One idea for deep space probes that resurfaces every few years is the inflatable sail. We’ve seen keen interest especially since Breakthrough Starshot’s emergence in 2016 in meter-class sails, perhaps as small as four meters to the side. But if we choose to work with larger designs, sails don’t scale well. Increase sail area and problems of mass arise thanks to the necessary cables between sail and payload. An inflatable beryllium sail filled with a low-pressure gas like hydrogen avoids this problem, with payload mounted on the space-facing surface. Such sails have already been analyzed in the literature (see below).

Roman Kezerashvili (City University of New York), in fact, recently analyzed an inflatable torus-shaped sail with a twist, one that uses compounds incorporated into the sail material itself as a ‘propulsive shell’ that can take advantage of desorption produced by a microwave beam or a close pass of the Sun. Laser beaming also produces this propulsive effect but microwaves are preferable because they do not damage the sail material. Kezerashvili has analyzed carbon in the sail lattice in solar flyby scenarios. The sail is conceived as “a thin reflective membrane attached to an inflatable torus-shaped rim.”

Image: This is Figure 1 from the paper. Credit: Kezerashvili et al.

Inflatable sails go back to the late 1980s. Joerg Strobl published a paper on the concept in the Journal of the British Interplanetary Society, and it received swift follow-up in a series of studies in the 1990s examining an inflatable radio telescope called Quasat. A series of meetings involving Alenia Spazio, an Italian aerospace company based in Turin, took the idea further. In 2018, Claudio Maccone, Greg Matloff, and NASA’s Les Johnson joined Kezerashvili in analyzing inflatable technologies for missions as challenging as a probe to the Oort Cloud.

Indeed, working with Joseph Meany in 2023, Matloff would also describe an inflatable sail using aerograpahite and graphene, enabling higher payload mass and envisioning a ‘sundiver’ trajectory to accelerate an Alpha Centauri mission. The conception here is for a true interstellar ark carrying a crew of several hundred, using a sail with radius of 764 kilometers on a 1300 year journey. So the examination of inflatable sails in varying materials is clearly not slowing down.

The Inflatable Starshade

But there are uses for inflatable space structures that go beyond outer system missions. I see that they are now the subject of a NIAC Phase I study by John Mather (NASA GSFC) that puts a new wrinkle into the concept. Mather’s interest is not propulsion but an inflatable starshade that, in various configurations, could work either with the planned Habitable Worlds Observatory or an Extremely Large Telescope like the 39 m diameter European Extremely Large Telescope now being built in Chile. Starshades are an alternative to coronagraph technologies that suppress light from a star to reveal its planetary companions.

Inflatables may well be candidates for any kind of large space structure. Current planning on the Habitable Worlds Observatory, scheduled for launch no earlier than the late 2030s, includes a coronagraph, but Mather thinks the two technologies offer useful synergies. Here’s a quote from the study summary:

A starshade mission could still become necessary if: A. The HWO and its coronagraph cannot be built and tested as required; B. The HWO must observe exoplanets at UV wavelengths, or a 6 m HWO is not large enough to observe the desired targets; C. HWO does not achieve adequate performance after launch, and planned servicing and instrument replacement cannot be implemented; D. HWO observations show us that interesting exoplanets are rare, distant, or are hidden by thick dust clouds around the host star, or cannot be fully characterized by an upgraded HWO; or E. HWO observations show that the next step requires UV data, or a much larger telescope, beyond the capability of conceivable HWO coronagraph upgrades.

So Mather’s idea is also meant to be a kind of insurance policy. It’s worth pointing out that coronagraphs are well studied and compact, while starshade technologies are theoretically sound but untested in space. But as the summary mentions, work at ultraviolet wavelengths is out of the coronagraph’s range. To get into that part of the spectrum, pairing Habitable Worlds Observatory with a 35-meter starshade seems the only option. This conceivably would allow a relaxation of some HWO optical specs, and thus lower the overall cost. The NIAC study will explore these options for a 35-meter as well as a 60-meter starshade.

I mentioned the possibility of combining a starshade with observations through an Extremely Large Telescope, an eye-widening notion that Mather proposed in a 2022 NIAC Phase I study. The idea here is to place the starshade in an orbit that would match position and velocity with the telescope, occulting the star to render the planets more visible. This would demand an active propulsion system to maintain alignment during the observation, while also making use of the adaptive optics already built into the telescope to suppress atmospheric distortion. The mission is called Hybrid Observatory for Earth-like Exoplanets.

Image: Artist concept highlighting the novel approach proposed by the 2025 NIAC awarded selection of Inflatable Starshade for Earthlike Exoplanets concept. Credit: NASA/John Mather.

As discussed earlier, mass considerations play into inflatable designs. In the HOEE study, Mather referred to his plan “to cut the starshade mass by more than a factor of 10. There is no reason to require thousands of kg to support 400 kg of thin membranes.” His design goal is to create a starshade that can be assembled in space, thus avoiding launch and deployment issues. All this to create what he calls “the most powerful exoplanet observatory yet proposed.”

You can see how the inflatable starshade idea grows out of the hybrid observatory study. By experimenting with designs producing the needed strength, stiffness, stability and thermal requirements and the issues raised by bonding large sheets of materials of the requisite strength, the mass goals may be realized. So the inflatable sail option once again morphs into a related design, one optimized as an adjunct to an exoplanet observatory, provided this early work can lead to a solution that could benefit both astronomy and sails.

The paper on inflatable beryllium sails is Matloff, G. L., Kezerashvili, R. Ya., Maccone, C. and Johnson, L., “The Beryllium Hollow-Body Solar Sail: Exploration of the Sun’s Gravitational Focus and the Inner Oort Cloud”, arXiv:0809.3535v1 [physics.space-ph] 20 Sep 2008. The later Kezerashvili paper is Kezerashvili et al., “A torus-shaped solar sail accelerated via thermal desorption of coating,” Advances in Space Research Vol. 67, Issue 9 (2021), pp. 2577-2588 (abstract). The Matloff and Meany paper on an Alpha Centauri interstellar ark is ”Aerographite: A Candidate Material for Interstellar Photon Sailing,” JBIS Vol. 77 (2024) pp. 142-146. Abstract. Thanks to Thomas Mazanec and Antonio Tavani for the pointer to Mather’s most recent NIAC study.