So, this is something that's got me wondering; The CCCP landed crafts on Venus, and more recently, NASA landed the Huygens on Titan. Both of these worlds are shrouded in clouds that block any visible light from coming back from the surface, though. But to send a lander, you obviously need some crucial data, like surface temperature, knowledge of the presence of corrosive elements (like sulpheric acid), surface pressure, air density at the surface for descent parachutes (or any landing system), if certain wavelengths of radar would work to get an altimetry measurement for descent, etc. So... To design these landers to work, they had to know a lot about the surface of these worlds that nothing had ever been to before, and that they couldn't see the surface of. How did they get this information? I understand IR light can pierce Titan's clouds, so I assume earlier fly-bys of unmanned missions could compare IR imagery of the surface with other wavelengths to measure the extent of the atmosphere, and from there spectral analysis and thermal readings could be used to build a model, but what about Venus? How did the Soviets know how to land there? I assume they would've had to know that Venus has a surface pressure of 90 bar and 900*F+ degrees surface temperature, and of course, that it actually had a solid surface that was reachable (ie, that the atmosphere didn't extend a thousand miles at 90+ bar, or even a hundred miles past the 1 bar mark, before reaching the surface). They also would've had to know roughly the surface air density to know terminal velocity and control final descent. So... How did they get all that information?... Assuming they did?