Centauri Dreams
Imagining and Planning Interstellar Exploration
Are Supernovae Implicated in Mass Extinctions?
As we’ve been examining the connections between nearby stars lately and the possibility of their exchanging materials like comets and asteroids with their neighbors, the effects of more distant events seem a natural segue. A new paper in Monthly Notices of the Royal Astronomical Society makes the case that at least two mass extinction events in our planet’s history were forced by nearby supernova explosions. Yet another science fiction foray turned into an astrophysical investigation.
One SF treatment of the idea is Richard Cowper’s Twilight of Briareus a central theme of which is the transformation of Earth through just such an explosion. Published by Gollancz in 1974, the novel is a wild tale of alien intervention in Earth’s affairs triggered by the explosion of the star Briareus Delta, some 130 light years out, and it holds up well today. Cowper is the pseudonym for John Middleton Murry Jr., an author I’ve tracked since this novel came out and whose work I occasionally reread.
Image: New research suggests at least two mass extinction events in Earth’s history were caused by a nearby supernova. Pictured is an example of one of these stellar explosions, Supernova 1987a (centre), within a neighbouring galaxy to our Milky Way called the Large Magellanic Cloud. Credit: NASA, ESA, R. Kirshner (Harvard-Smithsonian Center for Astrophysics and Gordon and Betty Moore Foundation), and M. Mutchler and R. Avila (STScI).
Nothing quite so exotic is suggested by the new paper, whose lead author, Alexis Quintana (University of Alicante, Spain) points out that supernova explosions seed the interstellar medium with heavy chemical elements – useful indeed – but can also have devastating effects on stellar systems located a bit too close to them. Co-author Nick Wright (Keele University, UK) puts the matter more bluntly: “If a massive star were to explode as a supernova close to the Earth, the results would be devastating for life on Earth. This research suggests that this may have already happened.”
The conclusion grows out of the team’s analysis of a spherical volume some thousand parsecs in radius around the Sun. That would be about 3260 light years, a spacious volume indeed, within which the authors catalogued 24,706 O- and B-type stars. These are massive and short-lived stars often found in what are known as OB associations, clusters of young, unbound objects that have proven useful to astronomers in tracing star formation at the galactic level. The massive O type stars are considered to be supernova progenitors, while B stars range more widely in mass. Even so, larger B stars also end their lives as supernovae.
Making a census of OB objects has allowed the authors to pursue the primary reason for their paper, a calculation of the rate at which supernovae occur within the galaxy at large. That work has implications for gravitational wave studies, since supernova remnants like black holes and neutron stars and their interactions are clearly germane to such waves. The distribution of stars within 1 kiloparsec of the Sun shows stellar densities that are consistent with what we find in other associations of such stars, and thus we can extrapolate on the basis of their behavior to understand what Earth may have experienced in its own past.
Earlier work by other researchers points to supernovae that have occurred within 20 parsecs of the Sun – about 65 light years. A supernova explosion some 2-3 million years ago lines up with marine extinction at the Pliocene-Pleistocene boundary. Another may have occurred some 7 million years ago, as evidenced by the amount of interstellar iron-60 (60Fe), a radioactive isotope detected in samples from the Apollo lunar missions. Statistically, it appears that one OB association (known as the Scorpius–Centaurus association) has produced on the order of 20 supernova explosions in the recent past (astronomically speaking), and HIPPARCOS data show that the association’s position was near the Sun’s some 5 to 7 million years ago. These events, indeed, are thought by some to have produced the so-called Local Bubble, the low density cavity in the interstellar medium within which the Solar System currently resides.
Here’s a bit from the paper on this:
An updated analysis with Gaia data from Zucker et al. (2022) supports this scenario, with the supernovae starting to form the bubble at a slightly older time of ∼14 Myr ago. Measured outflows from Sco-Cen are also consistent with a relatively recent SN explosion occurring in the solar neighbourhood (Piecka, Hutschenreuter & Alves 2024). Moreover, there is kinematic evidence of families of nearby clusters related to the Local Bubble, as well as with the GSH 238+00 + 09 supershell, suggesting that they produced over 200 SNe [supernovae] within the last 30 Myr (Swiggum et al. 2024).
