On March 17, after a roundabout, nearly seven-year journey, NASA's MESSENGER probe became the first spacecraft to orbit Mercury, the closest planet to the sun.

MESSENGER, which stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging, launched in 2004 on an inward-spiraling path through the inner solar system that covered nearly eight billion kilometers and included a number of planetary flybys before reaching Mercury orbit. With MESSENGER reaching its destination, humankind now has orbiters in place around Mercury, Venus, Earth, Mars, Saturn and the moon.

Among MESSENGER's tasks during its one-year primary science mission: making topographic maps of Mercury and characterizing the planet's magnetic field and geologic history, including the role of volcanism in its relatively recent past. The science phase of the mission is scheduled to begin April 4, after mission scientists have had a chance to check on the status of MESSENGER, which is operating in an environment of intense solar radiation, and power up the spacecraft's instruments. But MESSENGER has already told planetary scientists a great deal about Mercury, which had only been partially mapped prior to the spacecraft's three flybys en route to orbit in 2008 and 2009. Those flybys filled in most of the cartographic gaps and provided some tantalizing hints of the science to come.

"Despite its proximity to Earth, the planet Mercury has for decades been comparatively unexplored," Sean Solomon of the Carnegie Institution of Washington, MESSENGER's principal investigator, said in a prepared NASA statement. "For the first time in history, a scientific observatory is in orbit about our solar system's innermost planet. Mercury's secrets, and the implications they hold for the formation and evolution of Earth-like planets, are about to be revealed."

Photo of Mercury as seen by MESSENGER in 2008: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington