NASA launched its Mercury Surface Space Environment Geochemistry and Ranging (MESSENGER) spacecraft on August 3, 2004. Since that launch date, the spacecraft has made flybys of Earth and Venus.

The first flyby by MESSENGER of Earth was on August 2, 2005; the next planetary body MESSENGER flew by was Venus on October 24, 2006; and the craft passed Venus again on June 5, 2007. Eventually the MESSENGER spacecraft will  orbit Mercury.

Earlier this week the MESSENGER spacecraft made its first flyby of Mercury (PDF) from a distance of 200km above the planet’s surface. During the flyby, MESSENGER snapped the first images of Mercury’s surface since NASA’s Mariner 10 probe that flew past Mercury three times between 1974 and 1975. Mariner 10 was able to map about 45% of Mercury’s surface during its trio of flybys.

Dr. Sean C. Solomon, MESSENGER principal investigator said, "The MESSENGER Science Team is extremely excited about this flyby. We are about to enjoy our first close-up view of Mercury in more than three decades, and a successful gravity assist will ensure that MESSENGER remains on the trajectory needed to place it into orbit around the innermost planet for the first time."

During the flyby, the instruments on MESSENGER took readings and snapped over 1200 images of Mercury. The instrumental payload of MESSENGER intends to meet mission goals including: mapping the elemental and mineralogical composition of Mercury’s surface, imaging the entire surface of Mercury at resolutions of 100 meters or better, determining the structure of the planet’s magnetic field, measuring the planet’s gravitational field, and characterizing the exospheric neutral particles and magnetoshperic ions and electrons.

One of the main points of interest on the surface of Mercury is the Caloris basin, which is an impact crater about 808 miles in diameter making it one of the largest impact basins in the solar system. By comparison the largest visible impact crater on Earth, the Barringer Meteorite Crater in Arizona, is only about a mile wide.

Before its final orbital insertion around Mercury on March 18, 2011, MESSENGER will make a second flyby of the planet scheduled for October 6, 2008, and a third flyby on September 29, 2009. Currently MESSENGER is about half way through its 4.9 billion mile trek.