If we chose to launch an interstellar probe using near-term technologies, it would almost surely be a solar sail, unfurled on a close approach to the Sun and flung outward at speeds that could get it to Alpha Centauri in perhaps 1000 years. That’s the thinking of Gregory Matloff, author of Deep Space Probes: To the Outer Solar System and Beyond (Springer, 2005) and a leading proponent of interstellar studies. Later technologies might also use sails, driven by laser or microwave or even particle beams. But as we take incremental steps toward an interstellar future, solar sails have to be developed first.

On that score, it’s interesting to note the contributions of the Deutschen Zentrum für Luft- und Raumfahrt (DLR), Germany’s aerospace research center and space agency. An early consolidation of sail studies, for example, came from a joint NASA/DLR feasibility study conducted in 1996-97. The working group recommended a demonstration mission that could help develop the basic principles of sail fabrication, packaging, storage, deployment, and control.

The DLR successfuly deployed a 20m² sail structure in December of 1999, a ground-based demonstrator that consisted of four booms made of carbon fiber reinforced plastic. In August of 2000, DLR proposed a dedicated mission to explore near-Earth asteroids (NEAs) using a solar sail-driven spacecraft called ENEAS. The mission concept was to rendezvous with an object called 1996FG3 with a payload (not counting the sail assembly) of 75kg.

Such proposals run into financial difficulties familiar to anyone who recalls NASA’s early solar sail work for a Halley’s Comet mission that never flew. Tight budgets have kept ambitious sail missions on the ground both within ESA and NASA, but concepts continue to multiply. A new paper by Bernd Dachwald and Wolfgang Seboldt at DLR proposes a mission called ENEAS-SR, a 70m² sail designed to return a sample from NEA 1996FG3 within ten years of launch, carrying a 300 kg payload including lander and sample return capsule.

Why, other than to shake out a new technology, should we go to a near-Earth asteroid? In addition to pure science, NEAs are significant because they are probably fragments of main belt asteroids, and therefore representative of them. The larger class that contains them is the group called NEOs, those asteroids and comets with orbits that intersect or pass near the Earth’s orbit. We need, say Dachwald and Seboldt, to know much more about them. From the paper:

It is today widely accepted that NEO impacts have caused at least one mass extinction (65 million years ago at the Cretaceous/Tertiary boundary), and they are suspected to have caused several global catastrophes before. Even NEOs that do not intersect the orbit of Earth may evolve into Earth-crossers, because their orbits are chaotic, having a relatively short dynamical lifetime of about 107 years. One day, it will become necessary to prevent a specific object from impacting the Earth by nudging it out of its orbit. To be able to do this, the bulk properties of NEOs (material strengths, composition, structure, moments of inertia, etc.) should be determined as soon as possible.

Solar sails seem ideal for this purpose. Once at NEA 1996FG3, the sail-driven craft can take advantage of its sail in novel ways. Even close in to such a small object, the solar radiation pressure on the sail would exceed the asteroid’s gravitational pull; the spacecraft would therefore be placed into a hovering position in the hemisphere opposite the Sun (the sail can be turned to vary the photon pressure upon it, making this possible). Although ultimately unstable, these hovering positions could be maintained for extended periods of time, long enough for the release of the lander and its Earth return capsule.

Dachwald and Seboldt go on to envision an even more sophisticated triple NEA rendezvous mission with a 75kg payload and speculate on the properties of sails up to 140m² in size. Missions like these make outstanding sense — they take us to objects like NEOs with highly inclined orbits, the kind of target difficult to reach with conventional spacecraft, and they allow us to develop space experience with lightweight structures for future propulsion systems. The paper is “Multiple near-Earth asteroid rendezvous and sample return using first generation solar sailcraft,” in Acta Astronautica 57 (2005), pp. 865-875.