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January 16, 2008—The first of many planned images from NASA's MESSENGER spacecraft is showing astronomers a side of Mercury no one's ever seen before.
Mercury is tough to view from Earth, since it's so close to the sun. And when the Mariner 10 probe flew past the innermost planet in 1974 and 1975, only one side of the body was facing sunlight.
That's because Mercury rotates three times during every two orbits, so the same side of the planet is lit up every other time it is nearest to the sun—including during all of Mariner's flybys.