Centauri Dreams
Imagining and Planning Interstellar Exploration
A New Class of Interstellar Object?
Peculiar things always get our attention, calling to mind the adage that scientific discovery revolves around the person who notices something no one else has and says “That’s odd.” The thought is usually ascribed to Asimov, but there is evidently no solid attribution. Whoever said it in whatever context, “that’s odd” is a better term than “Eureka!” to describe a new insight into nature. So often we learn not all at once but by nudges and hunches.
This may be the case with the odd objects turned up by the Japanese infrared satellite AKARI in 2021. Looking toward the Scutum-Centaurus Arm along the galactic plane, the observatory found deep absorption bands of the kind produced by interstellar dust and ice. No surprise that a spectral analysis revealed water, carbon dioxide, carbon monoxide and organic molecules, given that interstellar ices in star-forming regions are rich in these chemicals, but the ‘odd’ bit is that these two objects are a long way from any such regions.
Image: Molecular emission lines from mysterious icy objects captured by the ALMA telescope. The background image is an infrared composite color map, where 1.2-micron light is shown in cyan and 4.5-micron light is in red, based on infrared data from 2MASS and WISE. Credit: ALMA (ESO/NAOJ/NRAO), T. Shimonishi et al. (Niigata Univ).
Interstellar ices are produced as submicron-sized dust grains, rich in carbon, oxygen, silicon, magnesium and iron, gather materials that adhere to their surfaces in cold and dense regions of the galaxy. Such ices are thought to be efficient at producing complex organic molecules, more so than chemical reactions that form in gaseous states, so they’re of high astrobiological interest.
The team performed follow-up observations using ALMA (this is the Atacama Large Millimeter/submillimeter Array in Chile) at a wavelength of 0.9 mm, useful because radio, as opposed to infrared, can be used to analyze the motion and composition of such gases. The data showed that what is being observed doesn’t exhibit the characteristics of any previously known interstellar objects in the vicinity of such ices. Instead, the researchers found molecular emission lines of carbon monoxide and silicon monoxide distributed in a tight region of less than one arcsecond. The expected submillimeter thermal emission from interstellar dust was not detected.
So what’s going on here? Takashi Shimonishi, an astronomer at Niigata University, Japan and lead author of the paper on this work, notes that the two objects are roughly 30,000 to 40,000 light years away. Interestingly, they show different velocities, indicating that they’re distinct and moving independently. Says the scientist:
“This was an unexpected result, as these peculiar objects are separated by only about 3 arcminutes on the celestial sphere and exhibit similar colors, brightness, and interstellar ice features, but they are not linked [to] each other.”
Let’s take a closer look at the odd energy distribution here. We would expect objects surrounded by ices would be embedded in interstellar dust, which should make for a bright signal in the far-infrared to submillimeter wavelength range. But ALMA detected no submillimeter radiation from either object. No previously known icy objects correspond to this signature.
Image: Energy distribution of one of the mysterious icy objects (black) compared with those of known interstellar icy objects. Interstellar ices are detected in protostars (green), young stars with protoplanetary disks (cyan), and mass-losing evolved stars (brown), but the spectral characteristics of the mysterious icy object do not match any of these known sources. Credit: T. Shimonishi et al. (Niigata Univ).
We learn from the paper that strong shockwaves seem to have disrupted interstellar dust in these bodies, based on the ratio of silicon monoxide to carbon monoxide. And the final oddity, at least so far: The size of the gas and dust clouds associated with these objects – determined by comparison of ALMA emission data with the AKARI absorption data – shows that both range from 100 to 1000 AU, which makes them compact in relation to typical molecular clouds.
So we have objects that don’t correspond to stars in the early stages of formation or stars shielded by dense molecular clouds. We seem to be looking at a new class of interstellar object altogether. The paper concludes:
These characteristics, i.e., (i) rich ice-absorption features, (ii) large visual extinction, (iii) lack of mid-infrared and submillimeter excess emission, (iv) very compact source size, (v) SiO-dominated broad molecular line emission [silicon monoxide], and (vi) isolation, cannot easily be accounted for by any of known interstellar icy sources. They may represent a previously unknown or rare type of isolated icy objects. Future high-spatial-resolution and high-sensitivity observations as well as detailed SED modeling is required. An upcoming near-infrared spectroscopic survey with SPHEREx (M. L. N. Ashby et al. 2023) may detect more similar sources.
Let’s hope so, because insight into oddities is a key part of interstellar exploration. Clearly we haven’t heard the last of these mysterious bodies.
The paper is Takashi Shimonishi et al., “ALMA Observations of Peculiar Embedded Icy Objects,” Astrophysical Journal 981 (2025), 49 (full text).
Shaping the Sail: Metamaterials and the Manipulation of Light
Experimenting on beamed energy and sailcraft is no easy matter, as I hope the previous post made clear. Although useful laboratory experiments have been run, the challenges involved in testing for sail stability under a beam and sail deployment are hard to surmount in Earth’s gravity well. They’re also demanding in terms of equipment, for we need both the appropriate lasers and vacuum chambers that can approximate the conditions the sail will be subjected to in space. But this space is being explored now more than ever before. Jim Benford has pointed me to an excellent bibliography on lightsail studies at Caltech that I recommend to anyone interested in following this further.
