The science fiction trope that often comes to mind in conjunction with the interstellar ark idea is of the crew that has lost all sense of the mission. Brian Aldiss’ Non-Stop (1958), published in the US as Starship, is a classic case of a generation ship that has become the entire world. The US title, of course, gave away the whole plot, which is sort of ridiculous. Have a look at the British cover, which leaves the setting mysterious for most of the book, and the American one, which blatantly tells you what’s going on. I wonder what Aldiss thought of this.

Be that as it may, interstellar arks are conceived as having large crews and taking a lot of time to move between stars, usually on the order of thousands of years. We can trace the concept in the scientific literature back to Les Shepherd’s famous 1952 paper on human interstellar travel, a key paper in the evolution of the field. An interesting adaptation of the paper appeared in Science Fiction Plus in April of the following year (see The Worldship of 1953). Alan Bond and Anthony Martin, whose names will always evoke Project Daedalus, likewise discussed interstellar arks, and Greg Matloff, whose presentation we looked at in the last post, has been working the numbers on these craft for much of his career.

Let’s look, then, at what Matloff and Joseph Meany say in their paper on aerographite. Here we’re talking about a sailcraft driven by solar photons rather than beamed energy, one that is based on an inflatable, hollow-body sail (itself a concept that goes back at least to the 1980s). Working with Roman Kezerashvili, Matloff has in the past addressed hollow-body sails made of beryllium as well as graphene, last discussing the latter in an interstellar ark concept in 2014. Here he and Meany set up an aerographite-graphene variant using a 90% absorptive and 10% reflective layer of aerographite that effectively pushes against the Sun-facing surface of graphene.

We’re in the realm of big numbers again. The radius of the sail is 764 kilometers, with the sail massing 5.49 X 106 kilograms. The as yet unpublished paper on this work shows that the payload mass (2.56 X 107 kilograms) is considerably higher than would be possible using a hollow-body sail made only of graphene. It’s interesting that for the close solar pass envisioned in the ‘sundiver’ maneuver for the sail, Matloff chooses a perihelion of 0.1 AU in order to hold down the g forces for the human crew. The point came up in the question session after his Montreal talk, for there do seem to be technologies for sustaining (for a short period) g-forces of 3 g and beyond, which would allow for a closer perihelion pass.

In any case, our ark reaches Alpha Centauri in about 1375 years carrying a crew of several hundred. If that figure seems exasperatingly high, consider that in the past few decades we’ve gone from the routine assumption that an interstellar mission would take millennia to the now plausible suggestion that we can get it down to a century or so. Massive arks, of course, take much longer, but this number isn’t bad. NASA’s Les Johnson told me in 2003, when I mentioned a thousand-year mission, that he would love to see a plan for one that could make the journey that fast. But here we are, discussing materials and techniques that can go well below that for unmanned probes. And then there is that Breakthrough Starshot concept of 20 percent of lightspeed…

We are, in other words, making progress. But so much remains to be done with regard to this particular material. Indeed, the work on graphene reminds us how little we know about the physical properties of aerographite. The paper lays out some large questions:

  • Will what we know of aerographite’s properties be sustained when we reduce the thickness to a single micron?
  • Will aerographite be stable at the temperatures demanded by our perihelion calculations?
  • Will aerographite equal the performance of graphene when exposed to the space environment?
  • What about trajectory adjustment for a non-reflecting surface like aerographite?

Thus the paper’s conclusion:

It is wise to consider the above discussion as very preliminary. There are many unknowns regarding aerographite and graphene that must be addressed before the missions discussed can be considered feasible.

One unknown is the closest feasible perihelion distance. Just because the Parker Solar Probe will likely survive its close perihelion pass does not mean that a carbon hyper-thin sail will do the same. It is necessary for some researcher to perform an exhaustive study of the effects of the near-Sun space environment upon these substances. A good consideration of the issues to be addressed is the exhaustive study for beryllium solar-photon sails performed by Kezerashvili [9].

One last note on early aerographite sails: What interesting problems they pose for tracking. We’d have to use infrared to follow their course, and a space-based telescope to do that because of infrared absorption in Earth’s atmosphere. Heller and team figured out in their aerographite paper that JWST could track a 1 m aerographite sail out to 2 AU. But swarm configurations (and we’ll be talking about this concept again in the near future) produce a signature that could be detected well beyond the Kuiper Belt. An onboard laser would greatly simplify the problem if we could find ways to power it up aboard the highly miniaturized craft that would be our first experiments.

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 2014 paper on graphene arks is “Graphene Solar Photon Sails and Interstellar Arks,” JBIS Vol. 67 (June 2014), 237-246 (abstract). The paper on beryllium sails by Roman Kezerashvili is “Thickness Requirements for Solar Sail Foils,” Acta Astronautica 65 (2009), 507-518 (abstract).