An Era of Discovery
The summer of 1976 marked the beginning of an exciting new dimension in Mars exploration: studying the planet from its surface rather than from above. On July 20, an American spacecraft called Viking 1 separated from its orbiter, parachuted down, and landed on Mars in an area known as Chryse Planitia. As the lander was programmed to do, it immediately photographed one of its footpads and sent the picture back to mission control. The image showed that the footpad was resting on the surface, rather than buried in deep Martian dust. The spacecraft continued transmitting photos, and as scientists studied them they were ecstatic. For the first time in history, they were staring at close-up color images of the surface of Mars. Carl Sagan, who was part of the Viking mission team, described his reaction:
I remember being transfixed by the first lander image to show the horizon of Mars. This was not an alien world, I thought. I knew places like it in Colorado and Arizona and Nevada. There were rocks and sand drifts and a distant eminence, as natural and unselfconscious as any landscape on Earth. Mars was a place. . . . One way or another, I knew, this was a world to which we would return. 26
Searching for Martian Life
Viking 1 's successful landing was the culmination of more than nine years of planning and development. As with all American Mars missions, two identical spacecraft were sent, but because they were composed of both an orbiter and a lander, these spacecraft were different from any the United States had previously launched. The landers, each encased in a protective pod, sat atop their orbiters. When the spacecraft were fully fueled, each weighed more than six thousand pounds—nearly three times heavier than any spacecraft before them. As a result of this massive weight, the Viking journey took ten months, instead of the typical six or seven months for prior Mars missions.
The Viking mission had several important goals, one of which was to obtain images of the Martian surface that could answer some questions raised by the Mariner photos. The spacecraft would also analyze the atmosphere and make inferences about the planet's interior. But Viking 's most important objective, as stated in a NASA mission report, was to search for evidence of living things: "The question of life on Mars has been [speculated on] for a hundred years. There is no conclusive way to determine its existence other than direct search by landing a vehicle on the planet. The Viking mission will do that." 27
A key factor in the landers' search for life was hunting for water, since scientists knew living things could not survive without it. Both were equipped with cameras that could easily identify anything that moved nearby and instruments designed to scoop up Martian soil. Once soil samples had been obtained, analyzing instruments would test for current organic life, as well as signs that life may have existed in the past. As the landers gathered data, they transmitted it to the orbiters, which in turn sent it back to Earth. Between the orbiters and the landers, any form of life on Mars would surely be found, as NASA scientist Gerald Soffen quips:
Many of the Viking instruments could have detected life. The orbiter camera could have seen cities or the lights of civilizations. The infrared mapper could have found an unusual heat source from concentration of life forms. The water vapor sensor could have detected watering holes or moisture from some great metabolic source. . . . Seismometers [which measure quake activity] could have detected a nearby elephant. 28
Exploring from the Surface and the Sky
When the Viking 1 orbiter reached Mars on June 19, 1976, its first priority was to scout the surface for
Over the course of their mission, the Viking spacecraft provided scientists with a wealth of information. Together, the landers returned more than four thousand photographs, and the orbiters transmitted more than fifty thousand. The spacecraft continued sending data back to Earth until 1982—far longer than the three or four months originally projected. Based on their findings, scientists learned that the Martian soil was composed of iron-rich clay and that the polar caps were at least partially made of water ice with a covering of carbon dioxide. The high concentration of atmospheric CO 2 was discovered, along with the slight traces of oxygen and water vapor, and the atmosphere was confirmed to be much thinner than Earth's. Also, atmospheric ozone was found to be almost nonexistent, which told scientists there was nothing to protect the Martian surface from the sun's deadly ultraviolet rays.
Despite all the Viking's valuable discoveries, however, the mission did not find what NASA had long been hoping for: tangible signs of Martian life. Notrace of surface water was found, and the various experiments and analyses failed to confirm any sign of organic molecules or substances, or any indication at all of living things. Some chemical activity was detected in the soil, so most scientists agreed that Martian life could have existed in the past—but only in the past. Yet according to biologist Norman Horowitz, even though the mission dashed all hopes for finding life on Mars, its discoveries were extremely valuable: "It now seems certain that the Earth is the only inhabited planet in the solar system. We have come to the end of the dream. We are alone—we and the other species that share the planet with us. If the Viking findings can make us feel the uniqueness of the Earth and thereby increase our determination to prevent its destruction, they will have contributed more than just science." 29
Failure and Success
Seventeen years passed before the United States attempted another journey to Mars. The Viking mission had cost a staggering $1 billion, and NASA needed to address other priorities such as developing a space shuttle, which consumed the agency's time and budget. Then on September 25, 1992, Mars exploration took off again when an orbiter called Mars Observer was launched. Unlike all prior Mars-bound spacecraft, this one made the journey alone, without a companion.
