We have obtained a second epoch observation of the Cassiopeia A supernova remnant ( SNR ) with the Chandra X-ray Observatory to measure detailed X-ray proper motions for the first time .
Both epoch observations are 50 ks exposures of the ACIS-S3 chip and they are separated by 2 years .
Measurements of the thin X-ray continuum dominated filaments located around the edge of the remnant ( that are identified with the forward shock ) show expansion rates from 0.02 % yr ^ { -1 } to 0.33 % yr ^ { -1 } .
Many of these filaments are therefore significantly decelerated .
Their median value of 0.21 % yr ^ { -1 } is equal to the median expansion of the bright ring ( 0.21 % yr ^ { -1 } ) as measured with Einstein and ROSAT .
This presents a conundrum if the motion of the bright ring is indicative of the reverse shock speed .
We have also re-evaluated the motion of the radio bright ring with emphasis on angle-averaged emissivity profiles .
Our new measurement of the expansion of the angle-averaged radio bright ring is 0.07 \pm 
0.03 % yr ^ { -1 } , somewhat slower than the previous radio measurements of 0.11 % yr ^ { -1 } which were sensitive to the motions of small-scale features .
We propose that the expansion of the small-scale bright ring features in the optical , X-ray , and radio do not represent the expansion of the reverse shock , but rather represent a brightness-weighted average of ejecta passing through and being decelerated by the reverse shock .
The motion of the reverse shock , itself , is then represented by the motion of the angle-averaged emissivity profile of the radio bright ring .