Because we had to go there for the Rosetta scientific objectives very far away -- four times the distance of the Earth to the sun -- and also at a much higher velocity than we could achieve with fuel, because we'd have to take six times as much fuel as the whole spacecraft weighed.
Parce que nous devions y aller pour les objectifs scientifiques de Rosetta, très loin, quatre fois la distance entre la Terre et le soleil, et aussi beaucoup plus vite que l'essence ne le permet. car le carburant pesait six fois plus que l'astronef lui-même.