Paul Gilster's Blog, page 6

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.

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. Preprint.

A Three-Dimensional Look at an Exoplanet Atmosphere

Some 900 light years away in the constellation Puppis, the planet WASP-121b is proving an interesting test case as we probe ever deeper into exoplanetary atmospheres. As has been the case with so many early atmosphere studies, WASP-121b, also known as Tylos, is a hot-Jupiter, with a year lasting about thirty Earth hours, in a vise-like tidal lock that leaves one side always facing the star, the other away. What we gain in two new studies of this world is an unprecedented map of the atmosphere’s structure.

At stake here is a 3D look into what goes on as differing air flows move from one side of the planet to the other. A jet stream moves material around its equator, but there is a separate flow at lower altitudes that pumps gas from the hottest regions to the dark side. “This kind of climate has never been seen before on any planet,” says Julia Victoria Seidel (European Southern Observatory), lead author of a paper that appears today in Nature. Seidel points out that we have nothing in the Solar System to rival the speed and violence of the jet stream as it crosses the hot side of Tylos.

The astronomers used the European Southern Observatory’s Very Large Telescope, combining all four units to parse out the movement of chemical elements like iron and titanium in the weather patterns produced by these layered winds. What’s particularly significant here is the fact that we are now able to delve into an exoplanet atmosphere at three levels, analyzing variations in altitude as well as across varying regions on the world, and finally the interactions that produce weather patterns, form clouds and induce precipitation. Such 3D models take us to the greatest level of complexity yet.

“The VLT enabled us to probe three different layers of the exoplanet’s atmosphere in one fell swoop,” says study co-author Leonardo A. dos Santos, an assistant astronomer at the Space Telescope Science Institute in Baltimore. Tracking the movements of iron, sodium and hydrogen, the researchers could follow the course of winds at different layers in the planet’s atmosphere. A second paper, published in Astronomy & Astrophysics, announced the discovery of titanium in the atmosphere.

Image: Structure and motion of the atmosphere of the exoplanet Tylos. Astronomers have peered through the atmosphere of a planet beyond the Solar System, mapping its 3D structure for the first time. By combining all four telescope units of the European Southern Observatory’s Very Large Telescope (ESO’s VLT), they found powerful winds carrying chemical elements like iron and titanium, creating intricate weather patterns across the planet’s atmosphere. The discovery opens the door for detailed studies of the chemical makeup and weather of other alien worlds. Credit: ESO.

Note what we have in the image above. The paper describes it this way:

…a unilateral flow from the hot starfacing side to the cooler space-facing side of the planet sits below an equatorial super-rotational jet stream. By resolving the vertical structure of atmospheric dynamics, we move beyond integrated global snapshots of the atmosphere, enabling more accurate identification of flow patterns and allowing for a more nuanced comparison to models.

And that’s the key here – refining existing models to pave the way for future work. Digging into the 3D structure of the atmosphere required the VLT’s ESPRESSO spectrograph, collecting four times the light of an individual instrument to reveal the planet as it transited its star, a F-class star with mass and radius close to that of the Sun. Planet Tylos is named after the ancient Greek name for Bahrain, as part of the project. The host star bears the name Dilmun after the ancient civilization emergent on a trade route in the region after the 3rd millennium BC.

The Seidel et al. paper notes that existing Global Circulation Models (3D) do not fully capture what is observed at WASP-121b, making scenarios like these valuable testbeds for advancing the state of the art. Extremely Large Telescopes now under development will be able to put these refined models to work as they broaden the study of exoplanet atmospheres in extreme conditions:

The discrepancy between GCMs and the provided observations highlight the impact of high signal-to-noise ratio data of extreme worlds such as ultra-hot Jupiters in benchmarking our current understanding of atmospheric dynamics. This study marks a shift in our observational understanding of planetary atmospheres beyond our solar system. By probing the atmospheric winds in unprecedented precision, we unveil the 3D structure of atmospheric flows, most importantly the vertical transitions between layers from the deep sub-to-anti-stellar-point winds to a surprisingly pronounced equatorial jet stream. These benchmark observations made possible by ESPRESSO’s 4-UT mode serve as a catalyst for the advancement of global circulation models across wider vertical pressure ranges thus significantly advancing our knowledge on atmospheric dynamics.

The papers are Seidel et al., “Vertical structure of an exoplanet’s atmospheric jet stream,” Nature 18 February 2025 (abstract) and Prinoth et al., “Titanium chemistry of WASP-121 b with ESPRESSO in 4-UT mode,” in process at Astronomy & Astrophysics (preprint).

What Would Surprise You?

Someone asked me the other day what it would take to surprise me. In other words, given the deluge of data coming in from all kinds of observatories, what one bit of news would set me back on my heels? That took some reflection. Would it surprise me, my interlocutor persisted, if SETI fails to find another civilization in my lifetime?

