to achieve escape from the Solar System.
Image: This and the animations below are courtesy of David Kipping.
Spinning Up TARS
Don’t get wed to the idea of those sails as paddles; as we’ll see, other options emerge. The nod toward Breakthrough Starshot is evident in the choice of a payload built around microelectronics, but in this case we give up the laser array and use the power of the Sun rather than the collected energies of nuclear reactors to power up the craft. Also like Breakthrough Starshot, we can envision such tiny spacecraft being hurled in swarm formations so that they can network with each other during their journeys. After all, this is a remarkably economical system, capable of launching swarm missions to targets near and far.
So we’re talking about gathering rotational kinetic energy. As Kipping points out, even at 1 AU, Earth receives solar energy of 1.36 kilowatts per meter squared, so if we can tap that energy efficiently, we don’t need to beam our sail. The TARS concept gets around the inverse square law, the fact that solar photons push a sail outwards even as their efficiency plummets. Go twice as far from the Sun and solar energy is reduced not by two but four times. Whereas the spinning TARS stores energy in something analogous to a flywheel while remaining in its orbit. It then releases that energy in a single fling.
The question of TARS’ orbit is an interesting one. Kipping refers to the concept of a quasite, which he developed some years back, though only recently finding a use for it in this new idea. In an email this afternoon, he distinguishes his TARS orbit from the better known statite:
If we could engineer a sufficiently light (and reflective) sail, it is possible that the outward force caused by radiation pressure upon the sail precisely equals the inward gravitational force of the Sun. Such an object need not rely on orbits for stability, it could be placed wherever you want – hanging out in inertial space just motionless. A quasite is not quite so extreme as this. Yes it’s still a sail, but now the gravitational force exceeds the radiative force. Hence, it wants to fall into the Sun (but less so than a non-sail object).
To avoid TARS indeed migrating inwards, we give it a well-calculated nudge such that its tangential velocity is sufficient to keep a constant altitude from the Sun at all times. Although all conventional orbits do this too, the tangential velocity here is less than that of the Earth or indeed any other orbiting object. Hence it’s in what we’d call a sub-Keplerian orbit, and indeed dust particles can do this too since they too can feel strong radiative forces. This engineered quasite thus is a Solar sail which doesn’t recede (or migrate) from the Sun, it stays at the same separation which is crucial for TARS being able to build up angular momentum over time. A consequence of its slower tangential nudge is that it orbits the Sun slower than the Earth does (if at 1AU).
Shape and Material
TARS in its simplest form can be reduced to a single ribbon-like structure, where there is no tether, and the two paddles simply meet at the midpoint. The shape arrived at in the image below is optimum for ensuring rotational stability. The paper considers the use of carbon nanotube sheets, given that this material is more readily available in the market. Tapering the ribbon improves performance, with a segment at the end containing the payload, which can be reflective enough to gain an additional boost as it recedes from the Sun.
Image: For the purposes of calculation, Kipping works with a TARS that is seven meters wide and 63 meters long. The thickness is 2.8 microns, using carbon nanotube sheets, sprayed on one side with nanostructure silver and carbon deposition on the other. This thickness allows a microchip to be attached flush at the two ends, as per the illustration. This is light in weight (1.6 kilograms), so rideshare payloads are hardly a problem. As with solar sails, the device would have to be unfurled once it reaches space. Animation credit: David Kipping.
The calculations referred to above see a three-year spin up time and ejection of the payload at 12.1 kilometers per second – this limit is dependent on the tensile strength of the TARS nanotube sheets. Moving in its quasite orbit, TARS already has 28.3 kilometers per second. Kipping calculates in this configuration that the payload chip would leave TARS at 40.4 kilometers per second. This is just over Solar System escape velocity, making TARS an interstellar option. No beamed energy, no onboard propulsion, just solar energy collected and deployed.
So we have a payload roughly the size of a smartphone that can escape from the Solar System, but velocities can be increased depending on materials used – graphene creates a clear improvement, one that could be further tweaked with a gravity assist. An Oberth effect ‘sundiver’ maneuver is a possibility. And as Kipping notes, the payload can be reflective enough to serve as a small solar sail, acquiring additional velocity as it departs the inner Solar System.
A Magnetic Option to Boost Velocity
To go beyond these tweaks, applying an equal and opposite charge to each tip would create a rotating magnetic dipole. Out of this we get a magnetic field, which in turn yields electromagnetic radiation. A system like this, calculated in the paper, is capable of a critical speed in the range of 1000 kilometers per second. 0.3 percent of c. Meanwhile, the use of TARS to create magnetic shielding for uses in the Solar System can hardly be discounted. Kipping mentions in his video the prospects of using numerous TARS orbiters at Mars to provide radiation shielding for colonies on the surface.
