A new research paper argues that solar sail efficiency would drop significantly at around 75% of the speed of light.
The idea behind solar sails is this. You create a spacecraft with a sail on which a laser left behind at or near the home planet shines. It will then propel the spacecraft to enormous speeds. Photons carry small momentum, and we've already conducted experiments showing they can provide propulsion.
Enormous technological challenges remain. We know of no way we can remove heat quickly enough from materials that can withstand continuous laser bombardment. Some solar sail ideas don’t require them. Instead, spacecraft could be propelled by the energy of stars, but it severely limits the amount of cargo they could carry.
A new research paper argues that at around 75% of the speed of light, laser-driven solar sail propulsion would lose efficiency significantly; scattered photons would no longer propel the spacecraft but would instead create drag.
Propulsion from photons hitting a solar sail from lasers can be divided into three categories. The first is incident light, which produces raw momentum as it hits the solar sail. The second is specular reflection, when photons bounce off the solar sail perfectly. Diffuse scattering provides the weakest push, as photons are absorbed and re-emitted in random directions.
At lower speeds, all three types of photon interactions push the spacecraft forward, but as the solar sail speeds up, the Doppler shift becomes stronger and reduces the frequency and energy of incoming photons. This makes achieving faster speeds increasingly inefficient.
At around 75%, diffuse scattering begins to act like drag because absorbed and re-emitted photons produce a recoil force opposite the direction of emission. The solar sail would still move forward and gain speed, but less efficiently.
The study didn’t account for all problems associated with solar sails and what happens when they reach relativistic speeds. These effects are serious, including collisions with dust and gas. They could cause further drag or, at our current level of technological sophistication, disintegrate the spacecraft. The study also doesn’t discuss the earlier-mentioned material limits for building a solar sail.
On the other hand, some studies have shown that aberration-induced drag can be exploited using materials such as photonic crystals and engineered metasurfaces. They could even stabilize the solar sail's orientation and trajectory.
It will be a long way before we overcome all the challenges of solar sails and can manufacture prototypes that can travel in outer space to far-off destinations impossible to reach with conventional propulsion systems.