A solar sail is a thin aluminum sail (approximately 0.1µm thick) used in space to propel a spacecraft. Unlike sails on Earth which require air, solar sails use the solar radiation pressure emitted by the Sun in space. It is a new concept of generating power and is still in its developmental phase but has a promising future. Although these sails are slower than spacecrafts propelled by fuels, their possible applications are huge: control of the attitude of the spacecraft, communications and remote sensing creating artificial lagrangian points, exploration of the solar system, planetary defense from asteroids and comets (by varying their trajectory using sails) or integrating solar panels on them to generate solar power. These are only some of the multiple applications that solar sails can present.
Moreover, the first mission strictly dedicated to prove the functionality of these sails was the Sunjammer, which was intended to be launched in November 2014, the largest solar sail ever. However, it was cancelled before launch.
The big question is, will solar sails meet the expectations that they have already created? To answer this, we will have to wait a little while longer.
Founder of Tech For Space