I sometimes hear from readers frustrated by the magnitude of the interstellar challenge. Even Breakthrough Starshot’s 20 percent of lightspeed takes too long for them, and they think we should put all our efforts into attempts to move faster than light. But progress is incremental in most cases, and whether or not we ever achieve breakthroughs like Alcubierre warp drive, we still push the envelope of what is practical today.
Progress is not just individual but civilizational. This is valuable near-term thinking that extends our capabilities one step at a time, and like TARS offers multiple uses within our System and beyond. One step at a time is the nature of the game, and these steps are taking us slowly but inexorably toward the sea.
The paper is Kipping & Lampo, “Torqued Accelerator using Radiation from the Sun (TARS) for Interstellar Payloads,” accepted at Journal of the British Interplanetary Society (preprint).
ETI in our Datasets?
A recent workshop at Ohio State raises a number of interesting questions regarding what is being referred to as ‘high energy SETI.’ The notion is that places where vast energies are concentrated might well attract an advanced civilization to power up projects on a Kardashev Type II or III scale. We wouldn’t necessarily know what kind of projects such a culture would build, but we might find evidence that these beings were at work, perhaps through current observations or, interestingly enough, through scans of existing datasets.
Running June 23-24, the event was titled “Bridging Multi-Messenger Astronomy and SETI: The Deep Ends of the Haystack Workshop.” ‘Multi-messenger astronomy’ refers to observations that take in a wide range of inputs, from electromagnetic wavelengths to gravitational waves, from X-rays through gamma ray emissions. Extend this to SETI and you’re looking in all these areas, the broad message being that a SETI signature might show up in regions we have only recently begun to look at and may have prematurely dismissed.
Notice that such ‘signals’ don’t have to imply intended communication. We might well turn up evidence of advanced engineering through astronomical plates taken a century ago and only now recognized as anomalous. This kind of search is deliberately open-ended, acknowledging as it does that civilizations perhaps millions of years ahead of us in their history might be far more occupied in their own projects than in trying to talk to species in their infancy.
As I mentioned in SETI at the Extremes, Brian Lacki (Oxford University) and Stephen DiKerby (Michigan State) have produced a white paper on the workshop, an overview that puts the major issues in play. The high-energy bands that we have been talking about recently have seldom been explored with SETI in mind, given the natural predisposition to think that life would be something rather like ourselves, and certainly not capable of existing on, say, a neutron star. High-energy SETI pushes the idea of astrobiology into these realms anyway, but equally significant, makes the point that whatever their makeup, advanced aliens might exploit high-energy sources whether or not they had evolved on them. Thus these energy resources become SETI targets, in the hope that activity affecting them will throw a signature.
Image: The area around Sgr A* contains several X-ray filaments. Some of these likely represent huge magnetic structures interacting with streams of very energetic electrons produced by rapidly spinning neutron stars or perhaps by a gigantic analog of a solar flare. Scattered throughout the region are thousands of point-like X-ray sources. These are produced by normal stars feeding material onto the compact, dense remains of stars that have reached the end of their evolutionary trail – white dwarfs, neutron stars and black holes. Because X-rays penetrate the gas and dust that blocks optical light coming from the center of the galaxy, Chandra is a powerful tool for studying the Galactic Center. This image combines low energy X-rays (colored red), intermediate energy X-rays (green) and high energy X-rays (blue). Credit: NASA/CXC/UMass/D. Wang et al.
Let’s acknowledge our ignorance by recognizing that the motivations of any off-Earth civilization are unknown to us, and for all our logic, we have no notion of what such a culture wants to do. It’s a helpful fact that technosignature searches don’t require futuristic off-planet observatories. Reams of observations have been recorded that have seldom if ever been actively mined. Thus high-energy SETI, exotic as it is, can proceed with existing materials, even as ongoing astrophysical research continues to produce new data that add to the mix.
As the authors note, high-energy radiation has many sources, from nuclear processes, from gamma ray emissions and neutrinos to relativistic particles, which include not only cosmic rays but particles thrown out by jets and the interaction of electrons and positrons. We can study compact sources like neutron stars and black holes (ideal for energy extraction) and relativistic flows from energetic transients. Gravitational waves might be used to bind together elements of a galactic network. How exactly might ETI modify any of these?
