Which spacecraft was the first to orbit Mercury?

MESSENGER was the first spacecraft to orbit Mercury, entering orbit around the planet on March 18, 2011.

NASA launched MESSENGER in 2004, but reaching Mercury was difficult because the Sun’s gravity greatly accelerates spacecraft travelling inward. To conserve fuel, the probe used a carefully choreographed sequence of gravity assists: one at Earth, two at Venus, and three at Mercury before its final orbital insertion.

MESSENGER’s name is an acronym for Mercury Surface, Space Environment, Geochemistry, and Ranging, while also evoking Mercury’s role as a messenger god in Roman mythology. During its orbital mission, the spacecraft studied Mercury’s composition, geology, magnetic field, exosphere, and polar regions. It helped confirm deposits of water ice in permanently shadowed polar craters.

Mariner 10 is the main source of confusion: it was the first spacecraft to visit Mercury and made three flybys in the 1970s, but it never entered orbit. MESSENGER completed its mission in 2015 by impacting Mercury after its fuel was exhausted.