Paul Gilster's Blog, page 23

Freeman Dyson’s Gravitational Machines

What an intriguing thing to find Freeman Dyson’s “Gravitational Machines” paper popping up on arXiv. This one is yet another example of Dyson’s prescience, for in it he examines, decades before the actual event, how gravitational waves could be produced and detected, although he uses neutron stars rather than black holes as his focus. Fair enough. When this was written, in 1962, black holes were far more conjectural than they appear in most of the scientific literature today.

But what a tangled history this paper presents. First of all, how does a 1962 paper get onto arXiv? A quick check reveals the uploader as David Derbes, a name that should resonate with Dyson purists. Derbes (University of Chicago Laboratory Schools, now retired) is the power behind getting Dyson’s lectures on quantum electrodynamics, first given at Cornell in 1951, into print in the volume Advanced Quantum Mechanics (World Scientific Publishing, 2007). He’s also an editor on Sidney Coleman’s Lectures on Relativity (Cambridge University Press, 2022) and has written a number of physics papers.

“Gravitational Machines” has been hard to find. Dyson wrote it, according to my polymath friend Adam Crowl, for the Gravitational Research Foundation in 1962; Centauri Dreams regular Al Jackson corroborates this in an email exchange, noting that the GRF was created by one Roger Babson, who offered a prize for such papers. Astrophysicist Alastair G. W. Cameron added it to his early SETI tome Interstellar Communications: A Collection of Reprints and Original Contributions (W. A. Benjamin, 1963). The paper, a tight six pages, does not appear in the 1996 volume Selected Papers of Freeman Dyson with Commentary (American Mathematical Society, 1996).

So we can be thankful that David Derbes saw fit to post it on arXiv. Al Jackson noted in his email that Greg Benford and Larry Niven have used Dyson’s gravitational concepts in their work, so I suspect “Gravitational Machines” was a paper known to them at this early stage of their career. A recent phone call with Jim Benford also reminded me of the Dyson paper’s re-emergence. Listen to Dyson’s familiar voice in 1962:

The difficulty in building machines to harness the energy of the gravitational field is entirely one of scale. Gravitational forces between objects of a size that we can manipulate are so absurdly weak that they can scarcely be measured, let alone exploited. To yield a useful output of energy, any gravitational machine must be built on a scale that is literally astronomical. It is nevertheless worthwhile to think about gravitational machines, for two reasons. First, if our species continues to expand its population and its technology at an exponential rate, there may come a time in the remote future when engineering on an astronomical scale will be both feasible and necessary. Second, if we are searching for signs of technologically advanced life already existing elsewhere in the universe, it is useful to consider what kinds of observable phenomena a really advanced technology might be capable of producing.

There’s the Dysonian reach into the far future, sensing where exponential technology growth might lead a civilization, and speculating at the most massive scale on the manipulation of matter as a form of engineering. But here too is the Dyson of ‘Dyson Sphere’ fame, tackling the question of whether or not such a project would be observable if undertaken elsewhere in the cosmos, just as he would go on to bring numerous other ideas on ‘technosignatures’ to our consciousness. Hence the term ‘Dysonian SETI,’ which I’ve often used here on Centauri Dreams.

Dyson goes on to speculate on the nature of eclipsing white dwarf binaries and their output of gravitational radiation, working the math to demonstrate the strength of such systems in terms of gravitational wave output, and finding that the output might be detectable. However, what catches his eye next is the idea of neutron star binaries, although he notes that at the time of writing, these objects were entirely hypothetical. But if they did exist (they do), their gravitational output should be “interesting indeed.”

…the loss of energy by gravitational radiation will bring the two stars closer with ever-increasing speed, until in the last second of their lives they plunge together and release a gravitational flash at a frequency of about 200 cycles and of unimaginable intensity.

It’s interesting that at the time Dyson wrote, Joseph Weber was mounting what must be the first attempt to detect gravitational waves, although he seems to have found nothing but instrumental noise. The LIGO (Laser Interferometer Gravitational-Wave Observatory) team would go on to cite Weber’s work following their successful detection of GW170817 in 2017, produced just as Dyson predicted by a neutron star binary. Calling such waves “a neglected field of study,” the 1962 paper adds this:

…the immense loss of energy by gravitational radiation is an obstacle to the efficient use of neutron stars as gravitational machines. It may be that this sets a natural limit of about 108 cm/sec to the velocities that can be handled conveniently in a gravitational technology. However, it would be surprising if a technologically advanced species could not find a way to design a nonradiating gravitational machine, and so to exploit the much higher velocities which neutron stars in principle make possible.

At the end of the paper posted on arXiv, David Derbes adds a useful note, pointing out Dyson’s prescience in this field, and adding that he had secured Dyson’s permission to publish the article before the latter’s death. But as typical of Dyson, he also stressed that he wanted Weber’s contribution to be noted, which Derbes delivered on by inserting a footnote to that effect in the text. We can all thank David Derbes for bringing this neglected work of a masterful scientist back into wider view.

In the next post, I want to talk about how these gravitational wave energies might be exploited by the ‘machines’ Dyson refers to in the title of the paper. The paper is Dyson, “Gravitational Machines,” now available on arXiv.

The Prevalence of ‘Jupiters’ around Larger Stars

Work on the Centauri Dreams internals continues, with the unwelcome result that the site has been popped offline twice because of a possible security problem. Needless to say, this has to be resolved before I can move forward on other aspects of the rebuild. While I deal with that issue, let me respond to a few backchannel questions about yesterday’s post on gas giants in red dwarf planetary systems. What I’m being asked about is my comment that gas giants like Jupiter, at similar distances and installation, around other classes of stars are common compared to what we see at red dwarfs.

