29 March
1974: On 29 March 1974, the first probe to Mercury, Mariner 10, arrived at the planet.
Mariner 10 eventually made three flybys of Mercury from 1974 to 1975 before running out of attitude control gas. The probe revealed Mercury as a heavily cratered world with a mass much greater than thought. This would seem to indicate that Mercury has an iron core which makes up 75 percent of the entire planet.