It’s natural to ask whether X-ray astronomy has implications for SETI. Bursts of emission using X-rays for communication, exploiting less diffraction and the ability to produce tighter beams, might be detected if aimed specifically at us, making something like a flash at these frequencies from a nearby star an anomalous event worth studying. Or consider signals more general in nature:
Non-directional X-ray communication can be effected by dropping an asteroid onto a neutron star [4]. When it hits, it releases a burst of energy detectable at interstellar distances. The cosmos also has a number of compact high-energy “signal lamps”. X-ray binaries (XRBs) are systems with a neutron star or black hole accreting from a donor star, having luminosities of up to 105 suns. Even a kilometer-scale object passing in front of the hotspots of an XRB can easily modulate its luminosity, serving as a technosignature [4, 16]. A subplanetary-scale lens is potentially capable of creating a brief flash visible even in nearby galaxies without any power input of its own. Credit: NASA/CXC/UMass/D. Wang et al.
We don’t have a handle on how to use neutrinos for communication, although there have been experiments along these lines given the ability of neutrinos to pass right through obstacles and thus probe, for example, the oceans of icy moons. But perhaps we can home in on industrial activities, which as the authors point out, could involve not just energy collection to power scientific experiments but interstellar propulsion through antimatter rockets. The interactions between a relativistic spacecraft and the interstellar medium could become apparent through gamma rays, while X-ray binaries might show oddities in their proper motion indicative of their use as stellar engines.
This possibility, studied at some length by Clément Vidal under his ‘stellivore’ concept, stands as a particularly detectable phenomenon:
What are the limits of life, broadly defined? At the very least, complex processes require a thermodynamic gradient to feed them. In his reflections on the future of the cosmos, Dyson suggested that this is the only absolute requirement, and that long after the stars have gone out, life could still thrive in the chilly atmospheres of cooled compact objects [7]. A contemporary test of this admittedly extreme idea might be found with today’s compact objects. The accretion hotspots of XRBs have some of the greatest sustained power densities around in the contemporary universe. If thermodynamics really is the only prerequisite factor for complexity and ETIs can withstand the incredibly hostile environments, they may find the energy gradients in XRBs attractive [29].
If we look not at the stellar but the galactic level, the actual lack of X-ray binaries could be a marker, with the deficiency being a sign their energies are being exploited to some purpose. For that matter, high-energy flare activity from an individual star or the source of a gamma ray burst may point us at locations where an advanced civilization can use its technologies to deflect these energies to avoid the threat. If we push speculation to the extreme, we’re talking once again about Robert Forward territory, wondering whether environments like neutron stars can sustain their own kinds of life.
Image: HEAO-1 All-Sky X-ray Catalog: Beginning in 1977, NASA launched a series of very large scientific payloads called High Energy Astronomy Observatories (HEAO). The first of these missions, HEAO-1, carried NRL’s Large Area Sky Survey Experiment (LASS), consisting of 7 detectors. It surveyed the X-ray sky almost three times over the 0.2 keV – 10 MeV energy band and provided nearly constant monitoring of X-ray sources near the ecliptic poles. We’ve been examining high-energy targets for quite a while now and have numerous datasets to consult. Image credit: NASA.
Several things to keep in mind as we consider ideas that are on the face of things fantastic. First, the very practical fact that high-energy SETI need not be expensive, given our growing sophistication at using machine intelligence to analyze existing astronomical data (I’ve always nursed the wonderful idea that some day we’ll make a SETI detection and it will be corroborated by a century-old astronomical plate taken at Mt. Wilson Observatory). Second, existing facilities monitoring things like gamma ray bursts and detecting neutrinos are capable of full-sky monitoring and are doing good science. Our search for high-energy anomalies, then, takes a free ride on existing equipment.
So while it’s completely natural to find this approach well outside our normal ideas of astrobiology, their improbable nature should elicit a willingness to keep our eyes open. It would be absurd to miss something that has been in our data all along. And filtering incoming data as an add-on investigation into astrophysical processes may turn up anomalies that advance high-energy physics even if they never do resolve themselves into a SETI detection.
The paper is Lacki & DiKerby, “Possibilities for SETI at High Energy,” submitted for 2025 NASA DARES [Decadal Astrobiology Research and Exploration Strategy] RFI and available as a preprint.