This has been a problematic issue, and the matter is a long way from achieving a consensus among researchers. A moment’s reflection yields the reason: Finding gas giants in outer system orbits around a star like the Sun is no easy matter. Radial velocity is most sensitive when dealing with large planets in tight orbits, which is why the first detections in main sequence stellar systems, beginning back in 1995 with 51 Pegasi b, were of the ‘hot Jupiter’ variety. That in itself offered new insights into planetary formation and dynamics. As physicist Isidor Isaac Rabi cogently asked when the muon was first detected, “Who ordered that?”

We’re making all kinds of advances in radial velocity as we use ever more sophisticated instruments to measure the motion induced by orbiting bodies around distant stars, but if we back out to, say, 5 AU, Jupiter’s distance from the Sun, we’re still dealing with extremely tiny effects. Transits are problematic because a planet on a five-year orbit obviously transits its host on long timeframes. Gravitational microlensing is an interesting prospect, because here we can detect planets at the needed distances, but even so the catalog isn’t large and there is much we don’t know.

Fortunately, resources like the California Legacy Survey (719 stars over three decades) are available and have produced data on what we can call ‘cold giants.’ I made my comment because of a paper in the Astrophysical Journal Supplement Series that I learned about through the Pass et al. paper we looked at in the previous post. This is from Caltech’s Lee Rosenthal and colleagues, and it examines the combination of small rocky planets with outer gas giants using the CLS for the bulk of its data. The result is a look at the occurrence of close-in planets with outer giant companions.

The Rosenthal paper addresses radial velocity work on F-, G-, K- and M-class stars and targets both categories of planets, finding that roughly 41 percent of systems with a close-in small planet also host an outer giant. By close-in small planet, the authors mean planets orbiting from 0.023–1 AU with a mass twice to 30 times that of Earth. And the giant planets examined are from 0.23–10 AU and 30 to 6000 Earth masses.

The implication is that stars hosting small inner planets are more likely to have an outer gas giant, for the number is roughly 17 percent for stars irrespective of small planet presence. There is much to be done with data from the California Legacy Survey (the baseline of RV observations goes back to 1988, and is invaluable), but studies like these lead to the conclusion that planets in Jupiter-like orbits are not uncommon among F-, G- and K-class stars. As to the M-dwarfs, the Pass paper indicates the scarcity of gas giants around them, with all that may imply about inner planet habitability. Note that the CLS is made up mostly of FGK stars, with 98% of stars in the sample having stellar masses above 0.3 solar masses..

I haven’t had time to dig into a previous paper using the California Legacy Survey data, this one from Benjamin Fulton (Caltech) with Rosenthal as a co-author, but do note that the authors find that the occurrence of planets less massive than Jupiter (from 30 Earth masses up to 300 as per RV data) is enhanced near 1–10 AU “in concordance with their more massive counterparts.” The complete citation is below.

We still have much to learn about exoplanet system architectures, but we’re making progress as the inflowing current of high-quality data grows ever more powerful.

The paper is Rosenthal et al., “The California Legacy Survey. III. On the Shoulders of (Some) Giants: The Relationship between Inner Small Planets and Outer Massive Planets,” Astrophysical Journal Supplement Series, Vol. 262, No. 1 (17 August 2022), 262 1 (abstract). The Fulton paper is “California Legacy Survey. II. Occurrence of Giant Planets beyond the Ice Line,” Astrophysical Journal Supplement Series Vol. 255, No. 1 (9 July 2021), 255, 13 (abstract).

A Scarcity of ‘Jupiters’ in Red Dwarf Systems

Gas giant worlds like Jupiter may be uncommon around red dwarf stars, as a number of recent studies have found. It would be useful to tighten up the data, however, because many of the papers on this matter have used stellar samples at the high end of the mass range of M-dwarfs. At the Center for Astrophysics | Harvard & Smithsonian (CfA), Emily Pass and colleagues have gone to work on the question by looking at lower-mass M-dwarfs and working with a lot of them, some 200 in their sample, all within 15 parsecs.

The question is not purely academic, for some scientists suggest that the presence of a Jupiter-class planet – not uncommon around G-class stars like the Sun – is a factor in the development of life. Migrating inward from a formation in the first few hundred million years of the Solar System’s existence, Jupiter would have stirred up plenty of icy cometary bodies through gravitational interactions. Impacts from this infall into the inner system likely delivered a great deal of water and organic molecules to the young Earth, thus becoming a factor in the development of life.

Thus a system like TRAPPIST-1, with its seven rocky planets orbiting a nearby red dwarf, raises the question of whether such a system would have gone through this kind of mixing. No one knows whether life would have begun on Earth without these effects, but the suggestion that systems without a gas giant are barren is plausible. So just how common are red dwarf systems with gas giants equivalent to Jupiter based on what we know so far? It’s telling that only two of the known gas giants orbiting a red dwarf occur around stars of less than 30 percent of the Sun’s mass: LHS 252 b and GJ 83.1 b.

Image: A gas giant around an M-class dwarf, as visualized by artist Melissa Weiss, CfA.

What Pass and team deliver is a statistical analysis, using spectroscopic surveys and radial velocity data on nearby M-dwarfs in the mass range of 0.10–0.30 stellar masses. The data are presented in a paper now in process at The Astronomical Journal. The results confirm the belief that red dwarfs are seldom the hosts for Jupiter-class worlds. In fact, in the entire sample, not a single Jupiter-equivalent planet occurred, allowing the authors to conclude that Jupiter analogues must be found in fewer than 2 percent of low-mass red dwarf systems:

Planets that are Jupiter-like in mass and instellation are rare around low-mass M dwarfs, consistent with expectations from core accretion theory. Compared with previous radial-velocity and microlensing studies that consider broader distributions of M-dwarfs with higher mean stellar masses, our results are consistent with a decrease in giant planet occurrence with decreasing M-dwarf mass…

The authors note the complications of comparing occurrence rate between the various surveys that have so far attempted it, but add:

…the picture of giant planet occurrence from microlensing is still unclear. If Poleski et al. (2021) are correct in their assertion that every microlensing star has a wide-orbit giant planet, our results imply that the distribution of giant planets around low-mass M dwarfs must differ dramatically from more massive stars, whose giant planets are more prevalent near the water snow line than on wide orbits.

