Mapping Spiral Structure on the far side of the Milky Way
  Little is known about the portion of the Milky Way lying beyond the Galactic
center at distances of more than 9 kilo-parsec from the Sun. These regions are
opaque at optical wavelengths due to absorption by interstellar dust, and
distances are very large and hard to measure. We report a direct trigonometric
parallax distance of 20.4_{-2.2}^{+2.8} kilo-parsec obtained with the Very Long
Baseline Array to a water maser source in a region of active star formation.
These measurements allow us to shed light on Galactic spiral structure by
locating the Scutum-Centaurus spiral arm as it passes through the far side of
the Milky Way, and to validate a kinematic method for determining distances in
this region based on transverse motions.
