Centauri Dreams

I’ve focused on aerographite these past several days because sail materials are a significant determinant of the kind of missions we can fly both in the near-term and beyond. The emergence of a new ‘contender’ to join graphene as a leading candidate for deep space missions is worthy of note. Whether or not this ultra lightweight material produced by teams at the Technical University of Hamburg and the University of Kiel lives up to its promise will depend upon a thorough investigation of its properties as adapted for sails, one which has already begun.

Sail materials matter because we have already begun flying spacecraft with these technologies, so that as we climb the learning curve in terms of design and engineering, we need to be thinking about how to increase performance to allow ambitious missions, and perhaps even audacious ones like Breakthrough Starshot, though the authors of the first paper on aerographite for sails are skeptical about whether the material could withstand the demands of the Starshot laser push.

Aerographite seems to allow strikingly fast missions using solar photons alone — the paper discusses reaching Pluto orbit in less than 5 years, for example — but the authors of this paper are fully aware of the number and complexity of the issues that need to be addressed to make such a thing happen. The idea is to advance the concept, bat the ideas around, learn from laboratory experiment, and try this relatively inexpensive material out in space, starting probably with near-term projects on the International Space Station and going from there.

Image: This is Figure 1 from the aerographite discovery paper. Note: TEM = Transmission electron microscopy. The caption: Overview of different Aerographite morphologies by controlled derivations of synthesis. a) Photograph of macroscopic Aerographite. b–d) 3D interconnected structure of closed-shell graphitic Aerographite in different magnifications and TEM inset of wall. e–h) Hierarchical hollow framework configuration of Aerographite in different magnifications. i–l) Other variants of Aerographite. i) Aerographite network in low aspect bubble-like configuration. j-k) Aerographite with nano porous graphite filling. l) Hollow corrugated pipe design of Aerographite surface by detailed adoption of template shape. Credit: Mecklenburg et al.

Among the issues I didn’t have time to discuss yesterday is the sail’s absorption of photons from a high-energy beam, like Starshot’s projected ground-based array. An aerographite sail that absorbs solar photons may have trouble under an intense beam, considering that the material melts at 400 C — remember, we are talking about a black sail that works through absorbance rather than reflectivity. 400 C is a figure that the Hamburg team measured under atmospheric conditions — consider it a combustion temperature, as Alex Tolley reminds me, rather than a melting point — so we would still need to learn about the situation in space. This would be a matter of simple experiment but a crucial issue to resolve if we’re interested in beamed sails.

A larger problem emerges with the fact that aerographite is an electrical as well as a heat conductor. That means an aerographite sail will accumulate charge from solar UV radiation or possibly the solar wind, as the paper is quick to note. Launching an aerographite sail from low Earth orbit could result in deflection of the sail’s trajectory by Lorentz forces induced by the Earth’s magnetic field. Interplanetary and interstellar magnetic fields pose a challenge depending on the mission.

Thus we have a navigation issue to investigate, one that would likely need to be resolved, says lead author René Heller (Max Planck Institute for Solar System Research, Göttingen) by an active, autonomous on-board computer “…and some form of photon “wings” or “rudders” to trim the sail.” The effect of charge upon aerographite can be viewed in a video: https://www.youtube.com/watch?v=Oh8skH1oQDE&feature=emb_title.

Heller said in an email discussion that before we start talking about interstellar missions, we need to figure out how to move an aerographite sail in predictable ways:

At this point, we are introducing the basic equations and some sample trajectories to show that interstellar escape is possible in principle. But steering is very complicated. There are so many unknowns that would affect a sail trajectory such as interplanetary magnetic fields (leads to deflection if the sail gets electrically charged, e.g. by cosmic particle hits), the solar wind, interstellar magnetic fields, limited knowledge of the actual position and velocity (“proper motion”) of the target star etc. that I can’t see at this point how one could passively steer an aerographite hollow sphere or cone or parachute web – whatever – to a star at 4 light years. That’s why we entitled our paper an “interstellar precursor”. Even aiming at Mars would be hard with a passive sail, which is why we talk about reaching the orbit of Mars and the orbit of Pluto rather than Mars and Pluto themselves.

