Forty years ago this month, Viking 1 left Earth on a technologically and scientifically challenging mission designed to answer the question: Are we alone in the universe?
The search for life on Mars started as soon as technology allowed. Relatively close to Earth, with a more approachable climate than Venus, humans had been intrigued by the possibility of life on Mars for hundreds of years. From early telescopic observations that uncovered “irrigation channels” on the surface of the planet to the shifting polar caps that indicated Mars was evolving and changing much like Earth, each new piece of information reinforced the notion that life existed 140 million miles away.
Getting there was the problem. The Soviets were the first to try: they sent multiple probes toward Mars in the early 1960s and 1970s with little success. In 1965, NASA’s Mariner 4 became the first spacecraft to fly past the planet, giving humans an initial glimpse of its surface. But it wasn’t until Mariner 9, in 1971, that a spacecraft successfully entered into orbit around Mars. Over the course of its original and extended missions, Mariner 9 mapped the entire planet, providing the clearest images yet and returning data about the atmosphere, surface composition, gravity, and topography.
The next goal, landing on the red planet, was realized five years later when Viking 1 became the first American spacecraft to touch down on Mars. The Viking mission—which included two spacecraft, Viking 1 and Viking 2—was an extraordinary success. But it wasn’t easy.
There were multiple dimensions to the challenges encountered in designing and developing the mission. The ambitious scope of the project—Viking 1 and Viking 2 each consisted of two parts, an orbiter and a lander—stretched the limits of NASA’s experience. The agency had never operated four spacecraft simultaneously. Furthermore, they would be doing so in relatively unfamiliar territory: despite successful flyby and orbital experience, little detail was actually known about the planet. Novel solutions were needed to novel problems, such as how to safely send a vehicle through the Martian atmosphere, how to conduct a soft landing on poorly understood terrain, and where to touch down in order to maximize the science return while minimizing the risks to the mission.
The specifics of the spacecraft designs were challenging as well. The large Viking orbiters—each, including fuel, weighing more than 5,000 pounds—were based heavily on Mariner technology. But unlike the Mariner orbiters, these vehicles had to transport large landers, which extended their transit time from Earth to Mars by six months. This in turn increased their power requirements, including attitude control gas and solar panel size.
The Viking landers presented other technological problems. Each consisted of a core body, which served as the platform for science instruments and operational subsystems, supported by three legs. A bioshield protected the body from biological contamination on Earth and during transit while an aeroshell heat shield protected it during entry into the Martian atmosphere. In contrast to the orbiters, there was little existing technology on which the lander design could be based. The most recent NASA unmanned lander, the lunar Surveyor, did not have much in common with the proposed Viking vehicles. Viking Lander 1 was twice as heavy as the Surveyor, carried a suite of sophisticated science instruments, and needed to travel much farther from Earth before landing in a poorly understood environment. Furthermore, the landers had to address the rigorous demands of the mission’s scientific goals while maintaining a high degree of reliability. This was due, in part, to the fact that they would be supported by only 70 watts of radioisotope-thermoelectric-generated power, which meant radio transmitters could only be used sparingly. Viking Landers 1 and 2 would need to function autonomously without reliance on Earth-based guidance.
Both the orbiters and the landers carried science instruments designed to pull back the curtain on the mysteries of Mars. Viking Orbiter 1 had two vidicon cameras for imaging, an infrared spectrometer to map water vapor, and infrared radiometers for thermal mapping from orbit. Key instruments on Viking Lander 1 were designed to determine whether life currently existed, or had ever been present, on Mars. The biology experiments featured a soil sample retrieval arm and a laboratory to analyze its findings. The lander also contained a gas chromatograph-mass spectrometer, which would look for compounds produced by microbes to determine whether life had ever existed on Mars—or might evolve there in the future.
On August 20, 1975, Viking 1 left Cape Canaveral on a Titan/Centaur launch vehicle. It took ten months for the spacecraft to reach Mars, where it was inserted into orbit on June 19, 1976. The plan called for Viking Lander 1 to touch down on Martian soil on July 4, the day of the U.S. Bicentennial. But things did not go as planned.
The landing site for Viking 1, Chryse, had been tentatively selected based on Mariner 9 data. But before making a final decision about where to land, the team intended to confirm the appropriateness of the site after viewing images from Viking Orbiter 1. Those images astounded the team. Craters, dry river beds, islands, knobs: for the first time ever, the Martian surface was revealed in detail. It was immediately apparent that the Mariner 9 images, while ground breaking, were merely suggestive of the landscape compared with the incredible clarity provided by Viking 1. Unfortunately, that clarity immediately made clear that the proposed landing site was untenable. What looked crater-free and hospitable on the Mariner 9 images was actually rocky and crevassed according to Viking 1. The team scrambled to find a better site. They ultimately chose an area northeast of their original goal: Chryse Planitia. The second Viking lander was redirected to the other side of the planet, closer to the north pole, in a region called Utopia Planitia.
On July 20, 1976, the seventh anniversary of the first human landing on the moon, Viking Lander 1 touched down. It ultimately spent more than six years collecting and returning data on the red planet. Together with Viking Lander 2, it transmitted more than 4,500 images, while the two orbiters sent back more than 50,000 pictures. The landers retrieved surface samples that were immediately analyzed for signs of life and studied the composition of the Martian atmosphere. They revealed that the terrain of Mars is far more varied than that of Mercury or the moon. Volcanoes, canyons, and craters were apparent, as were signs of surface water, including evidence of enormous floods at some point in the planet’s history. The landers assessed the planet’s surface material and measured temperatures and pressure changes.
The mission was a resounding technological and scientific success, but it failed to do one thing: detect signs of carbon-based life, either past or present. One biology experiment initially appeared to indicate the presence of metabolic activity in the soil, but that was later deemed a false positive. Still, scientists were—and remain—divided on the significance of these results. Some believed Viking 1 and 2 proved that Mars is barren. Some believed the Viking experiments were not equipped to reveal Mars-specific life forms. Others noted that there might still be evidence of life closer to the poles, where the moisture content in the soil and atmosphere is greater; or that chemical activity in Martian soil may have destroyed any evidence of organic compounds. Regardless of their conclusions, people agreed that the vast quantity of hard data returned by the Viking mission ended the era of speculation about Mars and heralded an era of deeper understanding about the planet, which continues today with NASA’s journey to Mars.
Originally intended to last 90 days after reaching the surface of the planet, the mission continued until May 21, 1983. Viking Orbiter 2 was the first of the four spacecraft to shut down, which it did on July 25, 1978, when it ran out of the fuel needed to orient it toward the sun in order to fuel its solar panels. Viking Lander 2 returned its final data on April 12, 1980, and Viking Orbiter 1 shut down later that same year. But Viking Lander 1 continued exploring the planet for two and a half more years, providing information about weather and sending occasional images to ground control before broadcasting its final transmission to Earth in November 1982.
Read more about the Viking mission in On Mars: Exploration of the Red Planet 1958-1978, from the NASA History Office.