The International Rosetta Mission is the Planetary Cornerstone Mission in ESA's long-term programme in space science, Horizons 2000. The mission's main goal is a rendezvous with Comet 46P/Wirtanen, but it is also intended to study two asteroids during close flybys on route to the comet, most likely asteroids Otawara and Siwa. Rosetta will study the nucleus of Comet Wirtanen and its environment in great detail for a period of nearly two years; the near-nucleus phase starting at a heliocentric distance of nominally 3.25 AU, from the onset of activity through to perihelion, close to 1 AU. The orbiter will carry twelve experiments and the Rosetta Lander. The payload will have unprecedented capabilities for studying the composition of both the volatile and refractory material released by the cometary nucleus with high resolution. The remote-sensing instrument suite will allow high resolution characterisation of the nucleus surface in a wide range of wavelengths (UV to mm) from close orbits (2-10 nucleus radii). The OSIRIS Narrow-Angle Camera, for example, will achieve a resolution of better than 10 cm on the nucleus surface. To complement the remote-sensing and composition-analysis instruments and to provide for the proper monitoring of the comet environment and its interaction with the solar wind, a Dust Flux Analyser and a Plasma Instrument Package have been included in the payload complement.