Which spacecraft became the first to orbit Mercury in 2011?

MESSENGER became the first spacecraft to orbit Mercury on 18 March 2011.

NASA’s MESSENGER mission entered orbit after a long trajectory using several planetary flybys and engine burns. Mercury’s proximity to the Sun makes orbital insertion difficult because a spacecraft must lose substantial orbital energy rather than simply fly inward from Earth.

During more than four years in orbit, MESSENGER mapped Mercury’s surface, measured its magnetic field, studied its thin exosphere, and found strong evidence of water ice in permanently shadowed polar craters. The spacecraft also showed that Mercury has a surprisingly large iron core and a geologically active past.

Mariner 10 had made three flybys of Mercury in the 1970s, but it never entered orbit. Europe and Japan’s BepiColombo mission later followed as the second mission intended to orbit Mercury.