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Past Missions

For nearly 50 years, our civilization has been attempting to send spacecraft to Mars. By the beginning of the 21st century, only 10 missions out of 33 tries had been a complete success. Journey with us down the rocky road of Mars exploration.


Viking 1: Aug. 20, 1975
Viking 2: Sept. 9, 1975

The Viking Project began in 1968 when the race for the Moon was in high gear, but its twin spacecraft did not arrive at the launch pad until 1975. A sagging U.S. economy and an expensive Vietnam War forced the government to cut deeply into NASA's budget. While the Viking mission was put on hold, the Soviet Union eagerly snapped up the 1973 launch opportunity with four probes. The race to put a successful lander on Mars looked like an easy win for the USSR. But it was Viking that was destined to take that honor as well as the distinction of being one of the most elaborate missions in the history of Martian exploration.

The goal of the Viking Project was to send two landers to the Martian surface to look for signs of life. However, there was a problem. By 1975, Mars was beginning to look like the bully who wouldn't let anyone tread on its turf. The Soviets had lost four probes attempting to land on the Red Planet. Strangely enough, they were able to make successful landings in the high-pressure, oven-like world of Venus. American engineers were concerned that there was something about Mars that made a lander mission a risky business. Members of the Viking team would think and rethink their selection of landing sites for NASA's billion-dollar project. A final decision would not be made until more information was obtained from the very orbiters that would deliver the landers to the surface of Mars.

Each Viking spacecraft carried 14 instruments. The twin orbiters were to photograph the Martian surface, find safe landing sites, act as a communications link for their landers, and carry out a variety of experiments on the Martian atmosphere. The identical Viking landers carried television cameras, a small laboratory for the life experiments, and instruments to monitor Martian weather and record any earthquake activity.

Viking 1 reached Mars on June 19, 1976. Viking 2 followed on Aug. 7, 1976. The first lander was to have a 4th of July touchdown in honor of America's bicentennial. That changed when scientists saw images of the original site at Chryse Planitia. It was much too rough and, at the last minute, a new landing site was selected to the west of the same region. The Viking 1 lander touched down on July 20. The landing site for Viking 2 was also changed. The probe settled onto the Martian surface at Utopia Planitia on Sept. 3, 1976.

After gathering several images of the surface of Mars, scientists were eager to carryout the biology experiments. Each lander had a "soil sampler" -- an arm that extended from the body of the probe and scooped up a small sample of soil for delivery to a test chamber.

The Gas Exchange Experiment checked for organisms in the soil by saturating them with a mixture of nutrients and water vapor. A build-up of gasses would mean microbes in the test sample had kick-started their metabolism to gorge themselves in the new environment.

The Carbon Assimilation Experiment looked for organisms that would thrive on carbon dioxide -- a major component of the atmosphere of Mars. Carbon dioxide and carbon monoxide gasses tagged with radioactive carbon-14 were added to a chamber that contained a soil sample and some Martian atmosphere. If "carbon eating" organisms were present, they would become radioactive.

The Labeled Release Experiment tested for organisms in the soil by monitoring the release of radioactive gas. A drop of nutrient solution containing carbon-14 was added to the soil sample. The idea was that organisms would eat the nutrients and get rid of the radioactive gas.

A few days after the biology experiments had begun, a burst of oxygen was released in the Gas Exchange Experiment. The Labeled Release Experiment showed a build-up of radioactive carbon. It looked like Viking discovered life on Mars and a press conference soon followed. But some scientists were uneasy about the test results. The reaction in the Gas Exchange Experiment happened too quickly. Organisms would need longer to process the nutrients. Scientists began to think that they could have received a false positive caused by the soil itself rather than the detection of living organisms. The "oxidant theory" arose as an explanation. Scientists realized that Martian soil is full of oxidants. When water is introduced, these oxidants fizz like soda and mimic the action of microbes. The results of the Labeled Release Experiment were also called into question when the level of carbon-14 quickly subsided. Scientists felt that if organisms were present, they would multiply and increase the amount of radioactive carbon in the chamber. The results obtained from the Carbon Assimilation Experiment were also attributed to chemical reactions in the soil.

After all the expense, time, effort, and analysis, the best conclusion seemed to be that there was no compelling evidence for life in the near-surface soil of Mars. But while Mars had finally allowed visitors to poke and prod at its soil, it still managed to retain more than a modicum of mystery.

