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This is a question about early planetary missions. It looks like there was only one early (unsuccessful) landing mission to Mars and that subsequently Venus became the target for interplanetary landings. Here's my question:

Why was Venus prioritized when (at least with modern technology) the surface of Mars seems way easier to explore, and e.g. send back amazing images? Does this have something to do with the difficulty of getting there? Something with the fact that we didn't know about the extremeness of Venus' atmosphere? Or was there a non-technology related reason that deemed Venus as the more exciting destination?

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The reason is delta-v, which is a crucial concept in Spaceflight. It means change in velocity, and is the primary 'currency' that space mission have to expend in order to reach places in the solar system. On earth, if you want to go anywhere, you can get there at any speed, it just takes longer. Unfortunately, that is not how it works in space, because the body you come from and the body you want to reach both cycle the sun at very large speeds. After getting into orbit, you need to accelerate by another 3.21 kilometers per second to escape the gravity influence of the earth completely.

In order to get to Mars from this point, you would need to accelerate another 1.06 km/s, so that the orbit of your spacecraft crosses the orbit of Mars. Alternatively, to achieve the same with Venus you would need to accelerate by only 640 meters per second. Already, it is cheaper to go to Venus than Mars, but it gets better:

As I said above, your spacecraft's orbit now crosses the orbit of your target planet in one point. However, the orbital speed is still quite different. With Mars, the spacecraft would crash right into the surface, and you need to accelerate another 4.28 kilometers per second in order to reach the surface safely*. If you went to Venus, you can enter the planet's dense atmosphere to cushion the impact. As a result, you don't need to carry any rocket fuel to make a landing.

This makes Venus a low hanging fruit. At a favorable constellation, Venus is the planetary body that takes the least delta-v to reach. That even includes our moon. Heck, it takes less delta-v to reach than geostationary orbit.

In fact, the Russian Venera Probes slammed right into the Venerean atmosphere from an interplanetary trajectory (11km/s). This caused a deceleration exceeding 300G and a heat shield temperature of 11,000 °C, but that didn't destroy those probes.

The diagram below shows different places in the solar system and rough estimates of how much delta-v you need to reach them. It all still depends on the exact constellation and the maneuver you use, so use these as ballpark estimates.

enter image description here

Copyright as noted in the image.

*Actually a bit less, because Mars also has some atmosphere that can help you with the landing.

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    $\begingroup$ The little arrows mean that, as Dietrich Epp correctly pointed out, that aerobraking is available. 27000 is a very theoretical value, that takes into account the losses of a rocket escaping the thick atmosphere of Venus $\endgroup$ – Rikki-Tikki-Tavi Apr 4 '16 at 21:04
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    $\begingroup$ Think about the enormous rockets you need just to get from Earth's surface to LEO. That's the 9400 right at the bottom of the diagram. But you clearly don't need those enormous rockets to get from LEO back to the surface. You don't event need rockets 10% of the size. You just need to nudge yourself into the atmosphere and let aerobraking take care of the rest. Same with Venus. $\endgroup$ – BenM Apr 4 '16 at 21:49
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    $\begingroup$ Well, I'm rather confused now about what this data even means, tbh. Rather than continue in the comments here, I've decided to open another question, because I am that confused by it at this point. $\endgroup$ – Octopus Apr 4 '16 at 22:36
  • $\begingroup$ Here's your citation: Does a mission to Venus orbit require less propellant than a similar mission to Mars? $\endgroup$ – Mazura Apr 8 '16 at 3:55
  • $\begingroup$ More frequent launch windows and shorter flight time could also have been considered when time was of the essence during the space race. No one knew how hostile Venus surface was. $\endgroup$ – LocalFluff Apr 8 '16 at 11:20
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We didn't know how hostile Venus's surface was, until we had landed there. The atmosphere of Venus makes it easier to land there than Mars. From Wikipedia, we learn:

Before radio observations in the 1960s, many believed that Venus contained a lush, Earth-like environment.

While there was some concept that Venus was hot, and had a high pressure, the exact amount as to how hostile it was wasn't really known until we had sent a lander there.

