Viking 1

• 1. Introduction

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  On the 20th of July 1976, Viking 1 became the first lander ever to work successfully on Mars. It was also the first time the United States had even attempted to set a spacecraft down on the Martian surface.

  Viking 2 flew alongside it, and each of the two spacecraft in NASA's Viking program carried both an orbiter and a lander. Once Viking 1 was on the ground, it refused to quit. It stayed active on the surface longer than any mission beyond Earth, until the Opportunity rover broke that record on the 19th of May 2010.

  Three questions run through this story. Why did the landing slip past the date NASA wanted? What did a robot find when it tested Martian soil for signs of life? And how did a single faulty command from Earth bring the whole mission to an end?

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• 2. A Missed Anniversary

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  Viking 1 left Earth on the 20th of August 1975, carried aloft by a Titan/Centaur launch vehicle. An 11-month cruise followed. About five days before the orbiter slipped into Mars orbit, it began sending back images of the whole planet.

  Orbit insertion came on the 19th of June 1976. Two days later, on the 21st of June, controllers trimmed the path to a loop of 1,513 by 33,000 kilometers that took 24.66 hours to complete. This was a site certification orbit, flown to inspect where the lander might safely go.

  The 4th of July 1976 was the original landing target, chosen to coincide with the United States Bicentennial. Pictures of the primary landing site ruined that plan. The ground there was too rough for a safe touchdown, so the lander stayed with the orbiter while planners searched for smoother terrain.

  The new date fell on the seventh anniversary of the Apollo 11 Moon landing. The lander separated from the orbiter at 08:51 UTC, and the descent began.

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• 3. Descent to Chryse

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  Traveling at about 5 kilometers per second, the lander and its aeroshell began their fall toward the planet. Retrorockets on the aeroshell fired to start the de-orbit maneuver. The craft then spent a few hours at an altitude near 300 kilometers before being turned around for atmospheric entry.

  An ablative heat shield slowed the craft as it plunged into the thin air. During that plunge, a retarding potential analyzer, a mass spectrometer, and sensors for pressure, temperature and density ran entry science experiments.

  At an altitude of 6 kilometers and a speed of about 250 meters per second, a parachute 16 meters wide opened. Seven seconds later the aeroshell fell away. Eight seconds after that, the three landing legs extended. Within 45 seconds the parachute had cut the speed to 60 meters per second.

  At 1.5 kilometers up, the lander's own retrorockets ignited. Forty seconds later it reached the ground at about 2.4 meters per second, with only a light jolt. Honeycomb aluminum shock absorbers in the legs softened the contact.

  Those retrorockets used an unusual design with 18 nozzles, which spread hydrogen and nitrogen exhaust over a wide area. NASA calculated that the surface would warm by no more than 1 degree Celsius (1.8 degrees Fahrenheit). It also expected the exhaust to move no more than 1 millimeter of material. A simpler engine would not have served, because most of the experiments were aimed at surface material.

  Touchdown came at 11:53:06 UTC, or 16:13 local Mars time, in western Chryse Planitia, a name that means Golden Plain. The coordinates were 22.480 degrees north and 47.967 degrees west. About 22 kilograms of propellant remained, and the lander was already starting to switch itself on.

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• 4. First Pictures

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  Twenty-five seconds after landing, the first image of the surface began to transmit, and the transfer took about four minutes. During that stretch the lander activated itself. It raised a high-gain antenna aimed at Earth for direct communication and swung out a boom carrying weather sensors.

  In the next seven minutes a second picture was taken, part of a 300 degree panoramic scene. On the day after landing, the camera captured the first color picture of the Martian surface.

  A five-day delay was the price of a stuck locking pin on the sampler arm, which took that long to shake free. The seismometer also failed to uncage. Every other experiment worked normally.

  Two routes carried data home. One was a direct link to Earth, and the other was a relay through the orbiter. The orbiter could transmit to Earth on S-band at 2,000 to 16,000 bits per second, depending on the distance between the two planets. The lander could send to the orbiter at 16,000 bits per second. The relay link carried about ten times as much data as the direct link.

