  The extended TeV gamma-ray source HESS J1023-575 is coincident with the
massive, young stellar cluster Westerlund 2 (Wd2) and its surrounding HII
region RCW 49. On the basis of an analysis of the CO emission and 21 cm
absorption along the line of sight to Wd2, it is argued that this cluster, and
by assumption the TeV source as well, must be associated with a giant molecular
cloud in the far side of the Carina arm with a mass of 7.5 x 10^5 Mo. Analysis
of the spatial and velocity structure of the cloud reveals clear evidence of
interaction with Wd2. The cloud's kinematic distance of 6.0 +/- 1.0 kpc is
shown to be consistent with distances inferred both from the radius-linewidth
relation of molecular clouds and from the foreground gas column derived from
230 X-ray sources in Wd2.
