The first Rosetta asteroid flyby

The International Rosetta Mission, a cornerstone mission of the European Space Agency Scientific Programme, was launched on 2 March 2004 on its 10 years journey towards a rendezvous with comet Churyumov-Gerasimenko. Once reached the comet nucleus in summer 2014, Rosetta will orbit it for about 1.5 years down to distances of a few kilometres and deliver a lander onto its surface. In the long cruise to its target, Rosetta performs four gravity assist manoeuvres, three times with Earth and once with Mars. During this flight Rosetta will cross twice the main asteroid belt, and in both occasions its trajectory is designed and controlled to flyby an asteroid at close distance. On 5 September 2008 the first of the two asteroid flybys has been conducted, with the spacecraft approaching asteroid 2867-Šteins at a minimum distance of about 800 km, and a relative velocity of 8.6 km/s. Šteins is an E-type asteroid with a diameter of about 5 km. This paper continues the period reporting of the operational experience from the Rosetta mission operations. Preparation and execution of the flyby activities are described including the first experience in optical navigation that will prove of utmost importance for the next asteroid flyby and for the comet approach phase.