The Lightsail 2 project concluded on 17 November as the craft finally re-entered and disintegrated in the Earth’s atmosphere. The project was crowdfunded by the Planetary Society to test solar sail technology for small spacecraft. Solar sailing activities managed to slow orbital decay by about 42%.

A spokesperson for the space exploration advocacy group said, “The re-entry completes a mission of nearly three-and-a-half years, during which LightSail 2 showed that it could change its orbit using the gentle push of sunlight, a technique known as solar sailing. LightSail 2 demonstrated that small spacecraft can carry, deploy, and utilise relatively large solar sails for propulsion.”

Lightsail 2 was a shoebox sized craft with 32sq.m. sails. It was launched into space in June 2019 aboard a SpaceX Falcon Heavy rocket. It began operation at 720km where it still had to fight against atmospheric drag. Within each 100-minute orbit of the Earth, it spent 67 minutes either in eclipse or moving towards the sun. This left 33 minutes in which it could ‘sail’, of which five minutes were spent turning to the desired orientation. This left 28 minutes per orbit to capture the momentum of solar photons to change velocity.

Light has no mass, but it has momentum that can be transferred to a reflective solar sail. The resulting push is small but continuous, allowing a spacecraft like LightSail 2 to change its orbit. When randomly oriented, average orbital decay was 34.5m per day. While solar sailing, the average decay was 19.9m per day.

“We have braved the harbor of Earth and found that a small craft can sail and steer,” said Bruce Betts, LightSail program manager and chief scientist for The Planetary Society. “Best wishes to those who sail similar craft into the vast ocean of space – we look forward to an exciting future of exploration, proud that we have played a role. Sail on.”