The light curve of asteroid 2867 Steins measured by VIRTIS-M during the Rosetta fly-by

Kurzfassung

On 5 September 2008, the Rosetta spacecraft encountered the asteroid 2867 Steins on its way to the comet 67P/Churyumov-Gerasimenko. This was the first of two planned asteroid fly-by sperformed by the probe, the second fly-by being with the much larger asteroid 21 Lutetia in July 2010. The VIRTIS imaging spectrometer (IFOV 0.250 mrad, overall spectral range 0.25–5.1 mm) onboard Rosetta acquired data of Steins already before the closest approach phase, when the target was spatially unresolved, in order to obtain a lightcurve of the asteroid in the infrared spectral range extending up to 5 mm, that was never explored before. The VIRTIS lightcurve campaign started at 11:30 UTC onboard time, when the spacecraft was about 221,377 km away from the target, and ended at 17:58 UTC, at a distance of 20,741 km away from Steins. During this time frame, the solar phase angle of the asteroid was roughly constant, ranging from 38.21° to 36.31°. Assuming the most recent value derived for the rotational period of Steins (Lamy et al., 2008), the VIRTIS observations covered slightly more than one rotation of the asteroid. In this interval, VIRTIS collected 8 hyperspectral cubes where Steins was captured 119 times, both in the visual and in the infrared range. Given the low signal and the unresolved appearance of the source, for which the instrument was not designed, only a small subset of wavelengths turned out to be suitable to sample the lightcurve. Nevertheless, in both the VIS and NIR ranges we find a similar trend, with two different maxima and minima during one rotational period, and amplitudes consistent with the results in the visual range obtained in previous works, including the data set acquired by the OSIRIS camera onboard Rosetta. We also report the presence of a new broad feature centered at approximately 0.81–0.82 mm, which is seen in the visual data throughout the rotation of the asteroid.