Structure and Dynamics of the Milky Way: an Astro2010 Science White
  Paper
  Recent advances in radio astrometry with the VLBA have resulted in near
micro-arcsecond accurate trigonometric parallax and proper motion measurements
for masers in star forming regions. We are now poised to directly measure the
full 3-dimensional locations and motions of every massive star forming region
in the Milky Way and for the first time to map its spiral structure. Such
measurements would also yield the full kinematics of the Milky Way and
determine its fundamental parameters (Ro and To) with 1% accuracy. Coupled with
other observations this would yield the distribution of mass among the various
components (including dark matter) of the Milky Way.
