A spacecraft with a second life
Mariner 5 began as a backup. NASA had built two Mariner craft for the Mars flyby mission of 1964, and the spare, once the mission of Mariner 4 had succeeded, was not needed. The agency chose to modify it for Venus. Changes were required because Venus is closer to the Sun than Earth, and the spacecraft would be exposed to much stronger sunlight. Engineers adapted the thermal control and added sun-shielding for the journey.
It was launched on 14 June 1967 and flew on a path that carried it past Venus on 19 October 1967, passing at a distance of roughly 4,000 kilometres from the planet. It was a flyby, not an orbiter: the spacecraft had one pass and then continued in orbit around the Sun.
Why Venus was such a puzzle
In the 1960s, Venus was close to a mystery. Its thick clouds hid the surface from telescopes, and ideas about conditions below them ranged widely. Some scientists thought the planet might be warm and swampy, and others thought it might be a hot desert. Earlier, in 1962, Mariner 2 had passed Venus and indicated that the planet was very hot, but its measurements left many questions open.
By 1967 radio observations from Earth also suggested high temperatures, though the interpretation was debated. Mariner 5 was meant to add direct measurements from close range.
How it measured the atmosphere
The craft carried several instruments, but its best-known result came from a radio experiment. As Mariner 5 went behind Venus, as seen from Earth, its radio signal passed through the planet’s atmosphere before being cut off by the planet itself. The way the signal was bent and weakened, and how that changed as the spacecraft moved in and out of view, allowed scientists to estimate the density and temperature of the gas at various heights. This technique is called radio occultation.
Other instruments looked for a magnetic field and for charged particles near the planet, and an ultraviolet instrument studied the very high atmosphere.
What the results suggested
The findings pointed to a hot, dense atmosphere. Analysis of the radio data indicated surface pressures many times those of Earth, with figures often quoted in the range of roughly 75 to 100 times Earth’s sea-level pressure, though the uncertainty was large. The atmosphere was concluded to consist mainly of carbon dioxide. Temperatures at the surface were thought to be several hundred degrees Celsius, far too hot for liquid water.
Mariner 5 found no strong magnetic field of the type Earth has, suggesting that Venus did not have a comparable internally generated one. It also detected an extended layer of hydrogen high above the planet.
Taken together, the results ended many of the more hopeful ideas about a gentle Venus.
The Soviet comparison
One day before Mariner 5’s pass, the Soviet probe Venera 4 released a capsule that descended by parachute through the atmosphere, transmitting readings as it went. It was the first time a probe had sent data from within another planet’s atmosphere. Venera 4 reported mostly carbon dioxide too. At first the Soviet team believed the capsule had reached the surface, but its reported pressure was lower than Mariner 5’s figures implied.
The discrepancy was puzzling at the time. Scientists compared the two sets of results, and the explanation that came to be accepted was that Venera 4 had stopped transmitting while still above the surface, at a height where the pressure was lower than the surface pressure. That reconciled the two sets of numbers.
The Soviet Union and the United States competed intensely in the broader space race, but this episode shows the two programmes producing complementary data about the same target, within a day of one another.
What happened next
Mariner 5 continued to transmit for a limited time after the flyby, and contact ended not long afterward. The findings shaped later missions, as the Soviet Venera landers were redesigned to survive far more severe conditions, and in 1974 Mariner 10 flew past Venus on its way to Mercury and took pictures of the cloud tops.
For most people in 1967, Venus attracted less attention than the Moon. Even so, Mariner 5 gave a first reasonably confident picture of an inhospitable neighbour, and it became a reference point for later study of Venus’s greenhouse-like atmosphere.