The Wide Angle Camera of the OSIRIS system onboard the Rosetta ESA mission

Rosetta is one of the cornerstone missions of the European Space Agency (ESA) dedicated to the exploration of the periodic 67P/Churyumov-Gerasimenko comet. The main mission objective is to study the physical evolution of the nucleus of the comet and of its dust and gas environment from 3-4 AU to perihelion. The Rosetta spacecraft is experiencing a rather long interplanetary journey, about 10 years, with also one Mars and three Earth gravity assists, before reaching the comet in 2014. The spacecraft has also the chance to flyby two main belt asteroids: (2867) Steins in September 2008 and (21) Lutetia in July 2010. The scientific payload of the mission consists of both remote and in situ instruments. One of the remote sensing instruments on board the satellite is OSIRIS, realized by a cooperation among several European institutes [1]. OSIRIS is a double camera comprising a narrow angle camera, NAC, and a wide angle camera, WAC. The NAC [2], having the higher spatial resolution, is able to study the comet nucleus surface properties in great detail; the WAC, thanks to its large field of view, is able to image the limb and the coma to study the evolution of weak coma structures near the bright nucleus, providing information on gas and dust outflow and jets directly above the comet nucleus surface [3]. The WAC camera adopts an all reflecting, unvignetted and unobstructed two mirrors configuration, which allows to cover a 12° x 12° Field of View with an F/5.6 aperture and a geometrical Ensquared Energy better than 80% inside approximately 20 arcsec. The WAC is equipped with 14 filters in the 230-720 nm wavelength range, they have been especially selected to study the gas and dust comet emissions. The flight model of the WAC has been successfully integrated and tested in our laboratories and finally has been integrated on the Rosetta spacecraft. The optimal performance of the camera has also been proved and exploited in-flight during the Mars swing-by (February 2007), Earth swing-bys in November 2007 and in November 2009, when impressive images of the red planet and of our Earth-Moon system have been successfully taken. Many images had also been taken during the asteroids Steins flyby on the 5th of September 2008, enabling us to study the surface photometrical and structural properties, to date the asteroid starting from cratering history and also to derive a 3-dimensional model of the asteroid itself [4].