Graphene Sails with Phased Array Optical Drive - Towards More Practical Interstellar Probes

Abstract: A spacecraft pushed by radiation has the major advantage that the power source is not included in the accelerated mass, making it the preferred technique for reaching relativistic speeds. There are two main technical challenges. First, to get significant acceleration, the sail must be both extremely light weight and capable of operating at high intensities of the incident beam and the resulting high temperatures. Second, the transmitter must emit high power beams through huge apertures, many kilometers in diameter, in order to focus radiation on the sail across the long distances needed to achieve high final speeds. Existing proposals for the sail use carbon or aluminum films, but aluminum is limited by a low melting point, and both have low mechanical strength requiring either a distributed payload or complex rigging. We propose here a graphene sail, which offers high absorption per unit weight, high temperature operation, and the mechanical strength to support simple rigging to a lumped mass payload. For the transmitter, existing proposals use a compact high power source, and focus the energy with a large (hundreds to thousands of km) space-based lens. Existing optical drive proposals also require launch from the outer solar system, have severe pointing restrictions, and require difficult maneuvering of the beam source. Instead we propose an active Fresnel lens, allowing smaller apertures of less mass, easier pointing with fewer restrictions, and probe launch from the inner solar system. The technologies for both the sail and the transmitter are already under development for other reasons. Worked examples, physically smaller and less massive than those suggested so far, range from a 1kg payload launched to 10\% of the speed of light by a transmitter only 25 times the mass of ISS, to a larger system that can launch a 1000 kg payload to 50\% of the speed of light.