SETI at the Extremes
Science fiction has always provoked interesting research. After all, many of the scientists I’ve spoken with over the years have been science fiction readers, some of whom trace their career choices to specific novels (Poul Anderson’s Tau Zero is frequently mentioned, but so is Frank Herbert’s Dune, and there are many others). This makes sense because there is a natural tension in exoplanet studies growing out of the fact that in most cases, we can’t even see our targets. Instead, we detect them through non-visual methods. True, we can analyze planetary atmospheres for some gas giant planets, but we’re only beginning to drill down to the kind of biosignature searches that may eventually flag the presence of life.
But fiction can paint a planet’s physics and visually explore its surface, modeling worlds in vast variety and sometimes spurring directions of thought that would otherwise remain unexplored. Consider Hal Clement, whose forays into planet-building included the remarkable Mesklin, a fast-rotating oblate world with an 18-minute day and surface gravity varying from 700 g at the poles to an almost bearable 3 g at the equator. Mission of Gravity, published as a serial in Astounding Science Fiction in 1953, involves an indigenous race’s interactions with a human crew at the equator. The encounter dazzled readers and led some into astrophysics.
These are unconventional aliens, and were particularly so in 1953, when communications between humans and tiny, flattened insect-like creatures seemed more at home in works of fantasy than what would become known as ‘hard science fiction’ (i.e., SF with a scrupulous reliance on proven physics). Clement’s novel was well received and spurred correspondence between the author and Robert Forward, who carried on the idea of extreme habitats in his novel Dragon’s Egg (1980). Both continued to ponder life in utterly extreme environments.
Gary Westfahl, the author of numerous titles of science fiction criticism including Hugo Gernsback and the Century of Science Fiction (McFarland, 2007) has dissected the hard science fiction genre in an essay in Science Fiction Studies. Westfahl makes the case that Mission of Gravity was “the first SF novel built on actual observational data involving another possible solar system.”
When I first read that, my thought was that it referred to Peter van de Kamp’s studies of Barnard’s Star at Swarthmore College’s Sproul Observatory in the 1930s and later. The detection of planets there proved erroneous, but so did a ‘detection’ at 61 Cygni. Clement seems to have used that supposed exoplanet as he modeled his world Mesklin. He wrote about his process in Astounding‘s issue of June, 1953 in which Mission of Gravity continued to be serialized.
I checked my collection of old magazines to find that issue, where he describes exactly how he built his planet. The details are fascinating, and available in some editions of Mission of Gravity. He’s not totally convinced that the 61 Cygni find is actually a planet — the object could not be seen, and the ‘detection’ was based on astrometry using photographs of this binary system. The paper, by Kaj Aage Strand, was painstaking, although the supposed planet turned out to be a chimera. Clement is not sure, but he accepts it as a planet for the purposes of the story: He writes:
If we assume the thing to be a planet, we find that a disk of the same reflecting power as Jupiter and three times his diameter would have an apparent magnitude of twenty-five or twenty-six in 61 C’s location; there would be no point looking for it with present equipment. It seems, then that there is no way to be sure whether it is a star or a planet, and I can call it whichever I like without too much fear of losing points in the game.
Image: Reproduction of diagrams by Hal Clement, originally published in his article “Whirligig World”, Astounding Science Fiction, June 1953. Top: Diagram of the cross-sectional shape of Mesklin, with approximate values for the effective surface gravity at various latitudes (in multiples of Earth gravity). The dashed lines are polar circles. The shaded circle in the middle represents the size of Earth on the same scale. Bottom: Diagram of Mesklin’s orbit, with approximate isotherms and times of crossing them. Credit: Wikimedia Commons.
These days we have to say that the first novel built on observational data of other stellar systems would have to be limited to a time after 1992, which is when Aleksander Wolszczan and Dale Frail found planets around the neutron star PSR B1257+12. Readers are welcome to name the novel (I don’t know the answer). This was, after all, the first time planets beyond our Sun were detected and confirmed, even if it would be another three years before we found 51 Pegasi b, the first planet around a main sequence star.
Robert Forward’s Dragon’s Egg takes astrobiology into even more extreme territory. He had been talking to Frank Drake, the first practitioner of SETI, who in 1973 was already thinking about life in highly unusual places, including settings on a neutron star. Let’s pause with Drake for a moment, because this is an interesting period in the history of science fictional ideas. Drake is quoted in Astronomy Magazine for December of 1973 as saying that life might well evolve in such a place.
