Paul Gilster's Blog, page 20

An Alternative Take on Fusion Fuel

Let’s talk about fusion fuels in relation to the recent discussion of building a spacecraft engine. A direct fusion drive (DFD) system using magnetic mirror technologies is, as we saw last time, being investigated at the University of Maryland in its Centrifugal Mirror Fusion Experiment (CMFX), as an offshoot of the effort to produce fusion for terrestrial purposes. The initial concept being developed at CMFX is to introduce a radial electric field into the magnetic mirror system. This enhances centrifugal confinement of the plasma in a system using deuterium and tritium as fusion fuel.

Out of this we get power but not thrust. However, both UMD’s Jerry Carson and colleague Tom Bone told the Interstellar Research Group’s Montreal gathering that such a reactor coupled with a reservoir of warm plasma offers prospects for in-space propulsion. Alpha particles (these are helium nuclei produced in the fusion reaction) may stay in the reactor, further energizing the fuel, or they can move upstream, to be converted into electricity by a Standing Wave Direct Energy Converter (SWDEC). A third alternative: They may move downstream to mix with the warm plasma, producing thrust as the plasma expands within a magnetic nozzle.

Image: The fusion propulsion system as shown in Jerry Carson’s presentation at IRG Montreal. Thanks to Dr. Carson for passing along the slides.

We also know that fusion fuel options carry their own pluses and minuses. We can turn to deuterium/deuterium reactions (D/D) at the expense of neutron production, something we have to watch carefully if we are talking about powering up a manned spacecraft. The deuterium/tritium reaction (D/T) produces even more neutron flux, while deuterium/helium-3 (D/He3) loses most of the neutron output but demands helium-3 in abundances we only find off-planet. Tom Bone’s presentation at Montreal turned the discussion in a new direction. What about hydrogen and boron?

Here the nomenclature is p-11B, or proton-boron-11, where a hydrogen nucleus (p) collides with a boron-11 nucleus in a reaction that is aneutronic and produces three alpha particles. The downside is that this kind of fusion demands temperatures even higher than D/He3, a challenge to our current confinement and heating technologies. A second disadvantage is the production of bremsstrahlung radiation, which Bone told the Montreal audience was of the same magnitude as the charged particle production.

The German word ‘bremsen’ means ‘to brake,’ hence ‘bremsstrahlung’ means ‘braking radiation,’ a reference to the X-ray radiation produced by a charged particle when it is decelerated by its encounter with atomic nuclei. So p-11B becomes even more problematic as a fuel, given the fact that boron has five electrons, creating a fusion plasma that is a lively place indeed. Bone’s notion is to take this otherwise crippling drawback and turn it to our advantage by converting some of the bremsstrahlung radiation into usable electricity. To do this, it will be necessary to absorb the radiation to produce heat.

Bone’s work at UMD focuses on thermal energy conversion using what is called a thermionic energy converter (TEC), which can convert heat directly into electricity. He pointed out that TECs are a good choice for space applications because they offer low maintenance and low mass coupled with high levels of efficiency. TECs operate off the thermionic emission that occurs when an electron can escape a heated material, a process Bone likened to ‘boiling off’ the electron. An emitter and collector in the TEC thus absorb the heat from the bremsstrahlung radiation to produce electricity.

Image: A screenshot from Dr. Bone’s presentation in Montreal.

I don’t want to get any deeper in the weeds here and will send you to Bone’s presentation for the details on the possibilities in TEC design, including putting the TEC emitter and collector in tight proximity with the air pumped out between them (a ‘vacuum TEC’) and putting an ionized vapor between the two (a ‘vapor TEC’). But Bone is upfront about the preliminary nature of this work. The objective at this early stage is to create a basic analytical model for p-11b fuel in a propulsion system using TECs to convert radiation into electricity, with the accompanying calculations to balance power and efficiency and find the lowest bremsstrahlung production for a given power setting.

The scope of needed future work on this is large. What exactly is the best ratio of hydrogen to boron in this scenario, for one thing, and how can the electric and magnetic field levels needed to light this kind of fusion be reduced? “It’s not an easy engineering problem,” Bone added. “It’s certainly not a near-term challenge to solve.”

True enough, but it’s clear that we should be pushing into every aspect of fusion as we learn more about confining these reactions in an in-space engine. Experimenting with alternate fusion fuels has to be part of the process, work that will doubtless continue even as we push forward on the far more tractable issues of deuterium/tritium.

A Fusion Drive Using Centrifugal Mirror Technologies

I want to drop back to fusion propulsion at this point, as it bears upon the question of a Solar System-wide infrastructure that we looked at last time. We know that even chemical propulsion is sufficient to get to Mars, but clearly, reducing travel times is critical if for no other reason than crew health. That likely puts the nuclear thermal concept into play, as we have experience in the development of the technology as far back as NERVA (Nuclear Engine for Rocket Vehicle Application), and this fission-based method shows clear advantages over chemical means in terms of travel times.

It’s equally clear, though, that for missions deep into the Solar System and beyond, the high specific impulse (ISP) enabled by a theoretical direct fusion drive sets the standard we’d like to meet. In his presentation at the Interstellar Research Group’s Montreal symposium, Jerry Carson discussed the ongoing work at the University of Maryland on creating fusion conditions using deuterium/deuterium (D/D) and deuterium/tritium (D/T) fuel with centrifugal mirror confinement. D/T fusion will likely drive our first fusion engines, but its higher neutron flux will spotlight the advantages of helium-3 when the latter becomes widely available, as shielding the crew on a fusion-powered spacecraft will be a critical factor.

Image: The Centrifugal Mirror Fusion Experiment at the University of Maryland at Baltimore (principal investigator Carlos Romero-Talamás, University of Maryland, Baltimore County). The plan is to achieve fusion conditions (D/D) by 2025. Credit: UMD.

Let’s dig into the centrifugal mirror (CM) concept. The beauty of plasma is that it is electrically conductive, and hence manageable by magnetic and electric fields. Hall thrusters use plasma (though not fusion!), as do concepts like Ad Astra’s VASIMR (Variable Specific Impulse Magnetoplasma Rocket). In a centrifugal mirror, the notion is to confine, compress and heat the plasma as it is spun within a fusion chamber, as opposed to the perhaps more familiar compression methods of inertial fusion, or the magnetic field structures within tokamaks. Carson argues that the CM makes for a more compact reactor and greatly reduces radiation and momentum loss.

The Maryland work implements this effect using magnetic ‘mirrors’ to create the rapid spin that imposes radial and axial forces on the plasma, confining it into a ‘well’ where fusion can be attained. The fuel is bouncing back and forth along the lines of force between the two magnets, a method first explored in the 1950s, when research indicated that mirrors of this kind are leaky and cannot maintain the plasma long enough to ignite fusion. Carson said that it is the addition of an electric field via a central electrode in the UMD design that spins the ‘doughnut’ around its axis, so that the plasma is held in place both axially as well as radially. The basic diagram is below.

Image: Centrifugal mirror confinement of a high energy plasma. Credit: UMD.

The ongoing work at Maryland grows out of an experimental effort in the 2000s that has led to the current Centrifugal Mirror Fusion Experiment (CMFX). The latter is designed with terrestrial power generation in mind, so we are talking about adapting a power-generating technology into a spacecraft drive. To do that, we fire up a centrifugal mirror fusion reactor in tandem with warm plasma (likely a reservoir of hydrogen, though other gasses are possible), so that high-energy fusion products escape the reactor downstream and deposit their energy in the plasma, causing it to expand as it passes through a magnetic nozzle to produce thrust. The reactor also uses energy leaving the upstream mirror to continue its own operations.

