A material called aerographite offers options for solar sails that transcend the capabilities of both beryllium and graphene, the latter being the most recent candidate for fast sail missions outside the Solar System. Developed at the Technical University of Hamburg and refined by researchers at the University of Kiel, aerographite came to the attention of the interstellar community in 2020 thanks to a groundbreaking paper by René Heller (Max Planck Institute for Solar System Research, Göttingen), working with co-authors Guillem Anglada-Escudé (Institut de Ciencies Espacials, Barcelona), Michael Hippke (Sonneberg Observatory, Germany) and Pierre Kervella (Observatoire de Paris).
I’ve written about aerographite before, in Aerographite: An Advance in Sail Materials with Deep Space Implications and Solar Sails: Deeper into the Aerographite Option, both of which are in the archives along with several other posts on the subject. But here I need to pause for a brief administrative moment: The recent changes to the website inadvertently resulted in a data overwrite in the archives that replaced some specialized characters used in scientific notation with question marks. Not good! I have a crack programmer working the fix using my backups, but at the moment the articles I’ve just mentioned do contain several missing characters. This will be remedied soon.
Back to aerographite, where I’m pleased to see this work receiving the further scrutiny it deserves, for this is a highly unusual material, not what you would expect when conceiving deep space missions. As Gregory Matloff and Joseph Meany explain in a new paper discussed at the Interstellar Research Group’s Montreal symposium, aerographite is both extremely low density and utterly opaque. The normal assumption is that an effective solar sail will be reflective (and indeed, graphene concepts include ways to introduce reflectivity, which could be achieved by adding substrates or doping graphene with alkali metals, thus increasing mass).
Image; 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.
But the startlingly black aerographite so effectively absorbs photons that in sail configuration it will be pushed into interstellar space. Indeed, Guillem Anglada-Escudé told me three years ago that absorbance works quite well for solar sailing, less effective than a highly reflective material by no more than a factor of 2. As Matloff (New York City College of Technology, CUNY) and Meany (Savannah River National Laboratory) explain in the paper growing out of their work, aerographite is produced by a chemical vapor deposition process that yields a synthetic foam connected by carbon microtubes, one whose opacity is complemented by its light weight. Indeed, the teams that developed it called aerographite “the lightest known material.”
At Montreal, Matloff explored how the material might be deployed in two classes of interstellar missions, looking at such factors as the maximum temperature of the sail at various perihelion distances (for possible ‘sundiver’ missions), the sail’s thermal emissivity, and the peak acceleration that can be achieved, along with payload mass limitations for a 1-micron spherical sail shell and a thin-film payload. The work also probes the characteristics of aerographite under laser beaming conditions, and goes on to examine how it might be deployed in futuristic manned interstellar ‘arks.’ You can see Matloff’s presentation at Montreal here.
Aerographite’s visible photon absorption approaches 100 percent, with high tensile strength and an extremely high melting point. Matloff and Meany’s research involves a hypothetical sail with maximum operational temperature of 3,500 K and a payload mass that is one-tenth of the sail’s. For the purposes of their calculations, they lower the sail’s absorptivity to sunlight to a perhaps more realistic 0.9. Here Matloff’s experience in graphene sails comes in handy, allowing him to use the same analytical tools he and colleague Giovanni Vulpetti have worked out over years of solar sail analysis. Of particular note is the ‘lightness factor,’ which measures solar radiation against acceleration, and which for aerographite works out to an exceptionally high value.
An aerographite sail, in other words, is extremely efficient at converting sunlight into acceleration. The numbers are striking in comparison to previous estimates for solar sailing (as opposed to beaming) technologies. The performance figures in the table below are for an interstellar probe whose sail is unfurled at perihelion during a close solar approach. If you check the perihelion figures used for the analysis, you’ll see that the 0.04 AU figure matches the closest approach of an existing spacecraft, the Parker Solar Probe. And it turns out that 0.06 AU is close to the closest perihelion distance assumed for a beryllium sail. Matloff’s previous analysis of graphene (in a 2014 paper) had assumed a 0.1 AU perihelion for a graphene sail in the same kind of mission.
Our probe reaches Proxima Centauri within a millennium for all cases, with the 0.04 AU perihelion probe cutting the travel time to two centuries, a striking figure for a solar sail. The further good news is that according to these calculations, the aerographite at no point exceeds its melting point. Note the huge peak acceleration for the 0.04 AU perihelion pass: 319 g! A sail that makes it through the perihelion pass at 0.04 AU achieves an interstellar cruise velocity of roughly 0.02 c, which we can then stack up against a laser-launched sail along the lines of what Breakthrough Starshot envisions.
Here we run into trouble. From the paper:
It is not clear that an aerographite sail could withstand the enormous accelerations necessary to propel a Project Starshot terrestrial-launched laser-photon sail. Also, such a sail must either have an appropriate curvature to remain within the beam because the beam source moves with Earth’s rotation or be implanted with an appropriate diffraction pattern to optically simulate an appropriately curved sail surface. Also, because aerographite is absorptive rather than reflective, the enormous required beam power on the sail to achieve an ~0.2c interstellar cruise velocity might be fatal.
Which is why Matloff and Meany studied the effects of a sail powered by the beam from a space-based laser array rather than a terrestrial one, using a 100 meter sail for the analysis. I will send you to the paper (or the video) for the details of these calculations, but a laser transmitter of approximately 1.8 kilometers is modeled, with the Sun-orbiting laser at 1 AU from the Sun. Here the craft achieves a velocity of 0.033c given the constraints applied to the beaming technology, which the authors note may be fewer than those imposed on the Starshot array. Indeed:
Constructing sail, sunlight-collection optics and the laser/transmitter are challenging as is the necessity of keeping the sail within the beam during the ~3-hour acceleration run. But these challenges are considerably less than is the case for the Project Starshot relativistic-velocity sails accelerated by a terrestrial laser array.
Those who know Greg Matloff’s work know how he rejoices in stretching ideas out to their maximum potential, much in the mode of Robert Forward. Thus it’s no surprise that the next idea considered here is an aerographite sail capable of carrying humans aboard an interstellar ark. That’s a discussion in itself, and so is the question of the best path forward for aerographite research, two subjects I’ll be taking up in the next post.
The paper is Matloff & Meany, ”Aerographite: A Candidate Material for Interstellar Photon Sailing,” submitted to JBIS and ultimately to be published as part of the proceedings of the Interstellar Research Group’s 2023 symposium. The Heller, Anglada-Escudé, Hippke & Kervella paper is “Low-cost precursor of an interstellar mission,” Astronomy & Astrophysics Vol. 641 (September 2020), A45 (abstract).