  We consider the evolution of a viscous vortex dipole in $R^2$ originating
from a pair of point vortices with opposite circulations. At high Reynolds
number $Re >> 1$, the dipole can travel a very long way, compared to the
distance between the vortex centers, before being slowed down and eventually
destroyed by diffusion. In this regime we construct an accurate approximation
of the solution in the form of a two-parameter asymptotic expansion involving
the aspect ratio of the dipole and the inverse Reynolds number. We then show
that the exact solution of the Navier-Stokes equations remains close to the
approximation on a time interval of length $O(Re^\sigma)$, where $\sigma < 1$
is arbitrary. This improves upon previous results which were essentially
restricted to $\sigma = 0$. As an application, we provide a rigorous
justification of an existing formula which gives the leading order correction
to the translation speed of the dipole due to finite size effects.
