This work describes a discontinuous or segmented mirror whose overall flatness is less dependent on the limited tension that can be supplied by the booms. A solar sail is a large, nominally flat sheet of extremely thin reflectorized film rigidly attached to a spacecraft, enabling propulsion via solar radiation pressure. Rip-stop fibers embedded in the backside of the film — with diameters ≈100× the thickness of the film — are commonly used to arrest tear propagation, which can easily occur in the handling and/or deployment of these gossamer-thin structures. Typically, the thin film or membrane that is the sail is systematically folded to enable both volumetrically compact transportation to space and mechanized deployment. It is the aggressive folding and creasing of the thin film that limits the ultimate flatness that can be achieved.
The Lunar Flashlight (LF) Cubesat requires its solar sail to closely approximate a plano mirror. This is a somewhat unusual requirement insofar as sails are typically optimized on their propulsive performance rather than their optical performance. Conventional sail designs typically involve aggressive folding and creasing of the thin reflecting film, causing many flatness-upsetting effects, including wrinkling and billowing. While the impact of these flatness errors on the propulsive performance of the sail is small, its impact on its performance as an illuminating mirror is large. It is unlikely that extant sail designs will meet the LF “80% of reflected light will be contained in a 3 deg cone” requirement.
The deployed solar sail is made more plano by segmenting the film into tiles joined together via a fiber-supported hinge design. Deployment includes systematic unfolding of the stowed sail via telescopic booms. These booms also support sail-flattening tension in the membrane.
This work is motivated specifically by LF science requirements. To detect water ice in darkened areas of the Moon, an IR spectrometer is proposed that will gather reflectance spectra from the lunar surface using sunlight reflected from the solar sail. This high-level requirement flows down to a surface illumination requirement in watts/m2, which in turn drives flatness and stability requirements on the sail.
This work was done by Eric B. Hochberg of Caltech for NASA’s Jet Propulsion Laboratory. NPO-49474