Late yesterday evening, eastern U.S. time—after 6½ years in space, almost 8 billion kilometers traveled, six orbit-altering flybys of planets, and 15 minutes of blasting its main engine, the MESSENGER spacecraft finally slipped into orbit around Mercury. The smallest, innermost planet is the last of the classical, naked-eye planets to get an orbiter. Tagged with the most contorted of acronyms (standing for MErcury Surface, Space ENvironment, GEochemistry, and Ranging), MESSENGER will probe everything from the mineral elements the sun continually blasts off the planet's surface to its relatively huge metallic core, which occupies nearly half of its volume. Also of interest are the subsurface ice deposits thought to linger near the poles of the sun's nearest neighbor. MESSENGER's seven scientific instruments, battened down for the rocket burn (after taking images during three earlier Mercury flybys), will be turned on and checked out for the start of science observations on 4 April. Team members expect the orbiter to return its first images later this month.
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