  The Newman-Penrose constants of the spacetime corresponding to the
development of the Brill-Lindquist initial data are calculated by making use of
a particular representation of spatial infinity due to H. Friedrich. The
Brill-Lindquist initial data set represents the head-on collision of two
non-rotating black holes. In this case one non-zero constant is obtained. Its
value is given in terms of the product of the individual masses of the black
holes and the square of a distance parameter separating the two black holes.
This constant retains its value all along null infinity, and therefore it
provides information about the late time evolution of the collision process. In
particular, it is argued that the magnitude of the constants provides
information about the amount of residual radiation contained in the spacetime
after the collision of the black holes.
