A spacecraft that's revealed a completely different side of Mercury is about to pay another call on the little planet. It'll skim within about 150 miles of Mercury's battered surface late tonight.
The spacecraft is called Messenger, and this will be its third encounter with Mercury.
Before Messenger, only one spacecraft had visited Mercury -- Mariner 10. It flew by the planet three times back in the 1970s. But Mariner saw the same side of Mercury each time it flew past. So that gave us a good view of half the planet, but left the other half in the dark.
Messenger has changed that. During its earlier encounters, it photographed most of the half of Mercury that Mariner 10 missed. It also mapped the composition of the rocks and measured the strength and contours of the magnetic field. During the last encounter, back in January, it discovered a 400-mile-wide basin whose floor is covered with rocks that are older than those seen elsewhere on the planet. And it found that the planet's magnetic field is a lot more complicated than anyone had thought.
Messenger won't get to fill in the missing sliver of Mercury this time around -- it'll be on the other side of the planet. But this won't be its last look at Mercury. In fact, the best is yet to come. Messenger is scheduled to enter orbit around Mercury in March of 2011. Over the following year, it'll map the entire planet -- providing the first complete look at this rocky little world.
Script by Damond Benningfield, Copyright 2009
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