  We present a characterization of the asymptotics of all asymptotically flat
stationary vacuum solutions of Einstein's field equations. This
characterization is given in terms of two sequences of symmetric trace free
tensors (we call them the `null data'), which determine a formal expansion of
the solution, and which are in a one to one correspondence to Hansen's
multipoles. We obtain necessary and sufficient growth estimates on the null
data to define an absolutely convergent series in a neighbourhood of spatial
infinity. This provides a complete characterization of all asymptotically flat
stationary vacuum solutions to the field equations.
