  We study the collision of two flat, parallel end-of-the-world branes in
heterotic M-theory. By insisting that there is no divergence in the Riemann
curvature as the collision approaches, we are able to single out a unique
solution possessing the local geometry of (2d compactified Milne)/Z_2 x R_3,
times a finite-volume Calabi-Yau manifold in the vicinity of the collision. At
a finite time before and after the collision, a second type of singularity
appears momentarily on the negative-tension brane, representing its bouncing
off a zero of the bulk warp factor. We find this singularity to be remarkably
mild and easily regularised. The various different cosmological solutions to
heterotic M-theory previously found by other authors are shown to merely
represent different portions of a unique flat cosmological solution to
heterotic M-theory.
