Ground based science of ESA's Rosetta mission targets: (21) Lutetia and (2867) Steins

International audienceIn the framework of ground-based science campaign dedicated to the encounter with Rosetta spacecraft, the mineralogies of the asteroids (21) Lutetia and (2867) Steins were investigated. Near-infrared (NIR) spectra of the asteroid in the 0.8-2.5 mum spectral range have been obtained with SpeX/IRTF in remote-observing mode from Meudon, France, and Cambridge, MA, in March, April and December 2006 and in January and March 2007. A chi2 test using meteorite spectra from the RELAB database was performed in order to find the best fit of complete visible infrared (VNIR) spectra of Lutetia. To constrain the possible composition of Steins' surface, we constructed a simple mixing model using a linear (areal) mix of three components obtained from the RELAB database. A space-weathering model was applied to the aubrite ALH-78113 spectrum. In the case of (21) Lutetia we find a clear spectral variation (slope), and a good correspondence between spectral variations and rotational phase. The surface mixing model places Steins in a subdivision of the E-type class with objects like Angelina, Eger, and Nereus, a group not sampled by the current collection of aubrite meteorites