But the authors of this paper focus on much earlier events. Earth has experienced five mass extinctions, and there is coincidental evidence for the effects of a supernova in the late Devonian and Ordovician extinction events (372 million and 445 million years ago respectively). Both are linked with ozone depletion and mass glaciation.
A supernova going off within a few hundred parsecs of Earth would have atmospheric effects but little of significance. But the authors point out that if we close the range to 20 parsecs, things get more deadly. The destruction of the Earth’s ozone layer would be likely, with resultant mass extinctions a probable result:
Two extinction events have been specifically linked to periods of intense glaciation, potentially driven by dramatic reductions in the levels of atmospheric ozone that could have been caused by a near-Earth supernova (Fields et al. 2020), specifically the late Devonian and late Ordovician extinction events, 372 and 445 Myr ago, respectively (Bond & Grasby 2017). Our near Earth ccSN [core-collapse supernova] rate of ∼2.5 per Gyr is consistent with one or both of these extinction events being caused by a nearby SN.
So this is interesting but highly speculative. The purpose of this paper is to examine a specific volume of space large enough to draw conclusions about a type of star whose fate can tell us much about supernova remnants. This information is clearly useful for gravitational wave studies. The supernova speculation in regard to extinction events on Earth is a highly publicized suggestion that grows out of this larger analysis. In other words, it’s a small part of a solid paper that is highly useful in broader galactic studies.
Supernovae get our attention, and of course such discussions force the question of what happens in the event of a future supernova near Earth. Only two stars – Antares and Betelgeuse – are likely to become a supernova within the next million years. As both are more than 500 light years away, the risk to Earth is minimal, although the visual effects should make for quite a show. And we now have a satisfyingly large distance between our system and the nearest OB association likely to produce any danger. So much for The Twilight of Briareus. Great book, though.
The paper is Van Bemmel et al., “A census of OB stars within 1 kpc and the star formation and core collapse supernova rates of the Milky Way,” accepted at Monthly Notices of the Royal Astronomical Society (preprint).
Quantifying the Centauri Stream
The timescales we talk about on Centauri Dreams always catch up with me in amusing ways. As in a new paper out of Western University (London, Ontario), in which astrophysicists Cole Gregg and Paul Wiegert discuss the movement of materials from Alpha Centauri into interstellar space (and thence to our system) in ‘the near term,’ by which they mean the last 100 million years. Well, it helps to keep our perspective, and astronomy certainly demands that. Time is deep indeed (geologists, of course, know this too).
I always note Paul Wiegert’s work because he and Matt Holman (now at the Harvard-Smithsonian Center for Astrophysics) caught my eye back in the 1990s with seminal studies of Alpha Centauri and the stable orbits that could occur there around Centauri A and B (citation below). That, in fact, was the first time that I realized that a rocky planet could actually be in the habitable zone around each of those stars, something I had previously thought impossible. And in turn, that triggered deeper research, and also led ultimately to the Centauri Dreams book and this site.
Image: (L to R) Physics and astronomy professor Paul Wiegert and PhD candidate Cole Gregg developed a computer model to study the possibility that interstellar material discovered in our solar system originates from the stellar system next door, Alpha Centauri. Credit: Jeff Renaud/Western Communications.
Let’s reflect a moment on the significance of that finding when their paper ran in 1997. Wiegert and Holman showed that stable orbits can exist within 3 AU of both Alpha Centauri A and B, and they calculated a habitable zone around Centauri A of 1.2 to 1.3 AU, with a zone around Centauri B of 0.73 to 0.74 AU. Planets at Jupiter-like distances seemed to be ruled out around Centauri because of the disruptive effects of the two primary stars; after all, Centauri A and B sometimes close to within 10 AU, roughly the distance of Saturn from the Sun. The red dwarf Proxima Centauri, meanwhile, is far enough away from both (13,000 AU) so as not to affect these calculations significantly.