These are interesting findings especially in terms of habitability. Rather than assuming that red dwarf planets are unlikely to have life, they could just as easily point to the differences between these systems and our own as offering other avenues for life to develop. CfA’s David Charbonneau makes the point explicitly: “We don’t think that the absence of Jupiters necessarily means rocky planets around red dwarfs are uninhabitable.”

What we do have are planetary systems different enough from ours to encourage speculation on what factors might produce life in different ways than our own system. Consider that the lack of gas giants also indicates more raw material for planetary formation on the scale of smaller rocky worlds. Given the proximity of red dwarf stars with rocky planets, they’ll be at the forefront of astrobiological investigation as we develop the ability to study their atmospheres. The possibilities remain open, and perhaps exotic, as we continue the hunt for life elsewhere. Adds Pass:

“We have shown that the least massive stars don’t have Jupiters, meaning Jupiter-mass planets that receive similar amounts of starlight as Jupiter receives from our Sun. While this discovery suggests truly Earth-like planets might be in short supply around red dwarfs, there still is so much we don’t yet know about these systems, so we must keep our minds open.”

The paper is Pass et al., “Mid-to-Late M Dwarfs Lack Jupiter Analogs,” in process at The Astronomical Journal (preprint).

 

Catching Up

Centauri Dreams began as a website back in August of 2004. I’m startled to realize, looking through the stats that my site’s software provides, that in the subsequent nineteen years, there have been 4,659 posts, along with close to 100,000 comments. The irony is that I started the site simply as a research venue for myself, thinking to keep up with the latest news by building a collection of articles and scientific papers. It took about a year before I even switched on the comments function.

One of the benefits of publishing for such a length of time is perspective, as the interstellar research scene has grown and changed over the past two decades. But one thing I didn’t do is keep up with the software. Always focused on content, I’ve kept writing but have let too many generations of internal programming stay mired in older iterations. The dangers of this are obvious. A site with obsolete internals is all too open to hacking. And now, completely normal upgrades to some of the site’s functionality threaten to break some of the older software. Something has to be done.

What’s now happening is a thorough re-doing of the internals of Centauri Dreams, one that will solve the immediate problems and allow upgrades to some of the external programs I use. The most obvious change to readers will be the site theme, although things should remain pretty familiar. I want Centauri Dreams to continue with its basic layout, and that means no advertising, no pop-up windows, no annoyances to distract from the text. Behind the scenes, the site will be rendered more secure and also more efficient, with less chance of an errant move on my part bringing things down.

Please bear with me as the work proceeds. The new look comes with significantly tightened security. Work behind the scenes will continue on a number of issues I want to resolve. I’ll tweak the look and feel around the edges, but let’s get through the transition first. This should be done within the next day or two. If any late-arriving comments get lost along the way, I’ll get those restored as soon as I can. Anticipating problems – and they always turn up, no matter what – should help to deflect them.

 

SETI: Asking the Right Questions

Did Carl Sagan play a role in the famous Arecibo message transmitted toward the Hercules Cluster in 1974? I’ve always assumed so, given Sagan’s connection with Frank Drake, who was then at Cornell University, where Sagan spent most of his career. But opinion seems to vary. Artist/scientist Joe Davis, who now has affiliations with both MIT’s Laboratory of Molecular Structure and Harvard Medical School, noted in an email this morning that Sagan’s widow, Ann Druyan, supports the connection, but according to Davis, Drake himself denied Sagan’s role in the composition or transmission of the message.

I mention all this because of Tuesday’s post on the simulated SETI signal being sent via ESO’s Mars ExoMars Trace Gas Orbiter, as a kind of work of art in its own right as well as a test case in building public involvement in the decoding of an unusual message. The idea of doing that irresistibly recalled Joe Davis because in 1988 Davis performed his own act of scientific art involving SETI, one that involved the Arecibo message and raised the question of whether any recipients would recognize it, much less decode it.

Image: A color-coded version of the Arecibo message highlighting its separate parts. The binary transmission itself carried no color information. Credit: Arne Nordmann / Wikimedia Commons. CC BY-SA 3.0.

The project, called “A Message in Many Bottles,” was set up at MIT’s Hayden Library in 1988. Davis used 1679 ‘Boston round’ 16 ounce glass bottles arrayed in a set of partitioned racks that were displayed in stacks. This is remarkably clever stuff: Each of 18 aisles in the library contained racks of bottles mounted, as Davis told me, 23 across. Empty bottles served as 0s in this digital message, while bottles filled with water represented the 1s. The whole thing reproduced the 1974 Arecibo message.

Now remember, this is MIT. You would think that if there is any place where a population of scientists, academics and students might puzzle out an enigmatic artifact like this, it would be here. Davis puts it this way in his email:

Hayden Library is MIT’s science library and contains all of the information needed to decode the message, all information the message refers to, and supposedly, better-than-average terrestrial intelligence. To the best of my knowledge, nobody decoded it. Instead, there were arguments…about whether or not the racks of bottles constituted works of art.

Image: An evidently baffled student contemplates the “Message in Many Bottles.” Credit: Joe Davis.

