Assigned ID: 607 ----- Attempting Submission 1 (Fri Feb 25 20:40:24 GMT 2011) ----- The jet of M87 remains a pivotal object in our understanding of high-energy processes at the nuclei of galaxies and in radio galaxies. Visible from radio to X-rays, it remains the most easily observed archetypical outflow of relativistic material from a supermassive black hole. By utilizing a time sequence of observations extending over ten years on one instrument, the WFPC-2/PC, and major technical advances in astrometric techniques for HST, we propose to map the complete velocity field of the M87 jet in the optical continuously and over the entire visible jet. With very conservative accuracies of 0.1 pixel, we reach 0.1 c, sufficient to detect even transverse motions in the magnetic helical instability scenario as well as bulk motion of the "slow" components of the outer jet. This will provide the superb data for comparison with theoretical work on the structure and dynamics of the jet and its interaction with the ambient medium. Additionally, we will push the techniques to their limit and strive to achieve an additional factor 10 in velocity precision (which has been done successfully in other astrometric programs). With such limits, we can measure velocities to about 3000 km/s opening up completely new areas for dynamical and MHD analysis and modeling, including rotary and spiral motions as well as acceleration and deceleration. With continuous coverage and conservative limits we straightforwardly discriminate between motion, variability, “aliasing” and non-uniform motion to provide a powerful new suite of insights into an astrophysically fundamental phenomenon.

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