  The lander's toolkit included two facsimile cameras and a gas chromatograph-mass spectrometer. It also held an x-ray fluorescence spectrometer, sensors for pressure, temperature and wind velocity, and a three-axis seismometer. A magnet on the sampler arm was watched by the cameras, and various engineering sensors completed the kit. Three further analyses, for metabolism, growth or photosynthesis, were built to ask the biggest question of all.

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• 5. The Orbiter's Long Watch

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  Two vidicon cameras served as the orbiter's eyes. An infrared spectrometer mapped water vapor, and infrared radiometers mapped heat across the planet.

  The primary mission ended on the 5th of November 1976, when solar conjunction began. The extended mission started on the 14th of December 1976, after the conjunction. In February 1977 the orbiter made close approaches to Phobos. On the 11th of March 1977, its lowest orbital point, called the periapsis, was brought down to 300 kilometers.

  Small orbit adjustments followed over the years. Most were meant to change the walk rate, which is how fast the longitude beneath the orbiter shifted with each orbit. On the 20th of July 1979, the periapsis was raised to 357 kilometers.

  By the 7th of August 1980, the orbiter was running low on attitude control gas. Controllers lifted its orbit from 357 by 33,943 kilometers to 320 by 56,000 kilometers. The aim was to keep it from striking Mars and possibly contaminating the planet before the year 2019.

  Operations ended on the 17th of August 1980, after 1,485 orbits. More than 57,000 images had reached Earth by then. A 2009 analysis could not rule out an impact on Mars, but it judged that the orbiter was most likely still circling.

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• 6. Search for Life

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  The biology package weighed 15.5 kilograms (34 pounds) and held three subsystems. They were the pyrolytic release experiment, the labeled release experiment, and the gas exchange experiment. Separately, the gas chromatograph-mass spectrometer could measure the makeup and abundance of organic compounds in the soil.

  Four instruments gave their verdicts, and only one said yes. The spectrometer, the pyrolytic release experiment and the gas exchange experiment were all negative. The labeled release experiment was positive. In 2009, Viking scientist Patricia Straat stated, "Our LR experiment was a definite positive response for life, but a lot of people have claimed that it was a false positive for a variety of reasons."

  Ice discovered near the surface close to the Viking landing zone may be softening the view that most scientists hold, which blames inorganic soil chemistry for the positive result. Some scientists still believe living reactions produced it.

  No organic chemicals turned up in the soil at all. Dry areas of Antarctica show no detectable organic compounds either, yet organisms live in the rocks there. Mars has almost no ozone layer, so ultraviolet light sterilizes the surface and creates highly reactive chemicals such as peroxides. Those would oxidize any organic material.

  The Phoenix Lander later found the chemical perchlorate in Martian soil. Perchlorate is a strong oxidant, so it may have destroyed organic matter on the surface. If it is widespread on Mars, carbon-based life would have a hard time surviving at the soil surface.

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• 7. Cold Air and Relativity

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  Midday air temperatures at the landing site climbed as high as minus 14 degrees Celsius (7 degrees Fahrenheit). Before dawn in summer, the reading dropped to minus 77 degrees Celsius (minus 107 degrees Fahrenheit). The robotic sampler arm scooped up soil and fed it to instruments such as the gas chromatograph-mass spectrometer.

  General relativity predicts gravitational time dilation, in which time passes more slowly in regions of lower gravitational potential. Scientists used the lander to test that idea. They sent radio signals to it and had it send signals back, sometimes along paths that passed close to the Sun.

  The observed Shapiro delays of those signals matched the predictions of general relativity. The same radio link that carried them would one day fall silent.

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• 8. Silence

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  In January 1981, the lander was renamed the Thomas Mutch Memorial Station, in honor of Thomas A. Mutch, who had led the Viking imaging team. It had by then worked for 2,245 sols, which is about 2,306 Earth days.

  On the 11th of November 1982, ground controllers sent a command meant to upload new battery charging software. The lander's battery capacity had been deteriorating. The command was faulty, and it accidentally overwrote data used by the antenna pointing software. Contact was lost.

  For the next four months, controllers kept trying to reach the lander, aiming at the antenna position they presumed it held. None of those attempts succeeded.

  In 2006, the Mars Reconnaissance Orbiter photographed the Viking 1 lander sitting on the Martian surface.

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