Gradient-index solar sail and its optimal orbital control

Solar sails with the capability of generating a tangential radiation pressure at the sun-pointing attitude, such as refractive sails can provide more efficient methods for attitude and orbital control of sailcraft. This paper presents the concept of gradient-index sail as an advanced class of refractive sail, which operates by guiding the solar radiation through a structure made of graded refractive index material. The design of the sail refractive index distribution is achieved by transformation optics, which is then realized by the effective refractive index of non-resonant bulk metamaterials made of silica. The methods for fabrication of the proposed three-dimensional gradient-index sail were discussed. The performance of the sail was evaluated by using ray tracing for a broad spectrum of solar radiation under the normal incidence angle, which showed an efficiency of 90.5% for generation of a tangential radiation pressure. The orbital control of the tangential-radiation-pressure-generating (TRPG) sails was studied and it was shown that the full orbital control including the modification of orbital axes, eccentricity, and inclination can be applied by changing the attitude of the sail merely around the sun-sail axis, while the sail keeps the sun-pointing attitude at every point of the orbit.