  Fast coronal mass ejections (CMEs) generate standing or bow shocks as they
propagate through the corona and solar wind. Although CME shocks have
previously been detected indirectly via their emission at radio frequencies,
direct imaging has remained elusive due to their low contrast at optical
wavelengths. Here we report the first images of a CME-driven shock as it
propagates through interplanetary space from 8R_Sun to 120R_Sun (0.5AU), using
observations from the STEREO Heliospheric Imager (HI). The CME was measured to
have a velocity of ~1000 km/s and a Mach number of 4.1\pm1.2, while the shock
front stand-off distance (Delta) was found to increase linearly to ~20 R_Sun at
0.5 AU. The normalised standoff distance (Delta/DO) showed reasonable agreement
with semi-empirical relations, where D_O is the CME radius. However, when
normalised using the radius of curvature, Delta/R_O did not agree well with
theory, implying that R_O was under-estimated by a factor of ~3-8. This is most
likely due to the difficulty in estimating the larger radius of curvature along
the CME axis from the observations, which provide only a cross-sectional view
of the CME.
