A major challenge of spaceflight is the weight of fuel, which dramatically increases costs. The LightSail 2 satellite, launched in June, may have proved how to lighten the load. It’s currently flying free using only light as fuel. When photons from sunlight hit its silvery sail, they impart a small force that increases velocity without the need for an engine or thruster. On July 31, LightSail successfully accelerated enough to raise its orbit by 1.25 miles—not much, but a critical proof of concept. —Jeffrey Kluger