  We have performed, in October 2005, a deep Chandra X-ray observation of HH
154. Comparison with the previous (2001) Chandra observation allows to detect
proper motion down to the level predicted by models of X-ray emitting shocks in
the jet. The 2005 Chandra observation of HH 154 shows unexpected morphological
changes of the X-ray emission in comparison with the 2001 data. Two components
are present: a stronger, point-like component with no detectable motion and a
weaker component which has expanded in size by approximately 300 AU over the 4
years time base of the two observations. This expansion corresponds to
approximately 500 km/s, very close to the velocity of the X-ray emitting shock
in the simple theoretical models. The 2005 data show a more complex system than
initially thought (and modeled), with multiple components with different
properties. The observed morphology is possibly indicating a pulsed jet
propagating through a non-homogeneous medium, likely with medium density
decreasing with distance from the driving source. Detailed theoretical modeling
and deeper X-ray observations will be needed to understand the physics of this
fascinating class of sources.
