Can Sunlight Push a Spacecraft? Yes, Very Gently and Forever

Light carries momentum despite having no mass, which follows from electromagnetic theory and has been measured directly. When light strikes a surface and is absorbed, it delivers that momentum; when it is reflected, it delivers roughly twice as much, since the light departs in the opposite direction and the change in momentum is correspondingly larger. The force involved is extremely small, amounting at the distance of the Earth from the sun to roughly the weight of a large insect spread over a sheet the size of a tennis court. That is nothing against gravity near a planet and is a usable thrust in space, where it acts continuously and where nothing opposes it.

Continuous acceleration produces results that a rocket cannot:

• •No propellant is carried, so the mass does not fall as the mission proceeds
• •Acceleration continues for years rather than for minutes
• •Very high final speeds are reachable given enough time
• •Thrust can be steered by angling the sail relative to the sun
• •Orbits can be raised or lowered gradually with no fuel cost
• •A spacecraft can hold a position that is not a natural orbit at all

Building one presents difficulties that have taken decades to address. The sheet must be enormous and extremely light, which means a film a fraction of the thickness of household wrapping, aluminised on one side, which tears if handled and cannot support itself. It must be folded into a small volume for launch and deployed reliably in space without snagging, which is where several proposals failed. Booms must hold it flat without adding much mass. The film degrades under ultraviolet light and particle bombardment over years. Attitude control must handle a very large, very flexible structure whose centre of pressure shifts. And any wrinkle reduces the thrust and introduces a torque.

Directing a craft pushed by light requires exploiting the angle of the sail, and the reasoning is the same as for a sailing boat with one important difference. Light striking a sail at an angle reflects off at the mirror angle, and the resulting push acts perpendicular to the sail surface rather than along the direction the light came from, so angling the sail directs the thrust sideways as well as outwards. Tilting to push against the direction of orbital motion lowers the orbit and spirals inwards towards the sun, and tilting the other way raises it. Unlike a boat there is no water to push against, so the craft cannot sail directly towards the light source, and everything depends on gradually changing the orbit instead.

The idea dates to the nineteenth century as speculation and has flown several times recently. A Japanese mission launched in 2010 deployed a sail of twenty metres across on a trajectory towards Venus, demonstrating both deployment and acceleration by sunlight, and it remains the first full success. A privately funded American mission demonstrated deployment and controlled orbit raising in 2019. Several small missions have flown since, including one deploying a sail as a deorbiting device, which uses atmospheric drag rather than light. Proposals under study include missions to observe the sun from positions no conventional spacecraft can hold, and using ground-based lasers rather than sunlight to accelerate very small probes towards another star.

Light carries momentum and delivers roughly twice as much when reflected as when absorbed, giving a force comparable to the weight of an insect over a sheet the size of a tennis court. That is useless near a planet and valuable in space, where it acts continuously and needs no propellant. Extremely thin film, reliable deployment and controlling a large flexible structure are the hard parts, and a Japanese mission demonstrated it in 2010.