Mars Observer 's main goal was to scan the entire Martian surface, taking photographs that were in much finer detail than any taken before. The spacecraft carried a wide array of sophisticated instruments, one of which was designed to measure Mars's magnetic fields. It did not accomplish its mission, however. After traveling for eleven months, Mars Observer was lost just three days before it was scheduled to reach Mars. Investigations showed no evidence of a cause, but scientists believed that a clogged fuel line had likely ruptured, crippling the spacecraft and ending all transmissions.
The loss of the orbiter was a tremendous disappointment for NASA scientists. And at a cost of $980 million, it was an expensive failure. Still, the United States remained firm in its commitment to continue exploring Mars. Two years after the mission failed, NASA announced an aggressive exploration plan called the Mars Surveyor Program. Over the following decade, numerous spacecraft would be sent to Mars with the goal of gaining a more in-depth understanding of the red planet and its history. Future voyages would include orbiters as well as orbiter/lander combinations, with each mission more advanced than those before it.
The first of NASA's missions was the Mars Global Surveyor (MGS) orbiter, which was launched on November 7, 1996. With an immense amount of computing power and memory, MGS was the most powerful spacecraft ever sent to Mars. Yet in spite of its vast capabilities, the orbiter was tiny compared with its predecessor—just twenty-three hundred pounds. Its objective was to learn more about Mars's surface and atmosphere and gain a greater understanding of what had caused such drastic changes over the planet's history. During its orbit, MGS was programmed to monitor global climate
The orbiter reached Mars eleven months after launch and began to transmit data back to Earth. As of late 2003, MGS had taken more than 150,000 photos of Mars, many of which showed such intricate detail that objects no bigger than a school bus were visible. Among its many discoveries were crescent-shaped dunes and gullies, channels, and other indentations in the planet's surface that suggested the presence of water beneath the ground. It also photographed seasonal climate changes and weather-related phenomena such as massive, swirling storms of fine red dust.
One of the scientific instruments aboard MGS was a laser altimeter (an instrument that measures elevation), which gave scientists their first three-dimensional view of Mars's polar ice caps. Another instrument called a magnetometer was designed to measure the planet's magnetic field. As the spacecraft swooped within seventy miles of the surface, the instrument recorded peculiar pockets of magnetism in the most ancient Martian terrain. From this, scientists inferred that Mars once had a magnetic field; as the planet changed over time, remnants of the magnetism were preserved in fossilized rocks.
Mars Global Surveyor 's mission was scheduled to last four years, but its longevity far exceeded scientists' expectations. As of 2004 the spacecraft was still transmitting data, and over the course of its orbit, MGS collected more information about Mars than all previous missions combined.
A One-Ton Beach Ball
At the same time MGS was being developed, another Mars mission was in the works. Since the time of the Viking mission, the United States had not sent a lander to explore the Martian surface. NASA scientists wanted to resume the exploration, but they needed a solution that was more affordable than the Viking mission. Their answer was Pathfinder , a relatively low-cost spacecraft that would leave Earth via a launch vehicle, spiral millions of miles through space, and land on Mars without a companion to assist with the landing. Pathfinder would not make the voyage alone, however. A tiny roving robot named Sojourner was traveling along, and once the spacecraft had safely landed on Mars, the rover would set out on its own to explore the surface. The mission had both technological and scientific goals: to prove that relatively low-cost spacecraft could successfully land on their own and to explore the Martian surface and garner valuable data about the planet's history. Pathfinder 's success would also pave the way for future Martian exploration, including voyages that carried humans.
Figuring out how to land Pathfinder was the biggest challenge faced by NASA scientists. Without an orbiter to scout the surface, the right landing site had to be determined ahead of time. After screaming through space at sixteen thousand miles per hour, the lander would open a huge, billowing parachute and fire several retro-rockets to slow its descent through the Martian atmosphere. But even then, the two-thousand-pound craft would hit the surface at speeds of fifty to one hundred miles per hour. Something was needed to cushion the impact, and scientists decided to expand on technology used in automobiles: an air bag system that would inflate just before landing. Because the Martian terrain was known to be extremely rough, the air bags had to be strong enough to resist tearing or puncturing if the spacecraft landed on jagged rocks or boulders.