In the exterior layers of these objects, we don’t have atoms…, but we do have atomic nuclei. And we have more varieties of atomic nuclei in a neutron star than we have varieties of atoms on our Earth. And from what we know of nuclear physics, those nuclei might combine together to form enormous supernuclei, or macronuclei, analogous to the large molecules which make up Earth life. And so as far as we know, it is possibly feasible to reproduce exactly the evolution which occurred on Earth but substituting for atoms and molecules, nuclei and macronuclei. So indeed there could be creatures on neutron stars that are made of nuclei. The temperatures are just right to make the required nuclear reactions go.
The combination of Clement’s planet Mesklin and Drake’s musings on neutron star life propelled Forward to re-examine the whole question and further refine Drake’s ideas. In Dragon’s Egg, the surface gravity on the neutron star is 67 billion times that of Earth. The local species is called the cheela, who are creatures the size of sesame seeds. The novel follows the development of their civilization from its earliest technologies to actual communications with a human-manned spacecraft in orbit around the star. For as the humans come to realize, the cheela experience the life and death of numerous generations in the span of mere hours.
So we see civilizational change in minutes. Forward had help with the structure of the novel from science fiction writer and editor Lester Del Ray, then working at Ballantine. He would eventually refer to the book as something of a textbook on neutron star physics “disguised as a novel.” None of that takes away from the sheer readability of this encounter with a species that within days achieves physics breakthroughs beyond those of the humans that are observing them. As with 1984’s Rocheworld, Forward’s prose is a bit clunky but his science is tight and his plot gripping.
Image: An combined image from multiple instruments showing a neutron star in the Small Magellanic Cloud. The reddish background image comes from the NASA/ESA Hubble Space Telescope and reveals the wisps of gas forming the supernova remnant 1E 0102.2-7219 in green. The red ring with a dark centre is from the MUSE instrument on ESO’s Very Large Telescope and the blue and purple images are from the NASA Chandra X-Ray Observatory. The blue spot at the centre of the red ring is an isolated neutron star with a weak magnetic field, the first identified outside the Milky Way. Credit: ESO/NASA, ESA and the Hubble Heritage Team (STScI/AURA)/F. Vogt et al. Acknowledgments: Mahdi Zamani.
We could go on with life in extreme environments as envisioned by science fiction (and I might mention Stephen Baxter’s Raft (Gollancz, 2018), where a rip in spacetime takes a human crew into a universe where the force of gravity is one billion times stronger than ours). Other readers will have their own favorites. I notice that some exoplanet and SETI researchers are following the lead of these novelists and taking a hard look at places we would consider hostile to any forms of life. As witness a recent paper from Brian Lacki and Stephen DiKerby on SETI at high energy levels.
And why not? We’re learning to think outside our usual preconceptions when it comes to habitability, and if we take seriously the idea of Kardashev Type II or III civilizations, we might well look for places where vast power resides in small spaces. Clément Vidal continues to make this point. Here the reference is his essential The Beginning and the End: The Meaning of Life in a Cosmological Perspective, Springer 2014. This is a key text for anyone serious about Dysonian SETI.
Can we learn to be as imaginative as some of the great science fiction authors? I think the wild variety of exoplanets thus far discovered demands that response from anyone pondering what might exist on everything from gas giant moons to desert worlds just barely touching the habitable zone. Keith Cooper gets into these questions in his fine Amazing Worlds of Science Fiction and Science Fact (Reaktion Books, 2025), where the link between the literature of the fantastic and cutting edge astrophysics is explicitly studied. I’ll be reviewing this one soon in these pages.
As to Lacki and DiKerby, they’re interested in exploring parts of the SETI landscape that have seen little attention. While our thinking about astrobiology naturally flows out of life as we already know it (and thus on Earth), what about those off-the-wall places where humans would instantly perish if they were so unwise as to get too near? Is a neutron star a SETI target? The accretion disk of a black hole? A binary X-ray pulsar?
We can posit strange lifeforms like those of Clement and Forward, but we can also add that places of high energy could be exploited by advanced civilizations that developed on far different worlds, cultures that are mining these high energy sources to drive civilizational projects whose intent may remain unfathomable. So without any knowledge of whether exotic life can be possible in, say, stellar plasma or on a neutron star’s surface, we might consider just what technosignatures would be possible if we found a culture at work in the places where the most extreme energies are available.
Lacki (University of Oxford) is part of the Breakthrough Listen team, while DiKerby is an astrophysicist at Michigan State University. I want to go through their paper next time as they push SETI concepts to the limit and ask what the result would look like.
The paper on high energy SETI is Lacki & DiKerby, “Possibilities for SETI at High Energy,” a white paper for NASA DARES (NASA Decadal Astrobiology Research and Exploration Strategy). Available here.
