Mercury is probably the solar system's most extreme planet. It is the oldest of all, the densest, the smallest, the closest to the Sun and, ironically, the least studied. Except the Messenger spacecraft (or the Mercury surface, space environment, geochemistry and ranging spacecraft), only one probe has been ever sent to study it: Mariner 10, which made three fly-bys around the planet during the span of a year, before running out of fuel.

"The mission will make three fly-bys. The probe will eventually be inserted into an orbit around Mercury in March 2011," says Sean Solomon, mission principle investigator and director of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington. Solomon was the first to propose NASA that another spacecraft be put in the orbit of Mercury, and twelve years later his dream came true, with the first fly-by of the Messenger spacecraft.

During Mariner 10's mission, between March 1974 and March 1975, NASA researchers revealed that, unlike the other rocky bodies in the solar system, Mercury has an extremely weak magnetic field. "Although Mercury's magnetic field is thought to be a miniature version of Earth's, Mariner 10 didn't measure it well enough to characterize it. These basic uncertainties in the nature of its magnetic field have led to uncertainties in understanding the source of the magnetic field. Messenger's magnetometer will characterize Mercury's magnetic field in detail, from orbit over Mercury four years to determine its exact strength and how its strength varies with position and altitude," said Solomon.

Images taken during Messenger's first fly-by reveal some rather intriguing troughs radiating from the Caloris basin, a crater on Mercury's surface about 1,550 kilometers in diameter. Solomon believes that further study of the planet with the Messenger spacecraft will uncover the nature of these features, as well as clarify why the planet seems to be so dense, the nature of its small atmosphere and the chemical composition of the material in the polar craters.