  We give necessary and sufficient conditions for an affine deformation of a
Schottky subgroup of O(2,1) to act properly on affine space. There exists a
real-valued biaffine map between the cohomology of the Schottky group and the
space of geodesic currents on the corresponding hyperbolic surface S. For a
fixed cohomology class, this map is uniformly positive or uniformly negative on
the space of geodesic currents if and only if the corresponding affine
deformation is proper. As a corollary, the deformation space of proper affine
deformations is an open convex cone.