A Better Look at 3I/ATLAS
Just a short note, prompted by the release of new imagery of the intersellar object 3I/ATLAS by the Gemini North telescope in Hawaii. It’s startling how quickly we’ve moved from the first pinpoint images of this comet to what we see below, which draws on Gemini North’s Multi-Object Spectrograph to show us the tight (thus far) coma of the object, the gas and dust cloud enshrouding its nucleus. Changes here as the comet nears perihelion will teach us much about the object’s composition and size. Some early estimates have the cometary nucleus as large as 20 kilometers, considerably larger than both ‘Oumuamua and 2I/Borisov, the first two such objects detected. This is a figure that will doubtless be adjusted with continued observation.
Image: Using the Gemini North telescope, astronomers have captured 3I/ATLAS as it makes its temporary passage through our cosmic neighborhood. These observations will help scientists study the characteristics of this rare object’s origin, orbit, and composition. Credit: NSF NOIRLab.
3I/ATLAS also shows a more eccentric orbit than its predecessors. Remember that an eccentricity of 0 means an orbit that is completely circular, while as we move from 0 to 1, the orbit becomes drawn out, to the point where an orbit with eccentricity values of 1 or above doesn’t return to the Sun, but continues into interstellar space. The new comet’s orbital eccentricity is 6.2, considerably higher than ‘Oumuamua (1.2) and Borisov (3.6). Perihelion will come at the end of October at a distance of 210 million kilometers, which will place the object just inside the orbit of Mars. Amateur astronomers with a good telescope may just be able to get a glimpse of it late in 2025.
A New Horizons First for Interstellar Navigation
If you’re headed for another planet, celestial markers can keep your spacecraft properly oriented. Mariner 4 used Canopus, a bright star in the constellation Carina, as an attitude reference, its star tracker camera locking onto the star after its Sun sensor had locked onto the Sun. This was the first time a star had been used to provide second axis stabilization, its brightness (second brightest star in the sky) and its position well off the ecliptic making it an ideal referent.
The stars are, of course, a navigation tool par excellence. Mariners of the sea-faring kind have used celestial navigation for millennia, and I vividly remember a night training flight in upstate New York when my instructor switched off our instrument panel by pulling a fuse and told me to find my way home. I was forcefully reminded how far we’ve come from the days when the night sky truly was a celestial map for travelers. Fortunately, a few bright cities along the way made dead reckoning an easy way to get home that night. But I told myself I would learn to do better at stellar navigation. I can still hear my exasperated instructor as he pointed out one celestial marker: “For God’s sake, see that bright star? Park it over your left wingtip!”
Celestial navigation of various kinds can be done aboard a spacecraft, and the use of pulsars will help future deep space probes navigate autonomously. Until then, our methods rely heavily on ground-based installations. Delta-Differential One-Way Ranging (Delta-DOR or ∆DOR) can measure the angular location of a target spacecraft relative to a reference direction, the latter being determined by radio waves from a source like a quasar, whose angular position is well known. Only the downlink signal from the spacecraft is used in a precision technique that has been employed successfully on such missions as China’s Chang’e, ESA’s Rosetta and NASA’s Mars Reconnaissance Orbiter.
The Deep Space Network and Delta-DOR can perform marvels in terms of the directional location of a spacecraft. But we’ve also just had a first in terms of autonomous navigation through the work of the New Horizons team. Without using radio tracking from Earth, the spacecraft has determined its distance and direction by examining images of star fields and the observed parallax effects. Wonderfully, the two stars that the team chose for this calculation were Wolf 359 and Proxima Centauri, two nearby red dwarfs of considerable interest.
The images in question were captured by New Horizons’ Long Range Reconnaissance Imager (LORRI) and studied in relation to background stars. These twp stars are almost 90 degrees apart in the sky, allowing team scientists to flag New Horizons’ location. The LORRI instrument offers limited angular resolution and is here being used well outside the parameters for which it was designed, but even so, this first demonstration of autonomous navigation didn’t do badly, finding a distance close to the actual distance of the spacecraft when the images were taken, and a direction on the sky accurate to a patch about the size of the full Moon as seen from Earth. This is the largest parallax baseline ever taken, extending for over four billion miles. Higher resolution imagers, as reported in this JHU/APL report, should be able to do much better.