A direct fusion drive of this kind could, Carson said, make the round trip to Mars in 3 months, and reach Saturn in less than three years, a sharp contrast to nuclear electric methods. Even nuclear thermal methods would take over a year to make the Mars mission. Looking further out, the Uranus Orbiter and Probe (UOP), which is being considered as a flagship mission for the upcoming Decadal Survey, would make for a 12 year journey using chemical propulsion and a gravitational assist at Jupiter, while DFD-CM in these specs could do a considerably larger mission to more distant Neptune in as little as 3 years. A second generation Interstellar Probe (50 years to the heliopause in the NASA concept) could reach 1000 AU in 30-35 years using DFD-CM.

We’re not quite through with the University of Maryland, because Carson’s colleague Tom Bone has been analyzing a unique way to take advantage of otherwise problematic bremsstrahlung radiation, which complicates foperations with various kinds of fusion fuels. I’ll run through that work in the next post. Turning this challenging radiation into usable energy is conceivably a possibility, but requires fuel other than the deuterium/tritium combination examined for the DFD-CM drive. Bone’s choice is intriguing, to say the least, but more about this next time.

Infrastructure and the Interstellar Probe

The question of infrastructure haunts the quest to achieve interstellar flight. I’ve always believed that we will develop deep space capabilities not only for research and commerce but also as a means of defense, ensuring that we will be able to change the trajectories of potentially dangerous objects. But consider the recent Breakthrough Starshot discussion. There I noted that we might balance the images we could receive through Starshot’s sails with those we could produce through telescopes at the Sun’s gravitational focus.

Without the infrastructure issue, it would be a simple thing to go with JPL’s Solar Gravitational Lens concept since the target, somewhere around 600 AU, is so much closer, and could produce perhaps even better imagery. But let’s consider Starshot’s huge photon engine in the Atacama desert not as a one-shot enabler for Proxima Centauri, but as a practical tool that, once built, will allow all kinds of fast missions within the Solar System. The financial outlay supports Oort Cloud exploration, fast access to the heliopause and nearby interstellar space, and planetary missions of all kinds. Add atmospheric braking and we can consider it as a supply chain as well.

Robert Freeland, who has labored mightily in the Project Icarus Firefly design, told the Interstellar Research Group’s recent meeting in Montreal about work he is doing within the context of the British Interplanetary Society’s BIS SPACE project, whose goal is to consider the economic drivers, resources, transportation issues and future population growth that would drive an interplanetary economy. That Solar System-wide infrastructure in turn feeds interstellar capabilities, as it generates new technologies that funnel into propulsion concepts. A case in point: In-space fusion.

To make our engines go, we need fuel, an obvious point and a telling one, since the kind of fusion Freeland has been studying for the Firefly design is limited by our current inability to extract enough Helium-3 to use aboard an interstellar craft. Firefly would use Z-pinch fusion – this is a way of confining plasma and compressing it. An electrical current fed into the plasma generates the magnetic fields that ‘pinch,’ or compress the plasma, creating the high temperatures and pressures that can produce fusion.

I was glad to see Freeland’s slides on the fusion fuel possibilities, a helpful refresher. The easiest fusion reactions, if anything about fusion can be called ‘easy,’ is that of deuterium with tritium, with the caveat that this reaction produces most of its energies in neutrons that cannot produce thrust. Whereas the reaction of deuterium with helium-3 releases primarily charged particles that can be shaped into thrust, which is why it was D/He3 fusion that was chosen by the Daedalus team for their gigantic starship design back in the 1970s. Along with that choice came the need to find the helium-3 to fuel the craft. The Daedalus team, ever imaginative, contemplated mining the atmospheres of the gas giants, where He3 can be found in abundance.

The lack of He-3 caused Icarus to choose a pure deuterium fuel (DD). Freeland ran through the problems with DD, noting the abundance of produced neutrons and the gamma rays that result from shielding these fast neutrons. The reaction also produces so-called bremsstrahlung radiation, which emerges in the form of x-rays. Thus the Firefly design stripped down what would otherwise be a significant portion of its mass in shielding by going to what Freeland calls ‘distance shielding,’ meaning minimal structure that allows the radiation to escape into space.

A starship using deuterium and helium-3 minimizes the neutron radiation, so the question becomes, when do we close the gap in our space capabilities to the point that we can extract helium-3 in the quantities needed from planets like Uranus? I see BIS SPACE as seeking to probe what the Daedalus team described as a Solar System-wide economy, and to put some numbers to the question of when this capability would evolve. The question is given point in terms of interstellar probes because while Firefly had been conceived as a starship that could launch before 2100, it seemed likely that helium-3 simply wouldn’t be available in sufficient quantities. So when would it be?

To create an infrastructure off-planet, we’ll need human migration outward, beginning most likely with orbital habitats not far from Earth – think of the orbital environments conceived by Gerard O’Neill, with their access to the abundant resources of the inner system. Freeland imagines future population growth moving further out over the course of the next 20,000 years until the Solar System is fully exploited. In four waves of expansion, he sees the era of chemical and ion rocketry, and perhaps beamed propulsion, to about 2050, with the second generation largely using fission-powered craft, in a phase ending in about 2200. 2200 to 2500 taps fusion energies (DD), while the entire Solar System is populated after 2500, with mining of the gas giants possible.

Let’s pause for a moment on the human population’s growth, because the trends noted in the image below, although widely circulated, seem not to be widely known. We’re looking here at the growth rate of our species and its acceleration followed by its long decline. As Freeland pointed out, the UN expects world population to peak at between 10 and 12 billion perhaps before the end of this century. After that, increase in the population is by no means assured. So much for the scenario that we have to go off-planet because we will simply overwhelm resources here with our numbers.

Image: In both this and the image below I am drawing from Freeland’s slides.

You would think this Malthusian notion would have long ago been discredited, but it is surprisingly robust. Even so, orbital habitats near Earth can potentially re-create basic Earth-like conditions while exploiting material resources in great abundance and solar power, with easy access to space for moving the wave of innovation further out. BIS SPACE looks with renewed interest at these O’Neill habitats in its first wave of papers.

The larger scenario plays out as follows: In the second half of our century, we move development largely to high Earth orbit, with materials drawn mostly from the Moon, using transport of goods by nuclear-powered cargo ships. The third generation creates orbital habitats at all the inner planets (and Ceres) and perhaps near-Earth asteroids using DD fusion propulsion, while the fourth generation takes in the outer planets and their moons. At this point we can set up the kind of aerostat mining rigs in the upper gas giant atmospheres that would enable the collection of helium-3. Here again we have to make comparisons with other technologies. Where will beamed spacecraft capabilities be by the time we are actively mining He-3 in the outer Solar System?

I’ve simplified the details on expansion greatly, and send you to Freeland’s slides for the details. But I want to circle back to Firefly. Using DD fusion, Firefly’s radiator and coolant requirements are extreme (480 tonnes of beryllium coolant!) But move to the deuterium/helium-3 reaction and you drop radiation output by 75 percent while increasing exhaust velocity. Beryllium can be replaced with less expensive aluminum and the physical size of the vessel is greatly reduced. This version of Firefly gets to Alpha Centauri in the same time using 1/5th the fuel and 1/12th the coolant.

