The planet Mercury and a spacecraft will meet for a second time late tonight. It's a setup for a permanent meeting in 2011, when the craft will enter orbit around Mercury.
The craft is called Messenger. It first flew by Mercury in January. In just a few hours of observations, it helped scientists solve some mysteries about the planet.
For one thing, they found that the largest feature on Mercury, the Caloris Basin, was filled by lava from volcanoes around its edge. Until Messenger, the scientists weren't quite sure how the smooth plain that coats the basin floor was formed.
For another, they found that Mercury's iron-rich core is shrinking. This process powers the planet's magnetic field.
Scientists are hoping for similar discoveries from tonight's encounter. Messenger will fly within about 125 miles of Mercury's surface. It'll approach on the nightside, so its cameras won't see much at first. But other instruments will probe the magnetic field and the wisps of material that make up Mercury's thin atmosphere.
As Messenger passes to the dayside, its cameras will first see regions that were photographed in the 1970s by the only other craft to visit Mercury. Later, though, it'll see regions that have never been studied from close range.
The craft will use Mercury's gravity on this approach, and another next year, to line up for its final approach in 2011, when Messenger will enter orbit around the Sun's closest planet.
Script by Damond Benningfield, Copyright 2008
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