When I said we were in the early days of sail experimentation, I was drawing your attention to the fact that we’re only now learning how to produce and manipulate the metamaterials – structures that have electromagnetic properties beyond those we find in naturally occurring materials – that may be our best choices for sail material. Here I’m looking at a paper I cited last time, by Jadon Lin (University of Sydney) and colleagues. Lin points out that we need to put any sail materials through a battery of tests. Let me quote the paper on this, as the authors sum it up better than I could:
To name a few, tests are needed for: more complete characterization of linear and nonlinear optical properties of candidate materials over the broad NIR-Doppler band and MIR band [Near-Infrared and Mid-Infrared] and over the full range of temperatures a sail may encounter; initially small scale, then large scale nanostructure fabrication followed by complete optical characterization of their scattering; structural tests and; direct radiation-pressure measurements. In particular, low defect, high purity synthesis and nanostructuring of materials over square meter scales will require substantial technological advancements, especially for the intricate designs found by inverse design that are often unintuitive.
Image: An artist’s conception of a lightsail during acceleration by a ground-based laser array. Here the sail appears curved, as many early studies had indicated. Now we’re learning that flat sails have a new life of their own. Read on. Image credit: Masumi Shibata/Breakthrough Initiatives.
Quiet Revolution
Let’s back out to the big picture for a moment, because materials science is moving so rapidly. We went from flat sails to rotating parabolic shapes in early sail analysis, hoping to find a way to keep a sail inundated with a 50 GW beam stable under acceleration. The revolution now emerging with metamaterials is that we are returning to the flat sail concept. Instead of a shaped reflective surface, we’re envisioning using properties of the sail material, including photonic metagratings. Unlike a curved, lens-like surface, these are ‘scatterers’ at the nano-scale that direct and shape incoming light. Recent research shows we can use these to produce restoring forces and torques that keep a flat sail stable. This in its own way is something of a revolution.
We need to evaluate candidate materials and the methods of fabrication that will produce them, and then factor all of this into the design of the actual sail membrane. It’s telling that, according to Lin’s paper, the flexibility of the membrane and its relation to sail stability is in need of extensive testing, and so is the question of special relativity and sail dynamics, which when we’re discussing interstellar sails and their velocities must be considered. Gratings and metastructures, the authors believe, are the best ways to cope with distorted sail shapes or the effects of relativistic velocities. To some extent we can test these matters on the interplanetary level with small sails in space.
Let’s now wind this back to the Breakthrough Starshot concept, announced in 2016 and pursued through sail studies in following years. Here the idea is to use a ground-based laser array to push tiny payloads attached to lightsails up to 20 percent of the speed of light, making the journey to the nearest stars a matter of two decades. The technology could be applied to various systems, but of course Alpha Centauri is the obvious first candidate, and in particular Proxima Centauri b a prime target in the habitable zone.
Harry Atwater, a professor of applied physics and materials science at Caltech, has been at the forefront of sail research, focusing on the ultrathin membranes that will have to be developed to make such journeys possible. He and his colleagues have developed a platform for studying sail membranes that can measure the force that lasers exert on such sails, an example of the movement from theory to laboratory observation and measurement. Atwater sees the matter this way:
“There are numerous challenges involved in developing a membrane that could ultimately be used as lightsail. It needs to withstand heat, hold its shape under pressure, and ride stably along the axis of a laser beam. But before we can begin building such a sail, we need to understand how the materials respond to radiation pressure from lasers. We wanted to know if we could determine the force being exerted on a membrane just by measuring its movements. It turns out we can.”
Image: Caltech’s Harry Atwater. Credit: California Institute of Technology.
The Sail as Trampoline
The team’s paper on these early measurements of radiation pressure on lightsail materials appears in Nature Photonics (citation below). To measure these forces, the team creates a lightsail in miniature, tethered at the corners within a larger membrane. Electron beam lithography is the means of crafting a silicon nitride membrane a scant 50 nanometers thick, producing the result seen in the image below. As of now, silicon nitride seems to have the inside track as the leading material candidate
The Caltech researchers note their experiment’s similarity to a tiny trampoline – the membrane is a square some 40 microns wide and 40 microns long, and as the Caltech materials show, it is suspended at the corners by silicon nitride springs. So we have a tiny lightsail tethered within a larger membrane as the subject of our tests.
Image: A microscope image of the Caltech team’s “miniature trampoline,” a tiny lightsail tethered at the corners for direct radiation pressure measurement. Credit: Harry Atwater/Caltech.
The method is to subject the membrane to argon laser light at a visible wavelength, measuring the radiation pressure that it experiences by its effects on the motion of the ‘trampoline’ as it moves up and down. The tethering of the sail is itself a challenge, with the sail acting as a mechanical resonator that vibrates as the light hits it. Crucial to the measurement is to subtract the heat from the laser beam from the actual direct effect of radiation pressure. Ingeniously, the researchers quantified the motion induced by these long-range optical forces, measuring both the force and power of the laser beam.
