orbit of eccentricity ''e'' {{= 1}} (radial elliptic trajectory). This allows one to compute the free-fall time for two point objects on a radial path. The solution of this equation of motion yields time as a function of separation:
:

where
: is the time after the start of the fall
: is the distance between the centers of the bodies
: is the initial value of
: is the standard gravitational parameter.