  A spherically symmetrical ultra-relativistic blast wave is not an attractor
of a generic asymmetric explosion. Spherical symmetry is reached only by the
time the blast wave slows down to non-relativistic velocities, when the
Sedov-Taylor-von Neumann attractor solution sets in. We show however, that a
directed relativistic explosion, with the explosion momentum close to the
explosion energy, produces a blast wave with a universal intermediate
asymptotic -- a selfsimilar directed ultra-relativistic blast wave. This
universality might be of interest for the astrophysics of gamma-ray burst
afterglows.
