Multiple asteroid systems: Dimensions and thermal properties from Spitzer Space Telescope and ground-based observations

► Mid-IR spectra from 5.2 to 38 μm using the Spitzer/IRS of 28 asteroids. ► Analyzed using STM and a thermophysical model (TPM) if 3D-shape is available. ► Large (Deq > 130 km) binary asteroids are covered with a thick layer of thermally insulating regolith. ► Smaller (Deq < 17 km) asteroids have their surfaces are coarser, and therefore younger. ► Their average densities vary from 0.7-1.7 g/cm3 (P-, C-type) to ∼2 g/cm3 (S-type) and 3.2 g/cm3 (M-type).