NASA’s MESSENGER was the first mission to orbit Mercury, using gravity assists from Earth, Venus, and Mercury to reach the planet.
MESSENGER launched in 2004, but it did not travel directly to Mercury. Because Mercury is close to the Sun, a spacecraft approaching it gains substantial speed. The probe therefore performed a carefully planned sequence of planetary flybys to shed orbital energy and line up for capture.
It flew past Earth once in 2005, Venus twice in 2006 and 2007, and Mercury three times between 2008 and 2009. These encounters gradually altered its path before orbital insertion in March 2011. MESSENGER then mapped Mercury and investigated its geology, magnetic field, exosphere, and polar deposits.
Mariner 10 reached Mercury earlier, but it was a flyby mission rather than an orbiter. BepiColombo, a European-Japanese mission with NASA contributions, entered Mercury orbit in 2025, after MESSENGER had already completed the first orbital survey.