The answer to that is no, because I approach SETI without expectations. My guess is that intelligence in the universe is rare, but it’s only a hunch. How could it be anything else? So no, continuing silence via SETI does not surprise me. And while a confirmed signal would be fascinating news, I can’t say it would truly surprise me either. I can work out scenarios where civilizations much older than ours do become known.

Some surprises, of course, are bigger than others. Volcanoes on Io were a surprise back in the Voyager days, and geysers on Enceladus were not exactly expected, but I’m talking here about an all but metaphysical surprise. And I think I found one as I pondered this over the last few days. What would genuinely shock me – absolutely knock the pins out from under me – would be if we learn through future observation and even probes that Proxima Centauri b is devoid of life.

I’m using Proxima b as a proxy for the entire question of life on other worlds. We have no idea how common abiogenesis is. Can life actually emerge out of all the ingredients so liberally provided by the universe? We’re here, so evidently so, but are we rare? I would be stunned if Proxima b and similar planets in the habitable zone around nearby red dwarfs showed no sign of life whatsoever. And of course I don’t limit this to M-class stars.

Forget intelligence – that’s an entirely different question. I realize that my core assumption, without evidence, is that abiogenesis happens just about everywhere. And I think that most of us share this assumption.

The universe is going to seem like a pretty barren place if we discover that it’s wildly unlikely for life to emerge in any form. I’ve mentioned before my hunch that when it comes to intelligent civilizations, the number of these in the galaxy is somewhere between 1 and 10. At any given time, that is. Who knows what the past has held, or what the future will bring? But if we find that life itself doesn’t take hold to run the experiment, it’s going to color this writer’s entire philosophy and darken his mood.

We want life to thrive. Notice, for example, how we keep reading about potentially habitable planets, our fixation with the habitable zone being natural because we live in one and would like to find places like ours. Out of Oxford comes a news release with the headline “Researchers confirm the existence of an exoplanet in the habitable zone.” That’s the tame version of more lively stories that grow out of such research with titles like “Humans could live here” and “A Home for ET.” I’m making those up, but you know the kind of headlines I mean, and they can get more aggressive still. We hunger for life.

Here’s one from The Times: “‘Super-Earth’ discovered — and it’s a prime candidate for alien life.’” But is it?

Image: Artist’s depiction of an exoplanet like HD 20794 d in a conceivably habitable orbit. It may or may not be rocky. It may or may not be barren. How much do our expectations drive our thinking about it? Credit: University of Oxford.

That Oxford result is revealing, so let’s pause on it. HD 20794 d is about 20 light years from us, orbiting a G-class star like the Sun, which gives it that extra cachet of being near a familiar host. Three confirmed planets and a dust disk orbit this star in Eridanus, the most interesting being the super-Earth in question, which appears to be about twice Earth’s radius and 5.8 times its mass. The HARPS (High Accuracy Radial Velocity Planet Searcher) and ESPRESSO spectrographs at La Silla (Chile) have confirmed the planet, quite a catch given that the original signal detected in radial velocity studies was at the limit of the HARPS spectrograph’s capabilities.

Habitable? Maybe, but we can’t push this too far. The paper notes that “HD 20794 d could also be a mini-Neptune with a non-negligible H/He atmosphere.” And keep an eye on that elliptical orbit, which means climate on such a world would be, shall we say, interesting as it moves among the inner and outer edges of the habitable zone during its 647-day year. I think Oxford co-author Michael Cretignier is optimistic when he refers to this planet as an ‘Earth analogue,’ given that orbit as well as the size and mass of the world, but I get his point that its proximity to Sol makes this an interesting place to concentrate future resources. Again, my instincts tell me that some kind of life ought to show up if this is a rocky world, even if it’s nothing more than simple vegetation.

Because it’s so close, HD 20794 d is going to get attention from upcoming Extremely Large Telescopes and missions like the Habitable Worlds Observatory. The level of stellar activity is low, which is what made it possible to tease this extremely challenging planetary signal out of the noise – remember the nature of the orbit, and the interactions with two other planets in this system. Probing its atmosphere for biosignatures will definitely be on the agenda for future missions.

Obviously we don’t know enough about HD 20794 d to talk meaningfully about it in terms of life, but my point is about expectation and hope. I think we’re heavily biased to expect life, to the point where we’re describing habitable zone possibilities in places where they’re still murky and poorly defined. That tells me that the biggest surprises for most of us will be if we find no life of any kind no matter which direction we look. That’s an outcome I definitely do not expect, but we can’t rule it out. At least not yet.

The paper is Nari et al., “Revisiting the multi-planetary system of the nearby star HD 20794 Confirmation of a low-mass planet in the habitable zone of a nearby G-dwarf,” Astronomy & Astrophysics Vol. 693 (28 January 2025), A297 (full text).