Venera 5 and 6 were two identical spacecraft designed to penetrate Venus’ atmosphere and to transmit a variety of scientific data back to Earth during descent. Both spacecraft were targeted to reach Venus only a day apart, thus allowing some cross-calibration of data.
The bus was basically similar in design to the Venera 4 bus, but the 904-pound (410-kilogram) lander had some significant alterations. Because of data from Venera 4, the new lander was designed to survive pressures as high as 25 atmospheres and temperatures as high as 608 degrees Fahrenheit (320 degrees Celsius).
These changes forced a fairly significant increase in the structural strength of the lander, increasing its mass by 59.5 pounds (27 kilograms). This is why the mass of the spacecraft bus was reduced by about 31 pounds (14 kilograms) by removing the SG 59 magnetometer.
The main and drogue parachutes were also reduced in size, thus reducing the time to descend to the surface. The new lander weighed 904 pounds (410 kilograms) and was designed to endure g-loads as high as 450 (as compared to 300 for their predecessors).
After performing 73 communications sessions with ground control and completing one mid-course correction on March 14, 1966, Venera 5 approached the dark side of Venus on May 16, 1969 and detached its lander whose speed reduced from about 7 miles per second to 689 feet per second (11.17 kilometers per second to 210 meters per second) after it hit the Venusian atmosphere at 06:02 UT.
One minute later, controllers reestablished contact with the lander and began receiving data on pressure, temperature, and composition (sampled at least twice during descent) of the Venusian atmosphere for 52.5 minutes.
Contact was lost at an altitude of about 11 miles (18 kilometers) when the pressure exceeded 27 atmospheres. In other words, the probe probably cracked and became inert. Impact coordinates were 3 degrees south latitude and 18 degrees east longitude.
Information extrapolated from Venera 5’s data suggested that ground temperature and pressure at the Venusian surface was 140 atmospheres and 986 degrees Fahrenheit (530 degrees Celsius), respectively.
Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.