Mariner 10 

At maximum elongation, Mercury is only 28 degrees from the Sun, making observers only able to see it at daylight hours and short periods of time at dawn and dusk. Because of the difficulty in observing Mercury from Earth's telescopes, spacecraft were necessary to understand the planet in much greater detail. The Mariner 10 (launched in November 3, 1973) flew by Mercury three times in 1973 and 1974. On March 29, 1974, it flew by the planet at a distance of 705 kilometers from the surface. On September 21, 1974 and March 16, 1975, Mariner 10 flew by Mercury for its second and third visits, mapping out around 45% of the planet's surface. The rest of the planet remains uncharted due to the fact that the Hubble Space Telescope cannot safely image the planet due to its close range to the Sun.

On the visits to Mercury, Mariner 10 unexpectedly found a magnetic field. This indicates that the iron core inside of the planet is at least partially molten. It is important to understand the dynamo effect, which is "the generation of magnetic fields through the rotation of a conductive molten core the basis of many of the complex interactions such as the aurora that take place in our Solar System and throughout the Universe.