This is intriguing stuff. A lightsail in space is not going to stay perpendicular to a laser source beaming up from Earth, so the measurements have to angle the laser beam in various ways to approximate this effect. The tiny signal from the motion of the lightsail material is isolated through the use of a common-path interferometer that effectively screens out environmental noise. The interferometer was integrated into the microscope being used to measure the sail, with the whole apparatus contained within a vacuum chamber. The result: Measurements down to the level of picometers could be detected, and mechanical stiffness shown by the motion of the springs as the sail was pushed by the radiation pressure from the laser.
Image: This is Figure 1 from the paper. Caption: From interstellar lightsails to laboratory-based lightsail platforms. a, Concept of laser-propelled interstellar lightsail of 10 m2 in area and 100 nm or less in thickness. b, Laboratory-based lightsail platforms relying on edge-constrained silicon nitride membranes (left), linearly tethered membranes (middle) and spring-supported membranes (right). Removing the edge constraint allows to decouple the effects of optical force and membrane deformation, model lightsail dynamics, and study optical scattering from the edges. Suspending lightsails by compliant serpentine springs rather than linear tethers significantly increases its mechanical susceptibility to laser radiation pressure of the same power P, resulting in larger out-of-plane displacement Δz for more precise detection. Credit: Michaeli et al.
The crucial issue of stability takes in the spread of the laser beam at an angle, with some of the beam missing the sample, perhaps due to its hitting the edge of the sail, scattering some of the light. It will be imperative to control any sideways motion and rotation in the sail once it is under the beam through careful crafting of the metamaterials from which it is made. Co-author Ramon Gao, a Caltech graduate student in applied physics, summarizes it this way:
“The goal then would be to see if we can use these nanostructured surfaces to, for example, impart a restoring force or torque to a lightsail. If a lightsail were to move or rotate out of the laser beam, we would like it to move or rotate back on its own. [This work] is an important stepping stone toward observing optical forces and torques designed to let a freely accelerating lightsail ride the laser beam.”
Thus we take an early step into the complexities of sail interaction with a laser. The paper presents the significance of this step:
Our observation platform enables characterization of the mechanical, optical, and thermal properties of lightsail prototype devices, thus opening the door for further multiphysics studies of radiation pressure forces on macroscopic objects. Additionally, photonic, phononic [sound-like waves traveling through a solid] or thermal designs tailored to optimize different aspects of lightsailing can be incorporated and characterized. In particular, characterizing and shaping optical forces with nanophotonic structures for far-field mechanical manipulation is central to the emerging field of meta-optomechanics, allowing for arbitrary trajectory control of complex geometries and morphologies with light. Laser-driven lightsails require self-stabilizing forces and torques emerging from judiciously designed metasurfaces for beam-riding. We expect that their direct observation is possible using our testbed, which is an important stepping stone towards the realization of stable, beam-riding interstellar lightsails, and optomechanical manipulation of macroscopic metaobjects.
We are, in other words, doing things with light that are far beyond what the early researchers into lightsails would have known about. I think about Robert Forward and Freeman Dyson at the 1980 JPL meeting I referred to last time, working out the math on an interstellar lightsail. Imagine what they would have made of the opportunity to use metamaterials and nanostructures to craft the optimum beam-rider. It’s heartening to see how the current effort at JPL under Harry Atwater is progressing. Laboratory experimentation on lightsails builds the knowledge base that will ultimately help us craft fast sails for missions within the Solar System and one day to another star.
The Atwater paper on sail technologies is Michaeli et al., “Direct radiation pressure measurements for lightsail membranes,” Nature Photonics 30 April 2025 (abstract). Also referred to above is Lin et al., “Photonic Lightsails: Fast and Stable Propulsion for Interstellar Travel,” available as a preprint.
Experimenting on an Interstellar Sail
The idea of beaming a propulsive force to a sail in space is now sixty years old, if we take Robert Forward’s first publications on it into account. The gigantic mass ratios necessary to build a rocket that could reach interstellar distances were the driver of Forward’s imagination, as he realized in 1962 that the only way to make an interstellar spacecraft was to separate the energy source and the reaction mass from the vehicle.
Robert Bussard knew that as well, which is why in more or less the same timeframe we got his paper on the interstellar ramjet. It would scoop up hydrogen between the stars and subject it to fusion. But the Bussard ramjet had to light fusion onboard, whereas a sail propelled by a laser beam – a lightsail – operated without a heavy engine. The idea worked on paper but demanded a laser of sufficient size (Forward calculated over 10 kilometers) to make it a concept for the far future. His solution demanded very large lasers in close solar orbits, and thus an existing system-wide space infrastructure.
Forward’s article “Pluto – Gateway to the Stars” ran in the journal Missiles and Rockets in April of 1962 (and would later be confused with an article having a similar name that ran in Galaxy that year, though without the laser sail concept). The beauty of the laser sail was immediately apparent, one of those insights that have other theorists asking themselves why they hadn’t come up with it. Because a beamed sail greatly eases the inverse square law problem. The latter tells us that solar photons aren’t enough because they decrease with the square of our distance from the Sun. Make your laser powerful enough and its narrow beam can push much harder and further.
Image: This is the original image from the Missiles and Rockets article. Caption: Theoretical method for providing power for interstellar travel is use of a very large Laser in orbit close to sun. Laser would convert random solar energy into intense, very narrow light beams that would apply radiation pressure to solar sail carrying space cabin at distances of light years. Rearward beam from Laser would equalize light pressure. Author Forward observes, however, that the Laser would have to be over 10 kilometers in diameter. Therefore other means must be developed.
