The Messenger probe is nearing the halfway point of 7.9 billion kilometre journey, which when completed will make it the first man-made object to orbit the planet Mercury.

During the craft's first three years and three months in space since its launch in August, 2004, Messenger has flown by Earth once and Venus twice. Now Messenger is nearing the goal of its mission, when it will pass the closest planet to the Sun three times before attempting to lock into orbit.

Mariner 10 also flew past the planet three times but was only able to photograph 45 per cent of the surface and carried out no other scientific investigation. Messenger's instruments have a few more tricks up its sleeve.

Among the goals of the mission are mapping the elemental and mineralogical composition of Mercury's surface; global imaging of the surface at a resolution of hundreds of meters or better; determining the structure of the planet's magnetic field; and measuring its gravitational field structure.

"The halfway point for Messenger's cruise phase is more than a statistical milestone, because in less than two months we'll have our first close-up view of Mercury," said Sean Solomon, head investigator of the Messenger mission in a statement. "From then until the end of the mission, we'll be peeling back Mercury's mysteries, many of which have perplexed the planetary science community for more than three decades." ®

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