Q:  Compute the mean free path of the Sun for "close encounters."

                            1
           mfp  =   -------------------------------------------------------
                     pi * (4.2 x 10^(11) m)^2 *  ( 2.5 x 10^(-51) m^(-3) )


                =   1.8 x 10^(26) m


  Q:  What is the time between such encounters?


                         1.8 x 10^(26) m
           t_close  =  ---------------------
                          25 x 10^3 m/s


                    =   7.2 x 10^(21) s   =   2 x 10^(14) yr



          This is roughly 10,000 times longer than the current age
          of the universe.