All the work that began with Forward’s initial sail insights had been theoretical, with authors exploring laser concepts of varying sizes and shapes even as Forward offered fantastic mission designs that could take human crews to places like Epsilon Eridani while obeying the laws of physics. He believed a mission to Alpha Centauri could be launched as early as 1995, triggering interest from JPL’s Bruce Murray, who convened a workshop in 1980 to quantify Forward’s notions and find ways to return a payload to Earth. To my knowledge, the papers from this workshop have never been published, doubtless because the engineering demanded by such a mission was far beyond our reach. Still, it would be interesting to read the thoughts of workshop luminaries Freeman Dyson, Forward, Bussard and others on where we stood in that timeframe.
In 1999 NASA’s Advanced Concepts Office proposed a launch to Alpha Centauri in 2028, a notion that might have been furthered by Jim Benford and Geoffrey Landis’ proposal of using a carbon micro-truss (just invented in that year) that could withstand a microwave beam without melting. Now we begin to see actual laboratory experiments, and in the same year Leik Myrabo subjected carbon micro-truss material to laser beam bombardment to measure an acceleration of 0.15 gravities. See Benford’s A Photon Beam Propulsion Timeline for more on this period of sail laboratory work.
Image: Plan for the development of sails for interstellar flight, 1999. Credit: JPL/Caltech.
So laboratory work explicitly devoted to microwave- and laser-driven sails began 25 years ago and has lately resurfaced through work on sail materials that has developed through the Breakthrough Starshot initiative. Indeed, there are numerous recent papers scattered through the literature that we will be discussing in the future, some containing experimental results from Starshot-funded scientists. It would be helpful for the entire community if this work could be codified and presented in a single report.
But let’s go back to that early lab work. It was in April of 2000 that Benford showed, in experiments at JPL, that sails driven by a microwave beam could survive accelerations up to 13 gravities, while undergoing desorption when the sail reached high temperatures (desorption could have interesting propulsive effects of its own). The effects of spinning the sail were also examined, while Myrabo’s team in that same year experimented with carbon sails coated with molybdenum. By 2002, Benford and his brother Gregory demonstrated in work at UC-Irvine that a conical sail could be stable while riding a microwave beam.
While further work at the University of New Mexico under Chaouki Abdallah and team developed simulations confirming the stability of conical sails under a microwave beam, interest in sails primarily focused on materials in the work of scientists like Gregory Matloff and Geoffrey Landis. Landis’ work on dielectric films for highly reflective sails was particularly significant as materials science kept coming up with interesting candidates — Matloff proposed graphene as a sail material that can sustain high accelerations in 2012, and the examination of metamaterials for the task continues.
When Philip Lubin’s team at UC-Santa Barbara began their work on small wafer-sized spacecraft, it would feed into the concept of the Breakthrough Starshot initiative that was announced in 2016 (See Breakthrough Starshot: Early Testing of ‘Wafer-craft’ Design). Lubin’s work in turn grew out of the Project Starlight and DEEP-IN beamed energy studies his team pursued at UC-Santa Barbara, work that has now been collected in a two-volume set called The Path to Transformational Space Exploration.
A spacecraft on a chip can itself be a micro-sail, as Mason Peck (Cornell University) and team have pointed out in their examination of chips that could use solar photon pressure to move about the Solar System (see Beaming ‘Wafer’ Probes to the Stars). So the idea of miniaturizing a payload and exploiting the potential of laser beaming grafts readily onto the microchip research already underway. It’s interesting that the idea of incorporating the payload into the sail itself goes back to Robert Forward’s Starwisp concept, a kind of ‘smartsail’ whose surface contains the circuits that acquire data. Unfortunately, the Starwisp design had serious flaws, as Geoff Landis would later point out.
We’re still in the early stages in terms of laboratory work focused on sail materials for a lightsail that could carry any kind of payload. Let me quote an interesting new paper on this matter:
Most of the work discussed so far has been theoretical and numerical. Experimental verification of many aspects of lightsails, such as deployment and stability, are difficult to achieve in laboratories subject to Earth’s gravity, and may require extremely powerful lasers and extreme vacuum chambers. Many of the proposed structures are not yet able to be fabricated on the scales required, or rely on material properties that are insufficiently characterized.38 Thus, before full sails can be made, let alone tested, it is imperative that experimental characterizations that can be achieved on Earth be conducted.
This is from a paper by Jadon Lin (University of Sydney) and colleagues called “Photonic lightsails: Fast and Stable Propulsion for Interstellar Travel,” a preprint available here (thanks to Michael Fidler for the reference). We need to talk about the kind of tests needed, and I’ll begin with that next time. We’re headed for the interesting work performed at JPL under Harry Atwater that grows out of a concept some consider our best chance for reaching another star in this century.
Ernst Öpik and the Interstellar Idea
Some names seem to carry a certain magic, at least when we’re young. I think back to all the hours I used to haunt St. Louis-area libraries when I was growing up. I would go to the astronomy section and start checking out books until over time I had read the bulk of what was available there. Fred Hoyle’s name was magic because he wrote The Black Cloud, one of the first science fiction novels I ever read. So naturally I followed his work on how stars produce elements and on the steady state theory with great interest.
