Mariner 4

• 1. Introduction

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  Mariner 4 changed what humanity thought it knew about Mars. Launched on the 28th of November 1964, this spacecraft became the first to successfully fly past the red planet, sending back something no one had ever seen before: actual photographs of another world taken from deep space. Before those images arrived, Mars was a place of speculation and hope. After them, it was something more sobering. What the camera revealed, how those images nearly never made it back to Earth, and what the mission found beyond the craters are the questions this documentary will explore.

• 2. Built to Reach Mars

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  The physical body of Mariner 4 was an octagonal magnesium frame, 127 cm across a diagonal and 45.7 cm high. Four solar panels extended outward from the top, spanning 6.88 m end to end. Those panels held 28,224 solar cells, enough to generate 310 watts at the distance of Mars. For backup power, a 1,200 watt-hour silver-zinc battery handled maneuvers and emergencies.

  Seven scientific instruments were packed into and onto the frame. A helium magnetometer measured interplanetary and planetary magnetic fields. An ionization chamber paired with a Geiger counter tracked charged-particle intensity. A cosmic dust detector measured the momentum and direction of microscopic debris drifting through space. A solar plasma probe pointed 10 degrees from the solar direction to capture the particle flux streaming outward from the Sun.

  The camera itself was mounted on a scan platform at the bottom center of the spacecraft. It combined a Cassegrain telescope with a field of view just 1.05 by 1.05 degrees, a shutter assembly using red and green filters, a slow-scan vidicon tube, and electronics to convert the analog image into a digital bitstream. Exposure times were either 0.08 or 0.20 seconds. What the camera captured would be stored on a magnetic tape recorder with a capacity of 5.24 million bits, then transmitted home at a rate as slow as 8 and a third bits per second.

  Mariner 4 also had a navigational challenge no earlier probe had faced. Previous missions stayed near Earth, the Moon, or Venus, and could lock their orientation onto a bright planet. On this flight, both Earth and Mars would be too dim for that purpose. Engineers chose the star Canopus as a reference instead, making Mariner 4 the first spacecraft to rely on a star for navigational orientation. Canopus would later become a standard reference point across many subsequent missions.

• 3. From Cape Canaveral to Mars Transfer

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  Mariner 4's launch almost did not happen. Its predecessor, Mariner 3, was a total loss when the payload shroud failed to jettison, trapping the spacecraft inside its own fairing. Engineers at JPL concluded that a pressure differential between the inner fiberglass lining and the outer metal fairing had caused the spring-loaded separation mechanism to tangle and fail. Testing confirmed this failure mode, and the solution was a new, all-metal fairing.

  The replacement shroud was heavier, which reduced the lifting capacity of the Atlas-Agena rocket. Convair and Lockheed-Martin had to squeeze more performance out of the booster to compensate. There were genuine fears the work could not be finished before the 1964 Mars launch window closed. The new shroud was ready by November.

  At 14:27:23 UTC on the 28th of November 1964, Mariner 4 lifted off from Cape Canaveral Air Force Station Launch Complex 12. The protective shroud separated cleanly. The Agena engine fired twice: the first burn, from 14:28:14 to 14:30:38, placed the spacecraft into an Earth parking orbit. The second burn, from 15:02:53 to 15:04:28, pushed it onto a Mars transfer trajectory. Mariner 4 separated from the Agena at 15:07:09. By 15:15:00, the solar panels had deployed and the camera scan platform was unlatched. Sun acquisition followed sixteen minutes after that.

  Then the star tracker went looking for Canopus. It took more than a day of what mission controllers called star-hopping to find it. The sensor locked onto Alderamin, Regulus, Naos, and Gamma Velorum in turn before finally settling on Canopus. The tracker had been set to respond to any object between one-eighth and eight times the brightness of Canopus, and there were six other qualifying objects visible to it.

• 4. Seven Months, One Maneuver

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  Canopus lock proved unreliable in the early weeks of flight. It was lost six times in less than three weeks after launch, and each loss required a sequence of radio commands from Earth to reacquire the star. Investigators eventually concluded that small dust particles released from the spacecraft itself were drifting through the star sensor's field of view. Sunlight scattering off those particles mimicked the brightness signature of a star. When a particle exceeded eight times the brightness of Canopus, the spacecraft automatically dropped lock and began searching for a new star.

  On the 17th of December 1964, mission controllers sent a command that removed the upper brightness limit the sensor used to discard false targets. Canopus was never lost again, though roll transients continued occurring 38 more times before the Mars encounter.

  The one midcourse maneuver of the 7.5-month flight took place on the 5th of December 1964. It had been scheduled for the 4th, but a Canopus lock loss forced a one-day postponement. The maneuver consisted of a negative pitch turn of 39.16 degrees, a positive roll turn of 156.08 degrees, and a thruster firing lasting 20.07 seconds. The pitch and roll turns were completed with better than one percent accuracy. The velocity change was accurate to about 2.5 percent. After that, Mariner 4 was on its planned course for Mars.

