The Venera (, , which means "Venus" in Russian) program was the name given to a series of space probes developed by the Soviet Union between 1961 and 1984 to gather information about the planet Venus.
Thirteen probes successfully entered the Venusian atmosphere, including the two Vega program and Venera-Halley probes. Ten of those successfully landed on the surface of the planet. Due to the extreme surface conditions on Venus, the probes could only survive for a short period on the surface, with times ranging from 23 minutes to two hours.
The Venera program established a number of precedents in space exploration, among them being the first human-made devices to enter the atmosphere of another planet (Venera 3 on 1 March 1966), the first to make a soft landing on another planet (Venera 7 on 15 December 1970), the first to return images from another planet's surface (Venera 9 on 8 June 1975), the first to record sounds on another planet (Venera 13 on 30 October 1981), and the first to perform high-resolution radar mapping scans (Venera 15 on 2 June 1983).
The Venera probes
Venera 1 and 2
The first Soviet attempt at a flyby probe to Venus was launched on 4 February 1961, but failed to leave Earth orbit. In keeping with the Soviet policy at that time of not announcing details of failed missions, the launch was announced under the name Tyazhely Sputnik ("Heavy Satellite"). It is also known as Venera 1VA.
As with some of the Soviet Union's other planetary probes, the later versions were launched in pairs, with a second vehicle launched soon after the first.
Venera 1 and Venera 2 were intended to fly past Venus without entering orbit. Venera 1 was launched on 12 February 1961. Telemetry on the probe failed seven days after launch. It is believed to have passed within of Venus and remains in heliocentric orbit. Venera 2 launched on November 12, 1965, but also suffered a telemetry failure after leaving Earth orbit.
Several other failed attempts at Venus flyby probes were launched by the Soviet Union in the early 1960s, but were not announced as planetary missions at the time, and hence did not officially receive the "Venera" designation.
Venera 3 to 6
The Venera 3 to 6 probes were similar. Weighing approximately one ton, and launched by the Molniya-type booster rocket, they included a cruise "bus" and a spherical atmospheric entry probe. The probes were optimised for atmospheric measurements, but not equipped with any special landing apparatus. Although it was hoped they would reach the surface still functioning, the first probes failed almost immediately, thereby disabling data transmission to Earth.
Venera 3 became the first human-made object to impact another planet's surface as it crash-landed on 1 March 1966. However, as the spacecraft's data probes had failed upon atmospheric penetration, no data from within the Venusian atmosphere were retrieved from the mission.
On 18 October 1967, Venera 4 became the first spacecraft to measure the atmosphere of another planet. This spacecraft first showed that the major gas of Venus' atmosphere to be CO2. While the Soviet Union initially claimed the craft reached the surface intact, re-analysis, including atmospheric occultation data from the American Mariner 5 spacecraft that flew by Venus the day after its arrival, demonstrated that Venus's surface pressure was 75–100 atmospheres, much higher than Venera 4's 25 atm hull strength, and the claim was retracted.
Realizing the ships would be crushed before reaching the surface, the Soviets launched Venera 5 and Venera 6 as atmospheric probes. Designed to jettison nearly half their payload prior to entering the planet's atmosphere, these craft recorded 53 and 51 minutes of data, respectively, while slowly descending by parachute before their batteries failed.
Around that time it became increasingly known that Venus was unlikely to have liquid bodies of water, however the designs for the Soviet Venera probes still considered the possibility of a water landing as late as 1964.