Solar sails get their propulsion from the Sun, so they don't need to carry propellant, but they come with their own challenges. A sail has a large surface area but a low mass, which creates a huge moment of inertia and makes it difficult to control, especially with reaction wheels. A team of engineers has developed a reflectivity control device, where thin membranes on the sail change their reflectivity, producing less force and allowing the sail to change direction.