The Motion of Water Masers in the Pre-Planetary Nebula IRAS 16342-3814

We present high angular resolution observations, using the Very Long Baseline Array (VLBA) of the NRAO, of the high-velocity water masers toward the "water-fountain" pre-planetary nebula, IRAS 16342-3814. The detailed structure of the water masers appears to be that of bow shocks on either side of a highly collimated jet. The proper motions of the water masers are approximately equal to the radial velocities; the three-dimensional velocities are approximately +/-180 km/s, which leads to a very short dynamical time-scale of ~100 years. Although we do not find direct evidence for precession of the fast collimated jet, there may be indirect evidence for such precession.