Structural reinforcement may turn out to be necessary given the material’s relatively low tensile strength. All of these matters need investigation, but the beauty of aerographite is that it is available for demonstrator work near Earth, as co-author Pierre Kervella adds:

…an interstellar spacecraft would likely be very different from the presented concept. The spherical shell could be a very valuable demonstrator in the vicinity of the Earth. Once we have convincingly shown that interstellar velocities are realistically within reach, it will likely change the research landscape and boost the innovations in ultra-light materials. Aerographite has such remarkable properties, but it was not developed at all for being a solar sail! There is certainly a large margin for improvement through a dedicated research effort.

A black sail presents interesting challenges when we want to track it from Earth. We have an infrared signature to work with that would be compromised by the absorption of atmospheric water vapor, making space-based observation the key. The sail’s effective temperature will drop as it recedes from the Sun, making the wavelength of its peak emission increase. The authors calculate that the James Webb Space Telescope could track such a sail, and demonstrate this by calculating its temperature in thermal equilibrium with absorbed sunlight.

The result: JWST observations of an aerographite sail of 1 m radius are possible out to 2 AU. A sail 10 m in radius can be observed to 3 AU. But we have other options as we move into the outer Solar System. A swarm configuration of sails as discussed yesterday could, the authors believe, be tracked in deep space by ALMA (the Atacama Large Millimeter/submillimeter Array) at distances of 1000 AU and beyond as the sail’s temperature drops to 10 K.

We’re hoping, of course, for a communicative spacecraft, one relying on ultra lightweight instrumentation. Ideally we would want to implement a laser on-board to remain in contact with Earth. From the paper:

Miniaturization of electronic components has made great progress in the last few decades, but we focus on mass margins above 1 g because we do not expect sub-gram margins to be relevant for the foreseeable future. Commercial lithium-ion batteries weighing a few grams and with power densities > 1 kW kg^?1 (Duduta et al. 2018) as well as ultra-high-energy density supercapacitors with power densities of ?32 kW kg^?1 (Rani et al. 2019) are already available, allowing energy emission of a gram-sized power source of 32 W in theory.

And again:

A laser sending the proper time of the sail to Earth would allow distance and speed measurements through the relativistic Doppler effect. Measurements of gravitational perturbations (Christian & Loeb 2017; Witten 2020) under consideration of dust and gas drag as well as magnetic forces exerted from the interstellar medium (Hoang & Loeb 2020) could also be used to search for the suspected Planet Nine in the outskirts of the solar system. Its expected orbital semimajor axis is between about 380 AU and 980 AU (Batygin & Brown 2016; Brown & Batygin 2016).

An interesting thought! René Heller mentioned this as well in an email, talking about the prospect of hundreds or thousands of aerographite sails, each with a gram-sized on-board transmitter to allow tracking of distance and speed relative to Earth. A reconstruction of their individual trajectories could be used to look for gravitational perturbations leading to the putative Planet Nine. Remember, this is a low-cost material. The authors estimate that meter-sized aerographite spheres with a thickness in the ?m range could be produced in large numbers for roughly $1000 US. Breakthrough Starshot is using a per-sail cost estimate of $100 US.

Let’s look long-range again and consider an interstellar implication. If aerographite did allow a mission to, say, Proxima Centauri with a travel time of less than 200 years, would there be any way to decelerate it upon arrival? Obviously we could brake against the star’s light, but the problem with Proxima is that its light is relatively weak, and deceleration would be negligible. Heller and Hippke have in the past considered using Centauri A and B as buffers for deceleration, with the residual kinetic energy absorbed by Proxima Centauri itself (see By ‘Photogravitational Assists’ to Proxima b).

But I’m getting way out in front in going this route. What we need now is something we can deploy in the near-term, and here it’s conceivable that aerographite may become valuable even for a laser-beaming project like Breakthrough Starshot. From co-author Guillem Anglada-Escudé (Institut de Ciencies Espacials, Barcelona):

…we are aiming at something we can deploy immediately, at a low budget. As you know, space is very slow in demonstrating technology. But most unknowns can be sorted out if we can make it fly instead of theorising about it for two decades. That’s the spirit. All knowledge on operating sails is of high value anyway. The micro-instrumentation that Starshot needs can be installed, tested and begin producing science with these sails on Solar System exploration for example.

A rousing prospect, that. An early hollow sphere aerographite demonstrator with a diameter in the range of a few meters might be brought into space as a piggyback add-on to an existing interplanetary mission, adding little mass given the lightweight nature of the material. Add to the low cost of aerographite itself the fact that deep space missions, conceivably interstellar ones, can be implemented using solar photons alone, without the need for a massive laser installation and all the issues ground-based laser beaming introduces, and the economic justification for pursuing this research becomes obvious.

