First spacecraft to visit Mercury

The first spacecraft to encounter the planet Mercury was NASA's Mariner 10 robotic probe, which made its first fly-by of Mercury on 29 March 1974. It would go on to make two further passes close to the planet – one on 21 September 1974 and one on 16 March 1975. That last encounter brought the probe to within 327 km (203 mi) of the planet's surface. After completing its mission, the probe was powered down. It remains in orbit around the Sun to this day.

Launched on 3 November 1973, Mariner 10 was the last mission in the decade-long Mariner program, which had already sent fly-by probes and orbiters to Mars and Venus. Like several of its predecessors, it first cruised to Venus to study the planet's atmosphere. Instead of overshooting the planet and drifting into a heliocentric orbit afterwards, Mariner 10 manoeuvred past Venus on a path that used the planet's gravity to pull it inward towards the previously unexplored Mercury.

Gravity-assist manoeuvres like this had been done within the Earth-Moon system, but this was the first time a spacecraft had done something so challenging. The fact that the probe made it to Mercury is all the more impressive considering that mission control was battling technical problems all the way there. Electrical components failed, the antenna kept malfunctioning and the spacecraft's shiny paint kept chipping off. This last issue seems minor, but it was a major threat to the mission as the reflective flecks of paint would confuse the automatic star-tracking systems Mariner 10 used for navigation.

Mariner 10 took more than 2,800 photos of Mercury, mapping its barren and moon-like surface for the first time. Its instruments revealed the presence of a thin and unstable atmosphere, as well as an unexpectedly large iron core with a corresponding magnetic field. Owing to the extreme difficulty of reaching Mercury from Earth, Mariner 10 remained the only spacecraft to journey to the planet until the launch of the MESSENGER orbiter in 2004.