The 1960s were a busy time for Venus-bound spacecraft with a large number of missions from both the United States and the Soviet Union, unfortunately most of them either failed to launch or did not make it to Venus. It has to be remembered that these were the very early years of the space race and a lot was learned from those early failures that resulted in the later successes and ultimately the Apollo programme to the Moon at the end of the decade. Mariner 2 launched 27 August 1962 and on the 14 December passed as close as 34,773km to Venus. Mariner was the first interplanetary mission and it conducted a number of ground breaking obeservations including measurement of cosmic articles from outside of the Solar System. The mission also measured the temperature of the cloud tops of Venus as well as the surface temperature. The mission was able to confirm Earth-based measurements that Venus has a very high temperature in the upper atmosphere reaching 500 degrees Celsius. The spacecraft had a number of science missions including a radiometer, infrared, magnetometer, charged particle, plasma and micrometeorites experiments. The magnetometer detected no magnetic field around the planet.
Venera 4 and Mariner 5 were the next successful missions. Venera launched on 12 June 1967 and entered the atmosphere of Venus on 18 October 1967. They analysed some of the attributes of the lower atmosphere as the probe descended to the surface. The final temperature reading was 262 degrees and the pressure was recorded at 22 standard atmospheres. The atmospheric composition was also very interesting with 90-93% carbon dioxide, 7% nitrogen and oxygen only making up about 0.4-08%. On entry into the atmosphere the heat shield on the probe got as high as 11,000 degrees Celsius.
Mariner 5 was basically the same as Mariner 4 but with more sensitive instruments. It was launched 14 June 1967 and did its closest flypast of Venus on 19 October that year. In January 1969 the Soviets also launched Venera 5 and 6 which basically confirmed the findings of Venera 4 though with a bit more sensitivity. Both of these spacecraft released probes that parachuted through the Venusian atmosphere where they operated for 53 and 51 minutes respectively.
In 1970 the Soviets launched Venera 7 with a lander that made it to the surface but rolled and didn’t return very useful data. Then finally on 22 July 1972 Venera 8 made it to Venus with a successful landing. This probe operated for about 50 minutes and sent back valuable information about the surface conditions. It measured temperatures as high as 470 degrees and a pressure of 90 standard atmospheres. Under about 10km in altitude the extremely high winds of the atmosphere subsided to around 1 metre per second. Anther interesting aspect that the Venera lander showed was the cloud base of the atmosphere was quite high and the air was relatively clear nearer the surface. The next attempt by the United States was with Mariner 10, which was launched on 3 November 1973 and successfully made it to Venus on the 5 February 1974, taking some fantastic images of the planet. This mission was to measure attributes of both Mercury and Venus so after a successful trip to Venus it went to capture outstanding images of Mercury. Whilst passing Venus, Mariner 10 measured the weak magnetic field of the planet and was able to observe cloud rotation as well.
The next big step in our understanding of Venus came with the Soviet landers that managed to return pictures of the surface. Nothing satisfies our hunger more than actual pictures of an alien world and the pictures of Venus are remarkable. Both Venera 9 and then Venera 10 sent back the first images of Venus’ surface. They also both recorded the large area of clear atmosphere between the very high cloud base, 30-35km, and the surface. Both spacecraft were launched in June 1975 and successfully got their landers to the surface. Considering the massive pressure and extraordinary high temperatures it is remarkable that we got any pictures at all. Don Mitchell did some great work in putting together the Soviet images that were recovered, you can see them on his website.
The 1980s also had many more missions including the radar mapping of the surface. I suppose the interest in Venus started to wain because of the realisation that the planet was rather inhospitable for life and there was no real chance that humans could ever successfully survive there with current technology.
Akatsuki is the only mission that is currently looking at Venus and there are no more missions on the horizon so the pictures we have from the Soviet missions of the 70s and 80s are all we might have for quite some time.