Moving a small sample of aerographite with light in the laboratory is the next step in the research, a set of experiments now being devised. Breakthrough Starshot should be keeping an eye on this research.

The paper is Heller, Anglada-Escudé, Hippke & Kervella, “Low-cost precursor of an interstellar mission,” Astronomy & Astrophysics 7 July 2020 (abstract / preprint). The aerographite discovery paper is Mecklenburg et al., “Aerographite: Ultra Lightweight, Flexible Nanowall, Carbon Microtube Material with Outstanding Mechanical Performance,” Advanced Materials Vol. 24, Issue 26 (12 June 2012). Abstract.

Aerographite is an ultra lightweight material made of carbon microtubes, just the sort of thing that seizes the imagination in terms of material for space sails powered by solar photons or laser beam. Such materials are much in my thinking these days and have been for some time, ever since I first read some of Robert Forward’s papers on using laser beaming to boost enormous sails to a substantial fraction of lightspeed. What kind of materials would be used, and how could the mass be kept low enough to allow significant payloads to be deployed?

These days, we think in terms of much smaller sails with miniaturized payloads of the sort advocated by Breakthrough Starshot. But of course advances in sail technology enable a wide range of concepts, and the place to start is with laboratory experiment — this is where we are with aerographite right now — moving into space demonstrators that can be low-cost and near-term. The kinds of missions conceivable with aerographite include fast access to the outer Solar System and, with the help of a close solar pass, interstellar trajectories to nearby stars.

What we are examining in this series of posts is a concept paper that asks for the first time whether aerographite can become a sail material, noting its low cost and our ability to begin testing not just on the ground but in space to find out whether it can carry a payload and survive the stresses of deep space journeys. Lead author René Heller (Max Planck Institute for Solar System Research, Göttingen), with co-authors Guillem Anglada-Escudé (Institut de Ciencies Espacials, Barcelona), Michael Hippke (Sonneberg Observatory, Germany) and Pierre Kervella (Observatoire de Paris), offer a pointer to areas of investigation we will have to address.

Image: An aerographite sample from the Technical University of Hamburg now under study as its potential as a sail material is examined. Sample courtesy of B. Fiedler/H. Beisch (TU Hamburg). Image by R. Heller (MPS Göttingen).

Exploring Aerographite’s Potential

When the teams at the Technical University of Hamburg and the University of Kiel developed aerographite, they dubbed it “the lightest known material.” It is a synthetic foam connected by carbon microtubes with a density of 180 g/m³. This intriguing material has useful properties indeed, though some of these, as we’re about to see, are problematic. For now, let’s note that aerographite is ultra-black at the 1 mm scale considered in the Hamburg team’s paper (citation in yesterday’s post), though as Guillem Anglada-Escudé explained, its opacity is unclear as we go below that scale, a matter that will have to be addressed.

Aerographite’s opacity is important because we are talking about a sail that functions not through reflectivity but absorptivity. The sail concepts we’ve bandied about in these pages have for the most part revolved around reflection, so let’s pause on this. I was unsatisfied with my own description of an absorptive sail in a rough draft of this post, so I asked René Heller for his own take. An opaque sail, he replied, absorbs any light that impinges upon it:

Think of a shower of photons that bumps into an aerographite sample. The sample is almost entirely black, and so most of the photons will not be reflected but they will transfer their kinetic energy (and their momentum) to the sample. As a consequence, the sample will be heated and “pushed” into the same direction as the incoming photon shower was moving before it got absorbed. This is due to conservation of momentum. Now if the sample were reflective then the incoming photons would bounce back from the sample in some angle. If the sample were 100% reflective and if it had a perfectly flat surface, and if the photons were coming in in a perpendicular angle with respect to the surface, then the sample would receive twice the amount of the momentum that it received in comparison to the absorptive case described above. In other words, reflection is twice as effective as absorption.

And yet absorption in the right material can be enabling. While we begin deeper investigations into aerographite’s properties, it’s worth noting that the 1 mm shell thickness in the putative sail explored in these calculations is already useful for some mission concepts. Solar photons alone can push an aerographite sail of this thickness beyond Solar System escape velocity.

If laboratory work confirms that the shell can be reduced below 1 mm while remaining opaque, we can start talking about on-board technology like scientific instruments and the needed communications devices. We can also start talking about much faster transit times. As mentioned yesterday, Heller et al. find the orbit of Mars reachable within 60 days assuming an aerographite shell of 0.5 mm. That gets you to Pluto in 4.3 years, nicely halving the New Horizons travel time. As you can see, we are getting into interstellar precursor range.

