Mariner 10 swept past Mercury for the third time, closing out humanity's first reconnaissance of the solar system's innermost planet. Launched in 1973, the spacecraft used a pioneering gravity-assist maneuver past Venus to reach Mercury and photographed roughly 45 percent of its cratered, moon-like surface before its attitude-control gas ran out.
Mariner 10 was a spacecraft of firsts: the first to use a gravity assist from one planet (Venus) to reach another (Mercury), and the first and, for over three decades, only probe to visit Mercury at all. Launched in November 1973, it made its initial Mercury flyby in March 1974, revealing a heavily cratered world startlingly similar in appearance to Earth's Moon, along with a surprising, faint magnetic field that puzzled scientists expecting a geologically dead planet.
By the time of its third and final Mercury encounter on March 16, 1975, Mariner 10 had already made an unplanned bonus close approach in September 1974 thanks to clever orbital mechanics that returned it to Mercury without additional fuel. Engineers, running low on the attitude-control gas needed to keep the craft's cameras and antenna properly oriented, used the last flyby to gather as much high-resolution imagery as possible of the planet's surface and magnetic environment.
Eight days later, its supply of attitude-control gas exhausted, ground controllers switched off Mariner 10's transmitter, leaving the derelict probe in a solar orbit that likely still carries it past Mercury every few months, silent forever. It would not be until NASA's MESSENGER mission entered Mercury orbit in 2011 that scientists got another close look at the planet.