  A method is developed to calculate collision probability in this paper. Based
on the encounter geometric features of space objects, it is reasonable to
separate the radial orbital motions from that in the cross section for most
encounter events in near circular orbit. Therefore, the collision probability
caused by orbit altitude difference in the radial direction and the collision
probability caused by arrival time difference in the cross section are
calculated respectively. The net collision probability is expressed as an
explicit expression by multiplying the above two components. Numerical cases
are applied to test this method by comparing the results with the general
method. The results indicate that this method is valid for most near circular
orbital encounter events.