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    $\begingroup$ Many early (40s and 50s) Sci-fi had human colonies on Venus, described as having a tropical atmosphere. $\endgroup$ – GdD Apr 4 '16 at 16:28
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    $\begingroup$ Public perception is a powerful thing. If Venus is popular or cool in the public mind, it will be high on the list. $\endgroup$ – wedstrom Apr 4 '16 at 18:15
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    $\begingroup$ Wow, 455 °C to 475 °C and 75 to 100 atmospheres of pressure (I think that means 1,100 to 1,400 PSI) that's pretty hostile compared to the inside of an oven, would burn and crush my dinner $\endgroup$ – Xen2050 Apr 4 '16 at 18:30
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    $\begingroup$ Venus is quite nasty. But we didn't really know how nasty it was until we landed something there, so... $\endgroup$ – PearsonArtPhoto Apr 4 '16 at 18:31
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    $\begingroup$ The early craft were not sent unprepared! That it was hot enough to melt solder was known during the planning stages. radio observations, remember, not touching it and saying "ow". $\endgroup$ – JDługosz Apr 4 '16 at 22:02
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One picture is worth all your base to us.

Before we sent probes to Venus we had no pictures of its surface. Whether it lands or not you have to penetrate the atmosphere of Venus to take pictures of it, whereas the surface of Mars can be easily seen through its weak atmosphere from afar.

Most of the information about Venus has been derived from the intensive Soviet study of the planet. The only existing images from the surface were returned from four of their landing craft.unbelievable-facts.com

It should be noted that the explorations of Venus and Mars both began ca. 1960. Venus is just the one that you have to land on (soft or hard) to gather any meaningful data*:

enter image description here

** Pics, or it didn't happen.

Although I can't rule out militaristic reasoning, I think it's more about the Space Race as a whole. During which, other than landing twelve people on the moon, the Soviet space program (notable firsts) beat the Americans' at nearly every conceivable turn. Including as a shining example: to this date theirs is the only country on Earth to ever take pictures of Venus' surface.

We hadn't seen it before nor have we seen it since. That's pretty exciting to me. CCCP-1, USA-0.

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    $\begingroup$ I don't think it's arguable that the Soviet space program beat the Americans. Leaving out the moon, there were successful US Mars landers & orbiters (Soviet ones all basically failed, IIRC), the Pioneer & Voyager missions to the outer planets (no Soviet equivalent that I know of), Mariner 10 to Mercury, and successful Venus orbiter and landers (though they didn't take pictures): en.wikipedia.org/wiki/Pioneer_Venus_Multiprobe Indeed, going by Wikipedia's list of space probes, about the only thing the Soviets led in was failed missions. $\endgroup$ – jamesqf Apr 5 '16 at 4:59
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    $\begingroup$ @jamesqf "The Space Race [1957-1975] spawned pioneering efforts to launch artificial satellites, unmanned space probes of the Moon, Venus, and Mars, and human spaceflight in low Earth orbit and to the Moon." Voyger1 was launched in 1977; a day late and a dollar short. Besides, I'm counting achievements not failures. Here's a rather impressive list of the Soviet's Notable Firsts. $\endgroup$ – Mazura Apr 5 '16 at 5:09
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    $\begingroup$ Why, the OP asks? "... not because they are easy, but because they are hard." $\endgroup$ – Mazura Apr 5 '16 at 5:20
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    $\begingroup$ On July 17, 1975 the Space Race ended when an astronaut and a cosmonaut made "the first international handshake in space." The Soviet list goes on up till 1987. I'm having trouble finding a similar list for NASA's firsts... $\endgroup$ – Mazura Apr 5 '16 at 5:58
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    $\begingroup$ @jamesqf, first satellite in orbit? USSR. first living being in space? USSR. first man in space? USSR. first space walk? USSR. first satellite to orbit the moon? USSR. first soft lunar landing? USSR. first lunar mission to return samples to earth? USSR. first manned mission to the moon? USA (yay finally!! good work USA) $\endgroup$ – Octopus Apr 5 '16 at 23:12

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