Venera 4 entered the Venusian atmosphere and radioed back the first in-situ measurements of another planet's atmosphere, revealing Venus to be shockingly hostile: heavy carbon dioxide, scorching heat, and pressure far beyond what Soviet engineers had designed the probe to survive. The capsule stopped transmitting partway down, crushed before reaching the surface, but the data reshaped scientific understanding of Earth's twin.
Soviet scientists had built Venera 4 expecting Venus to have a atmosphere perhaps ten times denser than Earth's; the probe's parachute and structure were engineered around that estimate. As it descended, the true numbers came back and kept getting worse: the atmosphere was over 90 times as dense as Earth's, composed almost entirely of carbon dioxide, with surface temperatures eventually understood to exceed 450 degrees Celsius, hot enough to melt lead.
The capsule's signal cut out at an altitude Soviet engineers initially hoped was near the surface; later analysis showed it had actually been crushed by pressure roughly 15 miles above the ground, having transmitted only a fraction of the descent. Even so, this was the first time any human-made object had returned atmospheric data from another world, arriving just one day before the American Mariner 5 flew past Venus, setting off a small controversy over whose mission deserved more credit for unraveling the planet's brutal, hellish nature.