  Two images of Cassiopeia A obtained at 24 micrometer with the Spitzer Space
Telescope over a one year time interval show moving structures outside the
shell of the supernova remnant to a distance of more than 20 arcmin. Individual
features exhibit apparent motions of 10 to 20 arcsec per year, independently
confirmed by near-infrared observations. The observed tangential velocities are
at roughly the speed of light. It is likely that the moving structures are
infrared echoes, in which interstellar dust is heated by the explosion and by
flares from the compact object near the center of the remnant.
