NASA's MESSENGER mission to Mercury is in good shape to begin its primary mission to the innermost planet. Scheduled to drop into orbit next March 18, the plan is for the probe to study Mercury for one Earth year. It will be the most extensive examination of the planet yet.
Launched in 2004, MESSENGER has already flown by Mercury three times as it jockeyed for the right angle and speed to allow it to reach the desired orbit in 2011. Science done during those flybys has already changed our view of Mercury. It seems to have a magnetic field similar to Earth's, for example, which argues Mercury still has an active core-- a surprise to most planetary scientists. There also seems to be volcanism in the Mercurian past, another surprise. We also now know Mercury has an atmosphere, albeit a wispy one, which seemed quite unlikely for such a small planet so exposed to the powerful, stripping solar wind.
No doubt the Mercury that emerges by the end of the MESSENGER mission will be different again from the image we now have.
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