Mercury can be easy to overlook. As the closest planet to the Sun, it’s hard to spot on its fleeting visits to our skies. Over the past few decades, M however, our view of the innermost planet has been transformed by NASA’s MESSENGER space probe. The first spacecraft to orbit Mercury revealed that this mysterious world has a complex history of its own, with a distant volcanic past, a core much larger than that of any other planet relative to its size and an active magnetic field. But perhaps most intriguing of all is the evidence that this tiny planet has shrunk considerably since it formed.
“Mercury is the least understood of our Solar System’s four terrestrial worlds because until relatively recently it was extremely hard to image or visit,” says Dr Paul Byrne of Washington University in St Louis. “It’s deep in the Sun’s gravity well, and we simply didn’t know how to get a spacecraft into orbit around it until 1985.” As a result of these challenges, most of our information about Mercury came from a single NASA probe that made three flybys of the planet in 1974 and 1975. Mariner 10 flew in a solar orbit that intercepted Mercury’s, but the geometry of the two orbits meant that its encounters only revealed a little less than half of the planet’s surface.