  We have monitored the Didymos-Dimorphos binary asteroid in
spectropolarimetric mode in the optical range before and after the DART impact.
The ultimate goal was to obtain constraints on the characteristics of the
ejected dust for modelling purposes. Before impact, Didymos exhibited a linear
polarization rapidly increasing with phase angle, reaching a level of about 5%
in the blue and about 4.5 in the red. The shape of the polarization spectrum
was anti-correlated with that of its reflectance spectrum, which appeared
typical of an S-class asteroid. After impact, the level of polarization dropped
by about 1 percentage point (pp) in the blue band and about 0.5 pp in the red
band, then continued to linearly increase with phase angle, with a slope
similar to that measured prior to impact. The polarization spectra, once
normalised by their values at an arbitrary wavelength, show very little or no
change over the course of all observations, before and after impact. The lack
of any remarkable change in the shape of the polarization spectrum after impact
suggests that the way in which polarization varies with wavelength depends on
the composition of the scattering material, rather than on its structure, be
this a surface or a debris cloud.
