Mariner 4

On the night of 14 July 1965, engineers at the Jet Propulsion Laboratory listened to a whisper from a dozen light-minutes away. Mariner 4, a 261-kilogram spacecraft riding on four solar panels, was sweeping past Mars at 9,846 kilometres, its television camera clicking frame after frame onto a loop of magnetic tape. No machine had ever photographed another planet up close. Somewhere on that tape, a century of speculation about canals, vegetation, and Martian civilisations was about to end.

That the spacecraft existed at all was a feat of desperation. Its twin, Mariner 3, had suffocated inside a faulty launch fairing on 5 November 1964, and the Mars window was closing fast. In under three weeks, engineers designed, built, and tested an entirely new metal shroud, and Mariner 4 left Cape Canaveral on 28 November 1964. It then flew seven and a half months through interplanetary space, holding its course by locking onto the star Canopus, a navigation technique no probe had used before.

The pictures came home at eight and a third bits per second, each frame taking about eight hours to transmit. Too impatient to wait for the processed image, engineers pasted the raw numbers into strips and coloured them by hand with pastel crayons; their paint-by-numbers Mars, finished before the official version, still hangs at JPL. The 21 complete photographs that followed covered barely one percent of the planet and revealed a cratered, Moon-like world. The radio occultation experiment was crueller still: an atmosphere thinner than one percent of Earth's, and daytime readings near minus 100 Celsius.

The romantic Mars of Percival Lowell died in those frames, but a real planet took its place. Every future lander would need retrorockets as well as parachutes to survive the thin air, a lesson that shaped spacecraft from Viking to Perseverance. NASA kept contact with Mariner 4 until 31 December 1967, more than three years after launch, and the flyby reconnaissance it pioneered became the template for the first exploration of every planet in the solar system.