  Spectrally-resolved observations of three pure rotational lines of H$_2$,
conducted with the EXES instrument on SOFIA toward the classic bow shock HH7,
reveal systematic velocity shifts between the S(5) line of ortho-H$_2$ and the
two para-H$_2$ lines [S(4) and S(6)] lying immediately above and below it on
the rotational ladder. These shifts, reported here for the first time, imply
that we are witnessing the conversion of para-H$_2$ to ortho-H$_2$ within a
shock wave driven by an outflow from a young stellar object. The observations
are in good agreement with the predictions of models for non-dissociative,
C-type molecular shocks. They provide a clear demonstration of the chemical
changes wrought by interstellar shock waves, in this case the conversion of
para-H$_2$ to ortho-H$_2$ in reactive collisions with atomic hydrogen, and
provide among the most compelling evidence yet obtained for C-type shocks in
which the flow velocity changes continuously.