In 1997, a year after Sagan’s death, Davis reinstalled the display at MIT’s then new biology facility (Building 68), dedicating the work to the memory of Sagan. A short article on the matter in Nature (27 March 1997) noted the project as an homage to Sagan that accurately reproduced the Arecibo signal, going on to note:

Philip Sharp, chairman of MIT’s biology department, describes the exhibit as a “fitting tribute” to Sagan’s work. “It brings the abstraction of a radar message into an accessible, physical form,” says Sharp. He says he sees “numerous benefits” in having an artist who approaches issues from an unorthodox perspective working alongside more formally trained scientists.

Labeled as a tribute to Sagan and explained so that viewers could decode the message, “A Message in Many Bottles” served as an effective exhibit inhabiting the muzzy borderland where science meets art and creative minds translate research into shapes and forms that interrogate the meaning of our experiments. For that matter, was the Arecibo message itself not a kind of art, given that with a target 25,000 light years away, there was no conceivable way to see it as an actual communication?

Back in 2009 Joe Davis wrote “RuBisCo Stars” and the Riddle of Life for Centauri Dreams, presenting his own work at Arecibo, which wound up, on the 35th anniversary of the Arecibo message, in a new message based on molecular biology that was sent to three nearby stars. How he did this using, remarkably, an analog audio file on his iPhone interfacing with Arecibo’s technology is explained in the second part of his 2009 post, “RuBisCo Stars”: Part II. These two posts are, as everything involving Joe Davis’ work continues to be, invigorating and startlingly thought-provoking.

Image: At Arecibo, Joe Davis ponders transmission options as he holds the possible answer. Credit: Ashley Clark.

In fact, Davis notes in part II, in the midst of explaining to Arecibo’s then interim director Michael Nolan what his project is about, that “projects concerned with the search for extraterrestrial intelligence are really more about a search for ourselves; that they make us look much more intensely at ourselves than we look away into space and that nobody seems to see that part of it.” Nor could the myriad well-trained minds who encountered the Arecibo message in “A Message in Many Bottles” decode its meaning.

Science is so often about asking the right question. What are we staring at right now that we are not seeing? Are we asking the right questions about SETI?

Links for IRG Interstellar Symposium in Montreal

The preliminary program for the Interstellar Research Group’s 8th Interstellar Symposium in Montreal is now available. For those of you heading to the event, I want to add that the early bird registration period for attending at a discount is May 31. Registration fees go up after that date. Registering at the conference hotel can be handled here. Registration before the 31st is recommended to get a room within the block reserved for IRG.

First Contact: A Global Simulation

Now and again scientists think of interesting ways to use our space missions in contexts for which they were not designed. I’m thinking, for example, of the ‘pale blue dot’ image snapped by Voyager 1 in 1990, an iconic view that forcibly speaks to the immensity of the universe and the smallness of the place we inhabit. Voyager’s cameras, we might recall, were added only after a debate among mission designers, some of whom argued that the mission could proceed without any cameras aboard.

Fortunately, the camera advocates won, with results we’re all familiar with. Now we have a project out of The SETI Institute that would use a European Space Agency mission in a novel way, one that also challenges our thinking about our place in the cosmos. Daniela de Paulis, who serves as artist in residence at the institute, is working across numerous disciplines with researchers involved in SETI and astronautics to create A Sign in Space, the creation of an ‘extraterrestrial’ message. This is not a message beamed to another star, but a message beamed back at us.

The plan is this: On May 24, 2023, tomorrow as I write this on the US east coast, ESA’s ExoMars Trace Gas Orbiter, in orbit around Mars, will transmit an encoded message to Earth that will act as a simulation of a message from another civilization. The message will be detected by the Allen Telescope Array (ATA) in California, the Green Bank Telescope (GBT) in West Virginia and the Medicina Radio Astronomical Observatory in Italy. The content of the message is known only to de Paulis and her team, and the public will be in on the attempt to decode and interpret it. The message will be sent at 1900 UTC on May 24 and discussed in a live stream event beginning at 1815 UTC online.

The signal should reach Earth some 16 minutes after transmission, hence the timing of the live stream event. This should be an enjoyable online gathering. According to The SETI Institute, the live stream, hosted by Franck Marchis and the Green Bank Observatory’s Victoria Catlett, will feature key team members – scientists, engineers, artists and more – and will include control rooms from the ATA, the GBT, and Medicina.

Daniela de Paulis points to the purpose of the project:

“Throughout history, humanity has searched for meaning in powerful and transformative phenomena. Receiving a message from an extraterrestrial civilization would be a profoundly transformational experience for all humankind. A Sign in Space offers the unprecedented opportunity to tangibly rehearse and prepare for this scenario through global collaboration, fostering an open-ended search for meaning across all cultures and disciplines.”

The data are to be stored in collaboration with Breakthrough Listen’s Open Data Archive and the storage network Filecoin, the idea being to make the signal available to anyone who wants to have a crack at decoding it. A Sign in Space offers a Discord server for discussion of the project, while findings may be submitted through a dedicated form on the project’s website. For a number of weeks after the signal transmission, the A Sign in Space team will host Zoom discussions on the issues involved in reception of an extraterrestrial signal, with the events listed here.

Remembering Jim Early (1943-2023)

I was saddened to learn of the recent death of James Early, author of a key paper on interstellar sail missions and a frequent attendee at IRG events (or TVIW, as the organization was known when I first met him). Jim passed away on April 28 in Saint George, UT at the age of 80, a well-liked figure in the interstellar community and a fine scientist. I wish I had known him better. I ran into him for the first time in a slightly awkward way, which Jim, ever the gentleman, quickly made light of.

