Mission target asteroids: thermal-IR characterization from ground

Interplanetary missions to asteroids, like the Hayabusa, Hayabusa2, OSIRIS-REx, DESTINY+ or DART/Hera project, depend on a good knowledge of the target object prior to the critical mission phases. Ground-based IR observations (at different times, observing geometries, wavelengths) play a very important role in this context: Via radiometric studies, in combination with auxiliary observations in the visible or near-IR, it is possible to derive the object's size, shape, spin properties, albedo, and thermal properties. The ground-based IR measurements are often absolutely critical, even in cases where detections from IR space surveys or IR space observatories are available. The ground-based data, mainly in N-band, partially also in M and/or Q-band, are well suited to monitor rotational properties (on hour or day scale), phase or aspect angle studies (over weeks/months) and to obtain well-calibrated fluxes over several orders of magnitude (from mJy almost up to kJy) needed for the determination of high-quality thermophysical properties. In comparison with space-based observations, the ground-based observations are less restricted in accessible solar elongation range and non-sidereal tracking speeds, both aspects are very relevant for near-Earth object mission targets. The ground-based IR measurements also play an important role for connecting and interpreting the results from in-situ findings on small scale (available only for very few objects which are considered fundamental for the understanding of the formation and evolution of planetary systems) with global, disk-integrated measurements (available for most of the small bodies). The ground-based IR characterization of these objects is therefore an important contribution to the understanding of our Solar System. We will present examples of past, ongoing and planned ground-based IR measurements of interplanetary mission targets, the role of these measurements for the characterization of the objects, and their importance for Solar System research.