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.
Musieliśmy tam dotrzeć  dla celów naukowych Rosetty. To bardzo daleko, to czterokrotny dystans od Ziemi do Słońca. Musieliśmy dotrzeć tam szybciej, niż pozwala zwykłe paliwo. Paliwo ważyłoby sześć razy niż sam statek.