What happened was this. In 2012 I was researching damage that an interstellar sail mission might experience in the boost phase of its journey. Somewhere I had seen what I recall as a color image in a magazine (OMNI?) showing a battered, torn sail docked in what looked to be a repair facility at the end of an interstellar crossing. It raised the obvious question: If we did get a sail up to, say, 5% of the speed of light, wouldn’t even the tiniest particles along the way create significant damage to the structure? The image was telling and to this day I haven’t found its source.

I think of the image as ‘lightsail on arrival,’ and if this triggers a memory with anyone, please let me know. Anyway, although our paths crossed at the first 100 Year Starship symposium in Orlando in 2011, I didn’t know Jim’s work and didn’t realize he had analyzed the sail damage question extensively. When I wrote about the matter on Centauri Dreams a year later, he popped up in the comments:

I presented a very low mass solution to the dust problem at the 100 Year Starship Symposium in a talk titled “Dust Grain Damage to Interstellar Vehicles and Lightsails”. An earlier published paper contains most of the important physics: Early, J.T., and London, R.A., “Dust Grain Damage to Interstellar Laser-Pushed Lightsail”, Journal of Spacecraft and Rockets, July-Aug. 2000, Vol. 37, No. 4, pp. 526-531.

I was caught by surprise by the reference. How did I miss it? Researching my 2005 Centauri Dreams book, I had been through the literature backwards and forwards, and JSR was one of the journals I combed for deep space papers. Later, at a TVIW meeting in Oak Ridge, we talked, had dinner and Jim kidded me about my research methods. As I saw it, his paper was a major contribution, and I should have known about it. Yesterday I asked Andrew Higgins (McGill University) about the paper and he had this to say in an email:

Jim Early’s paper (written with Richard London in 1999) on dust grain impacts addressed one of the bogeys of interstellar flight: The dust grain impact problem when traveling at relativistic speeds. Their analysis showed—counterintuitively—that the damage caused by a dust grain on an interstellar lightsail actually decreases as the sail exceeds a few percent of the speed of light. While the grain turns into an expanding fireball of plasma as it passes through the sail, the amount of thermal radiation deposited on the sail decreases as the fireball is receding more quickly from the sail. This was a welcome result suggesting sails might survive the interstellar transit, and their study remains the seminal work on dust grain interactions with thin structures at relativistic speeds.

Image: Dinner after the first day’s last plenary session in Oak Ridge in 2014. That’s Jim Benford at far left, then James Early, Sandy Montgomery and Michael Lynch.

The family has set up a website honoring Jim and offering photos and an obituary. He got his bachelor’s degree in Aeronautics at MIT, following it with a master’s degree in mechanical engineering at Caltech, and a PhD in aeronautics and physics at Stanford University. He was involved with development activities for the Delta launch vehicle while obtaining his bachelor’s degree by working at NASA Goddard Space Flight Center in the summers and then at McDonnell-Douglas after finishing his master’s degree. He joined Lockheed and Hughes aircraft for a time before finally ending up at the Lawrence Livermore National Laboratory working on laser physics until he retired.

Sail in Flight

So let’s look at Jim’s paper on sails, a subject he continued to work on for the next two decades. Although Robert Forward came up with sail ideas that pushed as high as 30 percent of the speed of light (and in the case of Starwisp, even higher), Jim and his co-author Richard London chose 0.1 c for cruise velocity in their paper, which provides technical challenges aplenty but at least diminishes the enormous energy costs of still faster missions, and certainly mitigates the problem of damage from dust and gas along the way. Depending on the methods used, the sail as analyzed in this paper may take a tenth of a light year to get up to cruise velocity. It’s worth mentioning that the sail does not have to remain deployed during cruise itself, but deceleration at the target star, depending on the methods used, may demand redeployment. Breakthrough Starshot envisions stowing the sail in cruise after its sudden acceleration to 20 percent of c.

Early and London use beryllium sails as their reference point, these being the best characterized design at this stage of sail study, and assume a sail 20 nm thick. In terms of the interstellar medium the sail will encounter, the authors say this:

Local interstellar dust properties can be estimated from dust impact rates on spacecraft in the outer solar system and by dust interaction with starlight. The mean particle masses seen by the Galileo and Ulysses spacecraft were 2×10-12 and 1×10-12g, respectively. A 10-12g dust grain has a diameter of approximately 1 µm. The median grain size is smaller because the mean is dominated by larger grains. The Ulysses saw a mass density of 7.5×10-27g cm-3. A sail accelerating over a distance of 0.1 light years would encounter 700 dust grains/cm2 at this density. The surface of any vehicle that traveled 10 light years would encounter 700 dust grains/mm2. If a significant fraction of the particle energy is deposited in the impacted surface in either case, the result would be catastrophic.

The question then becomes, just how much of the particle’s energy will be deposited on the sail? The unknowns are all too obvious, but the paper adds that neither of the Voyagers saw dust grains larger than 1 μm at distances beyond 50 AU, while a 1999 study on interstellar dust grain distributions found a flat distribution from 10-14 to 10-12 g with some grains as large as 10-11 g. Noting that a 10-12 dust grain has a diameter of about 1-μm, the authors use a 1-μm diameter grain for their impact calculations.

The results are intriguing because they show little damage to the sail. Catastrophe averted:

At the high velocities of interstellar travel, dust grains and atoms of interstellar gas will pass through thin foils with very little loss of energy. There will be negligible damage from collisions between the nuclei of atoms. In the case of dust particles, most of the damage will be due to heating of the electrons in the thin foil. Even this damage will be limited to an area approximately the size of the dust particle due to the extremely fast, one-dimensional ambipolar diffusion explosion of the heated section of the foil. The total fraction of the sail surface damaged by dust collisions will be negligible.