But while that and subsequent work homed in on orbits in the habitable zone, the Wiegert and Gregg paper examines the gravitational effects of all three stars on possible comets and meteors in the system. The scientists ask whether the Alpha Centauri system could be ejecting material, analyze the mechanisms for its ejection, and ponder how much of it might be expected to enter our own system. I first discussed their earlier work on this concept in 2024 in An Incoming Stream from Alpha Centauri. A key factor is that this triple system is in motion towards us (it’s easy to forget this). Indeed, the system approaches Sol at 22 kilometers per second, and in about 28,000 years will be within 200,000 AU, moving in from its current 268,000 AU position.
This motion means the amount of material delivered into our system should be increasing over time. As the paper notes:
…any material currently leaving that system at a low speed would be heading more-or-less toward the solar system. Broadly speaking, if material is ejected at speeds relative to its source that are much lower than its source system’s galactic orbital speed, the material follows a galactic orbit much like that of its parent, but disperses along that path due to the effects of orbital shear (W. Dehnen & Hasanuddin 2018; S. Torres et al. 2019; S. Portegies Zwart 2021). This behavior is analogous to the formation of cometary meteoroid streams within our solar system, and which can produce meteor showers at the Earth.
The effect would surely be heightened by the fact that we’re dealing not with a single star but with a system consisting of multiple stars and planets (most of the latter doubtless waiting to be discovered). Thus the gravitational scattering we can expect increases, pumping a number of asteroids and comets into the interstellar badlands. The connectivity between nearby stars is something Gregg highlights:
“We know from our own solar system that giant planets bring a little bit of chaos to space. They can perturb orbits and give a little bit of extra boost to the velocities of objects, which is all they need to leave the gravitational pull of the sun. For this model, we assumed Alpha Centauri acts similarly to our solar system. We simulated various ejection velocity scenarios to estimate how many comets and asteroids might be leaving the Alpha Centauri system.”
Image: In a wide-field image obtained with an Hasselblad 2000 FC camera by Claus Madsen (ESO), Alpha Centauri appears as a single bright yellowish star at the middle left, one of the “pointers” to the star at the top of the Southern Cross. Credit: ESO, Claus Madsen.
This material is going to be difficult to detect, to be sure. But the simulations the authors used, developed by Gregg and exhaustively presented in the paper, produce interesting results. Material from Alpha Centauri should be found inside our system, with the peak intensity of arrival showing up after Alpha Centauri’s closest approach in 28,000 years. Assuming that the system ejects comets at a rate like our own system’s, something on the order of 106 macroscopic Alpha Centauri particles should be currently within the Oort Cloud. But the chance of one of these being detectable within 10 AU of the Sun is, the authors calculate, no more than one in a million.
There should, however, be a meteor flux at Earth, with perhaps (at first approximation) 10 detectable meteors per year entering our atmosphere, most no more than 100 micrometers in size. That rate should increase by a factor of 10 in the next 28,000 years.
Thus far we have just two interstellar objects known to be from sources outside our own system, the odd 1I/’Oumuamua and the comet 2I/Borisov. But bear in mind that dust detectors on spacecraft (Cassini, Ulysses, and Galileo) have detected interstellar particles, even if detections of interstellar meteors are controversial. The authors note that this is because the only indicator of the interstellar nature of a particle is its hyperbolic excess velocity, which turns out to be very sensitive to measurement error.
