Interstellar Solar Sails Hit a Drag Problem at 75% of Light Speed

Reaching another star within a practical span of time will need propulsion far beyond chemical rockets, and one of the leading concepts is a solar sail — a large reflective structure pushed by light. Powerful lasers aimed at such a sail could accelerate it well past what today's engines can manage, but a new study finds an unexpected problem once the sail is already travelling fast.

In a paper posted on the preprint server arXiv, Chao Shen and Jiaze Li of the Harbin Institute of Technology find that the same laser light used to push the sail could eventually create drag. The researchers split the forces from photons striking the sail into three parts: the raw push of incoming light, momentum from light reflecting cleanly off the sail, and a weaker push from light that is absorbed and re-emitted in random directions, called diffuse scattering.

As the sail races away from its laser source, the light reaching it is increasingly Doppler-shifted to a lower frequency, so all three forces weaken and further acceleration becomes less efficient. At around 75% of the speed of light, the researchers found that relativistic light aberration — the bending of a light beam's apparent direction at high speed — becomes significant: from the viewpoint of an observer on Earth, the diffusely scattered light starts being thrown forward, and by Newton's third law, this creates a backward force on the sail. Diffuse scattering, the weakest of the three forces, begins acting as drag once the sail passes roughly 75% of light speed, even though the laser's total push remains positive.

The study models the lightsail as an idealized mirror and considers only radiative forces, leaving out collisions with interstellar gas and dust, the thermal limits of real sail materials, and the curvature of spacetime over interstellar distances. Engineers are already exploring metamaterials and photonic crystals tuned to specific laser wavelengths, which the researchers say could potentially exploit the same aberration effect to help a lightsail stay centered in its laser beam.