Several past calculations of collision probabilties between pairs of bodies
on independent orbits have yielded inconsistent results. We review the
methodologies and identify their various problems. Greenberg's (1982)
collision probability formalism (now with a corrected symmetry assumption)
is equivalent to Wetherill's (1967) approach, expect that it includes a
way to avoid singularities near apsides. That method shows that the procedure
by Namiki and Binzel (1991) was accurate for those cass where singularities
did not arise.