Solar sails are based on the principle that light exerts a tiny pressure on solid objects. Spacecraft have been launched that deployed reflective metallic “sails” to test the idea of using the pressure of sunlight to provide propulsion for future spacecraft. Now researchers are going beyond the conventional materials used for solar sails.
Grover Swartzlander is a professor at the Rochester Institute of Technology's Chester F. Carlson Center for Imaging Science. He has proposed making solar sails with advanced photonic metamaterials. Metamaterials are a new type of manmade structure with unconventional properties. He wants to replace the current reflective metal sails with metamaterial diffractive metafilm sails. These new sails would do a better job of utilizing the push from photons generated by the sun or lasers to propel near-Earth, interplanetary or even interstellar spacecraft. Swartzlander said, “Diffractive films may also be designed to replace heavy and failure-prone mechanical systems with lighter electro-optic controls having no moving parts.”
Swartzlander’s research group has received phase one funding from the NASA Innovative Advanced Concepts program. The nine month, one hundred and twenty five-thousand-dollar award “encourages development of visionary technology with potential to revolutionize future space exploration.”
The Optical Society is hosting an “incubator” meeting on October 7-9 in Washington, D.C. The meeting is being called Metamaterial Films for In-Space Propulsion by Radiation Pressure. The purpose of the meeting is to produce a “roadmap” for advancing the use of metamaterial sails on low Earth-orbiting satellites called CubeSats. The name comes from the use of a cube four inches on a side as a standard unit for cataloguing small satellites.
Swartzlander will facilitate the meeting along with Les Johnson who is the manager of the In-Space Propulsion Technology Projects Office at NASA Marshall Space Flight and the principle investigator for the NASA Near-Earth Asteroid Scout mission or NEA Scout. Nelson Tabirian also helped to organize the meeting. He is the president of Beam Co. which specialized in optical technologies and materials.
Swartzlander said that “CubeSats are becoming of great national importance for science, security and commercial purposes. The potential to raise, de-orbit or station-keep hundreds of CubeSats from low Earth orbit would be a recognized game changer that would build enthusiasm and advocacy among the growing small-satellite community of students, entrepreneurs and aerospace scientists and engineers.”
Later this year, NASA is going to launch thirteen satellites to carry out science and technology research in low Earth orbit. The Exploration Mission-1 (EM-1) will be launched aboard one the new Space Launch System rocket. The NEA Scout will be one of the thirteen CubeSats launched as part of the EM-1 project. It will be the first CubeSat science mission that has solar sails attached to the satellite. The NEA Scout will deploy an aluminum coated polyimide sail to propel it during its two-year mission.
Swartzlander explained that diffractive metafilms can correct some of the known limitations of reflective metal sails such as overheating, inefficient use of photon and excessive tilt of the spacecraft. The new materials has a lower photon absorption rate which means that it will not heat up as much as the metallic sails. The new materials can “reuse” photons by converting them to solar-electric power or diffracting them twice to increase momentum. Reflective sails just reflect photons or absorb them. The new materials can improve orientation of a satellite. They can maintain a more efficient position facing the sun. This provides highly efficient propulsion and improve energy generation of embedded solar cells. Reflective sails work best when the spacecraft is tilted but this tilting reduces the projection of solar power upon the sail.
Swartzlander said that "Diffractive sails may also be designed for laser-based propulsion, a decades-old concept that has recently attracted significant interest from private investors, resulting in a program called Breakthrough Starshot.” This program is intended to send many small satellites to another star.