The collision and snapping of cosmic strings generating spherical impulsive gravitational waves

The Penrose method for constructing spherical impulsive gravitational waves is investigated in detail, including alternative spatial sections and an a rbitrary cosmological constant. The resulting waves include those that are generated by a snapping cosmic string. The method is used to construct an e xplicit exact solution df Einstein's equations describing the collision of two non-aligned cosmic strings in a Minkowski background which snap at thei r point of collision.