This is done through solar photons alone, without the considerable overhead of the massive laser array assumed by Breakthrough Starshot, with obvious (and enormous) cost savings. Starshot wins on speed, aiming for 20 percent of lightspeed and thus a transit time to Proxima Centauri of about 20 years. But put a 1 ?m thin aerographite hollow sphere at 0.04 AU (this is Parker Solar Probe territory) for a close solar pass and you achieve 6900 km/second, which gets you to Proxima Centauri in 185 years, well below the 1,000 year threshold thus far determined for a graphene sail.

How to make a sail out of this material? Anglada-Escudé explained in an email that aerographite:

…has structural integrity (it keeps its shape and recovers over deformation), and it is “relatively” simple to make. Think of it as a foam. You make a template of porous ZnO [zinc oxide], carbon fills in the gaps through a deposition process forming carbon fibres/tubes/sheets, etc. You remove the template material, and you have a block of very light aerographite foam with the shape of the original ZnO template.

The authors discuss hollow spheres as the sail’s shape, but a range of possibilities exist. The spherical shape was chosen for the paper because it provides the simplest solution at this stage of the evaluation of aerographite, but a cone is likewise feasible, and would likely provide stability — I think of this in terms of Jim and Greg Benford’s work on ‘beamriding’ sails, obviously an issue with Breakthrough Starshot.

The today typical ‘flat’ sail is likewise a possibility. The authors have discussed the matter with the Hamburg group behind aerographite, with the latter seeing no problem in manufacturing the material in arbitrary shapes and sizes. Adds Anglada-Escudé:

A spherical shell with a wall of 1 mm is already unbound to the Solar System, but if we could work with a more classic flat (or quasi flat) sail in ‘parachute’ configuration, we could work with up to 4 mm in thickness (which is pretty macroscopic) and add payload easily. I would think that the idea is to start the conversation and let the community evolve it to the most useful application. This is now mission planning, not speculative design! Deploying it would be so cheap… it is almost painful to think about why we have not done it yet.

Or we can get more exotic still, as lead author Heller told me, envisioning separate sail components connected by carbon nanotubes:

The shell (or hollow sphere) design that we focus on is just better than a solid sphere or a cube but not necessarily the optimal shape. I personally think a web of dozens or hundreds of cm-sized cones or parachutes, all of which automatically orient themselves to the solar radiation individually will be a more practical application if it really comes to moving payload through the solar system. Such a web of aerographite parachutes would be more resilient to failure (of one or a few parts) and it would allow larger mass margins for the payload. I wouldn’t pay too much attention to single-shell concepts for real applications, although hollow spheres make the math very simple, which is why we used them for this introductory paper. A web will be better for “large” (gram to kilogram) payloads.

Directions for Research

Aerographite gives the appearance of an ideal sail material if it can be used to carry a payload. The authors produce benchmark scenarios showing that the weight of the payload is 1000 times the mass of the transport system, quite a contrast with chemical rockets — think New Horizons and its Atlas V — where the transport system is 1000 times heavier than the payload.

The authors calculate payload mass in terms of shell thickness, using benchmarks of 1 ?m and 100 ?m, with this result:

…a 1 m radius hollow aerographite sphere with a shell thickness of 1 ?m (100 ?m) would weigh 2.3 mg (230 mg) and have a margin of 2.4 g (2.2 g) to get interstellar. Upon release to the solar radiation in interplanetary space at 1 AU from the Sun, a payload mass of 1 g would yield a terminal speed of 51 km s^?1 (41 km s^?1), which is 3 times (2.4 times) the terminal speed of Voyager 1. A travel to the orbit of Pluto would take 3.9 yr (4.7 yr). An increase of the sail radius to 5 m would allow payload masses of 10 g to reach the orbit of Pluto in almost half the time.

These are tiny payloads, which is why the authors consider the aerographite sail in terms of scalability. The notion of a swarm of aerographite spheres connected by carbon nanofibers — “the additional weight of which would be small compared to the mass of the aerographite shell” — implies a combined thrust that would allow larger payloads on the one hand, or faster travel time for a given payload. Payloads could thus be brought into the kg domain if sail sizes in the 100 meter range can be achieved.

