The light curve of asteroid 21 Lutetia measured by VIRTIS-M during the Rosetta fly-by

On 10 July 2010, the Rosetta spacecraft encountered the asteroid 21 Lutetia on its way to the comet 67P/Churyumov-Gerasimenko. This was the second of two planned asteroid fly-bys performed by the probe, the first fly-by being with the much smaller asteroid 2867 Steins in September 2008. The VIRTIS imaging spectrometer (angular resolution 0.250 mrad, overall spectral range 0.25–5.1 μm) onboard Rosetta acquired data of Lutetia well before the closest approach phase, when the target was spatially unresolved, in order to obtain a light curve of the asteroid in the infrared spectral range extending up to 5 μm. Two light curve sequences were obtained, separated by a gap of 3.25 h. During the first light curve, the solar phase angle of the asteroid had a constant value of 10.8°, while in the second light curve it decreased from 10.7° to 7.8°. ng the latest value derived for the rotational period of Lutetia (8.168270±0.000001 h, Carry et al., 2010), each of the light curve sequences covered roughly one rotation of the asteroid. In the overall campaign, VIRTIS collected a total of 16 cubes where Lutetia was captured 165 times, both in the visual and in the infrared range. Given the low signal and the unresolved appearance of the source in the first sequence and at the beginning of the second sequence, several wavelengths turn out to be unsuited to sample the light curve. Nevertheless, in both the VIS and IR ranges we find a similar trend, with two different maxima and minima during one rotational period. In the edge-on view with north pole tilted towards the spacecraft, we find that the near infrared spectrum of Lutetia in its northern hemisphere is essentially flat and grey, with no relevant absorption features in the range 1–3.5 μm within the sensitivity of the VIRTIS instrument. We find a marked spectral homogeneity throughout the two explored rotational periods within the 1−σ variability of the data. Consistent with the results obtained at closest approach by Coradini et al. (2011), we confirm the absence of OH features at 1.9, 2.7, and 3 μm, and no other absorption features are seen in the near infrared range 1–3.5 μm of VIRTIS data at northern latitudes.