Artist's impression of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft in orbit at Mercury.

That's when MESSENGER, which stands for "MErcury Surface, Space ENvironment, GEochemistry and Ranging," is expected to finally achieve its intended mission: to become the first-ever spacecraft to orbit Mercury, the solar system's innermost planet.

Entering that orbit is no easy task: to get MESSENGER where it wants it to go, NASA has had to rely on a complicated series of planetary flybys (also known as gravity assist maneuvers or gravitational slingshots). Launched on Aug. 3, 2004, MESSENGER swung by Earth again one year later, then got a couple of additional boosts from Venus during flybys in October 2006 and June 2007. After that, the target planet itself will help send MESSENGER in the right direction: two Mercury flybys have already passed (January and October of this year) and one remains (September 2009) before the craft will finally be ready to head for an orbit around the planet.

The big day is set to arrive in March 2011, when MESSENGER starts circling Mercury for a year-long data-gathering mission.

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