A solar sailing experiment operated by The Planetary Society is successfully soaring on sunlight in an orbit around Earth — a big milestone in the pioneering attempt to propel a spacecraft in space without using propellant. The solar sail, called LightSail 2, has large reflective sails to capture the momentum of light from the Sun and use it to push the spacecraft forward. This ingenious contraption has now reached its planned high-point in Earth orbit — its apogee — of 729 kilometres above the Earth on 5 August. This essentially means that a spacecraft has raised its orbit, using a method of propulsion that uses the Sun's photons and not chemical rockets that burn fuel carried on board.
LightSail 2 has managed a 3.2-kilometre climb from the orbit it was deployed into by SpaceX. It was launched on a Falcon Heavy, tucked inside a small, bread loaf-sized spacecraft called 'Prox 1', along with 23 other satellites, into low-Earth Orbit. Prox 1 housed the first-of-its-kind solar sail called Lightsail 2, a passion project of non-profit space organization The Planetary Society, headed by Bill Nye of Bill Nye the Science Guy fame.
On 2 July, Prox 1 deployed LightSail 2 in a near-perfect circular orbit around the Earth, at an altitude of 720 km. After checks and tests of the cameras and solar panels on board, on 23 July, the solar sail was finally unfurled. For the first ten days after stretching its wings, LightSail 2 was in "solar sailing mode" over 60 percent of the time. A solar sail spacecraft can continue to accelerate a spacecraft as long as there's sunlight available to give it a push. On 3 August, the mission switched gears into analysis and fine-tuning mode.
LightSail 2 has a "momentum wheel" that controls the sail's angle and tilt so it's always fully facing the sun. It is in this orientation that the spacecraft gains the most energy. Mission scientists have noticed that the LightSail 2's momentum wheel gets saturated multiple times a day — reaching its speed limit. When this happens, LightSail 2 needs to switch from "solar sailing mode" to "detumble mode", in which various mechanisms kick in to try and stabilize the spacecraft while allowing the wheel time to wind down, a recent update by the Planetary Society said. A software patch was uploaded to the spacecraft that allowed it to automatically switch to "detumble mode" when in Earth's shadow and cut away from access to direct sunlight. Since the update, the spacecraft has been able to stay in "solar sailing mode" throughout the sunlit duration in every orbit instead of losing time allowing the wheel to wind down.
The mission team is currently trying to tweak and optimize the way LightSail 2 manages its momentum and performs turns.
The Lightsail 2 project is the result of a crowdfunded experiment to use sunlight to power movement in space. Much like boats have sails that rely on wind to propel it in seas, a solar sail uses photons from the Sun to propel itself through space. The theory, now proven, is that sunlight carries enough momentum to impart motion to a spacecraft with a large enough surface area, in this case, the sail. There are obvious limitations. For one, the momentum imparted by photons of light is minuscule, which means that the spacecraft has to be very light or have a massive surface area. The energy imparted to the sail also reduces with distance, making it impractical to use a sail for deep space exploration, or even anything beyond the orbit of Mars.
On 1 August, LightSail 2 made history as the first small spacecraft to ever use a solar sail in space and the second-ever solar sail spacecraft to fly at all, after Japan's IKAROS, which snagged those bragging rights in 2010. The LightSail 2 project is also the first crowdfunded mission to successfully demonstrate a new form of propulsion.