In other words, the sooner we can build the infrastructure allowing us to mine the critical helium-3, the sooner we can drop the costs of interstellar missions and expand their capabilities using fusion engines. If such a scenario plays out, it will be fascinating to see how the population growth curves for the entire Solar System track given access to abundant new resources and the technologies to exploit them. If we can imagine a Solar System-wide human population in the range of 100 billion, we can also imagine the growth of new propulsion concepts to power colonization outside the system.

Reflections on Breakthrough Starshot

If we’re going to get to the stars, the path along the way has to go through an effort like Breakthrough Starshot. This is not to say that Breakthrough will achieve an interstellar mission, though its aspirational goal of reaching a nearby star like Proxima Centauri with a flight time of 20 years is one that takes the breath away. But aspirations are just that, and the point is, we need them no matter how far-fetched they seem to drive our ambition, sharpen our perspective and widen our analysis. Whether we achieve them in their initial formulation cannot be known until we try.

So let’s talk for a minute about what Starshot is and isn’t. It is not an attempt to use existing technologies to begin building a starship today. Yes, metal is being bent, but in laboratory experiments and simulated environments. No, rather than a construction project, Starshot is about clarifying where we are now, and projecting where we can expect to be within a reasonable time frame. In its early stages, it is about identifying the science issues that would enable us to use laser beaming to light up a sail and push it toward another star with prospects of a solid data return. Starshot’s Harry Atwater (Caltech) told the Interstellar Research Group in Montreal that it is about development and definition. Develop the physics, define and grow the design concepts, and nurture a scientific community. These are the necessary and current preliminaries.

Image: The cover image of a Starshot paper illustrating Harry Atwater’s “Materials Challenges for the Starshot Lightsail,” Nature Materials 17 (2018), 861-867.

We’re talking about what could be a decades-long effort here, one that has already achieved a singular advance in interstellar studies. I don’t have the current count on how many papers have been spawned by this effort, but we can contrast the ongoing work of Starshot’s technical teams with where interstellar studies was just 25 years ago, when few scientific conferences dealt with interstellar ideas and exoplanets were still a field in their infancy. In terms of bringing focus to the issue, Starshot is sui generis.

It is also an organic effort. Starshot will assess its development as it goes, and the more feasible its answers, the more it will grow. I think that learning more about sail possibilities will spawn renewed effort in other areas, and I see the recent growth of fusion rocketry concepts as a demonstration that our field is attaining critical mass not only in the research labs and academy but in commercial space ventures as well.

So let’s add to Atwater’s statement that Starshot is also a cultural phenomenon. Although its technical meetings are anything but media fodder, their quiet work keeps the idea of an interstellar crossing in the public mind as a kind of background musical riff. Yes, we’re thinking about this. We’ve got ideas and lab experiments that point to new directions. We’re learning things about lightsails and beaming we didn’t know before. And yes, it’s a big universe, with approximately one planet per star on average, and we’ve got one outstanding example of a habitable zone planet right next door.

So might Starshot’s proponents say to themselves, although I have no idea how many of those participating in the effort back out sometimes to see that broader picture (I suspect quite a few, based on those I know, but I can’t speak for everyone). But because Starshot has not sought the kind of publicity that our media-crazed age demands, I want to send you to Atwater’s video presentation at Montreal to get caught up on where things stand. I doubt we’re ever going to fly the mission Starshot originally conceived because of cost and sheer scale, but I’m only an outsider looking in. I do think that when the first interstellar mission flies, it will draw heavily on Starshot’s work. And this will be true no matter what final choices emerge as to propulsion.

This is a highly technical talk compressed into an all too short 40 minutes, but let’s just go deep on one aspect of it, the discussion of the lightsail that would be accelerated to 20 percent of lightspeed for the interstellar crossing. Atwater’s charts are worth seeing, especially the background on what the sail team’s meetings have produced in terms of their work on sail materials and, especially, sail shape and stability. The sail is a structure approximately 4 meters in diameter, with a communications aperture 1 meter in size, as seen in the center of the image (2 on the figure). Surrounding it on the circular surface are image sensors (6) and thin-film radioisotope power cells (5).

Maneuvering LEDs (4) provide attitude control, and thin-film magnetometers (7) are in the central disk, with power and data buses (8) also illustrated. A key component: A laser reflector layer positioned between the instruments that are located on the lightsail and the lightsail itself, which is formed as a silicon nitride metagrating. As Atwater covers early in his presentation, the metagrating is crucial for attitude control and beam-riding, keeping the sail from slipping off the beam even though it is flat. The layering is crucial in protecting the sailcraft instrumentation during the acceleration stage, when it is fully illuminated by the laser from the ground.

How to design lensless transmitters and imaging apertures? Atwater said that lensless color camera and steerable phased array communication apertures are being prototyped in the laboratory now using phased arrays with electrooptic materials. Working one-dimensional devices have emerged in this early work for beam steering and electronic focusing of beams. The laser reflector layer offers the requisite high reflectivity at the laser wavelength being considered, using a hybrid design with silicon nitride and molybdenum disulfide to minimize absorption that would heat the sail.

I won’t walk us through all of the Starshot design concepts at this kind of detail, but rather send you to Atwater’s presentation, which shows the beam-riding lightsail structure and its current laboratory iterations. The discussion of power sources is particularly interesting given the thin-film lightweight structures involved, and as shown in the image below, it involves radioisotope thermoelectric generators actually integrated into the sail surface. Thin film batteries and fuel cells were considered by Breakthrough’s power working group but rejected in favor of this RTG design.

So much is going on here in terms of the selection of sail materials and the analysis of its shape, but I’ll also send you to Atwater’s presentation with a recommendation to linger over his discussion of the photon engine, that vast installation needed to produce the beam that would make the interstellar mission happen. The concept in its entirety is breathtaking. The photon engine is currently envisioned as an array of 1,767,146 panels consisting of 706,858,400 individual tiles (Atwater dryly described this as “a large number of tiles”), producing the 200 gW output and covering 3 kilometers on the ground. The communications problem for data return is managed by scalable large-area ground receiver arrays, another area where Breakthrough is examining cost trends that within the decades contemplated for the project will drive component expenses sharply down. The project depends upon these economic outcomes.

Image: What we would see if we had a Starshot-class sailcraft approaching the Earth, from the image at two hours away to within five minutes of its approach. Credit for this and the two earlier images: Harry Atwater/Breakthrough Starshot.

Using a laser-beamed sail technology to reach the nearest stars may be the fastest way to get images like those above. The prospect of studying a planet like Proxima b at this level of detail is enticing, but how far can we count on economic projections to bring costs down to the even remotely foreseeable range? We also have to factor in the possibility of getting still better images from a mission to the solar gravitational lens (much closer) of the kind currently being developed at the Jet Propulsion Laboratory.

Economic feasibility is inescapably part of the Starshot project, and is clearly one of the fundamental issues it was designed to address. I return to my initial point. Identifying the principles involved and defining the best concepts to drive design both now and in the future is the work of a growing scientific community, which the Starshot effort continues to energize. That in itself is no small achievement.

