On its way to the actual target of Rosetta, the comet Churyumov-Gerasimenko, there will also take place two flybys at the asteroids Steins and Lutetia. Those rendezvous will enhance or understanding of the small bodies in the solar system considerably.
Asteroid Steins is relatively small (diameter is about 5 km), rotates with a period of about 6 hrs and is categorized as E-class. The surface of E-class asteroids is characterized by iron-poor silicates (like feldspar). So far, only around 20 such bodies are known, Steins will be the first to be visited by a spacecraft.
The flyby takes place September 5th, at 18:38 (UTC) at a distance of only 800 km. Most of the Rosetta instruments, including the camera, developed by the Max Plank Institute for Solar System Physics in Lindau-Katlenburg, will be switched on. So we may expect sptectacular data and images.
At the time of the flyby, Rosetta is at a distance of about 320 million kilometres from Sun and about 360 million kilometres (i.e. 20 light-minutes) from the Earth. In order to optimally point the instruments towards the asteroid, dedicated attitude manoeuvres of the spacecraft are necessary.
In the time between September 3rd and 6th, on board of Philae, three scientific instruments will be activated: The magnetometer (ROMAP) as well as SESAME and MUPUS. Of course, the primary goal of the Lander is the target comet, where the landing shall take place in 2014. Philae is commanded from DLR-MUSC in Köln-Porz. Rosetta is operated from ESOC (European Space Operations Center) in Darmstadt.