NASA’s MESSENGER spacecraft has become the first ever to orbit the planet Mercury. MESSENGER, which stands for MErcury Surface, Space ENvironment, GEochemistry and Ranging spacecraft, has been en route since its launch on 3 August, 2004. The long cruise came to a successful end today at 0:45 pm GMT, when MESSENGER fired its main thruster for approximately 15 minutes, reducing its speed enough to be captured by Mercury’s gravitational pull.

Although MESSENGER had been performing well during its journey, which included three fly pasts of Mercury as it gradually worked its way towards an approach to orbit, today’s success comes as a big relief for mission scientists well aware of the problems that can arise during such a maneuver. Just last December, for example, Japan’s Akatsuki probe failed in its bid to orbit Venus, and must spend six more years circling the Sun before it has a chance to try again.

MESSENGER has already returned interesting science during its previous flybys including images of an entire hemisphere of Mercury that was not observed by Mariner 10 – the only other spacecraft to approach the innermost planet – in the 1970’s. Prior to MESSENGER, Mercury’s unseen hemisphere was the largest contiguous piece of Solar System terrain not imaged by a spacecraft.

Now that it’s in orbit, MESSENGER will conduct a comprehensive one-year survey of the planet revealing topographic and compositional features as small as 18 metres across. It will address long standing puzzles about Mercury’s interior and early history and attempt to confirm radar studies that hint at the presence of ice within permanently shadowed craters near the north and south poles of the nearest planet to the Sun.

MESSENGER’s science mission is run out of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland.