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.

Added up, these factors have meant that although Mercury sits only about 57 million miles (92 million kilometers) away from Earth, for more than 30 years scientists have had almost no details about its other face.

But on Monday MESSENGER, the first mission to Mercury since the 1970s, snapped the first image of the "missing" half of the rocky world.

Among many new sights, the picture features the full Caloris Basin, a huge impact basin more than 620 miles (1,000 kilometers) across that sits on the border between the known and previously unknown regions of the planet.

More unprecedented images of the tiny planet are expected as the MESSENGER craft completes three flybys of Mercury before settling into orbit in March 2011.

From that point on, writes astronomer Phil Plait on the Bad Astronomy blog, "we'll get as many images of this tiny, hot, battered, dense and neglected planet as we can handle."

—Victoria Jaggard

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