The Rosetta mission is a joint ESA-NASA mission approved by ESA in 1993. The mission goal was initially set for a rendezvous with comet 46 P/Wirtanen. After postponement of the initial launch (January 2003) a new target was selected: Comet 67 P/Churyumov-Gerasimenko. The Rosetta spacecraft was launched on March 2, 2004 aboard an Ariane 5 G+ rocket. The trip to Comet 67 P/Churyumov-Gerasimenko will take ten years and include two asteroid flybys: Steins (5 September 2008) and Lutetia (10 July 2010). Physical properties of the asteroids can be found here for Steins and here for Lutetia.
Once the Rosetta reaches 67 P/Churyumov-Gerasimenko in 2014, the spacecraft will fly along with the comet for over a year, collecting data and landing a small probe on the surface of the comet. Rosetta will measure the comet's maximum activity during and after its perihelion passage in July 2015.