Mercury is a land of contrasts. The solar system’s smallest planet boasts the largest core relative to its size. Temperatures at noon can soar as high as 800 degrees Fahrenheit (425 degrees Celsius) — hot enough to melt lead — but dip as low as –290 F (–180 C) before dawn. Mercury resides nearest the Sun, and it has the most eccentric orbit. At its closest, the planet lies only 29 million miles (46 million kilometers) from the Sun — less than one-third Earth’s distance — but swings out as far as 43 million miles (70 million km). Its rapid movement across our sky earned it a reputation among ancient skywatchers as the fleet-footed messenger of the gods: Hermes to the Greeks and Mercury to the Romans.

Even though Mercury lies tantalizingly close to Earth, it is frustratingly hard to get to. Only two spacecraft have ever visited this barren world. But that is set to change October 19, when the international BepiColombo spacecraft begins a decade-long odyssey to unlock the secrets of a planet that seems to defy common sense.

The mission’s namesake — Italian scientist, mathematician, and engineer Giuseppe “Bepi” Colombo (1920–1984) — was instrumental in devising a means to deliver a spacecraft from Earth, via Venus, to Mercury. Scientists already knew that a planet’s gravitational field could bend the trajectory of a passing spacecraft and enable it to rendezvous with another celestial body. In the early 1970s, Colombo showed that if a spacecraft encountered Mercury, it would end up with a period almost twice that of the planet’s orbital period. He suggested that a precisely targeted flyby would present a possibility for an economical second encounter.

NASA confirmed the idea and used it to send the Mariner 10 spacecraft past the innermost planet three times. The probe encountered Mercury in March 1974, September 1974, and March 1975. Its photographs gave humanity its first close-up views of the world, and the last ones we would see for a generation.