Image: Location of NASA’s New Horizons spacecraft on April 23, 2020, derived from the spacecraft’s own images of the Proxima Centauri and Wolf 359 star fields. The positions of Proxima Centauri and Wolf 359 are strongly displaced compared to distant stars from where they are seen on Earth. The position of Proxima Centauri seen from New Horizons means the spacecraft must be somewhere on the red line, while the observed position of Wolf 359 means that the spacecraft must be somewhere on the blue line – putting New Horizons approximately where the two lines appear to “intersect” (in the real three dimensions involved, the lines don’t actually intersect, but do pass close to each other). The white line marks the accurate Deep Space Network-tracked trajectory of New Horizons since its launch in 2006. The lines on the New Horizons trajectory denote years since launch. The orbits of Jupiter, Saturn, Uranus, Neptune and Pluto are shown. Distances are from the center of the solar system in astronomical units, where 1 AU is the average distance between the Sun and Earth. Credit: NASA/Johns Hopkins APL/SwRI/Matthew Wallace.
Brian May, known for his guitar skills with the band Queen as well as his knowledge of astrophysics, helped to produce the images below that show the comparison between these stars as seen from Earth and from New Horizons. A co-author of the paper on this work, May adds:
“It could be argued that in astro-stereoscopy — 3D images of astronomical objects – NASA’s New Horizons team already leads the field, having delivered astounding stereoscopic images of both Pluto and the remote Kuiper Belt object Arrokoth. But the latest New Horizons stereoscopic experiment breaks all records. These photographs of Proxima Centauri and Wolf 359 – stars that are well-known to amateur astronomers and science fiction aficionados alike — employ the largest distance between viewpoints ever achieved in 180 years of stereoscopy!”
Here are two animations showing the parallax involving each star, with Proxima Centauri being the first image. Note how the star ‘jumps’ against background stars as the view from Earth is replaced by the view from New Horizons.
Image: In 2020, the New Horizons science team obtained images of the star fields around the nearby stars Proxima Centauri (top) and Wolf 359 (bottom) simultaneously from New Horizons and Earth. More recent and sophisticated analyses of the exact positions of the two stars in these images allowed the team to deduce New Horizons’ three-dimensional position relative to nearby stars – accomplishing the first use of stars imaged directly from a spacecraft to provide its navigational fix, and the first demonstration of interstellar navigation by any spacecraft on an interstellar trajectory. Credit: JHU/APL.
This result from New Horizons marks the first time that optical stellar astrometry has been applied to the navigation of a spacecraft, but it’s clear that our hitherto Earth-based methods of navigation in space will have to give way to on-board methods as we venture still farther out of the Solar System. Thus far the use of X-ray pulsars has been demonstrated only in Earth orbit, but it will surely be among the techniques employed. These rudimentary observations are likewise proof-of-concept whose accuracy will need dramatic improvement.
The paper notes the next steps in using parallactic measurements for autonomous navigation:
Considerably better performance should be possible using the cameras presently deployed on other interplanetary spacecraft, or contemplated for future missions. Telescopes with apertures plausibly larger than LORRI’s, with diffraction-limited optics, delivering images to Nyquist-sampled detectors [a highly accurate digital signal processing method], mounted on platforms with matching finepointing control, should be able to provide astrometry with few milli-arcsecond accuracy. Extrapolating from LORRI, position vectors with accuracy of 0.01 au should be possible in the near future.
The paper on this work is Lauer et al., “A Demonstration of Interstellar Navigation Using New Horizons,” accepted at The Astronomical Journal and available as a preprint.
3I/ATLAS: Observing and Modeling an Interstellar Newcomer
Let’s run through what we know about 3I/ATLAS, now accepted as the third interstellar object to be identified moving through the Solar System. It seems obvious not only that our increasingly powerful telescopes will continue to find these interlopers, but that they are out there in vast numbers. A calculation in 2018 by John Do, Michael Tucker and John Tonry (citation below) offers a number high enough to make these the most common macroscopic objects in the galaxy. But that may well depend on how they originate, a question of lively interest and one that continues to produce papers.
Let me draw on a just released preprint from Matthew Hopkins (University of Oxford) and colleagues that runs through the formation options. Pointing out that interstellar object (ISO) studies represent an entirely new field, they note that theoretical thinking about such things trended toward comets as the main source, an idea immediately confronted by ‘Oumuamua, which appeared inert even as it drew closer to the inner system and even appeared to accelerate as it departed. The controversy over its origin made 2I/Borisov a relatively tame object, it being clearly a comet. 3I/ATLAS looks a lot more like 2I/Borisov than ‘Oumuamua, though it’s larger than either.