The torn and battered lightsail in its dock, as seen in my remembered illustration, may then be unlikely, depending on cruise speed and, of course, on the local medium it passes through. Sail materials also turn out to offer excellent shielding for the critical payload behind the sail:

Interstellar vehicles require protection from impacts by dust and interstellar gas on the deep structures of the vehicle. The deployment of a thin foil in front of the vehicle provides a low mass, effective system for conversion of dust grains or neutral gas atoms into free electrons and ions. These charged particles can then be easily deflected away from the vehicle with electrostatic shields.

And because the topic has come up in a number of past discussions here, let me add this bit about interstellar gas and its effects on the lightsail:

The mass density of interstellar gas is approximately one hundred times that of interstellar dust particles though this ratio varies significantly in different regions of space. The impact of this gas on interstellar vehicles can cause local material damage and generate more penetrating photon radiation. If this gas is ionized, it can be easily deflected before it strikes the vehicle’s surface. Any neutral atom striking even the thin foil discussed in this paper will pass through the foil and emerge as an ion and free electron. Electrostatic or magnetic shields can then deflect these charged particles away from the vehicle.

Consequences for Sail Design

All of these findings have a bearing on the kind of sail we use. The thin beryllium sail appears effective as a shield for the payload, with a high melting point and, the authors conclude, the ability to be increased in thickness if necessary without increasing the area damaged by dust grains. Ultra-thin foils of tantalum or niobium offer higher temperature possibilities, allowing us to increase the laser power applied to the sail and thus the acceleration. But Early and London believe that the higher atomic mass of these sails would make them more susceptible to damage. Even so, “…the level of damage to thin laser lightsails appears to be quite small; therefore the design of these sails should not be strongly influenced by dust collision concerns.”

Dielectric sails would be more problematic, suffering more damage from heated dust grains because of their greater thickness, and the authors argue that these sail materials need to be subjected to a more complete analysis of the blast wave dynamics they will experience. All in all, though, velocities of 0.1 c yield little damage to a thin beryllium sail, and thin shields of similar materials can ionize dust as well as neutral interstellar gas atoms, allowing the ions to be deflected and the interstellar vehicle protected.

These are encouraging results, but the size of the problem is daunting, and given the apparent cost of the classically conceived interstellar probe, the prospect of impact damage calls for continued analysis of the medium through which the probe would pass. This is one of the advantages of sending not one large craft but a multitude of smaller ‘chipsat’ style vehicles in the Breakthrough Starshot model. Send enough of these and you can afford to lose a certain percentage along the way. I can only wish I could sit down with Jim Early again to kick around chipsat concepts, but what a fine memorial to know that your paper continues to influence evolving interstellar ideas.

The paper is Early, J.T., and London, R.A., “Dust Grain Damage to Interstellar Laser-Pushed Lightsail,” Journal of Spacecraft and Rockets, July-Aug. 2000, Vol. 37, No. 4, pp. 526-531.

Assembly theory (AT) – A New Approach to Detecting Extraterrestrial Life Unrecognizable by Present Technologies

With landers on places like Enceladus conceivable in the not distant future, how we might recognize extraterrestrial life if and when we run into it is no small matter. But maybe we can draw conclusions by addressing the complexity of an object, calculating what it would take to produce it. Don Wilkins considers this approach in today’s essay as he lays out the background of Assembly Theory. A retired aerospace engineer with thirty-five years experience in designing, developing, testing, manufacturing and deploying avionics, Don tells me he has been an avid supporter of space flight and exploration all the way back to the days of Project Mercury. Based in St. Louis, where he is an adjunct instructor of electronics at Washington University, Don holds twelve patents and is involved with the university’s efforts at increasing participation in science, technology, engineering, and math. Have a look at how we might deploy AT methods not only in our system but around other stars.

by Don Wilkins

A continuing concern within the astrobiology community is the possibility alien life is detected, then misclassified as built from non-organic processes. Likely harbors for extraterrestrial life — if such life exists — might be so alien, employing chemistries radically different from those used by terrestrial life, as to be unrecognizable by present technologies. No definitive signature unambiguously distinguishes life from inorganic processes. [1]

Two contentious results from the search for life on Mars are examples of this uncertainty. Lack of knowledge of the environments producing the results prevented elimination of abiotic origins for the molecules under evaluation. The Viking Lander’s metabolic experiments provide debatable results as the properties of Martian soil were unknown. An exciting announcement of life detection in the ALH 84001 meteorite is challenged as the ambiguous criteria to make the decision are not quantitative.

Terrestrial living systems employ processes such as photosynthesis, whose outputs are potential biosignatures. While these signals are relatively simple to identify on Earth, the unknown context of these signals in alien environments makes distinguishing between organic and inorganic origins difficult if not impossible.

The central problem arises in an apparent disconnect between physics and biology. In accounting for life, traditional physics provides the laws of nature, and assumes specific outcomes are the result of specific initial conditions. Life, in the standard interpretation, is encoded in the initial period immediately after the Big Bang. Life is, in other words, an emergent property of the Universe.

Assembly theory (AT) offers a possible solution to the ambiguity. AT posits a numerical value, based on the complexity of a molecule, that can be assigned to a chemical, the Assembly Index (AI), Figure 1. This parameter measures the histories of an object, essentially the complexity of the processes which formed the molecule. Assembly pathways are sequences of joining operations, from basic building blocks to final product. In these sequences, sub-units generated within the sequence combine with other basic or compound sub-units later in the sequence, to recursively generate larger structures. [2]

The theory purports to objectively measure an object’s complexity by considering how it was made. The assembly index (AI) is produced by calculating the minimum number of steps needed to make the object from its ingredients. The results showed, for relatively small molecules (mass 

Figure 1. A Comparison of Assembly Indices for Biological and Abiotic Molecules.

