On March 29, 1974, Mariner-10 executed the first-ever flyby of Mercury, shortly after it executed the first-ever gravity-assist maneuver by which it used the gravity of Venus to accelerate(slingshot) toward Mercury.
Below is an artist's conception of Mariner-10 approaching Mercury and an actual image taken of Mercury by Mariner 10's camera.
Marvel at what scientists accomplished 43 years ago today and imagine what we'll be doing in another half century.