On 15 December 1970, a Soviet probe called Venera 7 reached the surface of Venus and kept transmitting. It was the first time any human-made object had returned data from the surface of another planet. The transmission lasted about 23 minutes before the probe fell silent.

The achievement was almost missed. Venera 7’s parachute failed shortly before landing. The probe struck the surface at around 17 metres per second, roughly 60 kilometres per hour, and came to rest on its side. The hard landing left its antenna pointing the wrong way, and the signal that reached Earth was very weak. Controllers initially concluded that the lander had died on impact.

What changed the assessment was a careful re-examination of the recorded radio signal. Specialists going back through the tapes separated a faint trace from the background noise. It carried temperature telemetry, and it showed that Venera 7 had continued transmitting from the surface for about 23 minutes after touchdown. The mission had succeeded. It had simply succeeded quietly.

What came before it

Venera 7 was not a first attempt. It was the seventh numbered probe in a Soviet program that had been trying to reach Venus through most of the 1960s, and the record up to that point was largely a record of failure.

According to Encyclopaedia Britannica’s account of the program, radio contact was lost with Venera 1 in 1961 before it reached the planet. Venera 3 struck the surface in 1966, becoming the first spacecraft to hit another planet, but returned no data. Venera 4 sent back the first direct measurements from inside the Venusian atmosphere in 1967. Venera 5 and 6, atmospheric probes launched in 1969, were destroyed by heat and pressure before they could reach the surface.

Venera 7 was the design built to get the rest of the way down. Its descent capsule was a reinforced sphere, pre-cooled before atmospheric entry and built to hold together under conditions that had crushed its predecessors. It was heavier and more thermally protected than the probes before it, at the cost of carrying fewer instruments.

What the surface is like

The reason Venus is hard on hardware is not subtle. The surface temperature is close to 475 degrees Celsius. The atmospheric pressure at the surface is roughly 90 times that at sea level on Earth, comparable to the pressure about 900 metres underwater. The atmosphere is almost entirely carbon dioxide, with clouds of sulfuric acid above.

Venera 7 carried instruments to measure temperature and pressure, and that is most of what it was able to report. The 23 minutes of surface transmission ended not with a single dramatic failure but as the probe’s batteries gave out under the thermal load. There was no way to shed the heat. Every Venus surface mission since has run into the same limit. The planet starts a clock the moment a lander arrives, and the clock is short.

The record that still stands

The Venera program continued. Venera 8 returned data in 1972. Venera 9 and 10, in 1975, sent back the first photographs ever taken from the surface of another planet, with Venera 9 transmitting for about 53 minutes and Venera 10 for about 65 minutes. Venera 13 and 14 returned colour images of the surface in 1982. The twin Vega spacecraft deployed Venera-style landers in 1985. The Soviet Union remains the only space program to have placed working landers on Venus and the only one to have returned images from its surface. The full visual record of Venus’s surface still comes from only four Soviet landers.

No currently approved Western mission is a long-duration Venus surface lander in the Venera sense. The next wave of Venus exploration is being planned mostly from above. NASA’s DAVINCI mission would send a descent probe through the atmosphere, sampling temperature, pressure, winds and chemistry on its way down to the surface, rather than operating as a long-duration lander. The European Space Agency’s EnVision and NASA’s VERITAS are both orbiters. All three have been subject to shifting timelines and budget pressure, and none would do what the Venera landers did.

That is the context in which Venera 7’s 23 minutes still sit. More than half a century on, the surface of Venus has been reached by working spacecraft from one program only, and the next attempt to stand on it has not yet been built.