  HST ACS images of the young SN remnant Cas A are used to explore the
expansion and spatial distribution of its highest velocity debris. Proper
motions of over 1800 outlying ejecta knots are reported. The distribution of
transverse expansion velocities for these knots shows a striking bipolar
asymmetry with the highest velocity knots confined to nearly opposing northeast
and southwest `jets'. The jets appear kinematically and chemically distinct
with respect to the remnant's highest velocity debris seen in other directions.
Significant gaps in the spatial distribution of outlying ejecta lie in
directions which are approximately perpendicular to the jets. Extrapolations of
9 month proper motions for all outer ejecta knots and a subsample of 72 bright
and compact knots suggest explosion dates (assuming no knot deceleration) of
1662 +/- 27 and 1672 +/- 18, respectively. We find some evidence for
non-uniform deceleration in different directions with knots located along the
northwestern limb among the least decelerated ejecta suggesting a convergence
date of 1681 +/-19. The remnant's central X-ray point source lies some $7''$ to
the southeast of the estimated expansion center (PA = 169 deg) indicating a
projected motion of ~350 km/s toward the middle of the broad southern outer
ejecta knot gap.