The Viking mission was one of the greatest successes in the history of planetary exploration. More than 4,000 photographs were returned from the Viking landers as well as 52,000 images from the two orbiters that revealed 97 percent of the Martian surface. Millions of weather-related measurements were returned to Earth from meteorology experiments.

The Viking mission was designed to last for 90 days, but the lifetime of each orbiter and lander stretched into years. The Viking 1 lander lasted the longest. Its final transmission was on Nov. 11, 1982. An entire decade would pass before America would attempt to send another spacecraft to Mars.


Mars 4: July 21, 1973
Mars 5: July 25, 1973
Mars 6: Aug. 5, 1973
Mars 7: Aug. 9, 1973

The United States sent no spacecraft to Mars in 1973. Scientists and engineers focused their attention on the Viking Landers that would be launched in 1975. The Soviet Union, however, made an aggressive move toward Mars, perhaps spurred on by the looming shadow of a sophisticated new mission by the Americans. Instead of the usual pair of spacecraft, the USSR readied four probes for an extensive exploration of the Red Planet.

The 1973 launch window was not a good one; more propellant than usual was needed to reach the proper trajectory for Mars. More propellant meant less payload, so the Soviets were not able to combine an orbiter and a lander on one spacecraft as they had done in 1971. Mars 4 and 5 were designed to investigate the Martian surface and atmosphere as orbiters. Mars 6 and 7 were to carry landers for science experiments on the surface. But this bold initiative would be crippled by one bad decision that affected the guts of each spacecraft.

A power system malfunction during tests of onboard equipment before launch provided the first clue that something was terribly wrong. Soviet scientists discovered that a transistor had failed because of corrosion. It was revealed that a decision had been made two years earlier to replace gold components in the transistors with aluminum in order to save Soviet gold resources. Soviet scientists came to the horrific realization that the equipment in Mars 4, 5, 6 and 7 was peppered with transistors made with a material that could easily corrode. There was not enough time to replace them all. The spacecraft were set to launch in six months and it would take at least six months to correct the problem. To make matters worse, an analysis of the defective transistors showed that their failure rate increased approximately two years after production. This corresponded to the exact time the four spacecraft would arrive at Mars. After an extensive analysis, it was predicted that each probe had a 50-50 chance of survival. Soviet officials decided to press ahead with the launch anyway.

Mars 4 never went into orbit. Retro-rockets malfunctioned, probably due to the flawed transistors, and the craft sailed by the planet instead of being slowed into an orbit.

Mars 5 was the only spacecraft of the four to find success. It entered Mars orbit in February of 1974 and was able to return more than 60 images, as well as data about the Martian atmosphere. But its success was short-lived. Two weeks later, the probe failed to return any further information.

Mars 6 stopped relaying its telemetry information two months into its journey. This was probably caused by the failure of one or more of the defective transistors. Still, the spacecraft operated autonomously and released its lander as planned. Signals that were received from the descent module fell silent just before reaching the surface. The only scientific data transmitted were a few readings of the Martian atmosphere.

Mars 7 released its lander, but into the void of space rather than onto the surface of Mars. The faulty transistors struck again. The lander could not be placed on the proper trajectory for an encounter with the planet.

The Soviet Union would not try another mission to Mars for 15 years.


Kosmos 419: May 10, 1971
Mars 2: May 19, 1971
Mars 3: May 28, 1971

The 1971 launch window was a good one for Martian exploration. Earth and Mars were aligned in such a way that less energy was required to put a spacecraft on the proper trajectory for the Red Planet. If less energy was needed, a rocket could lift more weight off the launch pad. The Soviet Union took advantage of this favorable alignment of the heavens to launch three heavy spacecraft. The first was an orbiter loaded with science instruments. The second and third carried both an orbiter and a lander.

Had it been successful, Kosmos 419 probably would have been named Mars 2. But two days after the loss of America's Mariner 8, Kosmos 419 also suffered a launch problem. The powerful Proton rocket that lifted the spacecraft into a parking orbit around the Earth had been programmed incorrectly. The booster rocket that should have put the probe on a trajectory for Mars never ignited. This failure prompted the Soviets to give the craft a "Kosmos" designation. It was a name reserved for Earth orbiters. Two days after its launch, Kosmos 419 was destroyed as it reentered the Earth's atmosphere.

Mars 2 and 3 were identical spacecraft that were designed to orbit Mars and deliver the first landers to the planet's surface. Both orbiters carried science instruments that would take images of the surface as well as study its composition and topography. Other instruments would investigate the planet's temperature, atmosphere, and magnetic fields.