Willy Ley’s name was magic because he worked with Chesley Bonestell (another magic name) producing The Conquest of Space in 1949, and then the fabulous Rockets, Missiles, and Space Travel in 1957, a truly energizing read. Not to mention the fact that he had a science column in what I thought at the time was the best of the science fiction magazines, the ever-engaging Galaxy. It still stuns me that Ley died less than a month before Apollo 11 landed on the Moon.
My list could go on, but this morning I’ll pick one of the more obscure names, that of Ernst Öpik. Unlike Hoyle and Ley, Öpik (1893-1985) wasn’t famous for popularizing astronomy, but I would occasionally run into his name in the library books I was reading. An Estonian who did his doctoral work at that country’s University of Tartu, Öpik also did work at the University of Moscow but wound up fleeing Estonia in 1944 out of fear of what would happen when the Red Army swept into his country. He spent the productive second half of his career at the Armagh Observatory in Northern Ireland and for a time held a position as well at the University of Maryland.
Image: Ernst Öpik. Credit ESA.
Did I say productive? Consider that by 1970 Öpik had published almost 300 research papers, well over a hundred reviews and 345 articles for the Irish Astronomical Journal, of which he was editor from 1950 to 1981. He remained associate editor there until his death.
I found the references to Öpik in my reading rather fascinating, as I was reminded when Al Jackson mentioned him to me in a recent email. It turns out, as I had already found, that Öpik turns up in the strangest places. Recently I wrote about the so-called ‘manhole’ cover that some have argued is the fastest human object ever sent into space. The object is controversial, as it was actually a heavy cover designed to contain an underground nuclear blast, and rather spectacularly proven unsuccessful at that task. In short, it seems to have lifted off, a kind of mini-Orion. And no one really knows whether it just disintegrated or is still out there beyond the Solar System. See A ‘Manhole Cover’ Beyond the Solar System if this intrigues you.
Öpik’s role in the ‘manhole cover’ story grows out of his book The Physics of Meteor Flight in the Atmosphere, in which he calculated the mass loss of meteors moving through the atmosphere at various velocities. Although he knew nothing about the cover, Öpik’’s work turned out to be useful to Al as he thought about what would have happened to the cover. Because calculations on the potential speed of the explosively driven lid demonstrated that an object moving at six times escape velocity, as this would have been, would vaporize. This seems to put the quietus on the idea that the 4-inch thick iron lid used at the test detonation of Pascal B had been ‘launched’ into hyperbolic orbit.
But this was just a calculation that later became useful. In broader ways, Öpik was a figure that Al describes as much like Fritz Zwicky, meaning a man of highly original thought, often far ahead of this time. He turns out to have played a role in the development of the Oort Cloud concept. This would have utterly escaped my attention in my early library days since I had no access to the journals and wouldn’t have understood much if I did. But in a paper called “Note on Stellar Perturbations of Nearby Parabolic Orbits,” which ran in Proceedings of the American Academy of Arts and Sciences in 1932, the Estonian astronomer had this to say (after page after page of dense mathematics that are to this day far beyond my pay grade):
According to statistics by Jantzen, 395 comets (1909) showed a more or less random distribution of the inclinations, with a slight preponderance of direct motions over retrograde ones, with an age of from 109 to 3.109 years, this would correspond to an average aphelion distance of 1500-2000 a.u., or a period of revolution of 20000-30000 years. For greater aphelion distances the distribution of inclinations should be practically uniform, being smoothed out by perturbations.
Does this remind you of anything? Öpik was writing eighteen years before Jan Oort used cometary orbits to predict the existence of the cloud that now bears his name. Öpik believed there was a reservoir of comets around the Sun. There had to be, for a few comets were known to take on such eccentric orbits that they periodically entered the inner system and swung by our star, some close enough to throw a sizeable tail. Öpik was interested in how cometary orbits could be nudged by the influence of other stars. In other words, there must be a collection of objects at such a distance that were barely bound to the Sun and could readily be dislodged from their orbits.
I’m told that the Oort Cloud is, at least in some quarters, referred to as the Öpik/Oort Cloud, in much the same way that the Kuiper Belt is sometimes called the Edgeworth/Kuiper Belt because of similar work done at more or less the same time. But such dual naming strategies rarely win out in the end.
Being reminded of all this, I noticed that Öpik had done major work on such topics as visual binary stars (he estimated density in some of these), the distance of the Andromeda Galaxy, the frequency of craters on Mars, and the Yarkovsky Effect, which Öpik more or less put on the map through his discussions of Yarkovsky’s work. Studying him, I have the sense of a far-seeing man whose work was sometimes overlooked, but one whose contributions have in many cases proved to be prescient.
Naturally I was interested to learn whether Öpik had anything to say about our subject on Centauri Dreams, the prospect of interstellar flight. And indeed he did, in such a way that the sometimes glowering photographs we have of him seem to reveal something of his thinking on the matter (to be fair, some of us are simply not photogenic, and I understand that he was a kind and gentle man). Indeed, Armagh Observatory director Eric Lindsay described him thus:
…a “very human person with an understanding of, and sympathy for, our many frailties and, thank goodness, with a keen sense of humour. He will take infinite patience to explain the simplest problem to a person, young or old, with enthusiasm for astronomy but lacking astronomical background and training.”
