Light-Powered Space Vehicles
June 21, 2024
Space is a frictionless environment. While planes have the atmosphere to fly and cars have the ground to push off of, spacecraft must exert mass to move. For instance, a stranded astronaut would only be able to propel itself by exerting mass in the opposite direction according to Newton’s Third Law of Motion which states every force exerted in a specific direction will end up having an equal and opposite reactionary force. Propulsion of a spacecraft is a huge challenge for space corporations when dealing with deep space missions because it has a strict lifeline or weight capacity that limits the distance the spacecraft can travel. However, the rise of solar sails solves this problem by allowing continuous travel with help of the Sun.
As of now, Solar Sails are usually featured on CubeSat spacecraft, which are typically no larger than a shoebox. These CubeSats hold the solar sail which is made from materials like Mylar. On the LightSail2 spacecraft developed by the Planetary Society, the Solar Sail was around 0.0002 inches thick with an area of 344 feet. Most of the CubeSats utilized for solar sails today are 12-unit, 9x9x13inch CubeSats. When light reaches this solar sail, the photons come into contact with the solar sail and transfer their momentum which pushes the CubeSat spacecraft ever so slightly. Additionally, these photons experience a change in their momentum vector according to the Law of Reflection. The Law of Reflection states that the angle at which the photon strikes the solar sail (angle of incident) will be the same angle in which the photon changes direction and rebounds (angle of reflection). Therefore, while the energy is preserved, this rebound allows the sail to receive a second push, thereby traveling a bit further. And while other spacecraft technically harness this photon rebounding “fuel” as well, solar sails allow this method to achieve much greater efficiency as the aluminum metal reflective coating allows for better reflecting of photons, along with a solar sail providing much more surface area for the photons to push in comparison to a regular satellite or spacecraft.
Solar sails can come with different designs and are still a factor for improving and researching the capabilities of solar sails. Square solar sails are made up of four triangular (more like a rhombus, actually) sheets folded up and deployed from the CubeSat with the help of composite booms. This process is lengthy and the difficult process of preparation and packaging increases risk of failure; however, these traditional shapes provide a large flat surface for light to bounce off of. Heliogyro Sails are another type of solar sail design that originated from Richard MacNeal’s design proposal in the 20th century. They are lightweight in comparison to the square design and are similar to helicopter propellers–four long rectangles that are propelled from the CubeSat and can individually be angled to maximize efficiency. Spinning-Disk Sails are similar to square sails in that they are large, flat circles. However, they use centrifugal force to spiral outward similar to a windmill with the help of tip satellite cold gas thruster-powered composite booms until fully deployed.
As solar sailing is researched further by space corporations to help with missions (hopefully), many more designs/composition of the solar sail will be used. However, there have already been some projects revolving around the usage of solar sails to study its capabilities. Japan was the first to launch a spacecraft with the implementation of solar sails in 2010 aboard its H-2A rocket. It was called the Interplanetary Kitecraft Accelerated by Radiation Of the Sun (IKAROS) and it became the first ever flying solar sail-powered spacecraft that successfully traveled consistently in space. This would soon lead to more developments centered around fuel-less spacecraft using the Sun for thrust. The Planetary Society began utilizing CubeSat satellites as carriers to test the propulsion of solar sailing in space, with its most recent success being LightSail2. LightSail2 launched in 2019 from a Falcon 9 and returned to Earth’s atmosphere 3 years laters, proving the viability of solar sailing for deep space missions or satellites. NASA’s Advanced Composite Solar Sail System (ACS3) is the newest addition to solar sail research.