Mariner 4

Mariner 4 flew because its twin died. Mariner 3 had launched on 5 November 1964 and never shed its fiberglass shroud, so the second spacecraft, already stacked on the neighbouring pad at Cape Kennedy, inherited both the mission and an all-metal replacement fairing built from scratch in seventeen days. It lifted off at 14:22:01 UTC on 28 November 1964 with two days left in the Mars window. Two days after that, the Soviet Union launched Zond 2 on the same errand; it lost a solar panel at separation, then lost its voice altogether in May 1965, and drifted past Mars silent. The cruise was long and only intermittently calm. Attitude reference came from Sun sensors plus a tracker locked onto Canopus, the second-brightest star in the sky and conveniently placed near the south ecliptic pole, and the tracker spent about a day hopping across the wrong stars before it settled, then lost its grip six more times in three weeks. The suspected culprit was dust shaken loose from the spacecraft itself, glinting in sunlight as it drifted through the sensor's field of view. A single midcourse burn on 5 December 1964, 20.07 seconds after turns of 39.16 degrees in pitch and 156.08 in roll, set up the encounter. The crossing took 228 days from launch to Mars.

The encounter itself was over in less time than a lunch break. Planetary science mode came on at 15:41:49 UTC on 14 July 1965. The camera sequence started at 00:18:36 UTC on 15 July and ran about twenty-five minutes: a narrow-angle Cassegrain telescope firing through alternating red and green filters onto a slow-scan vidicon tube, each frame digitised to 200 lines of 200 pixels at six bits per pixel. Two of every three exposures were written to a 330-foot loop of quarter-inch magnetic tape, which is why the surviving images come in overlapping pairs. At 01:00:57 UTC Mariner 4 passed 9,846 km above the surface at about 7 km/s, then, at 02:19:11, slid behind the planet as seen from Earth and went quiet. That silence was itself an experiment. As the radio signal grazed the atmosphere going in and coming out at 03:13:04, its frequency, phase and amplitude were bent and dimmed in ways that could be read backwards into a density profile. Arvydas Kliore and colleagues turned that into what their paper called the first direct measurement of Mars's atmosphere and ionosphere: a surface pressure of 4.1 to 7.0 millibars, against the roughly 85 millibars most people had expected. Mars did not have a thin atmosphere. Mars had almost none.

Getting the pictures home took nineteen days. At 216 million km the usable link ran at eight and a third bits per second, so the 240,000 image bits in a frame needed exactly eight hours of clean signal, and in practice rather more, relayed to JPL through Deep Space Network dishes in California, Australia and South Africa. Playback began about eight and a half hours after the spacecraft reappeared, ran through 24 July, and then started over so the whole tape could be sent a second time and the two copies compared for errors, finishing on 3 August. Waiting for the first computer-processed frame was more than some engineers could stand. On 15 July, staff from JPL's Telecommunications Section took the strips of paper coming off the real-time data translator, on which each of the 40,000 pixels of frame one appeared as a number from 0 to 63, cut them up, stapled them side by side to a display panel, and coloured the numbers in by hand like a paint-by-numbers kit. Richard Grumm, who ran the tape recorder operations, is credited with the idea and with the run to a nearby art store for chalk pastels. His initials, RLG, are in the bottom right corner. The finished panel was framed and given to JPL director William Pickering, and it hangs at JPL still.

What the pictures showed was craters. Frame 11, which Leighton called one of the most remarkable scientific photographs of the age, is dominated by the basin the IAU named Mariner crater in 1967, about 170 km across in the planetary nomenclature gazetteer; across frames 5 through 15 the team counted about seventy craters ranging from a few kilometres to at least 120 km. In their Science paper of 6 August 1965 Leighton, Murray, Sharp, Allen and Sloan drew the obvious and devastating inference: a surface that densely cratered is extremely old, and neither a thick atmosphere nor oceans can have existed since it formed. Combined with 4 to 7 millibars of air, minus 100 C daytime temperatures, no detectable magnetic field and therefore no shield against solar and cosmic radiation, the Mars of 1965 looked less like a drier Earth and more like a slightly rusty Moon. A hundred years of imagined canals, seasonal vegetation and Lowell's dying civilisation went out with it. Percival Lowell's canals had already been abandoned by working astronomers decades earlier, but the popular Mars, the one Wells and Bradbury wrote for, survived until Mariner 4's strip of grey craters landed on the front pages. President Johnson unveiled the rest of the pictures at the White House on 29 July and said that life as we know it, with its humanity, is more unique than many have thought.

The verdict was also, in an important sense, wrong. Mariner 4 imaged roughly 1 percent of Mars, and the strip happened to cross the ancient southern highlands, the oldest and least interesting terrain on the planet. Valles Marineris, the Tharsis volcanoes, the layered polar deposits and the dried river networks were all outside the frame, and it took Mariner 9's orbital survey in 1971 and 1972 to put them back. For six years, though, the dead-planet reading held, and it reshaped what NASA planned next. The spacecraft outlived all of it. Routine contact ended at 22:05:07 UTC on 1 October 1965, when 309.2 million km and unfavourable antenna geometry finally beat the link; intermittent telemetry returned on 3 May 1966, and full data acquisition resumed in late 1967, when Mariner 4 was pressed back into service for attitude control tests supporting Mariner 5 at Venus. On 15 September 1967 its dust detector logged 17 hits in fifteen minutes, apparently a micrometeoroid shower that nudged the spacecraft and probably scarred its thermal shield. The attitude gas ran out on 7 December 1967. On 10 and 11 December another 83 hits knocked it off pointing and the signal faded. Controllers gave up on 21 December 1967. Mariner 4 is still circling the Sun, roughly 1.11 by 1.57 astronomical units, once every 567 days.