  We present htof, an open-source tool for interpreting and fitting the
intermediate astrometric data (IAD) from both the 1997 and 2007 reductions of
Hipparcos, the scanning-law of Gaia, and future missions such as the Nancy
Grace Roman Space Telescope (NGRST). htof solves for the astrometric parameters
of any system for any arbitrary combination of absolute astrometric missions.
In preparation for later Gaia data releases, htof supports arbitrarily
high-order astrometric solutions (e.g. five-, seven-, nine-parameter fits).
Using htof, we find that the IAD of 6617 sources in Hipparcos 2007 might have
been affected by a data corruption issue. htof integrates an ad-hoc correction
that reconciles the IAD of these sources with their published catalog
solutions. We developed htof to study masses and orbital parameters of
sub-stellar companions, and we outline its implementation in one orbit fitting
code (orvara, https://github.com/t-brandt/orvara). We use htof to predict a
range of hypothetical additional planets in the $\beta$~Pic system, which could
be detected by coupling NGRST astrometry with Gaia and Hipparcos. htof is pip
installable and available at https://github.com/gmbrandt/htof .