The interstellar flight paper was written in 1964 for the Irish Astronomical Journal. Here he dismissed interstellar flight out of hand. Antimatter was a problem – remember that at the time he was writing, Öpik had few papers on interstellar flight to respond to, and he doesn’t seem to have been aware of the early work on sail strategies and lasers that Robert Forward and György Marx were exploring. So he focused on two papers he did know, the first being Les Shepherd’s study of interstellar flight via antimatter, seeing huge problems in storage and collection of the needed fuel. Here he quotes Edward Purcell approvingly. Writing in A.G.W. Cameron’s Interstellar Communication in 1963, Purcell said:
The exhaust power of the antimatter rocket would equal the solar energy received by the earth – all in gamma rays. So the problem is not to shield the payload, the problem is to shield the earth.
Having dismissed antimatter entirely, Öpik moves on to Robert Bussard’s highly visible ramjet concept, which had been published in 1960. He describes the ramjet sucking up interstellar gas and using it for fusion and spends most of the paper shredding the concept. I won’t go into the math but his arguments reflect many of the reasons that the ramjet concept has come to be met with disfavor. Here’s his conclusion:
…the ‘ramjet’ mechanism is impossible everywhere, as well as inside the Orion Nebula – one must get there first. “Traveling around the universe in space suits – except for local exploration… belongs back where it came from, on the cereal box.” (E. Purcell, loc. cit.). It is for space fiction, for paper projects – and for ghosts. “The only means of communication between different civilizations thus seems to be electro-magnetic signals” (S. von Hoerner, “The General Limits of Space Travel”, in Interstellar Communication, pp. 144-159). Slower motion (up to 0.01 c is a problem of longevity or hereditary succession of the crew; this we cannot reject because we do not know anything about it.
I always look back on Purcell’s comment and muse that cereal boxes used to be more interesting than they are today. I do wonder what Öpik might have made of sail strategies, and I’m aware of but have not seen a paper from him on interstellar travel by hibernation, written in 1978. So he seems to have maintained an interest in what he elsewhere referred to as “our cosmic destiny.” But like so many, he found interstellar distances too daunting to be attempted other than through excruciatingly long journey times in the kind of generation ship we’re familiar with in science fiction.
Since Öpik’s day a much broader community of scientists willing to study interstellar flight has emerged, even if for most it is a sideline rather than a dedicated project. We have begun to explore the laser lightsail as an option, but are only beginning the kind of laboratory work needed, even if a recent paper out of Harry Atwater’s team at Caltech shows progress. An unmanned flyby of a nearby star no longer seems to belong on a cereal box, but it’s a bit sobering to realize that even with sail strategies now under consideration by interstellar theorists, we’re still a long, long way from a mission.
Öpik’s paper on what would come to be known as the Oort Cloud is “Note on Stellar Perturbations of Nearby Parabolic Orbits,” Proceedings of the American Academy of Arts and Sciences, vol. 67 (1932), p. 169. The paper on interstellar travel is “Is Interstellar Travel Possible?” Irish Astronomical Journal Vol. 6(8) (1964), p. 299 (full text). The Irish Astronomical Journal put together a bibliography covering 1972 until his death in 1985, which students of Öpik can find here. The Atwater paper on sail technologies is Michaeli et al., “Direct radiation pressure measurements for lightsail membranes,” Nature Photonics 30 April 2025 (abstract). More on this one shortly.
SETI’s Hard Steps (and How to Resolve Them)
The idea of life achieving a series of plateaus, each of which is a long and perilous slog, has serious implications for SETI. It was Brandon Carter, now at the Laboratoire Univers et Théories in Meudon, France, who proposed the notion of such ‘hard steps’ back in the early 1980s. Follow-up work by a number of authors, especially Frank Tipler and John Barrow (The Anthropic Cosmological Principle) has refined the concept and added to the steps Carter conceived. Since then, the idea that life might take a substantial amount of the lifetime of a star to emerge has bedeviled those who want to see a universe filled with technological civilizations. Each ‘hard step’ is unlikely in itself, and our existence depends upon our planet’s having achieved all of them.
Carter was motivated by the timing of our emergence, which we can round off at 4.6 billion years after the formation of our planet. He reasoned that the upper limit for habitability at Earth’s surface is on the order of 5.6 billion years after Earth’s formation, a suspicious fact – why would human origins require a time that approximates the extinction of the biosphere that supports us? He deduced from this that the average time for intelligent beings to emerge on a planet exceeds the lifespan of its biosphere. We are, in other words, a lucky species that squeezed in our development early.
Image: Two highly influential physicists. Brandon Carter (right) sitting with Roy Kerr, who discovered the Einsteinian solution for a rotating black hole. Carter’s own early work on black holes is highly regarded, although these days he seems primarily known for the ‘hard steps’ hypothesis. Credit: University of Canterbury (NZ).
