Mariner 9 — first spacecraft to orbit another planet

Late on 13 November 1971, California time, Mariner 9's engine lit for a long braking burn, and at 00:18 UT on 14 November it slipped into orbit around Mars. For the first time in history, a machine built by human hands was circling another planet. The triumph came with a cruel joke: Mars had disappeared. A dust storm of planetary scale, the largest ever observed, had swallowed every surface feature. The cameras saw a blank ochre disk with only four dark spots punching through the haze.

Mariner 9 was already carrying a double burden. Its twin, Mariner 8, had tumbled into the Atlantic minutes after launch that May, so this one spacecraft inherited both mission plans. Its saving grace was a reprogrammable onboard computer. JPL simply told the spacecraft to wait, conserving film of the cameras' targets until the dust settled. The Soviet Mars 2 and Mars 3 probes arriving weeks later had no such flexibility; locked into automatic sequences, they dispatched their landers straight into the storm.

When the atmosphere cleared in early 1972, the mapping campaign rewrote the textbook on Mars. Those four dark spots resolved into colossal volcanoes, among them Olympus Mons, rising far higher than Everest above a base some 600 kilometres wide. The cameras traced a canyon system over 4,000 kilometres long, later named Valles Marineris in the spacecraft's honour, and channels that looked unmistakably like dry riverbeds. In total Mariner 9 returned 7,329 images and mapped 85 percent of the surface.

The spacecraft transmitted until 27 October 1972, when its attitude-control gas finally ran out after nearly a year of operations. Mariner 9 had arrived at a planet believed to be a cratered, Moon-like fossil, and left it a world of volcanoes, canyons, polar caps, and ancient water, the world that Viking, Pathfinder, and every rover since went to explore. Its derelict hull was left circling Mars, slowly spiralling toward an eventual atmospheric entry.