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 8 R _ { \odot } to 120 R _ { \odot } ( 0.5 AU ) , using observations from the STEREO Heliospheric Imager ( HI ) .
The CME was measured to have a velocity of \sim 1000 km s ^ { -1 } and a Mach number of 4.1 \pm 1.2 , while the shock front stand-off distance ( \Delta ) was found to increase linearly to \sim 20 R _ { \odot } at 0.5 AU .
The normalised standoff distance ( \Delta / D _ { O } ) 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 \approx 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 .