It is, in fact, a key building block in the scientific edifice that will define the best options for achieving the interstellar dream. And while this is not the place to go into the complexities of scientific funding, suffice it to say that putting out the cash to enable these continuing studies is a catalytic gift to a field that has always struggled for traction both financial and philosophical. The Starshot initiative has a foundational role in defining the best technologies for interstellar flight that will lead one day to its realization.

Braking at Centauri: A Bound Orbit at Proxima?

One of the great problems of lightsail concepts for interstellar flight is the need to decelerate. Here I’m using lightsail as opposed to ‘solar sail’ in the emerging consensus that a solar sail is one that reflects light from our star, and is thus usable within the Solar System out to about 5 AU, where we deal with the diminishment of photon pressure with distance. Or we could use the Sun with a close solar pass to sling a solar sail outbound on an interstellar trajectory, acknowledging that once our trajectory has been altered and cruise velocity obtained, we might as well stow the now useless sail. Perhaps we could use it for shielding in the interstellar medium or some such.

A lightsail in today’s parlance defines a sail that is assumed to work with a beamed power source, as with the laser array envisioned by Breakthrough Starshot. With such an array, whether on Earth or in space, we can forgo the perihelion pass and simply bring our beam to bear on the sail, reaching much higher velocities. Of the various materials suggested for sails in recent times, graphene and aerographite have emerged as prime candidates, both under discussion at the recent Montreal symposium of the Interstellar Research Group. And that problem of deceleration remains.

Is a flyby sufficient when the target is not a nearby planet but a distant star? We accepted flybys of the gas giants as part of the Voyager package because we had never seen these worlds close up, and were rewarded with images and data that were huge steps forward in our understanding of the local planetary environment. But an interstellar flyby is challenging because at the speeds we need to reach to make the crossing in a reasonable amount of time, we would blow through our destination system in a matter of hours, and past any planet of interest in perhaps a matter of minutes.

Robert Forward’s ingenious ‘staged’ lightsail got around the problem by using an Earth-based laser to illuminate one part of the now separated sail ring, beaming that energy back to the trailing part of the sail affixed to the payload and allowing it to decelerate. Similar contortions could divide the sail again to make it possible to establish a return trajectory to Earth once exploration of the distant stellar system was complete. We can also consider using magsail concepts to decelerate, or perhaps the incident light from a bright target star could allow sufficient energy to brake against.

Image: Forward’s lightsail separating at the beginning of its deceleration phase. Laser sailing may turn out to be the best way to the stars, provided we can work out the enormous technical challenges of managing the outbound beam. Or will we master fusion first? Credit: R.L. Forward.

But time is ever a factor, because you want to reach your target quickly, while at the same time, if you approach it too fast, you’re incapable of creating the needed deceleration. Moreover, what is your target? A bright star gives you options for deceleration if you approach at high velocity that are lacking from, say, a red dwarf star like Proxima Centauri, where the closest terrestrial-class world we know is in what appears to be a habitable zone orbit. In Montreal, René Heller (Max Planck Institute for Solar System Research), a familiar name in these pages, laid out the equations for a concept he has been developing for several years, a mission that could use not only the light of Proxima itself but from Centauri A and B to create a deceleration opportunity. You can follow Heller’s presentation at Montreal here.

Remember what we’re dealing with here. We have two stars in the central binary, Centauri A (G-class) and Centauri B (K-class), with the M-class dwarf Proxima Centauri about 13000 AU distant. Centauri A and B are close – their distance as they orbit around a common barycenter varies from 35.6 AU to 11.2 AU. These are distances in Solar System range, meaning that 35.6 AU is roughly the orbit of Neptune, while 11.2 AU is close to Saturn distance. Interesting visual effects in the skies of any planet there.

Image: Orbital plot of Proxima Centauri showing its position with respect to Alpha Centauri over the coming millennia (graduations are in thousands of years). The large number of background stars is due to the fact that Proxima Cen is located very close to the plane of the Milky Way. Proxima’s orbital relation to the central stars becomes profoundly important in the calculations Heller and team make here. Credit: P. Kervella (CNRS/U. of Chile/Observatoire de Paris/LESIA), ESO/Digitized Sky Survey 2, D. De Martin/M. Zamani.

Using a target star for deceleration by braking against incident photons has been studied extensively, especially in recent years by the Breakthrough Starship team, where the question of how its tiny sailcraft could slow from 20 percent of the speed of light to allow longer time at target is obviously significant. Deceleration into a bound orbit at Proxima would be, of course, ideal but it turns out to be impossible given the faint photon pressure Proxima can produce. Investing decades of research and 20 years of travel time is hardly efficient if time in the system is measured in minutes.

In fact, to use photon pressure from Proxima Centauri, whose luminosity is 0.0017 that of the Sun, would require approaching the star so slowly to decelerate into a bound orbit that the journey would take thousands of years. Hence Heller’s notion of using the combined photon pressure and gravitational influences of Centauri A and B to work deceleration through a carefully chosen trajectory. In other words, approach A, begin deceleration, move to B and repeat, then emerge on course outbound to Proxima, where you’re now slow enough to use its own photons to enter the system and stay.

Working with Michael Hippke (Max Planck Institute for Solar System Research, Göttingen) and Pierre Kervella (CNRS/Universidad de Chile), Heller has refined the maximum speed that can be achieved on the approach into Alpha Centauri A to make all this happen: 16900 kilometers per second. If we launch in 2035, we arrive at Centauri A in 2092, with arrival at Centauri B roughly six days later and, finally, arrival at Proxima Centauri for operations there in a further 46 years. That launch time is not arbitrary. Heller chose 2035 because he needs Centauri A and B to be in precise alignment to allow the gravitational and photon braking effects to work their magic.

So we have backed away from Starshot’s goal of 20 percent of lightspeed to a more sedate 5.6 percent, but with the advantage (if we are patient enough) of putting our payload into the Proxima Centauri system for operations there rather than simply flying through it at high velocity. We also get a glimpse of the systems at both Centauri A and B. I wrote about the original Heller and Hippke paper on this back in 2017 and followed that up with Proxima Mission: Fine-Tuning the Photogravitational Assist. I return to the concept now because Heller’s presentation contrasts nicely with the Helicity fusion work we looked at in the previous post. There, the need for fusion to fly large payloads and decelerate into a target was a key driver for work on an in-space fusion engine.

Interstellar studies works, though, through multiple channels, as it must. Pursuing fusion in a flight-capable package is obviously a worthy goal, but so is exploring the beamed energy option in all its manifestations. I note that Helicity cites a travel time to Proxima Centauri in the range of 117 years, which compares with Heller and company’s now fine-tuned transit into a bound orbit at Proxima of 121 years. The difference, of course, is that Helicity can envision launching a substantially larger payload.

Clearly the pressure is on fusion to deliver, if we can make that happen. But the fact that we have gone from interstellar flight times thought to involve thousands of years to a figure of just over a century in the past few decades of research is heartening. No one said this would be easy, but I think Robert Forward would revel in the thought that we’re driving the numbers down for a variety of intriguing propulsion options.

The paper René Heller drew from in the Montreal presentation is Heller, Hippke & Kervella, “Optimized Trajectories to the Nearest Stars Using Lightweight High-velocity Photon Sails,” Astronomical Journal Vol. 154 No. 3 (29 August 2017), 115. Full text.

