During the first half of the 20th century, the presence of clouds, the size and proximity of Venus suggest that the planet has conditions very close to that on Earth. It is accepted that the temperature is slightly higher than that on Earth, but from there, some imagine deserts, oceans, swamps or lush forests.
The first radar, infrared and ultraviolet measurements change our knowledge by revealing a large amount of CO2 unsuitable for animal life as known on Earth. Microwave measurements in 1956 estimated its temperature at 350°C, but it is possible that Venus’ ionosphere corrupted this data. So a multitude of scenarios on environmental conditions on Venus still collide at the launch of the first probes. Like all planets, probes can only be sent during windows and those for Venus take place every 1 year and 7 months.