Protoplanetary disks are a possible source of interstellar debris, but so for that matter are the Oort-like clouds that likely surround most main sequence stars, and that would largely be released when their hosts complete their evolution. ‘Oumuamua has been analyzed as a fragment of a small, outer-system world around another star, or even as a ‘hydrogen iceberg,’ and I see there is one paper suggesting that ISOs may be a part of galactic renewal, contributing their materials into protoplanetary disks and nascent planets.
The Hopkins paper underlines the ubiquity of such objects:
A standard picture has emerged, in which planetesimals formed within a protoplanetary disk are scattered by interactions with migrating planets or via stellar flybys, early in the history of a system (Fitzsimmons et al. 2023). The number density inferred from observations of the first two ISOs, in addition to studies of scattering in our own Solar System, suggest that such events are common, with ≳ 90% of planetesimals joining the ISO population (Jewitt & Seligman 2023). Such objects spread around the Milky Way’s disk in braided streams (Forbes et al. 2024), a small fraction of which intersect our Solar System. The observed ISO population is thus truly galactic, rather than being associated with local stars and stellar populations.
Image: ESO’s Very Large Telescope (VLT) has obtained new images of 3I/ATLAS, an interstellar object discovered in recent weeks. Identified as a comet, 3I/ATLAS is only the third visitor from outside the Solar System ever found, after 1I/ʻOumuamua and 2I/Borisov. Its highly eccentric hyperbolic orbit, unlike that of objects in the Solar System, gave away its interstellar origin. In this image, several VLT observations have been overlaid, showing the comet as a series of dots that move towards the right of the image over the course of about 13 minutes on the night of 3 July 2025. The data were obtained with the FORS2 instrument, and are available in the ESO archive. Credit: European Southern Observatory.
I’m struck anew by how much our view of our Solar System’s place in the cosmos has changed. The size and density of the Kuiper Belt only swam into focus when the first KBO was discovered in 1992, although the belt had been hypothesized by Kenneth Edgeworth in the 1930s and Gerald Kuiper in 1951. The vast Oort Cloud of comets that envelops our entire system was posited by Jan Hendrik Oort in 1950. Now we’re looking at populations of objects at minute sub-planetary scale existing between the stars in unfathomable numbers.
Hopkins and team point out that the Rubin Observatory Legacy Survey of Space and Time (LSST) will dramatically increase the number of confirmed ISOs. So then, what do we have on 3I/ATLAS? The early work on the object identifies it as a comet with a compact coma, a cloud of gas and dust surrounding the nucleus. It’s also bigger than its two predecessors, perhaps as large as 10 kilometers, as opposed to ‘Oumuamua and Borisov’s roughly 0.1 kilometers, although a more precise number will emerge as we learn more about its composition and albedo. It enters the Solar System at a higher speed than the latter ISOs, but one well within the distribution model used in this paper.
Interestingly, the object shows high vertical motion out of the plane of the galaxy, ruling out the idea that it comes from the same star as ‘Oumuamua or Borisov. That velocity points to an origin in the Milky Way’s thick disk – stars above and below the disk within which the Solar System resides. It is the first object to be identified as such. Says Hopkins:
“All non-interstellar comets such as Halley’s comet formed with our solar system, so are up to 4.5 billion years old. But interstellar visitors have the potential to be far older, and of those known about so far our statistical method suggests that 3I/ATLAS is very likely to be the oldest comet we have ever seen.”
The team’s model (based on Gaia data, disk chemistry and galactic dynamics) was developed during Hopkins’ doctoral research. It emerges as the first real-time application of predictive modelling to an interstellar comet. It likewise predicts that 3I/ATLAS will have a high water content. We’ll be able to check on that as observations continue. Co-author Michele Bannister, of the University of Canterbury in New Zealand, points out that 3I/ATLAS is already showing activity as it warms during its approach to the Sun. The gases the comet produces as it moves toward perihelion at 1.36 AU in October will tell us more.
The paper is Hopkins et al., “From a Different Star: 3I/ATLAS in the context of the ̄Otautahi–Oxford interstellar object population model,” submitted to Astrophysical Journal Letters and available as a preprint. The paper on the density of the interstellar object population is Do, Tucker & Tonry, “Interstellar Interlopers: Number Density and Origin of ‘Oumuamua-like Objects,” Astrophysical Journal Letters Vol. 855 (6 March 2018), L10. Full text. Also be aware of a new paper by Avi Loeb at Harvard that I haven’t yet had time to review. It’s “Comment on “Discovery and Preliminary Characterization of a Third Interstellar Object: 3I/ATLAS” (preprint).
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