Context. Near-Earth binary asteroid (175706) 1996 FG3 is the
current backup target of the ESA MarcoPolo-R mission, selected for the
study phase of ESA M3 missions. It is a primitive (C-type) asteroid that
shows significant variation in its visible and near-infrared spectra.
Aims: Here we present new visible and near-infrared spectra of
1996 FG3. We compare our new data with other published
spectra, analysing the variation in the spectral slope. The asteroid
will not be observable again over the next three years at least.
Methods: We obtained visible and near-infrared spectra using DOLORES and
NICS instruments, respectively, at the Telescopio Nazionale Galileo
(TNG), a 3.6 m telescope located at El Roque de los Muchachos
Observatory in La Palma, Spain. To compare with other published spectra
of the asteroid, we computed the spectral slope S', and studied any
plausible correlation of this quantity with the phase angle (α).
Results: In the case of visible spectra, we find a variation in
spectral slope of ΔS' = 0.15 ± 0.10%/103
Å/° for 3°<α< 18°, which is in good
agreement with the values found in the literature for the phase
reddening effect. In the case of the near-infrared, there seems to be a
trend between the reddening of the spectra and the phase angle,
excluding one point. We find a variation in the slope of ΔS' =
0.04 ± 0.08%/103 Å/° for 6° <α
< 51°. Our computed variation in S' is in good agreement with the
only two values found in the literature for the phase reddening in the
near-infrared. Conclusions: The variation in the spectral slope
of asteroid 1996 FG3 shows a trend with the phase angle at
the time of the observations, both in the visible and the near-infrared.
It is worth noting that, to fully explain this spectral variability we
should take into account other factors, like the position of the
secondary component of the binary asteroid 1999 FG3 with
respect to the primary, or the spin axis orientation at the time of the
observations. More data are necessary for an analysis of this kind.