After passing Earth once and Venus twice, NASA’s MESSENGER is now set to orbit the Solar System’s innermost planet-Mercury.

If everything goes as expected, the craft will reach its goal on 18th March, dipping as close as 200 kilometres to Mercury’s surface, reports Nature.

“All of us are extremely excited to have reached this milestone and we are anxious to learn the secrets that Mercury will finally reveal to us,” said Sean Solomon, a geophysicist at the Carnegie Institution for Science in Washington DC and the mission’s principal investigator.

Earlier glimpses suggested that the planet is far from the inert lump of rock that scientists once imagined.

MESSENGER (Mercury surface, space environment, geochemistry and ranging) is only the second spacecraft to reach the planet. The first, Mariner 10, made three fleeting passes in 1974 and 1975 and glimpsed only 45 per cent of the planet’s surface.

The new mission has already transformed this view by imaging 98 per cent of Mercury during the 2008 and 2009 fly-bys. Once in orbit, MESSENGER will map the entire surface, recording its topography and composition on scales as small as a few tens of metres.

Over the one-year mission, MESSENGER will also measure changes in Mercury’s tenuous atmosphere of hydrogen, helium and metal ions, information that might reveal how the thin atmosphere is replenished as ions escape into space.

The oversized core is somehow responsible for Mercury’s weak but intriguing magnetic field. First observed by Mariner 10, the field could be a frozen remnant from a geologically active past. During its fly-bys, however, MESSENGER detected changes in the field, suggesting that some of Mercury’s core might be molten.

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