It was a delta-v opposite to the direction of travel and that slowed Messenger enough so the relatively weak gravity of Mercury could capture the spacecraft and put it into an elliptical orbit around the planet rather than the Sun.

BBC: Juno Jupiter probe gets British boost

Delta will run one daily flight in each direction, with the eastbound flight leaving Newark's Liberty Airport at 6:50 p.m. local time, and the westbound flight departing Charles DeGaulle Airport at 1:30 p.m. local.

WSJ: Delta launching flights between Newark, Paris