Figuring a G-class star like the Sun having a lifetime on the order of 10 billion years, most such stars would spawn planetary systems that never saw the evolution of intelligence, and perhaps not any form of life. Because an obvious hard step is abiogenesis, and although the universe seems stuffed with ingredients, we have no evidence yet of life anywhere else. The fact that it did happen here tells us nothing more than that, and until we dig out evidence of a ‘second genesis,’ perhaps here in our own Solar System inside an icy moon, or on Mars, we can form no firm conclusions.
There’s a readable overview of the ‘hard steps’ notion on The Conversation, and I’ll direct you both to that as well as to the paper just out from the authors of the overview, which runs in Science Advances (citation below). In both, Penn State’s Jason Wright and Jennifer Macalady collaborate with Daniel Brady Mills (Ludwig Maximilian University of Munich) and the University of Rochester’s Adam Frank to describe such ‘steps’ as the development of eurkarytic cells – i.e., cells with nuclei. We humans are eukaryotes, so this hard step had to happen for us to be reading this.
We could keep adding to the list of hard steps as the discussion has spun out over the past few decades, but it seems agreed that photosynthesis is a big one. The so-called ‘Cambrian explosion’ might be considered a hard step, since it involves sudden complexity, refinements to body parts of all kinds and specialized organs, and it happens quickly. And what of the emergence of consciousness itself? That’s a big one, especially since we are a long way from explaining just what consciousness actually is, and how and even where it develops. Robin Hanson has used the hard steps concept to discuss ‘filters’ that separate basic lifeforms from complex technological societies.
Whichever steps we choose, the idea of a series of highly improbable events leveraging each other on the road to intelligence and technology seems to make the chances of civilizations elsewhere remote. But let’s pause right there. Wright and colleagues take note of the work of evolutionary biologist Geerat Vermeij (UC-Davis), who argues that our view of innovation through evolution is inescapably affected by information loss. Here’s a bit on this from the new paper:
Vermeij concluded that information loss over geologic time could explain the apparent uniqueness of ancient evolutionary innovations when (i) small clades [a clade comprises a founding ancestor and all of its descendants] that independently evolved the innovation in question go extinct, leaving no living descendants, and (ii) an ancient innovation evolved independently in two closely related lineages, or within a short period of time, and the genetic differences between these two lineages become “saturated” to the point where the lineages become genetically indistinguishable.
In other words, as we examine life on early Earth, we have to reckon with incompleteness in our fossil record (huge gaps possible there), with species we know nothing about going extinct despite having achieved a hard step. The authors point out that if this is the case, then we can’t really describe proposed hard steps as ‘hard.’ Other possibilities exist, including that innovations do happen only once, but they may be so powerful that creatures with a new evolutionary trait quickly change their environment so that other lineages of evolution don’t have time to develop.
Image: Earth’s habitability is compromised by a Sun that will, about 5.6 billion years after its formation, become too hot to allow life. Image credit: Wikimedia Commons.
We’re still left with the question of why it has taken so much of the lifetime of the Sun to produce ourselves, a question that bothered Carter sufficiently in 1983 that it drove him to the hard steps analysis. Here the authors offer something Carter did not, an analysis of Earth’s habitability over time. It’s one that can change the outcome. For each of the hard steps sets up its own evolutionary requirements, and these could be met only as Earth’s environment changed. Consider, for example, that 50 percent of our planet’s history elapsed before modern eukaryotic cells had enough oxygen to thrive.
So maybe our planet had to pass certain environmental thresholds:
…we raise the possibility that there are no hard steps (despite the appearance of major evolutionary singularities in the universal tree of life) (51) and that the broad pace of evolution on Earth is set by global-environmental processes operating on geologic timescales (i.e., billions of years) (30). Put differently, humans originated so “late” in Earth’s history because the window of human habitability has only opened relatively recently in Earth history.
Suppose abiogenesis is not a hard step. Biosignatures, then, should be common in planetary atmospheres, at least on planets like Earth that are geologically active, in the habitable zone of their stars, and have atmospheres involving nitrogen, carbon dioxide and water. If oxygenic photosynthesis is a hard step, then we’ll find atmospheres that are low in oxygen, rich in methane and carbon dioxide and other ingredients of the atmosphere of the early Earth. If no hard steps exist at all, then we should find the full range of atmospheric types from early Earth (Archean) to present day (Phanerozoic). Our study of atmospheres will help us make the call on the very existence of hard steps.
Given a lack of hard steps, if this model is correct, then the evolution of a biosphere appears more predictable as habitats emerge and evolve. That would offer us a different way of assessing Earth’s past, but also imply that the same trends have emerged on other worlds like Earth. Our existence in that sense would imply that intelligent beings in other stellar systems are more probable than Carter believed.
The paper is Mills et al., “Reassessment of the “hard-steps” model for the evolution of intelligent life,” Science Advances. Vol. 11, Issue 7 (14 February 2025). Full text. Brandon Carter’s famous paper on the hard steps is “The Anthropic Principle and its Implications for Biological Evolution.” Philosophical Transactions of the Royal Society of London A 310 (1983), 347–363. Abstract.