Analyzing a molecule begins with basic building blocks, a shared set of objects, Figure 2. AI measures the smallest number of joining operations required to create the object. The assembly process is a random walk on weighted trees where the number of outgoing edges (leaves) grows as a function of the depth of the tree. A probability estimate an object forms by chance requires the production of several million trees and calculating the likelihood of the most likely path through the “forest”. Probability is related to the number of joining operations required or the path length traversed to produce the molecule. As an example, the probability of Taxol forming ranges between 1:1035 to 1:1060 with a path length of 30. In Figure 2, alpha biasing controls how quickly the number of joining operations grows with the depth of the tree.

Figure 2. Calculating Complexity

AT does not require extremely fine-tuned initial conditions demanded in the physics-based origins of life. Information to build specific objects accumulates over time. A highly improbable fine-tuned Big Bang is no longer needed. AT takes advantage of concepts borrowed from graph (networks of interlinked nodes) theory. [3] According to Sara Walker of Arizona State University and a lead AT researcher, information “is in the path, not the initial conditions.”

To explain why some objects appear but others do not, AT posits four distinct classifications, Figure 3. All possible basic building block variations are allowed in the Assembly Universe. Physics, temperature or catalysis are examples, constraining the combinations, eliminating constructs which are not physical in the Assembly Possible. Only objects that can be assembled comprise the Assembly Contingent level. Observable objects are grouped in the Assembly Observed.

Figure 3. The four “universes” of Assembly Theory

Chiara Marletto, a theoretical physicist at the University of Oxford, with David Deutsch, a physicist also at Oxford, are developing a theory resembling AT, the constructor theory (CT). Mimicking the thermodynamics Carnot cycle, CT uses machines or constructors operating in a cyclic fashion, starting at a original state, processing through a pattern until the process returns to the original state to explain a non-probabilistic Universe.

A team headed by Lee Cronin of the University of Glasgow and Sara Walker proposes AT as a tool to distinguish between molecules produced by terrestrial or extraterrestrial life and those built by abiotic processes. [4] AT analysis is susceptible to false negatives but current work produces no false positives. After completing a series of demonstrations, the researchers believe an AT life detection experiment deployable to extraterrestrial locations is possible.

Researchers believe AI estimates can be made using mass or infrared spectrometry. [5-6] While mass spectrometry requires physical access to samples, Cronin and colleagues showed a combination of AT and infrared spectrometry sensors similar to those on the James Webb Space Telescope could analyze the chemical environment of an exoplanet, possibly detecting alien life.

References

[1] Philip Ball, A New Idea for How to Assemble Life, Quanta, 4 May 2023,
https://www.quantamagazine.org/a-new-theory-for-the-assembly-of-life-in-the-universe-20230504?mc_cid=088ea6be73&mc_eid=34716a7dd8

[2] Abhishek Sharma, Dániel Czégel, Michael Lachmann, Christopher P. Kempes, Sara I. Walker, Leroy Cronin, “Assembly Theory Explains and Quantifies the Emergence of Selection and Evolution,”
https://arxiv.org/abs/2206.02279

[3] Stuart M. Marshall, Douglas G. Moore, Alastair R. G. Murray, Sara I. Walker, and Leroy Cronin, Formalising the Pathways to Life Using Assembly Spaces, Entropy 2022, 24(7), 884, 27 June 2022, https://doi.org/10.3390/e24070884

[4] Yu Liu, Cole Mathis, Michał Dariusz Bajczyk, Stuart M. Marshall, Liam Wilbraham, Leroy Cronin, “Ring and mapping chemical space with molecular assembly trees,” Science Advances, Vol. 7, No. 39
https://www.science.org/doi/10.1126/sciadv.abj2465

[5] Stuart M. Marshall, Cole Mathis, Emma Carrick, Graham Keenan, Geoffrey J. T. Cooper, Heather Graham, Matthew Craven, Piotr S. Gromski, Douglas G. Moore, Sara I. Walker, Leroy Cronin, “Identifying molecules as biosignatures with assembly theory and mass spectrometry,” Nature Communications volume 12, article number: 3033 (2021)
https://www.nature.com/articles/s41467-021-23258-x

[6] Michael Jirasek, Abhishek Sharma, Jessica R. Bame, Nicola Bell1, Stuart M. Marshall,Cole Mathis, Alasdair Macleod, Geoffrey J. T. Cooper!, Marcel Swart, Rosa Mollfulleda, Leroy Cronin, “Multimodal Techniques for Detecting Alien Life using Assembly Theory and Spectroscopy,” https://arxiv.org/ftp/arxiv/papers/2302/2302.13753.pdf

Game Changer: Exploring the New Paradigm for Deep Space

The game changer for space exploration in coming decades will be self-assembly, enabling the growth of a new and invigorating paradigm in which multiple smallsat sailcraft launched as ‘rideshare’ payloads augment huge ‘flagship’ missions. Self-assembly allows formation-flying smallsats to emerge enroute as larger, fully capable craft carrying complex payloads to target. The case for this grows out of Slava Turyshev and team’s work at JPL as they refine the conceptual design for a mission to the solar gravitational lens at 550 AU and beyond. The advantages are patent, including lower cost, fast transit times and full capability at destination.

Aspects of this paradigm are beginning to be explored in the literature, as I’ve been reminded by Alex Tolley, who forwarded an interesting paper out of the University of Padua (Italy). Drawing on an international team, lead author Giovanni Santi explores the use of CubeSat-scale spacecraft driven by sail technologies, in this case ‘lightsails’ pushed by a laser array. Self-assembly does not figure into the discussion in this paper, but the focus on smallsats and sails fits nicely with the concept, and extends the discussion of how to maximize data return from distant targets in the Solar System.

