The time evolution of HH~2 from four epochs of HST images
  We have analyzed four epochs of H$\alpha$ and [S~II] HST images of the HH~1/2
outflow (covering a time interval from 1994 to 2014) to determine proper
motions and emission line fluxes of the knots of HH~2. We find that our new
proper motions agree surprisingly well with the motions measured by Herbig \&
Jones (1981), although there is partial evidence for a slight deceleration of
the motion of the HH~2 knots from 1945 to 2014. We also measure the
time-variability of the H$\alpha$ intensities and the [S~II]/H$\alpha$ line
ratios, and find that knots H and A have the largest intensity variabilities
(in $1994\to 2014$). Knot H (which now dominates the HH~2 emission) has
strengthened substantially, while keeping an approximately constant
[S~II]/H$\alpha$ ratio. Knot A has dramatically faded, and at the same time has
had a substantial increase in its [S~II]/H$\alpha$ ratio. Possible
interpretations of these results are discussed.