Interstellar Path? Helicity’s Bid for In-Space Fusion

Be aware that the Interstellar Research Group has made the videos shot at its Montreal symposium available. I find this a marvelous resource, and hope I never get jaded with the availability of such materials. I can remember hunting desperately for background on talks being given at astronomical conferences I could not attend, and this was just 20 years ago. Now the growing abundance of video makes it possible for those of us who couldn’t be in Montreal to virtually attend the sessions. Nice work by the IRG video team!

There is plentiful material here for the interstellar minded, and I will be drawing on this resource in days ahead. But let’s start with fusion, because it’s a word that all too easily evokes a particular reaction in those of us who have been writing about the field for some time. Fusion has always seemed to be the flower about to bloom, even as decades of research have passed and the target of practical power generation hovers in the future. In terms of propulsion, I’ve long felt that if we had so much trouble igniting fusion on Earth, how much longer would it be before we could translate our knowledge into the tight dimensions of a drive for an interstellar spacecraft?

But if you’ll take a look at Helicity Space’s presentation at the Montreal symposium, you’ll learn about a tightly focused company that approaches fusion from a different direction. Rather than starting with fusion reactors designed to produce power on Earth, Helicity’s entire focus is building a fusion drive for space. NASA’s Alan Stern, of New Horizons fame but also a veteran of decades of space exploration, is senior technical advisor here, and as he puts it, “our goal is not to power up New York City but to push spacecraft.” Stern appeared at Montreal to introduce a panel including Stephane Lintner, the company’s CEO and chief scientist Setthivoine You.

Let’s assume for a moment that at some point in the coming years – we can hope sooner rather than later – we do develop in-space propulsion using fusion. The easiest approach is to contrast working fusion with the methods available to us today. New Horizons was, shortly after launch, the fastest spacecraft ever built; it crossed the orbit of the Moon in a scant 9 hours, but it took a decade to reach Pluto/Charon. We can use gravity assists to sling a payload to the Kuiper Belt, perhaps a craft like JHU/APL’s Interstellar Probe concept, but here we’re dependent on planetary geometry, which isn’t always cooperative, and doesn’t provide the kind of boost that fusion could.

Image: Setthivoine You, co-founder and chief scientist at Helicity Space.

I’m a great advocate of sails, both solar sails and so-called ‘lightsails’ driven by beamed energy, but if reaching a distant target fast is the goal, we’re constrained by the need for a large laser installation and the likelihood, in the near future anyway, of pushing sails that are tiny. New Horizons would take two centuries to reach the Sun’s gravitational lens, a target of JPL’s SGL mission, which will use tight perihelion passes of multiple craft to get up to speed. We continue to talk about decades of travel time, even within the Solar System. As for interstellar, Breakthrough Starshot’s tiny craft, perhaps sent as a swarm to Proxima Centauri, might reach the target in 20 years if all goes well, but data return is a huge problem, and the mission can only be a flyby.

Helicity boldly sketches a future in which we might combine Interstellar Probe and the SGL mission into a single operation using a craft capable of taking a 5 ton payload to the gravitational lens (roughly 600 AU) in a little over a decade. Pluto becomes reachable in some of the Helicity concepts in about a year, with a craft carrying far heavier payloads than New Horizons and abundant power for operations and communications. All this in craft that could be carried into space by existing vehicles like Falcon Heavy or the SLS. The vision reminds us of The Expanse – imagine 4 months to Mars carrying 450 tons of payload as one step along the way.

All of this is a science fictional vision, but the work coming out of Helicity’s labs is solid. It involves so-called peristaltic magnetic compression to compress plasma and raise energy density. The method relies on specific stable behaviors within the plasma, a type of ‘Taylor State’ – these describe plasma within strong magnetic fields – discovered by You. These confine the plasma, while magnetic reconnection heating preheats it in the first place. Setthivoine You describes all this in the video, noting that what Helicity is developing is a method that we can consider magneto-inertial confinement, one producing a plasma jet stabilized through shear flows within the plasma itself. The firm’s computer simulations, in collaboration with Los Alamos National Laboratories, show four jets merging in a double-helix fashion inside the magnetic nozzle. You’s slides, available in the video, are instructive and dramatic.

What emerges from these results is scalable performance, allowing a path forward that the firm hopes to test as early as 2026 in a demonstration flight in orbit, with the possibility of a functional spacecraft at TRL 9 within roughly a decade. The scalable design points to the possibility of compact, reusable spacecraft with the unique feature of continuous thrusting. Think of the Helicity system, Stern has suggested, as a kind of afterburner put onto an electric propulsion system, and one that can scale into a working fusion drive. The concept scales by adding more fusion jets, allowing higher and higher efficiency, until the engine eventually becomes self-sustaining – the energy required to sustain the thrust is less than what the fusion begins to generate.

Does Helicity’s technology have the inside track, or will a contender like the UK’s Pulsar Fusion get to in-space fusion first? The latter is working on a study with Princeton Satellite Systems to examine plasma characteristics in a fusion rocket engine. Helicity’s founders are appropriately cautious about their work and stress how much needs to be learned, but they are continuing to develop the concept in their prototypes. With its laboratory in Pasadena, the company is privately funded and maintains active collaborations with Caltech and Los Alamos as well as UMBC and Swarthmore. Grants from the Department of Energy and other sources have likewise contributed. Have a look at the Montreal video to get a sense of where this exciting research stands, and also a glimpse of the kind of future that in-space fusion propulsion could produce.

Save New Horizons

The idea that we might take an active, working spacecraft in the Kuiper Belt and not only repurpose it for a different task (heliophysics) but also dismiss the team that is now running it is patently absurd. Yet this appears to be a possibility when it comes to New Horizons, the remarkable explorer of Pluto/Charon, Arrokoth, and the myriad objects of the Kuiper Belt. NASA’s Science Mission Directorate, responding to a 2022 Senior Review panel which had praised New Horizons, is behind the controversy, about which you can read more in NASA’s New Horizons Mission Still Threatened.

So absurd is the notion that I’m going to assume this radical step, apparently aimed at ending the Kuiper Belt mission New Horizons was designed for on September 30 of 2024, will not happen, heartened by a recent letter of protest from some figures central to the space community, as listed in the above article from Universe Today. These are, among a total of 25 planetary scientists, past Planetary Society board chair Jim Bell, Lori Garver (past Deputy Administrator of NASA), Jim Green (Past Director of NASA’s Planetary Science Division), Candice Hansen-Koharcheck (Past chair of the American Astronomical Society’s Division of Planetary Sciences and Past Chair of NASA’s Outer Planets Assessment Group), author Homer Hickham, Wesley T. Huntress (Past Director of NASA’s solar system Exploration Group), astrophysicist Sir Brian May, and Melissa McGrath (past NASA official and AAS Chair of DPS).

Needless to say, we’ll keep a wary eye on the matter (and I’ve just signed the petition to save the original mission), but let’s talk this morning about what New Horizons is doing right now, as the science remains deeply productive. I like the way science team member Tod Lauer spoke of the spacecraft’s current position in a recent post on the team’s website, a place where “It’s still convenient to think of neat north and south hemispheres to organize our vista of the sky, but the equator is now defined by the band of the galaxy, not our spinning Earth, lost to us in the glare of the fading Sun.”

