Most of what scientists know about Mercury was learned during the three fly-bys by Mariner 10. Because the spacecraft was placed in an orbit around the Sun equal to one Mercurian solar day, it made each of its three passes when exactly the same half of the planet was in sunlight. Slightly less than the illuminated half, or about 45 percent of Mercury's surface, was eventually imaged. Mariner 10 also collected data on particles and magnetic fields during its flybys, which included two close nightside encounters and one distant dayside pass. Mercury was discovered to have a surprisingly Earthlike (though much weaker) magnetic field. Scientists had not anticipated a planetary magnetic field for such a small, slowly rotating body because the dynamo theories that described the phenomenon required thoroughly molten cores and rather rapid planetary spins. Even more rapidly spinning bodies such as the Moon and Mars lack magnetic fields. In addition, Mariner 10's spectral measurements showed that Mercury has an extremely tenuous atmosphere.
The first significant telescopic data about Mercury after the Mariner mission resulted in the discovery in the mid-1980s of sodium in the atmosphere. Subsequently, better Earth-based techniques enabled the variations of several of Mercury's atmospheric components to be studied from place to place and over time. Also, ongoing improvement in the power and sensitivity of ground-based radar resulted in intriguing maps of the hemisphere unseen by Mariner 10 and, in particular, the discovery of condensed material, probably water ice, in permanently shadowed craters near the poles.
In 2008 the Messenger probe made its first flyby of Mercury and obtained photos of more than a third of the hemisphere that was unseen by Mariner 10. The probe passed within 200 km (120 miles) of the planet's surface and saw many previously unknown geologic features. In 2011 Messenger is slated to enter Mercury's orbit and study it for one year.
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