The Penrose method for constructing spherical impulsive gravitational waves is investigated
in detail, including alternative spatial sections and an arbitrary cosmological constant.
The resulting waves include those that are generated by a snapping cosmic string. The
method is used to construct an explicit exact solution of Einstein’s equations describing the
collision of two nonaligned cosmic strings in a Minkowski background which snap at their
point of collision.
This is a pre-print. It is also available at: http://xxx.soton.ac.uk/abs/gr-qc/0001049.