WASHINGTON, Sept. 23 (Xinhua) -- NASA's MESSENGER spacecraft will fly by Mercury for the third and final time on Sept. 29, NASA said Wednesday. The spacecraft will pass less than 142 miles above the planet's rocky surface for a final gravity assist that will enable it to enter Mercury's orbit in 2011.

An image of the planet Mercury, made during the January 2008 flyby of the planet by the Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft is seen in this image released by NASA July 3, 2008. The image shows that volcanoes were involved in plains formation and suggest that its magnetic field is actively produced in the planet's core. (Xinhua/Reuters Photo) Photo Gallery>>>

    Determining the composition of Mercury's surface is a major goal of the orbital phase of the mission. The spacecraft already has imaged more than 90 percent of the planet's surface. The spacecraft's team will activate instruments during this flyby to view specific features to uncover more information about the planet.

    "This flyby will be our last close look at the equatorial regions of Mercury, and it is our final planetary gravity assist, so it is important for the entire encounter to be executed as planned," said Sean Solomon, principal investigator at the Carnegie Institution in Washington. "As enticing as these flybys have been for discovering some of Mercury's secrets, they are the hors d'oeuvres to the mission's main course -- observing Mercury from orbit for an entire year."

    The spacecraft may observe how the planet interacts with conditions in interplanetary space as a result of activity on the sun. During this encounter, high spectral- and high spatial-resolution measurements will be taken again of Mercury's tenuous atmosphere and tail.

    Launched Aug. 3, 2004, MESSENGER has completed nearly three-quarters of its 4.9-billion-mile journey to enter orbit around Mercury. It already has flown past Mercury twice.