  The black hole X-ray transient XTE J1550-564 has undergone a strong outburst
in 1998 and two relativistic X-ray jets have been detected years later with the
$\it Chandra$ X-ray observatory; the eastern jet was found previously to have
decelerated after its first detection. Here we report a full analysis of the
evolution of the western jet; significant deceleration is also detected in the
western side. Our analysis indicates that there is a cavity outside the central
source and the jets first traveled with constant velocity and then were slowed
down by the interactions between the jets and the interstellar medium (ISM).
The best fitted radius of the cavity is $\sim$0.31 pc on the eastern side and
$\sim$0.44 pc on the western side, and the densities also show asymmetry, of
$\sim$0.034 cm$^{-3}$ on the east to $\sim$0.12 cm$^{-3}$ on the west. The best
fitted magnetic fields on both sides are $\sim$0.5 mG. Similar analysis is also
applied to another microquasar system, H 1743-322, and a large scale low
density region is also found. Based on these results and the comparison with
other microquasar systems, we suggest a generic scenario for microquasar jets,
classifying the observed jets into three main categories, with different jet
morphologies (and sizes) corresponding to different scales of vacuous
environments surrounding them. We also suggest that either continuous jets or
accretion disk winds, or both may be responsible for creating these cavities.
Therefore X-ray jets from microquasars provide us with a promising method of
probing the environment of accreting black holes.
