Solar Sails May Face Drag at 75% of Light Speed

Interstellar solar sails hit a strange problem at 75% of light speed. Solar sails pushed by powerful lasers could reach incredible speeds, but relativity may eventually make the light itself work against them. At around 75% of the speed of light, scattered photons can begin creating drag, adding a surprising obstacle to interstellar travel.

Reaching another star within a practical amount of time will require something far more capable than chemical rockets. One of the leading ideas is a solar sail, a large reflective structure that can be pushed by light. If extremely powerful lasers are aimed at such a sail, they could accelerate it to speeds well beyond what current propulsion systems can achieve.

But there may be an unexpected problem once a lightsail begins traveling at a significant fraction of the speed of light. According to a new paper on arXiv by Chao Shen and Jiaze Li of the Harbin Institute of Technology, the same light used to propel the sail could eventually produce a drag effect. The researchers separate the forces produced by photons striking a solar sail into three categories.