How Did The CCCP See Past Venus' Clouds Prior to Landing?

[MattRob]

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?

 

[jedishrfu]

How about radar?

 

[Bandersnatch]

First Venus-bound probes were of one of two kinds: "fly by close enough to sneak a peek", or "drop it down and see what happens". Each successive(and successful) mission brought new data that informed further designs.

Best thing to do, is go here: http://solarsystem.nasa.gov/missions/profile.cfm?Sort=Target&Target=Venus&Era=Past
and read on each mission. Then click on "read more" down the page, to see given probe's details. Then google the instrumentation listed for relevant information on how those work. E.g., the first successful mission(US Marnier 2) will point you to microwave an IR interferometers, iirc.

 

[lpetrich]

In "The Cosmic Connection", Carl Sagan recalled an amusing interchange about the fate of one of the spacecraft with a colleague of his in the Soviet Union, A.D. Kuzmin.

In 1968, the Venera 4 spacecraft entered Venus's atmosphere, and its last data reading that it radioed back was 450 F (230 C) and 20 atmospheres. Soviet scientists concluded that this was what Venus's surface was like. However, Carl Sagan and some of his colleagues decided that Venus's surface must have been far below that point, because the spacecraft 's radioed temperature and pressure were much less than what they had estimated for those quantities. That was from Mariner 5's data and various ground-based radio and radar observations.

At an astronomers' meeting CS proposed that Venera 4 stopped transmitting about 15 miles (25 km) up from the surface. ADK argued that it had indeed landed. CS noted that there was a lot of evidence that Venus's surface was a long way down from that last transmission point. ADK responded that Venera 4 had landed on a high mountain. CS noted that there was not much evidence for mountains that high, and that it was very improbable that Venera 4 had landed on its peak or nearby. ADK asked CS how likely it was that the first German bomb dropped on Leningrad would kill the only elephant in the Leningrad Zoo. CS conceded that that was very unlikely, and ADK responded that that was indeed what had happened.

Later Soviet spacecraft designers decided not to press their luck, and they made their spacecraft much stronger. Venera 7 could resist as much as 180 times the Earth's surface pressure. It transmitted 20 minutes of data before getting fried. Venera 8 transmitted even longer.

 

[MattRob]

First Venus-bound probes were of one of two kinds: "fly by close enough to sneak a peek", or "drop it down and see what happens". Each successive(and successful) mission brought new data that informed further designs.

Best thing to do, is go here: http://solarsystem.nasa.gov/missions/profile.cfm?Sort=Target&Target=Venus&Era=Past
and read on each mission. Then click on "read more" down the page, to see given probe's details. Then google the instrumentation listed for relevant information on how those work. E.g., the first successful mission(US Marnier 2) will point you to microwave an IR interferometers, iirc.

Ooh, very nice little archive there. I have to say I'm surprised - I thought the CCCP were the first to Venus. I think they were the only ones to land there, though, but kind of neat to see NASA was the first one there.

In "The Cosmic Connection", Carl Sagan recalled an amusing interchange about the fate of one of the spacecraft with a colleague of his in the Soviet Union, A.D. Kuzmin.

In 1968, the Venera 4 spacecraft entered Venus's atmosphere, and its last data reading that it radioed back was 450 F (230 C) and 20 atmospheres. Soviet scientists concluded that this was what Venus's surface was like. However, Carl Sagan and some of his colleagues decided that Venus's surface must have been far below that point, because the spacecraft 's radioed temperature and pressure were much less than what they had estimated for those quantities. That was from Mariner 5's data and various ground-based radio and radar observations.

At an astronomers' meeting CS proposed that Venera 4 stopped transmitting about 15 miles (25 km) up from the surface. ADK argued that it had indeed landed. CS noted that there was a lot of evidence that Venus's surface was a long way down from that last transmission point. ADK responded that Venera 4 had landed on a high mountain. CS noted that there was not much evidence for mountains that high, and that it was very improbable that Venera 4 had landed on its peak or nearby. ADK asked CS how likely it was that the first German bomb dropped on Leningrad would kill the only elephant in the Leningrad Zoo. CS conceded that that was very unlikely, and ADK responded that that was indeed what had happened.

Later Soviet spacecraft designers decided not to press their luck, and they made their spacecraft much stronger. Venera 7 could resist as much as 180 times the Earth's surface pressure. It transmitted 20 minutes of data before getting fried. Venera 8 transmitted even longer.

Heh, that's pretty funny.

I guess what I'm really wondering at, though, is precisely how they figured this out. Radar can tell you the distance to a solid surface, then I guess using spectral analysis they could determine the composition, and thus molecular properties of the atmosphere. Then use IR to determine temperature, then other kinds of imagery to get a ballpark figure of density at a given altitude, then they build a model from there that matches the IR surface temperature readings?

Something like that is what I'm wondering about. How Sagan and his colleagues got their estimates for what Venus' surface must be like.

 

[lpetrich]

Observations and explorations of Venus - Wikipedia, http://www.agu.org/books/gm/v176/176GM12/176GM12.pdf, http://solarsystem.nasa.gov/missions/profile.cfm?Sort=Alpha&Letter=M&Alias=Mariner%2002, http://solarsystem.nasa.gov/missions/profile.cfm?Sort=Alpha&Letter=M&Alias=Mariner%2005, Mariner 2 - Wikipedia, Mariner 5 - Wikipedia, Venera 4 - Wikipedia, Venera 7 - Wikipedia

One can observe Venus with a radio telescope, and from its radio brightness deduce its surface temperature. In 1957, Mayer et al. found 620K ± 100 K at 3.15 cm, though A.D. Kuzmin found a lower temperature in millimeter-wave observations.

There was a big argument around 1960 about whether Venus's radio emissions were due to the planet having a hot surface or else due to some nonthermal mechanism in the planet's upper atmosphere.


In 1962, the Mariner 2 spacecraft observed Venus with a microwave radiometer at 19 mm and an infrared radiometer at 8.4 μm and 10.4 μm.

The microwave results:
Dark side: 490 ± 11 K
Terminator: 595 ± 12 K
Light side: 511 ± 14 K
A bit of "limb darkening", what would happen for going outward through an absorbing part of the atmosphere or a colder one.

The infrared results were nearly constant over Venus's surface and consistent with ground-based measurements.

In 1967, Mariner 5 flew by, and from flying behind Venus relate to the Earth, it was possible to get some clues about Venus's atmosphere from its radio transmissions. From its density, one could find a pressure of 75 to 100 atmospheres, and from the likely temperature profile a surface temperature of about 800 K.


In 1970, Venera 7 landed on Venus and radioed back a surface temperature of 750 K. That finally settled the question of Venus's surface temperature.