We always think of the vast distances between stellar systems, but this work reminds us that there is a connectedness that we have only begun to investigate, an exchange of materials that should be common across the galaxy, and of course much more common in the galaxy’s inner regions. All this has implications, as the authors note:
…the details of the travel of interstellar material as well as its original sources remain unknown. Understanding the transfer of interstellar material carries significant implications as such material could seed the formation of planets in newly forming planetary systems (E. Grishin et al. 2019; A. Moro-Martín & C. Norman 2022), while serving as a medium for the exchange of chemical elements, organic molecules, and potentially life’s precursors between star systems—panspermia (E. Grishin et al. 2019; F. C. Adams & K. J. Napier 2022; Z. N. Osmanov 2024; H. B. Smith & L. Sinapayen 2024).
The paper is Gregg & Wiegert, “A Case Study of Interstellar Material Delivery: α Centauri,” Planetary Science Journal Vol. 6, No. 3 (6 March 2025), 56 (full text). The Wiegert and Holman paper, a key reference in Alpha Centauri studies, is “The Stability of Planets in the Alpha Centauri System,” Astronomical Journal 113 (1997), 1445–1450 (abstract).
Spaceline: A Design for a Lunar Space Elevator
The space elevator concept has been in the public eye since the publication of Arthur C. Clarke’s The Fountains of Paradise in 1979. Its pedigree among scientists is older still. With obvious benefits in terms of moving people and materials into space, elevators seize the imagination because of their scale and the engineering they would require. But we needn’t confine space elevators to Earth. As Alex Tolley explains in today’s essay, a new idea being discussed in the literature explores anchoring one end of the elevator to the Moon. Balanced by Earth’s gravity (and extending all the way into the domain of geosynchronous satellites), such an elevator opens the possibility of moving water and materials between Earth and a lunar colony, though the engineering proves as tricky as that needed for a system anchored on Earth. Is it even possible given the orbital characteristics of the Moon? Read on.
by Alex Tolley
Image: Space elevator connecting the moon to a space habitat. Credit: coolboy.
It is 2101, and the 22nd century has proven the skeptics wrong about space. Sitting comfortably in an Aries-1B moon shuttle on its way to Amundsen City in the Aitken basin at the lunar south pole, I am enjoying a bulb of hot Earl Grey tea. Sadly, without artificial gravity, no one has worked out how to dunk a digestive biscuit in hot drinks. The viewscreen shows the second movie of our almost 24-hour trip. Having been transferred from a spaceplane (far better than those giant VTOL rockets that reduced the cost of space access and paved the way for our multi-planetary advances), the moon-bound shuttle has crossed the Clarke orbit, past the remaining telecomm birds that haven’t been obsoleted by the swarms of comsats in low earth orbit (LEO).
A strange string of bright glints catches my eye. They are arrayed in an arrow-straight line, looking like dew on an invisible spiderweb seeking infinity. Nothing seems to move, the objects just apparently hanging in space. The flight attendant notices my apparent confusion. “What are they?” I ask as she crouches down beside me. “That is the newly operational Spaceline, a geosynchronous orbit to lunar surface space elevator. Those lights are the elevator cars carrying supplies to and from the moon. “But they are not moving,” I say. “They are moving, but too slowly to notice at this distance”, she replies, smiling, as this was probably a common observational mistake by passengers on the Moon run. “We should be seeing the expanding set of facilities at the Earth-Moon Lagrange Point 1 (EML1) nearer the Moon. The captain usually puts a magnified image on the viewscreens as we pass.”
This post is about space elevators, but not the Earth to Geosynchronous orbit that is most known, but rather a lesser known lunar space elevator {LSE), and in particular the one that rises from the lunar surface and terminates somewhere between the Earth’s surface and the Earth-Moon Lagrange Point 1 (EML1).
Because the Moon is tidally locked to Earth, an LSE from its surface can hang over the Earth, with the cable tension maintained by the gravitational pull of the Earth. Deployment of the cable is similar to the Earth space elevator (SE) which uses the Clarke orbit as a stable position that keeps the same point on the Earth below it, but in the case of the Moon, that deployment position is at the gravitationally neutral EML1. The cable is then simultaneously unrolled towards the Moon’s surface, and balanced by cable unrolled towards Earth and pulled towards Earth by gravity.