Places like that evoke the poet in all of us. Lauer points out that the New Horizons main telescope is no more powerful than what amateur stargazers willing to open their checkbooks might use in their backyards. Part of the beauty of the situation, though, is that New Horizons moves in a very dark place indeed. While I have vivid memories of seeing a glorious Milky Way bisecting the sky aboard a small boat one summer night on Lake George in the Adirondacks, how much more striking is the vantage out here in the Kuiper Belt where the nearest city lights are billions of kilometers away?

Image: A Kuiper Belt explorer looks into the galaxy. Credit: Serge Brunier/Marc Postman/Dan Durda

The science remains robust out here as well. Consider what Lauer notes in his article. Looking for Kuiper Belt objects using older images, the New Horizons team found that in all cases, even objects far from the Milky Way’s band appeared against a background brighter than scientists can explain, even factoring in the billions of galaxies that fill the visible cosmos. The puzzle isn’t readily solved. Says Lauer:

…we then tried observations of a test field carefully selected to be far away from the Milky Way, any bright stars, clouds of dust – or anything, anything that we could think of that would wash out the fragile darkness of the universe. The total background was much lower than that in our repurposed images of Kuiper Belt objects – but by exactly the amount we expected, given our care at pointing the spacecraft at just about the darkest part of the sky we could find. The mysterious glow is still there, and more undeniable, given the care we took to exclude anything that would compete with the darkness of the universe, itself. You’re in an empty house, far out in the country, on a clear moonless night. You turn off all the lights everywhere, but it’s still not completely dark. The billions of galaxies beyond our Milky Way are still there, but what we measure is twice as bright as all the light they’ve put out over all time since the Big Bang. There is something unknown shining light into our camera. If it’s the universe, then it’s just as strong, just as bright, as all the galaxies that ever were.

Only New Horizons can do this kind of work. How far are we from Earth? Take a look at the animated image below, which is about as vivid a reminder as I can find.

Image: Two images of Proxima Centauri, one taken by NASA’s New Horizons probe and the other by ground-based telescopes. The images show how the spacecraft’s view, from a distance of 7 billion kilometers, is very different from what we see on Earth, illustrating the parallax effect. Credit: NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute/Las Cumbres Observatory/Siding Spring Observatory.

With 15 more dark fields slated for examination this summer, New Horizons remains active on this and many other fronts. In his latest PI’s Perspective, Alan Stern notes the eight-week period of observations taking place now and through September. include continued observations of KBOs, imaging of the ice giants, dust impact measurements, mapping of hydrogen gas in the outer heliosphere, analysis using the ultraviolet spectrometer to look for structures in the interstellar medium, and spatial variations in the visible wavelength background. Six months of data return will be needed to download the full set for the planetary, heliospheric and astrophysics communities.

According to Stern, new fault protection software will allow New Horizons to operate out to 100 AU. That assumes we’ll still be taking Kuiper Belt data. Given the utterly unique nature of this kind of dataset and the fact that this spacecraft is engaged in doing exactly what it was designed to do, it is beyond comprehension that its current mission might be threatened with a shakeup to the core team or a reframing of its investigations. I’ve come across the National Space Society’s petition to save the New Horizons Kuiper Belt mission late, but please take this chance to sign it.

Rethinking Planet 9

Trans-Neptunian Objects, or TNOs, sound simple enough, the term being descriptive of objects moving beyond the orbit of Neptune, which means objects with a semimajor axis greater than 30 AU. It makes sense that such objects would be out there as remnants of planet formation, but they’re highly useful today in telling us about what the outer system consists of. Part of the reason for that is that TNOs come in a variety of types, and the motions of these objects can point to things we have yet to discover.

Thus the cottage industry in finding a ninth planet in the Solar System, with all the intrigue that provides. The current ‘Planet 9 Model’ points to a super-Earth five to ten times as massive as our planet located beyond 400 AU. It’s a topic we’ve discussed often in these pages. I can recall the feeling I had long ago when I first learned that little Pluto really didn’t explain everything we were discovering about the system beyond Neptune. It simply wasn’t big enough. That pointed to something else, but what? New planets are exciting stuff, especially when they are nearby, as the possibility of flybys and landings in a foreseeable future is real.

Image: An artist’s illustration of a possible ninth planet in our solar system, hovering at the edge of our solar system. Neptune’s orbit is show as a bright ring around the Sun. Credit: ESO/Tom Ruen/nagualdesign.

But a different solution for a trans-Neptunian planet is possible, as found in the paper under discussion today. Among the active researchers into the more than 1000 known TNOs and their movements are Patryk Sofia Lykawka (Kobe University) and Takashi Ito (National Astronomical Observatory of Japan), who lay out the basics about these objects in a new paper that has just appeared in The Astronomical Journal. The astronomers point out that we can use them to learn about the formation and evolution of the giant planets, including possible migrations and the makeup of the protoplanetary disk that spawned them. But it’s noteworthy that no single evolutionary model exists that would explain all known TNO orbits in a unified way.

So the paper goes to work on such a model, and while concentrating on the Kuiper Belt and objects with semimajor axes between 50 and 1500 AU, the authors pin down four populations, and thus four constraints that any successful model of the population must explain. The first of these are detached TNOs, meaning objects that are outside the gravitational influence of Neptune and thus not locked into any mean motion resonance with it. The second is a population the authors consider statistically significant, TNOs with orbital inclinations to the ecliptic above 45 degrees. This population is not predicted by existing models of early Neptune migration.

The paper is generous with details about these and the following two populations, but here I’ll just give the basics. The third group consists of TNOs considered to be on extreme orbits. Here we fall back on the useful Sedna and other objects with large values for perihelion of 60 AU or more. These demand that we find a cause for perturbations beyond the four giant planets, possibly even a rogue planet’s passing. Any model of the outer system must be able to explain the orbits of these extreme objects, an open question because interactions with a migrating Neptune do not suffice.

Finally, we have a population of TNOs that are stable in various mean motion resonances with Neptune over billions of years – the authors call these “stable resonance TNOs” – and it’s interesting to note that most of them are locked in a resonance of approximately 4 billion years, which points to their early origin. The point is that we have to be able to explain all four of these populations as we evolve a theory on what kind of objects could produce the observable result.

What a wild place the outer Solar System would have been during the period of planet formation. The authors believe it likely that several thousand dwarf planets with mass in the range of Pluto and “several tens” of sub-Earth and Earth-class planets would have formed in this era, most of them lost through gravitational scattering or collisions. We’ve been looking at trans-Neptunian planet explanations for today’s Kuiper Belt intensely for several decades, but of the various suggested possibilities, none explained the orbital structure of the Kuiper Belt to the satisfaction of Lykawka and Ito. The existing Planet 9 model here gives way to a somewhat closer, somewhat smaller world deduced from computer simulations examining the dynamical evolution of TNOs.

What the authors introduce, then, is the possibility of a super-Earth perhaps as little as 1.5 times Earth’s mass with a semimajor axis in the range of 250 to 500 AU. The closest perihelion works out to 195 au in these calculations, with an orbit that is inclined 30 degrees to the ecliptic. If we plug such a world into this paper’s simulations, we find that it explains TNOs that are decoupled from Neptune and also many of the high-inclination TNOs, while being compatible with resonant orbit TNOs stable for billions of years. The model thus broadly fits observed TNO populations.

