In a pair of flybys by a robotic explorer last year, planetary scientists began to unravel some of the mysteries of Mercury, a planet that is difficult to study from Earth and that had not been visited by a spacecraft since the 1970s.

Today brings the third such near approach to the planet by the Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft, providing one last close-up view before MESSENGER enters orbit around Mercury in March 2011. Today's maneuver, which will harness the planet's gravity for a trajectory assist, culminates in a closest approach of roughly 230 kilometers at about 5:55 p.m. (Eastern Daylight Time).

Before MESSENGER's first flyby, in January 2008, less than half of Mercury's surface had even been photographed, and the planet had never known a dedicated orbiter. Humankind's only detailed look at the diminutive body, whose proximity to the sun largely stymies ground-based observation, had come from a series of flybys by Mariner 10 in 1974 and 1975.

Now MESSENGER has boosted the coverage of the planet's surface maps to more than 90 percent, and a few more gaps should be filled during today's flyby. But perhaps more importantly, the spacecraft's science team has used data gathered during the first two flybys to target their instruments today on interesting-looking craters and other surface features.

Photo of Mercury taken by MESSENGER en route to its third flyby: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington