Viking program

• 1. Introduction

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  The Viking program landed spacecraft on Mars in 1976, and for the next quarter-century it supplied nearly everything humanity knew about the red planet. Two identical probes, Viking 1 and Viking 2, each carried an orbiter and a lander, four spacecraft in total, all born from a mission effort that began in 1968 at NASA's Langley Research Center. The bill came to roughly one billion dollars in 1970s money. What the probes found when they got there changed the way scientists think about water, about soil chemistry, and about whether life might exist beyond Earth. How do you build a machine to answer a question that old? And when the results finally come in, what happens when the answer is yes and no at the same time?

• 2. The Machines Themselves

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  Viking 1 lifted off on the 20th of August 1975, riding a Titan IIIE rocket with a Centaur upper stage. Viking 2 followed on the 9th of September 1975. Each orbiter-lander pair at launch weighed 3,527 kilograms, of which 1,445 kilograms were propellant and attitude control gas. The orbiters were based on the earlier Mariner 9 spacecraft and shaped as an octagon roughly 2.5 meters across, with four solar panel wings stretching 9.75 meters from tip to tip. Those panels held 34,800 solar cells and generated 620 watts at Mars. When the orbiter and lander separated before descent, the lander settled at around 600 kilograms and the orbiter at roughly 900 kilograms.

  The landers were six-sided aluminium structures resting on three legs, each leg attached to one of the shorter sides. To shield Mars from Earth contamination, each lander was sealed inside a pressurized bioshield and then baked at 111 degrees Celsius for 40 hours before launch. The bioshield cap was jettisoned after the Centaur upper stage pushed the combination out of Earth orbit. Every significant redundancy added cost and mass, but a 1971 directive required that no single failure could halt more than one experiment on a device containing over 40,000 parts.

  The landers' Guidance, Control and Sequencing Computers each used two Honeywell HDC 402 24-bit processors with 18 kilowords of plated-wire memory. Power came from two radioisotope thermoelectric generators running on plutonium-238, each 28 centimeters tall and producing 30 watts of continuous power at 4.4 volts. The scientific payload aboard each lander weighed approximately 91 kilograms and covered biology, chemistry, meteorology, seismology, and imaging.

• 3. Getting Down Safely

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  Viking 1 reached Mars orbit on the 19th of June 1976. Viking 2 arrived on the 7th of August. Astronomer Carl Sagan helped choose the landing sites for both probes. Each orbiter circled Mars for more than a month, sending back images to help certify those sites before the landers were released.

  Descending to the surface unfolded in four distinct phases. First came a deorbit burn. Then atmospheric entry, with peak heating arriving a few seconds after friction with the Martian atmosphere began. At about 6 kilometers altitude, traveling at 900 kilometers per hour, the parachute deployed and the aeroshell fell away. At about 1.5 kilometers, three retrorocket engines fired. Those engines each had 18 nozzles specifically to disperse exhaust and limit disturbance to the ground below, and they were throttleable from 276 to 2,667 newtons. The hydrazine propellant had been specially purified to avoid depositing Earth microbes on the surface.

• 4. Water Everywhere

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  Images from the orbiters revealed huge river valleys cutting across many areas of Mars. Floods had broken through dams, carved deep valleys, eroded grooves into bedrock, and traveled thousands of kilometers. Large areas of the southern hemisphere held branched stream networks pointing to rainfall in the distant past. Some volcanic flanks resembled those produced by rainfall on Hawaiian volcanoes. Underground volcanism may have melted frozen subsurface ice; water then drained away and the ground collapsed into what geologists call chaotic terrain. The volume of water estimated to have moved through some of those channels was ten thousand times the flow of the Mississippi River.

  Many Martian craters also looked wrong. Normally, impact ejecta goes up and comes back down. On some Martian craters, material had clearly flowed outward and around obstacles, suggesting the impactor struck ground saturated with ice. The ice melted on impact, turning the soil briefly into mud. These orbiter findings drove a near-complete revision in scientific understanding of water on Mars, a revision that shaped planetary science through the late 1990s and early 2000s.

• 5. The Life Question

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  The most expensive single piece of the Viking program was the lander's life-detection unit, which cost about 60 million dollars at the time. Three separate scientific teams designed the biological experiments, under the direction of chief scientist Gerald Soffen of NASA. Each experiment tested Martian soil for signs of metabolic activity or organic chemistry.

  One experiment returned a positive result for the detection of metabolism, suggesting current life. The other two experiments failed to find any organic molecules in the soil. Most scientists concluded that the positive signal likely came from non-biological chemistry driven by highly oxidizing soil conditions rather than from microbes. NASA stated during the mission that the results did not demonstrate conclusive biosignatures at the two landing sites.

  The debate did not close there. The positive result came from the Labeled Release experiment, and its validity hinged on the absence of an oxidative agent in the soil. The Phoenix lander, arriving decades later, discovered perchlorate salts in 2008. Researchers found that perchlorate, when heated, destroys organic compounds and produces chloromethane and dichloromethane, which are precisely the chlorine compounds both Viking landers had detected when they ran their tests. New findings from re-examination of the Gas Chromatograph Mass Spectrometer results were published in 2018. On the 12th of April 2012, an international team of scientists published work based on complexity analysis of the Labeled Release data suggesting possible detection of extant microbial life. The question remains unresolved.

• 6. Seeing Mars

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  Thomas A. Mutch, a geologist at Brown University in Providence, Rhode Island, led the imaging team. Each lander carried two 360-degree cylindrical scan cameras. The cameras used a movable mirror to illuminate 12 photodiodes sensitive to different frequencies of light, including three narrowband diodes for color and three for infrared. The cameras scanned at five vertical lines per second, each line containing 512 pixels. A full 300-degree panorama required 9,150 lines. The scan rate was slow enough that during development testing, crew members who moved across the camera's field appeared multiple times in a single shot.

  The Viking camera system cost 27.3 million dollars to develop. Finding a manufacturer capable of building such an advanced design proved difficult. Program managers were later praised for resisting pressure to adopt a simpler system, a decision that paid off when the first images of the Martian surface arrived. The program did reduce costs by cutting a planned third lander and trimming some experiments.

• 7. End of Operations

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  The four spacecraft failed one by one over six years. The Viking 2 orbiter was the first to go, shut down on the 25th of July 1978 after a fuel leak in its propulsion system, having operated for 1 year, 11 months, and 18 days. The Viking 2 lander followed on the 11th of April 1980, lost to a battery failure after 3 years, 7 months, and 8 days. The Viking 1 orbiter was shut down on the 17th of August 1980, after attitude control fuel ran out; its orbit had been raised on the 7th of August to prevent an imminent impact with Mars, with surface impact now possible from 2019 onwards. The Viking 1 lander survived the longest. On the 13th of November 1982, a human error during a software update caused the lander's antenna to lose orientation, cutting power and communication after 6 years, 3 months, and 22 days of operation.

  The Viking program was formally declared ended on the 21st of May 1983. Each lander had carried a tiny microfilm dot bearing the names of the thousands of people who worked on the mission, a practice that later probes continued; the Perseverance rover, for instance, recognized almost 11 million people who submitted their names for inclusion.