One advantage of this concept is that it would aid in tracking the spacecraft, which is itself an issue we need to address. But so is the matter of how a payload is positioned, either attached to the sail itself or, depending on sail shape and configuration, drawn behind it in parachute fashion. This is a problem faced by Breakthrough Starshot as well as it seeks the right sail material and optimum configuration for ‘beam-riding’ with its proposed laser array. The authors do not address the question of payload configuration and attachment in this paper.

Nor are we through with significant issues. How would the aerographite sail behave in space, given its tendency to accumulate electrical charge? Navigation issues immediately arise that we need to talk about tomorrow, and we also have issues regarding aerographite’s tensile strength.

Let’s also talk in the next post about the kind of missions we might want to deploy, assuming a successful research project produces flight-testable sails. Interesting options open up long before we start talking about true interstellar flight, perhaps piggybacking on other missions as we learn both about the material in space as well as the formidable issues of tracking an utterly black sail. More on all this next time.

The paper is Heller, Anglada-Escudé, Hippke & Kervella, “Low-cost precursor of an interstellar mission,” Astronomy & Astrophysics 7 July 2020 (abstract / preprint).

Invented at the Technical University of Hamburg and developed with the aid of researchers at the University of Kiel, a new material called aerographite offers striking prospects for solar sail missions within the Solar System as well as interstellar precursor implications. Judging from the calculations in a just published paper in Astronomy & Astrophysics, aerographite conceivably enables a mission to Proxima Centauri with a flight time of less than two centuries. We are not talking about laser-driven missions here, but rather meter-scale craft that would be pushed to interstellar velocities by solar radiation; i.e., true solar sails.

But let’s focus near-term before going interstellar. I’ve been talking to René Heller (Max Planck Institute for Solar System Research, Göttingen) about the paper, along with co-authors Guillem Anglada-Escudé (Institut de Ciencies Espacials, Barcelona), Michael Hippke (Sonneberg Observatory, Germany) and Pierre Kervella (Observatoire de Paris). Just what are the prospects for aerographite, and what are its problems? The authors stress that aerographite for space applications implies a development path through laboratory work and near-term experimentation in space. “Before we run, we need to walk,” as Anglada-Escudé told me in an email that summarized how the idea grew.

Anglada-Escudé and Heller had been studying the conditions that would allow a sail pushed by solar radiation, as opposed to laser beaming, to leave the Solar System, developing calculations for the kind of material needed, and initially envisioning a sail made of graphene (about which more in a moment). As Hippke and Kervella joined the discussion, the connection with aerographite was made and the previous computations recalculated. Says Heller:

“Aerographite is both ultralight and opaque (= black) so that it effectively absorbs photons and overcomes the gravitational attraction from the Sun under certain circumstances. We find that a hollow sphere (or shell) with a diameter of a few meters and a shell thickness less than 1 mm would become unbound from the solar system if it were submitted to sunlight in interplanetary space. It could be brought into space as a piggyback mission to any interplanetary mission without adding significant amounts of mass to the payload (because aerographite is so ridiculously lightweight)”.

Adds Anglada-Escudé, on how a material might be ‘unbound’ from the Solar System:

It is known that small dust (mostly SiO₂ [silicon dioxide] balls, which are quite dense) in the solar system are blown away at some 100 nanometer sizes, so any material lighter than that should work. Also, one does not need to be reflective. Absorbance is also OK (only a factor of 2 worse than a fully reflective material).

The reference to Starshot is useful because it sets up some technology comparisons I want to make, while also driving home the point that as a near-term goal, something as relatively “local” as a demonstrator released from the International Space Station could move the ball forward. So what I want to do in the course of the next several posts is to examine the Astronomy & Astrophysics paper and consider the uses of aerographite in a variety of mission concepts as we begin to explore how such a sail could be constructed and flown.

But we also need some context, and a nod to a slightly earlier and itself promising sail material gets the story in motion.

Image; This graphic shows a detail of the world’s lightest material: aerographite. Open carbon tubes form a fine mesh and offer a low density of 0.2 milligram per cubic centimetre. The picture was taken with a scanning electron microscope (SEM). Credit: TUHH.

Sails for Deep Space Missions

Sails have been considered for long-duration, conceivably interstellar missions since the days of Robert Forward, and Centauri Dreams readers will also be aware of such seminal works as Greg Matloff’s 1981 paper “Solar Sail Starships—The Clipper Ships of the Galaxy,” which ran in the Journal of the British Interplanetary Society. What we are after is a thin sail that is temperature tolerant and rugged enough to endure its passage through the interstellar medium. Such a sail is usually assumed to be highly reflective, though aerographite will put a new spin on this. A ‘sundiver’ maneuver, taking a shielded sail close to the Sun for deployment at perihelion, was seen as offering travel time to the Centauri stars of perhaps a thousand years. And travel times like that seemed the best we could do with solar sails.

