  We consider the observational signatures of giant impacts between planetary
embryos. While the debris released in the impact remains in a clump for only a
single orbit, there is a much longer lasting asymmetry caused by the fact that
all debris must pass through the collision-point. The resulting asymmetry is
stationary, it does not orbit the star. The debris is concentrated in a clump
at the collision-point, with a more diffuse structure on the opposite side. The
asymmetry lasts for typically around 1000 orbital periods of the progenitor,
which can be several Myr at distances of ~50 AU. We describe how the appearance
of the asymmetric disc depends on the mass and eccentricity of the progenitor,
as well as viewing orientation. The wavelength of observation, which determines
the grain sizes probed, is also important. Notably, the increased collision
rate of the debris at the collision-point makes this the dominant production
site for any secondary dust and gas created. For dust small enough to be
removed by radiation pressure, and gas with a short lifetime, this causes their
distribution to resemble a jet emanating from the (stationary) collision-point.
We suggest that the asymmetries seen at large separations in some debris discs,
like Beta Pictoris, could be the result of giant impacts. If so this would
indicate that planetary embryos are present and continuing to grow at several
tens of AU at ages of up to tens of Myr.
