Venera 7 (Венера-7) was a Soviet spacecraft, part of the Venera series of probes to Venus. When it landed on the Venusian surface on 15 December 1970, it became the first spacecraft to soft land on another planet and the first to transmit data from there back to Earth.
The lander was designed to be able to survive pressure of up to and temperatures of . This was significantly greater than what was expected to be encountered but significant uncertainties as to the surface temperatures and pressure of Venus resulted in the designers’ opting for a large margin of error. The degree of hardening added mass to the probe which limited the amount of mass available for scientific instruments both on the probe itself and the interplanetary bus. The interplanetary bus carried a solar wind charged particle detector and a cosmic ray detector. On the lander there were temperature and pressure sensors as well as an accelerometer to measure atmospheric density. The probe also carried a radar altimeter.
The probe was launched from Earth on 17 August 1970, at 05:38 UTC. It consisted of an interplanetary bus, based on the 3MV system, and a lander. During the flight to Venus, two in-course corrections were made using the bus' on-board KDU-414 engine.
Venera 7 entered the atmosphere of Venus on 15 December 1970. The lander remained attached to the interplanetary bus during the initial stages of atmospheric entry, to allow the bus to cool the lander to −8 °C (17 °F) for as long as possible. The lander was ejected once atmospheric buffeting broke the interplanetary bus's lock-on with Earth. The parachute opened at a height of , and atmospheric testing began with results showing the atmosphere to be 97% carbon dioxide. The parachute was initially reefed down to , opening to 13 minutes later, when the reefing line melted as designed. Six minutes after the unreefing, the parachute started to fail, resulting in a descent more rapid than planned. The parachute eventually failed completely, and the probe entered a period of freefall.
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Venera 4 (Венера-4), also designated 4V-1 No.310, was a probe in the Soviet Venera program for the exploration of Venus. The probe comprised a lander, designed to enter the Venusian atmosphere and parachute to the surface, and a carrier/flyby spacecraft, which carried the lander to Venus and served as a communications relay for it. In 1967, it was the first successful probe to perform in-place analysis of the environment of another planet.
The atmosphere of Venus is primarily of supercritical carbon dioxide and is much denser and hotter than that of Earth. The temperature at the surface is 740 K (467 °C, 872 °F), and the pressure is , roughly the pressure found underwater on Earth. The Venusian atmosphere supports opaque clouds of sulfuric acid, making optical Earth-based and orbital observation of the surface impossible. Information about the topography has been obtained exclusively by radar imaging. Aside from carbon dioxide, the other main component is nitrogen.