The idea of an LSE is not new and may even predate that of the Earth space elevator with a 1910 note by Friedrich Zander. Unlike the Earth space elevator that cannot be built as no material available can support its own mass, existing high-strength plastics like Zylon® can in principle be used today to build an LSE. Prior work by Pearson in 1979 on the LSE, and in 2005, with collaborators Levin, Oldson and Wykes published a NASA report on the LSE for two LSE cases, one passing through EML1 and the other anchored on the lunar farside passing through EML2 (using centrifugal force to tension the cable). These works and others demonstrated that an LSE could be built that would reduce the cost of transporting water and regolith to and from the Moon.
Prior work assumed that the cable would terminate in the Earth direction with a mass to provide the tension. The shorter the EML1 to terminus length, the greater the mass, and also the total mass of the LSE. A design tradeoff.
Image: The Lunar Space Elevator concept. A tapered cable from the Moon’s surface, through EML1 and terminating inside the geosynchronous orbit.
A 2019 arXiv preprint by Penoyre and Sandford adds their calculations for an LSE that they call the “Spaceline”. The authors show that a cable can be created without any counterweight between the Earth and the Moon, with the end of the cable dipping inside the geosynchronous orbital height. The total length of their optimal design is about 340,000 km, with a total mass of 40,000 kg (40 MT). The deployment facility and payload carriers are extra mass. The total mass of the system would therefore be lower than a design with a shorter cable and terminus mass. The authors also indicated that with the cable terminating inside the geosynchronous orbit, it would be easier to reach the cable from Earth and therefore reduce the transport costs further.
As with prior work, this all-cable design ensures that maximum tension in the cable is at the EML1 point, and declines towards both ends. The design ensures that the cable cannot collapse onto the Moon, nor break under load and fall towards Earth.
Table 1. Materials with values for density (ρ), breaking stress (B), specific strength (S), and relative strength (α). Materials with relative strength greater than 1.0 can be used in an LSE. Zylon have the highest relative strength that can be mass-manufactured. Carbon nanotubes can only be made in very short lengths at present. Source – Penoyre & Sandford.
This paper is primarily an exercise in designing an all-cable system using fixed area cross-sections, tapered cross-sections, and a hybrid of these two designs to simplify manufacture, deployment, and operation. Figure 2 shows that a Zylon cable of uniform cross-section suffices as an LSE that neither collapses nor breaks. Their calculations show that the tapered design is the most efficient with total mass, but the hybrid design using mostly uniform cross-section cable is a good compromise that simplifies manufacture and operation that reaches geosynchronous orbit.
Image: The white area is a feasible space for a cable of a uniform cross-section. With a relative strength above a critical value, a cable length can be constructed that neither collapses back to the Moon nor breaks under its own mass. Zylon can achieve this, albeit not to the desired geosynchronous orbit height. Not shown are the cases for a tapered and hybrid cable that can reach geosynchronous orbit. Source modified from Penoyre & Sandford
No attempt is made to determine other masses to support, such as the crawlers to carry payloads, how many could be supported, and the deployment hardware that must be transported to EML1. The assumption is that scaling up the area or number of cables will allow for the payload masses to be carried.
The authors do not justify the construction of the cable beyond showing that the cost of delivering payloads to the Moon, as well as returning material to space or Earth, is significantly reduced using a cable compared to spacecraft requiring propellant to transport the payloads.
The cost savings are not new, and no doubt a cable would be built if there were no other issues. But as with the SE, some issues complicate the construction of an LSE. Other authors have analyzed the LSE in more detail including survivability to space hazards [Eubanks], payload capability, speed of crawlers, ROI, and even transport of materials to and from the lunar South Pole to the LSE base on the lunar surface [Pearson et al, 2005].