But there is a useful addition. In the passage below, the italics are mine:

…the results of the KBP scenario support the existence of a yet-undiscovered planet in the far outer solar system. Furthermore, this scenario also predicts the existence of new TNO populations located beyond 150 au generated by the KBP’s perturbations that can serve as observationally testable signatures of the existence of this planet. More detailed knowledge of the orbital structure in the distant Kuiper Belt can reveal or rule out the existence of any hypothetical planet in the outer solar system. The existence of a KBP may also offer new constraints on planet formation and dynamical evolution in the trans-Jovian region.

A testable signature is the gold standard for a credible hypothesis, which is not to say that we will necessarily find it. But if we do locate such a world, or a planet corresponding to the more conventional Planet 9 scenarios, we will have ramped up our incentive to explore beyond the Kuiper Belt, an incentive already given further impetus by our growing knowledge of the heliosphere and its interactions with the interstellar medium. But to the general public, interstellar dust is theoretical. An actual planet – a place that can be photographed and one day landed upon – awakens curiosity and the innate human drive to explore with a target that pushes all our current limits.

The paper is Lykawka & Ito, “Is There an Earth-like Planet in the Distant Kuiper Belt?,” Astronomical Journal Vol. 166, No. 3 (25 August 2023) 118 (full text).

Interstellar Probe: Into the G Cloud

We’re living in a prime era for studying the Solar Systems’ movement through the galaxy, with all that implies about stellar evolution, planet formation and the heliosphere’s interactions with the interstellar medium. We don’t often think about movements at this macro-scale, but bear in mind that the Sun and the planets are now moving through the outer edges of what is known as the local interstellar cloud (LIC), having been within the cloud by some estimates for about 60,000 years.

What happens next? I always think about Poul Anderson’s wonderful Brain Wave when contemplating such matters. In the classic 1954 tale, serialized the year before in Space Science Fiction during the great 1950s boom in science fiction magazines, Brain Wave depicts the Earth’s movement out of an energy-damping field it had moved through since the Cretaceous. When the planet moves out of this field at long last, everyone on the planet gets smarter. What will happen when we leave the LIC?

Nothing this dramatic, as far as anyone can tell, but we’re moving into the so-called G-cloud, a somewhat denser region we’ll get to know within a few thousand years (Alpha Centauri is already within the G-cloud). The LIC appears to be about 30 light years across, sporting somewhat higher density hydrogen than the interstellar medium around it and flowing from the direction of Scorpius and Centaurus. We seem to be moving through its outer edge.

Image: That’s a Richard Powers cover on the first edition of Poul Anderson’s Brain Wave, illustrating a novel Anderson considered one of his best.

The word ‘denser’ has to be kept in perspective. Consider that about half of the interstellar gas – hydrogen and helium for the most part – takes up 98 percent of the space between the stars and is thus extraordinarily low in density, The other half of this gas is compressed into 2% of the volume and is found in molecular clouds, mostly molecular hydrogen, that include some carbon monoxide and higher concentrations of dust than the surrounding ISM.

We’d like to know more about the G-cloud, and in particular such factors as its temperatures and density, for just how the heliosphere reacts to this changing environment, presumably through compression, could have effects upon its shape and thus its shielding effects from cosmic rays. Thus the importance of the Interstellar Probe mission (ISP) out of Johns Hopkins Applied Physics Laboratory that we’ve discussed often in these pages (I’ll point you to Mapping Out Interstellar Clouds for more on the subject). One thing I noted in an article last year was that there are alternative models to the Sun’s nearby cloud environment, some challenging the idea that the LIC and the G-cloud are distinct regions. Interstellar Probe would help in the investigation.

Image: Our solar journey through space is carrying us through a cluster of very-low-density interstellar clouds. Right now the Sun is inside of a cloud (Local cloud) that is so tenuous that the interstellar gas detected by the IBEX (Interstellar Boundary Explorer mission) is as sparse as a handful of air stretched over a column that is hundreds of light-years long. These clouds are identified by their motions, indicated in this graphic with blue arrows. Credit: NASA/Goddard/Adler/U. Chicago/Wesleyan.

The fate of Interstellar Probe is the hands of the National Academies of Sciences, Engineering, and Medicine, which prioritizes in its decadal studies where we are going in space exploration over ten year periods. We find out next year whether ISP has been selected among several strong candidates for the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Until then, I continue to scout the literature both for Interstellar Probe as well as JPL’s Solar Gravitational Lens (SGL) mission, a candidate for the same study. These are huge decisions in space science.

Be aware of a new paper tackling the issues of heliospheric and dust science that a mission like Interstellar Probe could examine, with a solid backgrounder on the kind of instrumentation that would make this first dedicated interstellar probe a game-changer in our understanding of the local interstellar environment. The paper, by lead author Veerle J. Sterken (ETH Zürich) and colleagues, was originally submitted to the heliophysics Decadal, and is here available in a modified version (citation below) that includes the new instrumentation discussion among other modifications.

I won’t run through the entire paper; suffice it to say that its discussion of interstellar dust in and around the heliosphere is comprehensive and its analysis of how best to study changing environments in the interstellar medium is highly informative. The benefit of a probe explicitly designed to examine the transition between dust and gas states along its trajectory are clear. From the paper, which notes here the particular importance of heliopause passage:

ISP will fly throughout approximately 16 years, more than a solar cycle, while passing through interplanetary space, the termination shock, the heliosheath, up to the heliopause and beyond, making it an optimal mission for studying the heliosphere-dust coupling and using this knowledge for other astrospheres. Beyond the heliopause, the tiny dust with gyro-radii of only a few to 100 AU (for dust radii < 0.1 μm…), will help study the interstellar environment (magnetic field, plasma) and may detect local enhancements of smaller as well as bigger ISD [interstellar dust]. The strength of the mission lies in flying through all of these diverse regions with simultaneous magnetic field, dust, plasma and pickup ion measurements. No mission so far has flown a dedicated dust dynamics and composition suite into the heliosheath and the vast space beyond.

Analyzing the interaction between the heliosphere and interstellar dust acts as a probe into the history of our own Solar System, since we know that some of this material was affected by near-Earth supernovae, still raining down in the form of dust today. Mid-sized dust particles in the range of 0.1 to 0.6 μm in radius can make it through the heliosphere into the Solar System. Some smaller particles (30-100 nm) as well may escape filtering, but the authors note the limitations of our knowledge: “The exact lower cut-off size and time-dependence of particles that can enter the solar system is not yet exactly known, but Ulysses and Cassini already have measured ISD particles with radii between 50 and 100 nm.”

Thus we have markers for stellar and galactic evolution as factors in this study. But I’ll also remind those interested in interstellar flight that assessing dust density and the size distribution of particles will play a major role when we reach the point where we can send craft at a significant fraction of the speed of light. Larger particles in the local interstellar environment could cause catastrophic failure, while Ian Crawford has pointed out that other properties of this region could inform propulsion concepts like the interstellar ramjet.

Something else I learned from Ian Crawford (University of London) and wrote about twelve years ago (see Into the Interstellar Void) is that a spacecraft moving at 0.1c could make daily measurements 17 AU apart, which is roughly half the radius of the Solar System. So as we develop the fastest probe we can manage today in the form of designs like Interstellar Probe, we can see a larger picture in which such studies assist as we develop future technologies capable of actually making a crossing to Alpha Centauri, sampling widely along the way two different interstellar clouds and the boundary between them.

The paper is Sterken et al., “Synergies between interstellar dust and heliospheric science with an Interstellar Probe,” submitted as white paper for the National Academies Decadal Survey for Solar and Space Physics 2024-2033 and available as a preprint here.

