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

htof: A new open-source tool for analyzing Hipparcos, Gaia, and future astrometric missions