Pandora: Exoplanet Atmospheres via Smallsat
I’ve been digging into NASA’s Small Spacecraft Strategic Plan out of continuing interest in missions that take advantage of miniaturization to do things once consigned to large-scale craft. And I was intrigued to learn about the small spacecraft deployed on Apollo 15 and 16, two units developed by TRW in a series called Particles and Fields Subsatellites. Each weighed 35 kilograms and was powered by six solar panels and rechargeable batteries. The midget satellites were deployed from the Apollo Command and Service Module via a spring-loaded container giving the units a four foot-per-second velocity. Apollo 15’s operated for six months before an electronics failure ended the venture. The Apollo 16 subsatellite crashed on the lunar surface 34 days into its mission after completing 424 orbits.
Here I thought I knew Apollo history backwards and forwards and I had never run into anything about these craft. It turns out that smallsats – usually cited as spacecraft with weight up to 180 kilograms – have an evocative history in support of larger missions, and current planning includes support for missions with deep space applications. Consider Pandora, which is designed to complement operations of the James Webb Space Telescope, extending our knowledge of exoplanet atmospheres with a different observational strategy.
JWST puts transmission spectroscopy to work, analyzing light from the host star as a transiting planet moves across the disk. A planet’s spectral signature can thus be derived and compared to the spectrum taken when the planet is out of transit and only the star is visible. This is helpful indeed, but despite JWST’s obvious successes, detecting the atmosphere of planets as small as Earth is a challenge. The chief culprit is magnetic activity on the star itself, contaminating the spectral data. The Pandora mission, a partnership between NASA and Lawrence Livermore National Laboratory, mitigates the problem by collecting long-duration observations at simultaneous visible and infrared wavelengths.
Image: A transmission spectrum made from a single observation using Webb’s Near-Infrared Imager and Slitless Spectrograph (NIRISS) reveals atmospheric characteristics of the hot gas giant exoplanet WASP-96 b. A transmission spectrum is made by comparing starlight filtered through a planet’s atmosphere as it moves across the star, to the unfiltered starlight detected when the planet is beside the star. Each of the 141 data points (white circles) on this graph represents the amount of a specific wavelength of light that is blocked by the planet and absorbed by its atmosphere. In this observation, the wavelengths detected by NIRISS range from 0.6 microns (red) to 2.8 microns (in the near-infrared). The amount of starlight blocked ranges from about 13,600 parts per million (1.36 percent) to 14,700 parts per million (1.47 percent). Credit: European Space Agency.
Stellar contamination produces spectral noise that mimics features in a planetary atmosphere, or else obscures them, a problem that has long frustrated scientists. Collecting data at shorter wavelengths than JWST’s shortest wavelengths (0.6 microns) helps get around this problem. Pandora’s visible light channel will track the spot-covering fractions of surface stellar activity while its Near-Infrared channel will simultaneously measure the variation in spectral features as the star rotates. A more fine-grained correction for stellar contamination thus becomes possible, and as the new paper on this work explains, the ultimate objective then becomes “…to robustly identify exoplanets with hydrogen- or water-dominated atmospheres, and determine which planets are likely covered by clouds and hazes.”
Pandora will operate concurrently with JWST, complementing JWST’s deep-dive, high-precision spectroscopy measurements with broad wavelength, long-baseline observations. Pandora’s science objectives are well-suited for a SmallSat platform and illustrate how small missions can be used to truly maximize the science from larger flagship missions.
The plan is for the mission to select 20 primary exoplanet host stars and collect data from a minimum of 10 transits per host star, with each observation lasting about 24 hours, producing 200 days of science observations. The lengthy data acquisition time for each star means an abundance of out-of-transit data can be collected to address the problem of stellar contamination. The primary mission has a lifetime of one year, which allows for a significant range of science operations in addition to the above.
Long-duration measurements like those planned for Pandora contrast with data collection on large missions like JWST, which often focus on one or a small number of transits per target. Such complementarity is a worthy goal, and a reminder of the lower cost and high adaptability of using the smallsat platform in conjunction with a primary mission. In addition, smallsats rely on standardized and commercial parts to reduce risk and avoid solutions specific to any single mission. Cost savings can be substantial.
Image: The Pandora observatory shown with the solar array deployed. Pandora is designed to be launched as a ride-share attached to an ESPA Grande ring [(Evolved Expendable Launch Vehicle) Secondary Payload Adapter ring]. Very little customization was carried out on the major hardware components of the mission such as the telescope and spacecraft bus. This enabled the mission to minimize non-recurring engineering costs. Credit: Barclay et al.
Operating at these scales has clear deep space applications. This is a fast growing, innovative part of spacecraft design that has implications for all kinds of missions, and I’m reminded of the interesting work ongoing at the Jet Propulsion Laboratory in terms of designing a mission to the Sun’s gravity lens. Smallsats and self-assembly enroute may prove to be a game-changer there.
For the technical details on Pandora, see the just released paper. The project completed its Critical Design Review in October of 2023 and is slated for launch into a Sun-synchronous orbit in the Fall of this year. Launch is another smallsat benefit, for many smallsats are being designed to fit into a secondary payload adapter ring on the launch vehicle, allowing them to be ‘rideshare’ missions that launch with other satellites.
The paper is Barclay et al., “The Pandora SmallSat: A Low-Cost, High Impact
Mission to Study Exoplanets and Their Host Stars,” accepted for the IEEE Aerospace Conference 2025. The preprint is here.
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