Diffractive solar sail

A diffractive solar sail, or diffractive lightsail, is a type of solar sail which relies on diffraction instead of reflection for its propulsion.^[1][2] Current diffractive sail designs use thin metamaterial films, containing micrometer-size gratings based on polarization or subwavelength refractive structures, causing light to spread out (i.e. diffract) and thereby exert radiation pressure when it passes through them.^[2][3]

The idea of using diffraction for a solar sail was first proposed in 2017 by researchers at the Rochester Institute of Technology.^[4] This was enabled in part by advances in material design and fabrication (particularly of gratings), and optoelectronic control.^[5] In 2019 a diffractive solar sail project from the Rochester Institute of Technlology suggested a solar polar orbit mission with diffractive sails that could reach a higher solar inclination angle and smaller orbital radius than one with reflective sails, reaching NASA's NIAC phase II.^[1][2][6] In 2022 the NIAC project reached phase III and gained US$2 million of support from NASA, with involvement of researchers from both Johns Hopkins University and the Rochester Institute of Technology.^[7][8]

Reflective solar sail designs tend to consist of large, thin reflective sheets. By the law of reflection, the forces acting on them will always be normal to the sheet surface; the sheets must therefore be tilted during navigation, which poses structure and control challenges, and reduces the power reaching the sail.^[2][5][7] These in turn can lower reliability, increase mass, and reduce acceleration.^[2] Furthermore, reflective sails tend to absorb a reasonable proportion of the light hitting them, causing them to heat up; this can cause structural problems, particularly when the sail is repeatedly heated and then allowed to cool.^[5] Also, each photon hitting the sail is used once, i.e. it's either reflected or absorbed.^[5]

On the other hand, in a diffractive sail the grating can redirect light even when the sheet directly faces the sun, allowing much more efficient control with maximum power hitting the sail.^[5][2] The diffractive film can be designed to allow for optoelectronic control of the gratings, thereby reducing mass and increasing reliability relative to mechanical control.^[2] Since the film is translucent, most of the light just passes through the sail, reducing overall heating.^[5] Photons can be reused: either by passing through a second diffraction grating for more thrust, or by going to a solar cell to provide electricity.^[8]