Asteroid exosphere: A simulation for the ROSETTA flyby targets (2867) Steins and (21) Lutetia

Asteroids (2867) Steins and (21) Lutetia are two flyby targets of ESA's cornerstone mission Rosetta. Since Rosetta is a cometary mission, some of the instruments are designed to investigate the surroundings of small bodies. To prepare the operation of these instruments, in our case the ROSINA instrument, for the asteroid flyby's, we adapted a Monte Carlo simulation code, initially developed to simulate the exosphere of Mercury. Modelled release processes are solar wind sputtering, micrometeorite impact vaporisation, photon stimulated desorption and in some cases thermal release. Released species were derived from estimations of the asteroid composition. This was done for both asteroids by using results from ground based observations and meteorite science. We used the estimated compositions and other known properties as input for the simulation. Our results suggest that neutral sodium and oxygen might be the best species to investigate by the means of mass spectrometry: We expect to be able to detect these species at least in the exosphere of (21) Lutetia.