After entering Mercury's orbit in March 2011, the MESSENGER probe finished its first full mapping campaign around this date, capturing images and data covering nearly the entire planet's surface for the first time. The mission revealed a shrinking, wrinkled world with a surprisingly large iron core and traces of water ice hidden in permanently shadowed polar craters.
Mercury had long been the least-explored terrestrial planet simply because it is so hard to reach — the sun's gravity makes braking into orbit around it fiendishly difficult, requiring years of looping trajectories past Venus and Mercury itself just to slow down enough. MESSENGER, launched in 2004, finally pulled off the feat in March 2011 after a six-and-a-half-year, 4.9-billion-mile journey.
Once in orbit, its cameras and spectrometers filled in gaps left by Mariner 10's brief flybys in the 1970s, which had only ever photographed about 45 percent of the surface. MESSENGER's completed map showed a planet crisscrossed by odd, spidery cracks called 'Pantheon Fossae' and craters holding frozen water despite Mercury's status as the sun's nearest neighbor.
The spacecraft continued observing Mercury for four more years before mission controllers deliberately crashed it into the planet's surface in 2015, when it ran low on fuel — a fittingly dramatic end for a mission that had spent a decade chasing the sun.