On December 4, 1996, Pathfinder blasted off and headed for Mars. Exactly seven months later, on the Fourth of July, it passed through the Martian atmosphere, shed its heat shield, and unfurled its enormous parachute. Ten seconds before it slammed into the surface, a massive cluster of air bags inflated and encased the lander in a protective cocoon. Like an enormous rubber ball, Pathfinder dropped to the ground and bounced about fifty feet into the air. After fifteen to twenty more bounces, the lander rolled around the surface and then finally came to a stop. When it transmitted an "A-OK" signal back to JPL headquarters, the packed control room erupted into applause, cheers, and tears over an event that was both incredible and historic.
Roaming the Surface
Once the air bags had deflated, Pathfinder 's three panels opened up like the petals of a flower and its camera began to take pictures. Scientists could see dark rocks, red dust, a pale yellow Martian sky—and the deflated air bags billowing out around the lander instead of retracting. With its off-ramps blocked, Sojourner was stuck on the lander and could not roll off. Ground engineers fixed the problem by directing one of the panels to open further, which allowed the air bags to fully retract. By the end of the second day (or sol, as a Martian day is known), Pathfinder 's camera was transmitting images that showed Sojourner rolling down the ramp and making its first tracks in the Martian soil, with ground controllers using remote control to "drive" it from Earth.
The tiny Sojourner was no bigger than a microwave oven and weighed just twenty-three pounds, but NASA scientist Ricaurte Chock describes it as the real star of the show: "This is the first time that anybody has operated a wheeled vehicle on another planet, and I'm pleased to tell you that it set a world speed record for the fastest vehicle ever to go on the world of Mars. The speed record was . . . about one-fiftieth of a mile per hour—but that is faster than anybody has ever gone on Mars before." 30 As Sojourner wheeled along the surface, it used tiny cameras to take photos of rocks that scientists whimsically named Yogi, Boo Boo, Stimpy, Froggy, Ratbert, Barnacle Bill, Marvin the Martian, and Snukums, among others. Using an X-ray spectrometer, the rover analyzed rocks and soil to determine their chemical properties and found that the darker rocks appeared to have a high amount of a hard, glassy mineral known as silica, while the lighter ones were rich in sulfur. Also, there were rocks of varied textures, as well as rounded cobbles (rock fragments), that hinted at a warmer and wetter past.
While Sojourner focused on the Martian surface, the lander used its built-in weather station to study climate and weather patterns. It found that ice clouds were common in the morning and then dissipated in the afternoon, and surface temperatures changed abruptly between day and night. Pathfinder also confirmed that dust absorbed solar radiation in the Martian atmosphere, which partially blocked solar energy from reaching the surface.
By the time the Pathfinder mission was over, the two spacecraft had transmitted 16,500 photos of the Martian surface and 8.5 million measurements of the planet's temperature and atmospheric pressure. Their longevity also far exceeded scientists' expectations; the lander operated nearly three times longer than expected and the rover operated twelve times longer than the projected seven days. The end eventually came, however. On September 27, 1997, ground engineers received the last transmission. Both spacecraft had functioned far longer than anyone dreamed they would, yet scientists felt a great sense of loss when the mission was over. Raeburn says Pathfinder was one of the most successful missions in history: "America's space exploration program
From Disappointment to Triumph
In addition to its many valuable discoveries, Pathfinder had proven something profound: Spacecraft could journey to Mars on their own, land safely, and accumulate a wealth of information as they explored the planet's surface. With anticipation of continued success, NASA scientists prepared for the next launch opportunity in 1999.
On January 3, a spacecraft called Mars Polar Lander blasted off, carrying two instruments called probes. Just before the lander entered the Martian atmosphere, the probes would fall to the surface and deeply penetrate the ground. Then they would start transmitting information to Mars Global Surveyor , which would send the data back to Earth. After the lander had touched down, it would scoop up soil samples and analyze them and record Martian sounds on a special microphone. None of these plans came to fruition, though, because the mission failed. Ground controllers lost all contact with the spacecraft just as it entered the Martian atmosphere; it was later assumed that faulty computer software had caused it to crash while attempting to land.