So far so good. Penoyre and Sandford have shown that an unweighted cable can be used as an LSE which can stretch from the lunar surface to geosynchronous orbit, a mere 36,000 km from the Earth’s surface. Not quite as complete as the web between Earth and Moon in Aldiss’ Hothouse, but close. To reach the start of the cable, a spaceplane needn’t have to maintain geosynchronous orbit, but rather make a ballistic trajectory with the apogee reaching the terminus of the cable, and be captured by it similar to that of a skyhook, but with less difficulty.
But wait. Isn’t the paper skipping some important issues that could make this LSE impractical?
With the SE the geosynchronous orbit is circular. Once the orbital station is constructed and the cables reeled out to Earth and the counterweight, the system is very stable. This is not the case for the LSE.
The Moon’s orbit, with an eccentricity of 0.055, varies in distance from the Earth over its period from 362,600 km at perigee to 405,400 km at apogee, a difference of 42,800 km. This will result in the EML1 point moving back and forth towards the lunar surface about 36,300 km over the orbit, or about 18,000 km back and forth over the average EML1 distance. As the semi-major axis distance of EML1 to the lunar surface is about 57,000km, this is about a 2/3rd change in distance. Therefore the extra mass of the cable from EML1 to the lunar surface at apogee must be balanced by an extra length of the cable from EML1 to geosynchronous orbit, and vice versa at perigee. The base station at EML1 must therefore reel in or out cable continuously over the Moon’s orbital period. This is a dynamic situation that cannot fail or the LSE will be destabilized and potentially break or collapse.
Eubanks [2016] calculated that the micrometeoroid impacts would break a cable of uniform circular cross-section within hours, effectively breaking the cable before it could be deployed. The longer the cable, especially the long section between EML1 and geosynchronous orbit, the more quickly the break. A break in the cable would result in the section attached to the lunar surface falling back onto the Moon, wrapping itself around the Moon as it continued its orbit.
The other section would fall towards Earth, crossing the lower orbits and probably having a perigee that would enter the Earth’s atmosphere and likely burn up on entry over some time. Eubanks calculated that making the cable with a flat cross-section would ensure a lifetime between possible breaks of 5 years. Penoyre and Sandford acknowledged the danger of such breaks and also suggested a flat cross-section, although this would be less effective where the cable tapered.
While the LSE is relatively free of satellites and other artifacts, the question arises why the Earth’s terminus of the cable is inside the geosynchronous orbit. Geosynchronous satellites have a relative velocity of about 3 km/s relative to the end of the cable, posing a hazard to both satellites and cable. This is made worse if the cable length is not adjusted fast enough as it would dip deeper into the orbits of satellites with corresponding higher impact velocities and increased numbers of possible impacts. All this for the advantage of easier access to the end of the cable.
While accepting that cables with moving payloads are a cheaper way to transport material to and from the lunar surface, the speed at which these payloads can be moved is also relevant. An analogy might be that while walking across a country might be the cheapest form of travel, it is far slower than taking powered transport and time is important for commercial transport.
Various authors have used different assumptions of travel speed on the cable, up to 3600 kph (1.0 kps) [Radley 2017]. A more realistic speed might be 100 kph. At this speed, a payload from geosynchronous orbit to the lunar surface would take about 20 weeks. This is the same order of time to reach Mars on a Hohman orbit and similar to transoceanic voyages in the age of square-rigged sailing ships, This is entirely unsuited for transportation of humans or goods that can perish or be damaged by radiation from the solar wind or galactic cosmic rays. It would be suited for carrying bulk materials and equipment.
If the cable were a constant area flat ribbon, probably woven into a Hoytether [Eubanks 2016, Radley 2017], the payloads may not need to be self-powered but simply attached to the cable and moved like a cable car. Transport from the lunar surface to a station at EML1 would take about 3-4 weeks. Therefore people and some food supplies would still need to be ferried by rocket to and from the Moon.
