The acceleration that a laser provides is proportional to the force it transmits to the sail and inversely proportional to the spacecraft s mass. The sail designs that have been proposed consist of a polished, thin metal film, most with some kind of backing for structural strength. The temperature a sail attains can be lowered, and so its acceleration increased, by coating its reverse side with materials that efficiently radiate heat. To reach very high velocities, a spacecraft must sustain its acceleration. The ultimate velocity achievable by a light sail is determined by how long the Earth-bound laser can illuminate its target efficiently.
Laser light has an important property known as coherence.
It means that the energy it can impart is undiminished by distance, up to a critical value known as the diffraction distance. Beyond it, the power delivered quickly becomes insignificant. The diffraction distance of a laser, and thus the ultimate velocity of a spacecraft it powers, is governed by the size of the laser s aperture. Very powerful lasers would probably consist of hundreds of smaller ones ganged together in an array. The effective aperture size is roughly the diameter of the entire array.