Although the loss of Mars Polar Lander was a huge setback for NASA, officials viewed it as a wake-up call: In an effort to cut costs, they had not spent enough time and money on adequate testing. The result was a complete reevaluation of the space exploration program, as well as improvements to ensure that such a failure would not happen again. With a firm resolve to press on with Mars exploration, the agency began to plan the next mission: an orbiter named Mars Odyssey.
For the first time since the Viking mission in 1976, this voyage would involve both orbiting and landing spacecraft. NASA scientist Bob Mase explains why:
Before you send any landers to Mars, you want to look at the planet as a whole. We call that "global reconnaissance." . . . Orbiters can't scrape rocks and look at them microscopically, and rovers cannot traverse and image the entire planet. So, the two types of missions really complement one another. It's difficult to communicate from the surface of Mars directly to Earth. You'd need a big antenna and a lot of power. It turns out that the rovers can more efficiently send the information up to the orbiters, which are better equipped to relay the data back to Earth. 32
Mars Odyssey was launched in April 2001 and reached Mars the following October. Its function was to survey the entire Martian surface during its orbit and to be a communication vehicle for the Mars Exploration Rover (MER) mission scheduled for launch
On June 10, 2003, the first MER spacecraft blasted off on its way to Mars, carrying a rover named Spirit. Four weeks later its twin took off, accompanied by a rover named Opportunity. After traveling for seven months, the spacecraft reached Mars three weeks apart and landed in much the same way as Pathfinder: Parachutes deployed, retro-rockets fired, a massive cluster of air bags inflated, and the landers bounced like giant rubber balls upon the Martian ground before finally rolling to a stop. Then the air bags deflated and retracted, the landers' petals opened up, and Spirit and Opportunity were released from their protective housing. After seven years, America was back on the surface of Mars.
Spirit and Opportunity landed on opposite sides of the planet. Scientists chose their landing sites based on where they believed there was once liquid water. The rovers' mission was to search for and study many different types of rocks and soil that might hold clues to past water activity, including searching for minerals that would not have formed unless water had been present. Unlike in past missions, these rovers would not be searching for life but for clues to an environment that may have supported life in the past.
Once they were on the Martian surface, Spirit and Opportunity resembled strange galactic creatures. Each weighed four hundred pounds and was about the size of a riding lawn mower, with a flat-topped equipment deck, six motorized wheels, and a rotating mast assembly that served as the "head and neck." Winglike solar panels provided power for up to four hours per sol, and two rechargeable batteries furnished back-up power when the sun was not shining.
To perform their experiments, both rovers were outfitted with instruments that allowed them to function as robotic geologists. Their masts were capable of rotating 360 degrees, and small panoramic cameras mounted on top of the masts would capture high-quality close-up images of the surface. Three more pairs of cameras located on the front, back, and mast of each rover enabled them to see their surroundings and navigate around obstacles. In addition, each rover had a robotic arm that could bend and move much the same way as a human arm: with a shoulder, elbow, and wrist. Attached to the arm was a magnifying camera that allowed scientists to examine the fine structure of rocks and soil; also attached was a drill called a Rock Abrasion Tool (RAT) that was designed to grind away the outer surfaces of rocks so that the interiors could be exposed for examination. Powerful antennas located on each rover's equipment deck served as both their voice and ears, facilitating two-way communication with Earth and with orbiters in the sky above them.
From the very first day that Spirit and Opportunity started roaming around the Martian surface, scientists at mission control were thrilled with their discoveries. Opportunity dug into the rocks in the enormous Eagle Crater, while Spirit focused on a large rock named "Humphrey." Collectively, they proved what scientists had long suspected: There was no question that Mars was once soaked with water, including the likelihood that there were once saltwater seas. The rovers found evidence of sulfates and other
Spirit and Opportunity were originally scheduled to explore Mars for three months. However, in April 2004 the rovers were operating well enough for the mission to be extended for at least five more months. According to Matt Wallace, JPL mission manager, it is hard to predict when the two rovers will actually quit working, but of course, he and other scientists must be prepared for that. As for how they will handle it, Wallace says they just have to step back and say, "Hey, this is a machine. That's why we send machines to these hazardous environments before we send humans. . . . But it's always tough at the end . . . no matter how it comes or when it comes." 33