  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.