Tidal Lock or Sporadic Rotation? New Questions re Proxima and TRAPPIST-1

Centauri Dreams regular Dave Moore just passed along a paper of considerable interest for those of us intrigued by planetary systems around red dwarf stars. The nearest known exoplanet of roughly Earth’s mass is Proxima Centauri b, adding emphasis to the question of whether planets in an M-dwarf’s habitable zone can indeed support life. From the standpoint of system dynamics, that often comes down to asking whether such a planet is not so close to its star that it will become tidally locked, and whether habitable climates could persist in those conditions. The topic remains controversial.

But there are wide variations between M-dwarf scenarios. We might compare what happens at TRAPPIST-1 to the situation around Proxima Centauri. We have an incomplete view of the Proxima system, there being no transits known, and while we have radial velocity evidence of a second and perhaps a third planet there, the situation is far from fully characterized. But TRAPPIST-1’s superb transit orientation means we see seven small, rocky worlds moving across the face of the star, and therein lies a tale.

The paper Dave sent, by Cody Shakespeare (University of Nevada Las Vegas) and colleague Jason Steffen, picks up on earlier work Shakespeare undertook that probes the differences between such scenarios. We know that conditions are right for a solitary planet, unperturbed by neighbors, to orbit with a spin rate synchronous with its orbital rate, the familiar ‘tidal lock.’ On such a world, we probe questions of climate, heat transport, the effects of an ocean and so on, to see if a planet with a star stationary in its sky could sustain life.

Image: This illustration shows what the TRAPPIST-1 system might look like from a vantage point near planet TRAPPIST-1f (at right). Credit: NASA/JPL-Caltech.

But as TRAPPIST-1 shows us in exhilarating detail, multi planet systems are not uncommon around this type of star, and now we have to factor in mean motion resonance (MMR), where the very proximity of the planets (all well within a fraction of Mercury’s orbit of our Sun) means that these effects can perturb a particular planet out of its otherwise spin-orbit synchronization. Call this ‘orbital forcing,’ which breaks what would have been, in a single-planet system, a system architecture that would inevitably lead to permanent tidal lock.

The results of this breakage produce the interesting possibility that planets like TRAPPIST-1 e and f may retain tidal lock but exhibit sporadic rotation (TLSR). Indeed, another recent paper referenced by the authors, written by Howard Chen (NASA GSFC) and colleagues, makes the case that this state can produce permanent snowball states in the outer regions of an M-dwarf planetary system. What is particularly striking about TLSR is the time frame that emerges from the calculations. Consider this, from the Shakespeare paper:

The TLSR spin state is unique in that the spin behavior is often not consistently tidally locked nor is it consistently rotating. Instead, the planet may suddenly switch between spin behaviors that have lasted for only a few years or up to hundreds of millennia. The spin behavior can occasionally be tidally locked with small or large librations in the longitude of the substellar point. The planet may flip between stable tidally locked positions by spinning 180°so that the previous substellar longitude is now located at the new antistellar point, and vice versa. The planet may also spin with respect to the star, having many consecutive full rotations. The spin direction can also change, causing prograde and retrograde spins.

Not exactly a quiescent tidal lock! Note the term libration, which refers to oscillations around the rotational axis of a planet. What Shakespeare and Steffen are analyzing is the space between long-lasting rotation and pure tidal lock. Indeed, the authors identify a spin scenario within the TLSR domain they describe as prolonged transient behavior, or PTB. Here the planet moves back and forth in a ‘spin regime’ that is essentially chaotic, so that questions of habitability become fraught indeed. Instead of a persistent climate, which we usually assume when assessing these matters, we may be looking at multiple states of climate determined by present and past spin regimes, and their necessary adaptation to the ever changing spin state.

Such global changes are reminiscent, though for different reasons, of Asimov’s fabled story “Nightfall,” in which scientists on a world in a system with six stars must face the social consequences of a ‘night’ that only appears every few thousand years. For here’s what Shakespeare and Steffen say about a scenario in which TSLR effects kick in, a world that had been tidally locked long enough for the climate to have become stable. The scenario again involves TRAPPIST-1:

Such a planet in the habitable zone around a TRAPPIST-1-like star could have an orbital period of around 4-12 Earth days – the approximate orbital periods of T-1d and T-1g, respectively. Due to the TLSR spin state, this planet may, rather abruptly, start to rotate, albeit slowly – on the order of one rotation every few Earth years. The previous night side of the planet, which had not seen starlight for many Earth years, will now suddenly be subjected to variable heat with a day-night cycle lasting a few years. The day side would receive a similar abrupt change and the climate state that prevailed for centuries would suddenly be a spinning engine with momentum but spark plugs that now fire out-of-sync with the pistons. In this analogy, the spark plugs and the subsequent ignition of fuel correspond to the input of energy from starlight. The response of ocean currents, prevailing winds, and weather patterns may be quite dramatic.

Not an easy outcome to model in terms of climate and habitability. The authors use a modified version of an energy release modeling software called 1D EBM HEXTOR as well as a model called the Hab1 TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI) Protocol as they analyze these matters. I send you to the paper for the details.

Science fiction writers take note – here is rich material for new exoplanet environments. Notice that the TLSR spin state is different from the one-way change that occurs when a rotating planet gradually becomes tidally locked over large timescales. This is a regime of sudden change, or at least it can be. The authors consider spin regimes lasting less than 100 Earth years, with the longest regimes (these are classified as ‘quasi-stable’) lasting for 900 years or more and perhaps reaching durations of hundreds of thousands of years. The point is that “TLSR planets are able to be in both long-term persistent regimes and PTB regimes -– where frequent transitions between behaviors are present.”

We learn that all tidally locked bodies experience libration to some degree even if no other bodies are found in the system being examined. Four spin regimes are found within the broader spin state TLSR. Tidal lock with libration can occur around the substellar point, as well as around the substellar or antistellar point, or as noted a planet may be induced into a slow persistent rotation. Much depends upon how long any one of these ‘continuous’ states lasts; given enough time, a stable climate could develop. The chaotic behavior of the fourth state, prolonged transient behavior (PTB), induces frequent transitions in spin. Such transitions would be expected to produce extreme changes in climate.

The spin history of a given system will depend upon that system’s architecture and the key parameters of each individual planet, an indication of the complexity of the analysis. What particularly strikes me here is how fast some of these changes can occur. Here’s a science fiction scenario indeed:

The more extreme change is in the temperature of different longitudes as the planet transitions from a tidally locked regime to a Spinning regime or after the planet flips 180 and remains tidally locked. Rotating planets experience temperature changes at the equator of 50K or more over a single rotation period. The exact effects require more robust climate models, like 3D GCMs [Global Circulation Model], to properly examine. However, using comparisons with climate changes on Earth, it is likely that erosion of land masses would increase and major climate systems would experience significant changes.

As if the issue of habitability were not complex enough…

The paper is Shakespeare & Steffen, “Day and Night: Habitability of Tidally Locked Planets with Sporadic Rotation,” in process at Monthly Notices of the Royal Astronomical Society and available as a preprint. The Chen paper referenced above is “Sporadic Spin-Orbit Variations in Compact Multi-planet Systems and their Influence on Exoplanet Climate,” accepted at Astrophysical Journal Letters (preprint).