The Mars 2 and 3 landers were supposed to image the surface of the Red Planet, and gather information about the temperature, wind speed, atmospheric pressure, and the composition of Martian soil. But the same massive dust storm that greeted Mariner 9 was also waiting for the two Soviet probes. Unfortunately, they could not be reprogrammed to wait out the storm. The descent capsules were released into an atmosphere churning with dust and very strong winds. Mars 2 released its lander on November 27, 1971. The probe malfunctioned and crashed onto the Martian surface. Mars 3 released its descent capsule on December 2 and the Soviets laid claim to a successful landing. But approximately two minutes after touchdown, the probe fell silent.

The Mars 2 and 3 orbiters continued to operate through August. They sent back 60 images, along with data about the planet's upper atmosphere, surface pressure and temperatures, and magnetic fields. The USSR had finally managed to arrive at Mars.


Mariner 8: May 8, 1971
Mariner 9: May 30, 1971

It was May of 1971 when the launch window for Mars rolled around again. The United States was still sending manned missions to the Moon and the USSR's Venera 7 became the first spacecraft to make a soft landing on Venus. Five spacecraft were poised for Martian exploration. Two of them were American - and Mars would try to leave them all in the dust.

Mariner 9Mariner 8 and 9 marked America's shift from probes that flew by the Red Planet to those that would go into orbit to gather information. Both spacecraft carried cameras, spectrometers and other science instruments to study the Martian atmosphere, its surface composition, gravity and topography. The plan was to have Mariner 8 map the planet from a polar orbit while Mariner 9 gathered information to explain the light and dark seasonal variations on Mars. But Mariner 8 failed during launch. The upper stage of the Atlas-Centaur started to gyrate and the officer in charge of safety at the site had no choice but to destroy the rocket and its payload. This was a major blow to the Mariner program. These two spacecraft were part of the most ambitious and expensive Mars program of its day and now one of them was in pieces in the Atlantic Ocean.

The days between the loss of Mariner 8 and the launch of Mariner 9 were intense. The launch window for Mars would only stay open for a few short weeks. Mission engineers scrambled to find the problem that caused the Atlas-Centaur to malfunction and then had to make sure the rocket that carried Mariner 9 did not suffer the same fate. The failure of the launch vehicle was traced to an electronic component in the part of the rocket's guidance system that controlled the vehicle's roll motions. The component was replaced in the Mariner 9 rocket and on May 30, just 22 days after Mariner 8 tumbled into the Atlantic, Mariner 9 soared into the heavens.

The spacecraft made a smooth exit from Earth, but six months later found a difficult arrival at Mars. Days before reaching its destination, Mariner 9's signal disappeared. Engineers assumed that it was having navigational problems and struggled to find the spacecraft using a different antenna. Regular communication was restored just in time to receive a rude welcome from Mars. A giant dust storm had enveloped the planet. Only four mysterious dark spots could be seen protruding from the surface. It took more than a month for the dust to settle, but it was worth the wait. In January of 1972, Mariner 9 cameras started to gather images. The four dark spots revealed themselves as the tops of gigantic volcanoes. One of these giants, Olympus Mons, was discovered to be the tallest volcano in the entire solar system. Mariner 9 also collected images of channels and ancient riverbeds, as well as a landscape slashed by a vast network of canyons to be called the Valles Marineris.

Mariner 9 recorded more than 7,000 images of Mars The entire surface of the planet was imaged, as well as the Martian moons Phobos and Deimos. The mission had gone from a rocky start to a resounding success. The abundance of features that seemed to have been carved by liquid water led scientists to believe that Mars was once a warmer and more watery world. The question of life on Mars arose once again. America's next mission to the Red Planet - the Viking program - would carry experiments that would look for signs of life. The seasonal "wave of darkening" that appeared across the planet's surface was found to be caused by dust storms that shifted the position of lighter and darker surface materials. Even the huge dust storm yielded discovery. Mariner 9 data showed that Martian surface temperatures dropped significantly when the planet was shrouded. This information led Carl Sagan to predict that, should there be a nuclear war on Earth, a thick blanket of tiny particles in the atmosphere would plunge our world into a "nuclear winter."

The Mariner 9 mission was designed to last 90 days, but the flow of information continued for almost a year. The probe's final transmission was on Oct. 27, 1972.

Web content editor/writer: Pamela R. Smith