Hard Landings

When your assignment is to put a space probe on another planet, be prepared to sweat.

The first picture taken by Viking 1 on the surface of Mars, July 20, 1976. (NASA)
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On June 19, 1976, Viking 1 went into orbit around Mars. Three days later, when the orbiter's first images were received at JPL, scientists were stunned--not only because the pictures showed more detail than any previous views of Mars, but because the planned landing site was far rougher than anyone had expected. So began a feverish effort to find a safe landing site, using the Viking 1 orbiter's cameras. Graduate students poured over each new image with magnifiers, tallying the smallest boulders and craters for hazard analysis. Scientists argued about how to interpret radar soundings made with Earth-based radio telescopes.

Viking's earliest possible landing date, by coincidence, had turned out to be July 4. Now NASA was counting on the landing to be part of the nation's bicentennial celebration. But as the hunt for a new landing site dragged on, Martin informed NASA administrator James Fletcher that the July 4th date was out of the question. By chance, Viking 1 was ready to land on a day that had its own significance: July 20, seven years to the day after the first humans had landed on the moon.

The moment of truth came just after 5 a.m. Pacific time, as the lander, encased in its heat-protective aeroshell, sped into the Martian atmosphere. Within minutes deceleration forces on the lander built to 8.4 Gs, then slacked off somewhat as the aeroshell sped toward Mars' Chryse Plains. At 21,000 feet, with Viking still traveling at supersonic speed, a mortar fired to deploy the parachute. Less than a minute later the parachute was cast off as the lander's three vernier engines ignited.

Like their Surveyor predecessors, Jim Martin and his people were reduced to waiting. Because Viking's radio signals took 19 minutes to reach Earth, it was a bit like sweating out the delayed broadcast of a crucial football game; by the time the first descent data appeared on their screens they knew that Viking would have already landed--or crashed. Then came exultant words: "Touchdown! We have touchdown!"

When the first image from the surface of Mars appeared on the monitors, NASA knew it had been lucky again: A boulder almost as big as the lander stood only 30 feet away. Less than seven weeks later, Viking 2 alighted safely on Mars' Utopia Plains, but instead of the sand dunes the geologists had promised, its pictures showed a jumble of rocks, much to Martin's chagrin. But no matter: The Mars landings had been more successful than almost anyone had dared hope.

Even so, another 16 years would pass before NASA decided to return to the surface of Mars. Not until 1992 did the space agency approve Mars Pathfinder as the first of a series of robotic landings. As project manager Tony Spear soon learned, Pathfinder was expected to live up to its name and test a new type of landing system. Spear's assignment got even tougher when he was given a budget ceiling of $150 million in 1992 dollars--five percent of Viking's cost. Says Spear, "We had no idea whether we could do that or not."

The Pathfinder used a risky new method of arrival -- bouncing to a halt inside a protective cocoon of airbags.

Whatever they lacked in certainty Spear's people made up for in motivation. Pathfinder represents a new generation of scientists and engineers, exemplified by 36-year-old Sam Thurman. An engineer who coordinated the design of Pathfinder's entry, descent, and landing systems, Thurman remembers being galvanized by the Viking landings as a teenager. When he came to JPL 10 years ago, nothing like a Mars lander was in the works. "I was really getting worried for a while that the most exciting era in the space program had come and gone, and I missed it," Thurman says. Pathfinder was the opportunity he had been waiting for.

Thurman and his teammates had only 38 months to redefine the art of landing on Mars, and they would have to do it with a fraction of Viking's workforce. They also had to stay motivated in the face of some healthy criticism from a NASA review panel that included Viking veterans like Jim Martin. (Engineers who have already braved the challenges of landing on another world can be tough to impress. At a 20th anniversary reunion last summer at the National Air and Space Museum, one Viking veteran expressed doubt that the public would be excited about Pathfinder or its rover, which he puckishly dismissed as "a Tonka toy." Gesturing toward a Viking replica, he beamed with pride. "Viking was big! That was a real lander.") "They were brutal," Pathfinder deputy project manager Brian Muirhead recalls wryly. "They were sharp guys and they were getting paid to beat us up. I think they sensed that it was for our benefit."

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