This is a discretionary time proposal to attempt detection of the lowest
pure rotational transition of hydrogen deuteride, HD, at 112.072 microns (um)
toward the Orion Bar. Before launch it was thought that this line would be
routinely detected, but that has not proven to be the case. Indeed, so far
ISO has failed to detect this important line in either emission or
absorption outside of the solar system, for a number of possible reasons.
This proposal therefore represents a last deep search, and indeed our last
chance before SOFIA or FIRST. Observation of this line will allow a
determination of the HD abundance, as well as probe the chemistry
of molecular clouds. When compared with observations of molecular hydrogen,
it will also allow a constraint to be placed on the D/H ratio, an important
cosmological parameter.