The Soviet Venera program, from 1961 to 1985, is one of the largest efforts ever undertaken to study another planet.
The fundamentals of interplanetary spacecraft design and remote sensing were first realized in the Venera missions.
Successes included 3 atmospheric probes, 10 landings, 4 orbiters, 11 flybys or impacts, and 2 balloon probes in the clouds.
Much of what is known today about Venus was discovered by these missions.
Venera-1
Venera-1: Inventing The Interplanetary Spacecraft (1957-1965)
Venera-4
Venera-4: Plumbing The Atmosphere (1967-1972)
Venera-9
Venera-9: Landers With Cameras (1975)