NAME: 
  num_rate
 PURPOSE:   (one line only)
  Gives the relative number vs. apparent motion rate(arcsec/hr) of KBOs.
 DESCRIPTION:
  This is a crude approximation of the number of objects as a
  function of distance, and rate is given as a proxy for distance.
  The intended range of the rate is from 2arcsec/hr to 5arcsec/hr, it
  is valid to evaluate it at any rate, but will be poorly representatibe
  outside of 2 to 5.  This function is meant to be probabalistically
  sampled with smplprb.pro.  The normalization of this function is
  arbritrary.
 CATEGORY:
  Miscellaneous
 CALLING SEQUENCE:
  array=num_rate(x)
 INPUTS:
  x        :An array or scalar value in arcsec/hr
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
  array    :Returns a float array of the same size as x.
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  2004/07/13, Written by Maureen Teyssier, REU Lowell Observatory
  2004/07/15, MWB, incorporated into library.
  2004/07/18, MWB, changed to a more realistic function based on real data.