With more attention now being focused on possible missions to an asteroid, we should keep in mind that DLR, the German Aerospace Center, has been looking into an asteroid mission via solar sail for some time now. One 2006 paper from DLR’s Institute of Space Simulation pondered a 70-meter sail for use in a projected mission to the Near-Earth Object 1996FG3 within ten years of launch. It’s an interesting notion, one that would involve the sail hovering over the NEA hemisphere opposite to the Sun, deploying a lander and return capsule.

DLR has been into serious sail studies for some time now, as the photo below attests. It’s a 1999 shot of the ground deployment of a square solar sail 20 meters to the side. As you can see, this is a square sail made up of four triangular sail segments, an exercise that could readily lead to a sail deployment in space if the European Space Agency opts for funding such a mission. Just what ESA has in mind for such technology was the subject of a presentation at the just concluded Second International Symposium on Solar Sailing in Brooklyn.

Image: DLR’s deployed solar sail, seen at the Center’s facility in Cologne. Credit: DLR.

I’m looking at the paper on “The 3-Step DLR-ESA Gossamer Road to Solar Sailing,” available in the proceedings of the conference, and enjoying the ‘gossamer road’ metaphor that is so reminiscent of the fabled Silk Road, that network of trade routes that took its name from the Chinese silk trade and reached across Asia to the Mediterannean, Europe and north Africa. Maybe the ‘gossamer road’ is an indicator of enthusiasm at DLR and ESA for renewing sail work, for late last year the two agreed on the road map to solar sailing presented here.

Three consecutive steps define the roadmap:

• Gossamer-1: A 5-meter square solar sail launched as a deployment demonstrator to a 320 kilometer Earth orbit. Documentation of the deployment is to be handled by two onboard cameras (which inevitably calls up the images of the IKAROS sail deployment, similarly tracked). This demonstrator mission would be launched in 2013.
• Gossamer-2: A 20-meter square sail launched to a 500 kilometer Earth orbit. Here the idea is to test orbit and attitude control of a sail built out of thinner materials than the 7.5 µm Kapton used in Gossamer-1. Launch in 2014.
• Gossamer-3: A 50m x 50m solar sail launched to a 10,000 kilometer Earth orbit, with testing of orbit and attitude control and, as with the earlier missions, documentation by onboard cameras. An acceleration > 0.1 mm/s² is sufficient for the sailcraft to leave the Earth’s gravitational field after a period of about 100 days.

As you see, the gossamer missions build into growing layers of complexity and, because of limited budgets and a tight time schedule, tap the technologies and materials already developed in earlier DLR and ESA sail work. The paper notes that DLR has already done extensive work not only on sail materials but on the boom technology that supports the sail.

Also supporting the Gossamer project is a Light Pressure Measurement Facility (LPMF) set up by the DLR Institute for Space Systems in Bremen and Berlin. This is a key issue, because the reflectivity of the sail materials determines the efficiency of the propulsion achieved, and a variety of processes during a mission can cause that reflectivity to degrade. DLR is also setting up a Complex Irradiation Facility, now being commissioned, to examine the effects of the solar wind and electromagnetic radiation on sail materials. The trick here is to extrapolate from short-period degradation caused by high intensity bombardment in the facility to the longer, slower processes that a sail will experience in the space environment.

It’s interesting to see that so much recent sail technology has revolved around CubeSats, miniaturized satellites weighing no more than one kilogram that typically work with off-the-shelf electronics. CubeSats were developed as a way for universities to become involved in space exploration, but their small size and inexpensive components make them ideal for experimentation of all kinds. These ‘nano-satellites’ play a role in the NanoSail-D and the Planetary Society’s Lightsail-1 projects as well as DLR’s Gossamer program, allowing early risks to be spread over a number of low cost missions. It’s satisfying to think that IKAROS will soon be joined by other experiments shaking out a future workhorse propulsion system.

The asteroid mission referenced above is discussed in Dachwald et al., “Multiple rendezvous and sample return missions to near-Earth objects using solar sailcraft,” Acta Astronautica 59 (2006), pp. 768-776.