Laser-beaming could conceivably change the equation, and the Breakthrough Starshot effort revolves around a massive, ground-based laser array that would drive small sails up to 20 percent of the speed of light for fast passage to Proxima Centauri or other stars. In any case, the question of materials figures prominently in sail literature. The most studied material to date has been beryllium, but in 2012 Greg Matloff revisited sails with graphene in mind. He described it as “a mono-molecular lattice of carbon atoms” and noted that materials experts and condensed matter physicists had graphene under intense investigation. Matloff saw prospects for graphene in terms of thin-film probes and much larger manned starships:

In its application to interstellar solar sailing, it seems that graphene can exceed the performance of beryllium with less extreme perihelion requirements, peak temperatures and maximum accelerations. Thousand-year transits to Alpha Centauri do not seem out of the question for probes and generation ships using this mode of acceleration and deceleration.

Matloff also noted the problems posed by graphene, pointing to the difficulty of large-scale preparation at high-purity levels and questions involving its performance during a close solar pass, a maneuver that colleague Roman Kezerashvili had analyzed for beryllium sails several years earlier. The question facing Heller, Anglada-Escudé, Hippke and Kervella was whether the newly discovered aerographite could significantly upgrade the performance of a graphene sail, allowing us to reduce travel times to something below that 1,000 year threshold.

Aerographite offered several clear advantages along with some properties that would need to be analyzed and accounted for. It is true that graphene’s extremely low mass per cross section ratio can theoretically enable high velocities. But graphene turns out to be all but transparent, with a reflectivity close to zero. And now we run into difficulties with creating a graphene sail that I want to illustrate by quoting the Heller et al. paper. In the passage that follows, sigma (?) stands for the mass per cross section ratio, which for graphene is 7.6 × 10^?7 kg m^?2:

The absorptive and reflective properties of graphene can be greatly enhanced by doping graphene monolayers with alkali metals (Jung et al. 2011) or by sandwiching them between substrates (Yan et al. 2012). But this comes at the price of greatly increasing ?. The limited structural integrity of a graphene monolayer requires additional material thereby further increasing ? and complicating the experimental realization. All of this ultimately ruins the beautiful theory of a pure graphene sail.

To be sure, Matloff and others working on the graphene sail concept are well aware of these issues and various papers have investigated the prospects for surmounting them. But the prospect of an aerographite sail, a material with its own strengths and question marks, gives us a new entrant in the sail arena, now with initial calculations provided by Heller and team. Specifically, what the Astronomy & Astrophysics paper does is to examine a hollow sphere at meter-scale, one made out of aerographite, that can be launched into interplanetary space by conventional rocket and released, allowing solar photon pressure to go to work.

Missions within the Solar System assuming such a sphere with a shell thickness of roughly 0.5 mm could, according to these calculations, reach the orbit of Mars within 60 days, arriving at Pluto’s orbit in 4.3 years. If the material proved capable of withstanding a close solar pass, there are mission prospects for Proxima Centauri in the range of 185 years of travel time.

But a Proxima mission gets way ahead of the game. We need to look at aerographite in terms of what we can learn about it in the near-term, with missions within the Solar System of various complexity part of the learning curve. In the next post, I want to go into how an early aerographite sail could be made, and how deep space sails might be configured, and we’ll consider whether in the near-term, solar as opposed to laser sailing may be our path toward true interstellar precursors. And either tomorrow or in a third post, we need to examine how to overcome some problems raised by this material in terms of both navigation and monitoring / communications.

The paper is Heller, Anglada-Escudé, Hippke & Kervella, “Low-cost precursor of an interstellar mission,” Astronomy & Astrophysics 7 July 2020 (abstract / preprint). The discovery paper for aerographite is Mecklenburg et al., “Aerographite: Ultra Lightweight, Flexible Nanowall, Carbon Microtube Material with Outstanding Mechanical Performance,” Advanced Materials Vol. 24, Issue 26 (12 June 2012). Abstract. Matloff’s paper on graphene is “Graphene: The Ultimate Interstellar Solar Sail Material?” JBIS 65, pp. 378-381 (2012) (full text).