  On the 5th of January 1965, now 36 days out from Earth and 10,261,173 km away, the spacecraft reduced its data transmission rate from 33 and a third bits per second to 8 and a third bits per second. This was the first autonomous action the spacecraft had taken since the midcourse maneuver, and it reflected the growing distance between Mariner 4 and home.

• 5. The Flyby and Its Images

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  Mariner 4 flew past Mars on the 14th and the 15th of July 1965. Its closest approach brought it within 9,846 km of the Martian surface at 01:00:57 UT on the 15th of July. At that moment, Earth was 216 million km away. The spacecraft was traveling at 7 km/s relative to Mars and 1.7 km/s relative to Earth.

  Planetary science mode switched on at 15:41:49 UTC on the 14th of July. The camera sequence began at 00:18:36 UT on the 15th. It captured 21 complete pictures using alternating red and green filters, plus 21 lines of a 22nd picture. The images covered a discontinuous swath of the planet's surface, from near 40 degrees north and 170 degrees east, down to about 35 degrees south, then across to the terminator at 50 degrees south. That swath represented roughly 1 percent of Mars.

  At 02:19:11 UTC, Mariner 4 slipped behind Mars as seen from Earth and the radio signal cut off. It reappeared at 03:13:04 UTC. Transmission of the recorded images began about 8.5 hours after signal reacquisition. Each photograph took approximately six hours to transmit. To protect against data loss, every image was sent twice. The full transmission ran until the 3rd of August 1965.

  Before the computer-processed images were ready, the team wanted confirmation the camera had actually worked. Using a pastel set purchased from an art supply store, they hand-colored a numerical printout of the raw pixel values, paint-by-numbers style. That hand-drawn image compared favorably with the final computer-processed version and became the first digital image from another planet to be hand-rendered by human beings.

• 6. What Mars Told Science

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  The total data returned by Mariner 4 amounted to 5.2 million bits, roughly 634 kilobytes. What that small archive contained upended decades of assumptions. The images showed a Moon-like cratered terrain. Scientists had not expected this, though an amateur astronomer named Donald Cyr had previously predicted craters on Mars.

  The atmospheric pressure at the Martian surface was estimated at 4.1 to 7.0 millibars. That figure meant an atmosphere far thinner than scientists had anticipated. No magnetic field was detected. No Martian radiation belts. No surface water. The picture that emerged was of a relatively inactive world, stripped of the shielding that might protect biology from the harshness of space.

  Life on Mars had been a subject of scientific speculation and science fiction for centuries. After Mariner 4, most researchers concluded that if life existed at all, it would be small and simple. A counterargument was also raised: a search for life on Earth, conducted at similar resolution using a comparable number of photographs, would fail to detect signs of life across the vast majority of those images. So the 22 photographs taken by Mariner 4 could not, by themselves, rule out life on Mars.

  Later missions would show that the cratered terrain Mariner 4 imaged was not typical of the whole planet. It represented only the more ancient regions. Bruce C. Murray used the photographs to trace Mars's geologic history. The solar wind measurements gathered during the flight were later compared with simultaneous records from Mariner 5, which traveled to Venus. One instrument, the Geiger-Muller tube component of the ionization chamber, failed in February 1965. A resistor failure in the plasma probe on the 8th of December 1964 degraded its performance, but experimenters recalibrated the instrument and recovered usable data.

• 7. The Long Silence and the End

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  Mariner 4 returned useful data continuously from launch until 22:05:07 UTC on the 1st of October 1965. By then, the spacecraft was 309.2 million km from Earth, and the combination of that distance and imprecise antenna orientation caused a communication blackout that lasted until 1967.

  When contact resumed in late 1967, the cosmic dust detector began registering hits in startling numbers. On the 15th of September, 17 hits arrived in a 15-minute span, part of what appeared to be a micrometeoroid shower that temporarily shifted the spacecraft's attitude and probably damaged its thermal shield. Scientists later speculated that Mariner 4 had passed through debris from the comet D/1895 Q1 (Swift), and may even have made an unplanned flyby of that comet's possibly shattered nucleus at a distance of 20 million km.

  The final days came quickly. On the 7th of December 1967, the attitude control system exhausted its nitrogen gas supply. Between the 10th and the 11th of December, 83 micrometeoroid hits were recorded, perturbing the spacecraft's orientation and degrading the signal. On the 21st of December 1967, mission controllers sent the last transmission and terminated contact. Mariner 4 now drifts in a superior heliocentric orbit, derelict but intact. The ultraviolet photometer that had to be removed before launch due to electrical interference with the camera was eventually flown on Mariner 5 in 1967, as a small footnote connecting the two missions that together measured both Mars and Venus for the first time.