Q:  What is the typical speed of hydrogen atoms?

                       3 * k * T 
           v  =  sqrt( --------- )  =   1,100   m/s
                          m(H)


  Q:  What is the mean free path of a dust grain?

                           1                               1
           mfp  =  --------------------  =  -----------------------------------
                   n(H) * (pi * (2*a)^2)    [60 x 10^(-6)] * [ pi * (2.0x10^(7))^2 ]


                =   1.6 x 10^(5)   m


  Q:  How long between each collision of an H atom and the grain?


                     mfp           1.6 x 10^(5) m
             t  =  --------  =   -----------------   =   144 s
                      v             1100 m/s


                =    about 2 minutes