The key to the Santi paper is swarm technologies, numerous small sailcraft placed into orbits that allow planetary exploration as well as observations of the heliosphere. We’re talking about payloads in the range of 1 kg each, and the intent of the paper is to explore onboard systems (telecommunications receives particular attention), the fabrication of the sail and its stability, and the applications such systems can offer. As you would imagine, the work draws for its laser concepts on the Starlight program pursued for NASA by Philip Lubin and the ongoing Breakthrough Starshot project.

Image: NASA’s Starling mission is one early step toward developing swarm capabilities. The mission will demonstrate technologies to enable multipoint science data collection by several small spacecraft flying in swarms. The six-month mission will use four CubeSats in low-Earth orbit to test four technologies that let spacecraft operate in a synchronized manner without resources from the ground. Credit: NASA Ames.

The authors argue that ground-based direct energy laser propulsion, with its benefits in terms of modularity and scalability, is the baseline technology needed to make small sailcraft exploration of the Solar System a reality. Thus there is a line of development which extends from early missions to targets like Mars, with accompanying reductions in the power needed (as opposed to interstellar missions like Breakthrough Starshot), and correspondingly, fewer demands on the laser array.

The paper specifically does not analyze close-pass perihelion maneuvers at the Sun of the sort examined by the JPL team, which assumes no need for a ground-based array. I think the ‘Sundiver’ maneuver is the missing piece in the puzzle, and will come back to it in a moment.

Breakthrough Starshot envisions a flyby of a planetary system like Proxima Centauri, but the missions contemplated here, much closer to home, must find a way to brake at destination in cases where extended planetary science is going to be performed. Thus we lose the benefit of purely sail-based propulsion (no propellant aboard) in favor of carrying enough propulsive mass to make the needed maneuvers at, say, Mars:

…the spacecraft could be ballistically captured in a highly irregular orbit, which requires at least an high thrust maneuver to stabilize the orbit itself and to reduce the eccentricity…The velocity budget has been estimated using GMAT suite to be ∆v ⋍ 900−1400 m s−1, depending on the desired final orbit eccentricity and altitude. A chemical thruster with about 3 N thrust would allow to perform a sufficiently fast maneuver. In this scenario, the mass of the nanosatellite is estimated to be increased by a wet mass of 5 kg; moreover, an increase of the mass of reaction wheels needs to be taken into account given the total mass increment.

Even so, swarms of nanosatellites allow a reduction of the payload mass of each individual spacecraft, with the added benefit of redundancy and the use of off-the-shelf components. The authors dwell on the lightsail itself, noting the basic requirement that it be thermally and mechanically stable during acceleration, no small matter when propelling a sail out of Earth orbit through a high-power laser beam. Although layered sails and sails using nanostructures, metamaterials that can optimize heat dissipation and promote stability, are an area of active research, this paper works with a thin film design that reduces complexity and offers lower costs.

We wind up with simulations involving a sail made of titanium dioxide with a radius of 1.8 m (i.e. a total area of 10 m2) and a thickness of 1 µm. The issue of turbulence in the atmosphere, a concern for Breakthrough Starshot’s ground-based laser array, is not considered in this paper, but the authors note the need to analyze the problem in the next iteration of their work along with close attention to laser alignment, which can cause problems of sail drifting and spinning or even destroy the sail.

But does the laser have to be on the Earth’s surface? We’ve had this discussion before, noting the political problem of a high-power laser installation in Earth orbit, but the paper notes a third possibility, the surface of the Moon. A long-term prospect, to be sure, but one having the advantage of lack of atmosphere, and perhaps placement on the Moon’s far side could one day offer a politically acceptable solution. It’s an intriguing thought, but if we’re thinking of the near term, the fastest solution seems to be the Breakthrough Starshot choice of a ground-based facility on Earth.

What we have here, then, could be described as a scaled-down laser concept, a kind of Breakthrough Starshot ‘lite’ that focuses on lower levels of laser power, larger payloads (even though still in the nanosatellite range), and targets as close as Mars, where swarms of sail-driven spacecraft might construct the communications network for a colony on the surface. A larger target would be exploration of the heliosphere:

…in this last mission scenario the nanosatellites would be radially propelled without the need of further orbital maneuvers. To date, the interplanetary environment, and in particular the heliospheric plasma, is only partially known due to the few existing opportunities for carrying out in-situ measurements, basically linked to scientific exploration missions [76]. The composition and characteristics of the heliospheric plasma remain defined mainly through theoretical models only partially verified. Therefore, there is an urgent need to perform a more detailed mapping of the heliospheric environment especially due to the growth of the human activities in space.

Image: An artist’s concept of ESA’s Swarm mission being deployed. This image was taken from a 2015 workshop on formation flying satellites held at Technische Universiteit Delft in the Netherlands. Extending the swarm paradigm to smallsats and nanosatellites is one step toward future robotic self-assembly. Credit: TU Delft.

Spacecraft operating in swarms optimized for the study of the heliosphere offer tantalizing possibilities in terms of data return. But I think the point that emerges here is flexibility, the notion that coupling a beamed propulsion system to smallsats and nanosats offers a less expensive, modular way to explore targets previously within reach only by expensive flagship missions. I’ll also argue that a large, ground-based laser array is aspirational but not essential to push this paradigm forward.

Issues of self-assembly and sail design are under active study, as is the question of thermal survival for operations close to the Sun. We should continue to explore close solar passes and ‘sundiver’ maneuvers to shorten transit times to targets both relatively near or as far away as the Kuiper Belt. We need test missions to firm up sail materials and operations, even as we experiment with self-assembly of smallsats into larger craft capable of complex operations at target. The result is a modular fleet that can make fast flybys of distant targets or assemble for orbital operations where needed.

The paper is Santi et al., “Swarm of lightsail nanosatellites for Solar System exploration,” available as a preprint.