While we think of the Moon as tightly locked facing the Earth, in practice it has a libration that would move the relative position of the lunar surface sideways back and forth over the month. This would send waves up the cable with a velocity dependent on the design of the cable. The dynamics of this oscillation would need to be investigated. Similarly, while Coriolis forces do not affect a static LSE, they will be a factor with the carriers moving on a cable from a low velocity at geosynchronous orbit to the Moon’s orbital velocity of about 1 kps. This is an order of magnitude higher than at the Earth terminus of the cable, and these forces will need to be determined for their effect on cable dynamics.
The authors also state that a base station of potentially immense size could be positioned at EML1 where the cable would be deployed. EML1 would be a convenient place to expand facilities making use of the zero gravity at that point. While Arthur Clarke had a manned telescope at EML1 in his novel A Fall of Moondust, EML1 is not a stable point or attractor, but rather unstable. This is made even worse by the orbit of the Moon which changes the position of this point and the surface over its orbit, as well as its position in orbit. As a result, the proposed Lunar Gateway space station is not placed at EML1 but rather in a Near Rectilinear Halo Orbit (NRHO) that requires far less fuel for station keeping. While the idea of a station at EML1 sounds attractive, it might be a costly facility to maintain, even with the advantage of having a cable to adjust its position.
Despite these caveats, there are good scientific and potential commercial reasons for reducing the cost of transporting mass to and from the Moon, as well as maintaining a facility close to the EML1. These have been explained in more detail by [Pearson 2005, Eubanks 2016]. I would add, as a fan of solar and beamed sails, that this could be a good place to deploy and launch these sails. There are no satellite hazards to navigate, and the low to zero gravity would allow these sails to reach escape velocity without needing the slow spiral out from Earth if started at LEO.
I propose that rather than having a cable reach geosynchronous orbit, it might be better to have a shorter weighted cable as proposed by Pearson even at the cost of a greater total mass of the LSE. Used in combination with a rotating tether in LEO (skyhook), transport to the tether could be achieved with an aircraft or suborbital rocket, attaching the payload to the skyhook, and having it launched into a high orbit to reach the end of the LSE. This would have to be a well-coordinated maneuver, reducing the costs even further albeit with the potential problems of skyhook and satellite impacts, especially with satellite swarms in LEO.
Our journey to the Moon has nearly ended. On the surface, I can see the long tracks of what looks like monorail lines. They were designed to launch packages of regolith or basic metal components into space, as originally envisaged by Gerard O’Neill in the late 20th century, to construct space solar power satellites and habitats. China’s AE Corp’s Spaceline (太空线) proved more economic, obsoleting the mass driver’s original purpose. They were repurposed to accelerate probes brought up from the Earth to the Moon, into deep space. One day their larger descendants will launch crewed spaceships on their journeys to the planets.
References
Penoyre, Z, Sandford E, (2019) The Spaceline: A Practical Space Elevator Alternative Achievable With Current Technology https://arxiv.org/abs/1908.09339
Pearson J, et al (2005) Lunar Space Elevators For Cislunar Space Development. Phase I Final Technical Report.
Eubanks, T. M. (2013). A space elevator for the far side of the moon. Annual Meeting of the Lunar Exploration Analysis Group, 1748, 7047. http://ui.adsabs.harvard.edu/abs/2013LPICo1748.7047E/abstract
Eubanks, T. M., & Radley, C. F. (2017). Extra-Terrestrial space elevators and the NASA 2050 Strategic Vision. Planetary Science Vision 2050 Workshop, 1989, 8172. https://ui.adsabs.harvard.edu/abs/2017LPICo1989.8172E/abstract
Eubanks, T. M., & Radley, C. F. (2016). Scientific return of a lunar elevator. Space Policy, 37, 97–102. https://doi.org/10.1016/j.spacepol.2016.08.005
Radley, C. F. (2017). The Lunar Space Elevator, a near term means to reduce cost of lunar access. 2018 AIAA SPACE and Astronautics Forum and Exposition. https://doi.org/10.2514/6.2017-5372
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.
Follow with RSS or E-Mail
