MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging) was the seventh Discovery-class mission, and the first spacecraft to orbit Mercury. NASA's Messenger probe is set to make history tomorrow night (March 17) when it becomes the first spacecraft ever to enter into orbit around the planet Mercury.

5 Mercury Secrets Revealed by MESSENGER After two years of doing the loop-the-loop around Mercury, MESSENGER has unveiled a bunch of surprises from Mercury — the closest planet to the Sun. It was the first mission to Mercury after Mariner 10, more than 30 years before. Five of Messenger’s Discoveries About Mercury APRIL 27, 2015 Once thought to be a dull round rock with craters, Mercury turned out to be much more interesting.

Its primary goal was to study the geology, magnetic field, and chemical composition of the planet. But the Mercury Surface, Space Environment, Geochemistry, and Ranging probe, aka MESSENGER, was a game changer, for it orbited Mercury for several years. In 2012, MESSENGER discovered that Mercury's core is probably about 85 percent of its radius, which is an immense fraction compared to Earth's core, which is 50 percent of the planet's radius. "Messenger completed the job of mapping the surface of Mercury, and it did so in amazing detail," says Prentice. With this long-term exploration, our little rocky planet had the mysterious veil that surrounded it lifted and has proven to be just as fascinating a place as any other in the solar system. NASA's Mariner 10 spacecraft was only able to image one side of the planet.