Mariner 10 swept within 703 kilometers of Mercury's surface, becoming the first spacecraft to visit the planet and returning over 2,700 images that revealed a heavily cratered, moon-like world. The flyby also discovered Mercury's surprisingly strong magnetic field, a finding that puzzled scientists for decades given the planet's small size and slow rotation.
Launched in November 1973, Mariner 10 used a pioneering gravity-assist maneuver around Venus to reach Mercury, the first time such a technique had been employed in interplanetary navigation—a trick that became standard practice for later missions to the outer planets. Its cameras revealed a cratered surface eerily similar to Earth's Moon, along with vast cliff-like structures called lobate scarps, evidence that the planet had shrunk as its iron core cooled.
The discovery of Mercury's magnetic field, roughly 1 percent as strong as Earth's, stunned researchers who had assumed the planet's core was solid and inert. It would take until the MESSENGER mission decades later to explain the field fully, tying it to a partially molten core.
Mariner 10 went on to make two more flybys of Mercury in 1974 and 1975 before running out of fuel, and its imagery, though covering less than half the planet, remained the primary source of Mercury data for over 30 years until MESSENGER's arrival in 2008.