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.
Maximum power is transferred when the array is packed as densely as possible.
We have a tessellated design that approaches 100 percent packing density. At the Jet Propulsion Laboratory in Pasadena, Calif., my team has studied the trade-offs in mission cost between the power of individual lasers and the size of an array. The aperture size required for an interstellar mission is enormous. Fortunately, planetary missions require much smaller apertures. This system could send a probe to the boundary between the solar wind and the interstellar medium in three to four years.