NASA is exploring a new take on the idea of solar sails. By putting diffraction gratings into their sails, they hope to overcome the key drawback existing sails suffer, their lack of flexibility in the direction of travel.

Sunlight exerts a force. On Earth, this is usually overwhelmed by air resistance and friction, aside from toys that spin inside a vacuum. In space it influences the movements of asteroids and the dust that makes up meteor showers, contributing to the challenges of predicting their paths. A century ago, engineers proposed spacecraft could use vast sails to coast to other planets without the need for fuel or propellant.

The idea has captured the public imagination so much that a small test sail was deployed after crowdfunding followed by a larger one. 

Although the practicality of the idea was demonstrated when the Japanese Space Agency flew the IKAROS craft to Venus this way, it has one major weakness – there is only limited control of the direction of travel. Sails like this do not take well to sudden changes of angle relative to the Sun in order to maneuver, let alone tack like sailors going against the wind. Rigging that could partially address this adds weight to missions that can't afford it.

However, some engineers believe they have found a way around this. Their work is convincing enough that NASA have invested $2 million to explore it further as part of a Phase III award.

“As we venture farther out into the cosmos than ever before, we’ll need innovative, cutting-edge technologies to drive our missions," said NASA Administrator Bill Nelson in a statement. "The NASA Innovative Advanced Concepts program helps to unlock visionary ideas – like novel solar sails – and bring them closer to reality.”

Instead, the new proposal is to embed fine gratings in the sail, through which light will diffract, as waves do when passing through gaps of similar width to their wavelength, Phase I and II awards have already demonstrated the practicality of making diffractive sails and having light bend through them.

“Unlike a reflective sail, a sun-facing diffractive sail experiences a component of force perpendicular to the sun line, thereby allowing navigation without sacrificing the amount of solar power on the sail,” Professor Grover Swartzlander of Rochester Institute of Technology said when phase II funding was announced. Swartzlander argues diffraction sails would also avoid overheating, which could be a problem for reflection sails.

Naturally, the idea works best where sunlight is strongest, so the first mission to use diffractive solar sails is likely to be one that takes a spacecraft over the Sun's unexplored poles. Ideally, however, the same techniques could prove effective further out for long-duration missions that can't carry the fuel for frequent changes of direction over a long time.

"Diffractive solar sailing is a modern take on the decades old vision of lightsails. While this technology can improve a multitude of mission architectures, it is poised to highly impact the heliophysics community’s need for unique solar observation capabilities," said team leader Amber Dubill of Johns Hopkins University. "With our team’s combined expertise in optics, aerospace, traditional solar sailing, and metamaterials, we hope to allow scientists to see the Sun as never before."

The idea is related to, but separate from, proposals to explore neighboring star systems by shining lasers on similar sails to accelerate them to large fractions of the speed of light. However, the two ideas could be merged, by using lasers to control the orbits of craft with diffractive sails.