The MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) space probe is a robotic NASA spacecraft in orbit around the planet Mercury. The 485-kilogram (1,070 lb) spacecraft was launched aboard a Delta II rocket in August 2004 to study the chemical composition, geology, and magnetic field of Mercury. It became the second mission after 1975's Mariner 10 to reach Mercury successfully when it made a flyby in January 2008, followed by a second flyby in October 2008, and a third flyby in September 2009. MESSENGER is furthermore the first spacecraft ever to orbit Mercury.
The instruments carried by MESSENGER were tested on a complex series of flybys – the spacecraft flew by Earth once, Venus twice, and Mercury itself three times, allowing it to decelerate relative to Mercury with minimal fuel. MESSENGER successfully entered Mercury's orbit on 18 March 2011, and reactivated its science instruments on 24 March, returning the first photo from Mercury orbit on 29 March. MESSENGER's formal science data collection mission began on 4 April 2011.
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