Solar Sail to the Heliopause

by | Dec 2, 2005 | Missions, Sail Concepts

Proposals for realistic interstellar missions are not a new thing; in fact, several concepts grew out of work in the early 1980’s at the Jet Propulsion Laboratory, starting with the ‘Thousand Astronomical Units’ (TAU) mission, and extending to recent studies on the mission commonly referred to as the Interstellar Probe. By ‘interstellar,’ I mean journeys not to a nearby star but (a much needed first step) a journey to the interstellar medium beyond the heliosphere, that region carved out by the influence of the Sun’s solar wind. We have one vehicle there now, as Voyager 1 seems to be crossing the heliopause into true interstellar space. What we need to ponder next is how to build a spacecraft specifically designed for heliopause studies.

A team of European researchers is now tackling the job. Designed as part of the European Space Agency’s excellent Technology Reference Studies, the Interstellar Heliopause Probe is put forth as a mission to reach 200 AU within 25 years, using a variety of near-term technologies including a solar sail.

Numerous questions about payload, communications and spacecraft autonomy come out of this work, but the big issue remains propulsion. ESA considered three candidate systems:

  • Chemical propulsion, using one or two Earth gravity assists to reach Jupiter, followed by a close solar flyby with propulsive maneuver at perihelion (rejected because of the high thermal requirements and lack of sufficient specific impulse to push a useful payload);
  • Nuclear electric propulsion, using a close Jupiter flyby and extended low-thrust cruise, with specific impulse in the range of 5000 to 20,000 seconds and a thrust sequence lasting as long as 20 years. This one was derailed by power issues, especially the mass requirements of the needed nuclear reactor.
  • Solar sailing, the method of choice, using the momentum of photons to achieve low acceleration without propellant. ESA is investigating both square and spinning disk sails for the job. To give an idea of the size requirement, the square sail demands about 260 x 260 meters at a sail thickness of 2 microns; the spinning sail allows a reduction to a radius of 140 meters, but demands a sail thickness of 1 micron. To achieve the needed acceleration to reach 200 AU in 25 years, the spacecraft would make a close Solar pass.

    • So where do we stand on making solar sailing a reality? The largest sail deployment has been on the ground, in the form of an ESA/DLR (Deutsches Zentrum für Luft- und Raumfahrt) tests, while spinning disk deployments have recently been demonstrated at the Jet Propulsion Laboratory. The Russian Znamya ‘mirror’ is the only space-based sail deployment, and it revealed significant issues in need of resolution. Clearly, we have much to learn about preventing ruptures and damage to delicate sail coatings, not to mention retaining the sail’s optical properties during a solar pass that closes to 0.25 AU.

    The paper is Lyngvi, A., Falkner, P. et al., “The interstellar heliopause probe,” Acta Astronautica 57 (2005), pp. 104-111.

    Centauri Dreams‘ take: The consensus about sail technologies for our interstellar precursor work seems to be firming up, and we are not so many years from seeing sail deployment tests in space. Note the key fact that the length of even the most optimistic missions (and 25 years to 200 AU is quite optimistic) presupposes a mission that effectively occupies the career of the researchers who design, build and fly it. Add the extensive testing and development needed to create such a spacecraft to the quarter century flight time and you can see why deep space research demands long-term thinking, a product in short supply given our ephemeral political and economic cycles. Probing interstellar space turns out to be a matter of will as much as technology.