MESSENGER became the first spacecraft to enter orbit around Mercury on March 18, 2011.
NASA launched MESSENGER on August 3, 2004. Reaching Mercury required several gravity-assist flybys because the planet’s proximity to the Sun makes it difficult for a spacecraft to lose enough orbital energy. MESSENGER flew past Earth once, Venus twice, and Mercury three times before orbital insertion.
The spacecraft mapped Mercury’s surface and measured its composition, magnetic field, thin exosphere, and internal structure. It confirmed that permanently shadowed polar craters contain water ice and other volatile material. MESSENGER also showed that Mercury has contracted as its interior cooled, leaving extensive cliff-like scarps across the planet.
Mariner 10 was the first spacecraft to visit Mercury, but it performed flybys rather than entering orbit. BepiColombo, a joint European-Japanese mission, reached Mercury later and is designed for detailed orbital studies. MESSENGER ended its mission by impacting Mercury on April 30, 2015.