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)
Air & Space Magazine

Five minutes. That's how long it will take for Mars Pathfinder to make this Fourth of July either a day of celebration or a day of sorrow for Tony Spear. Just five minutes after speeding into the Martian atmosphere at 17,000 mph, the Pathfinder lander will strike the Red Planet. If it survives, this robotic emissary, which was launched by a Delta rocket last December, will undertake the first exploration of Mars' surface in more than two decades. If it does not, Spear and his team of engineers and scientists--who labored for more than five years to create Pathfinder and send it to Mars--will shoulder one of NASA's greatest disappointments. Of those five minutes, Spear says: "I don't know how I'm going to stand it."

But Spear is only the latest in a long line of engineers to face the uncertainty that precedes every robotic landing on another world. It's an anxiety that began almost four decades ago, when the cold war was being waged in space. In the years after Sputnik, with the United States stinging from one Soviet space first after another, engineers at NASA's Jet Propulsion Laboratory in Pasadena, California, dreamed of scoring a first of their own. Although the Soviets had, in September 1959, already hit the moon with their Luna 2 probe, a true moon landing had yet to be achieved. That was the goal of JPL director William Pickering, who pushed for a mission to deliver a package of scientific instruments to the lunar surface. Early in 1960, NASA headquarters gave formal approval to JPL's Project Ranger.

In terms of physics, the concept for any lander is simple to describe: Just as it reaches its destination, it must undo the work of the launch rocket, canceling the kinetic energy the launch and the gravitational pull its destination give it. But Ranger project manager James Burke and his team found that executing this task was anything but simple. By the time Ranger reached the vicinity of the moon it would be traveling at 4,500 mph. Ideally the lander would settle onto the moon at only a few miles per hour. But this so-called "soft landing" was beyond the reach of existing technology. The best Ranger could do was fire a blast from a solid-fuel braking rocket to slow its descent before its lander simply fell to the surface--a "hard landing."

Solid-fuel rockets were already being used in military airlifts to supplement parachutes when tanks and other massive objects were dropped out of airplanes. But the JPL engineers knew that solid rockets, while simpler than liquid-fuel ones, were also more unpredictable. No one could be sure a solid rocket would deliver the amount of braking needed to counteract all of the lander's excess speed. Furthermore, nobody knew how to predict precisely how fast the spacecraft would be going relative to the moon, or even the exact location of the moon itself.

With all these uncertainties, Burke figured that Ranger might strike the moon at speeds up to 200 mph. He and his team began talking about developing a rugged spherical capsule capable of withstanding such an impact. If this "survival package" seemed a less than elegant plan for humanity's first landing on the moon, Burke didn't mind. "All we were thinking about," he says, "was 'Let's get a transmitter down so we can prove we're there.' "

But how to protect sensitive scientific instruments from a crash as violent as an Indy race car hitting a concrete wall? To identify the best energy absorber, a variety of materials, including aluminum honeycomb, cardboard, and, in Burke's words, "anything crushable," were subjected to tests such as being dropped from a helicopter and slammed around with laboratory equipment. The victor, by a surprisingly wide margin, proved to be blocks of balsa wood, oriented with the end grain radiating out around the sphere for maximum energy absorption.

By the summer of 1960, a 26-inch-diameter sphere weighing 92 pounds began to take shape at a division of the Ford Motor Company in Newport Beach, California. Attached to the capsule would be a solid-fuel retrorocket, which was to ignite when Ranger was 10 miles above the moon. Ten seconds later, after slowing the lander almost to a hover, the rocket would burn out and be cast off. Pulled by the moon's gentle gravity, the sphere would fall the remaining 1,100 feet to the surface, striking at a speed of about 75 mph. Cushioned by a six-inch layer of balsa wood, the lander would bump and roll to a stop. Inside, floating on a thin layer of water, a one-foot-diameter fiberglass sphere containing a seismometer, radio, and batteries would right itself and begin transmitting.

But the lander was never able to prove it was up to the task. Rangers 1 through 6, including the only three to carry survival packages, all failed. Several missed the moon because of malfunctions in the Atlas Agena launcher. Launch calculations showed that Ranger 4 reached the lunar surface, but the spacecraft malfunctioned shortly after leaving Earth in April 1962 and was unable to return data. Although Rangers 7 through 9 were successful, taking high-resolution photographs on the way down to a crash-landing on the moon, none carried survival packages--in part because JPL was by then working on a lunar lander called Surveyor.

Looking back, Burke sees the Ranger lander as a challenge undertaken before its time. "We were trying something that was too complicated," he says. "Our reach was exceeding our grasp."

But Surveyor made Ranger look simple; it would have to execute the first soft landing. Surveyor would build on Ranger's braking technology, using a solid-fuel rocket to slow from an initial descent at more than 6,000 mph to 240 mph. However, the lander would have to aim the thrust of its braking rocket directly along its flight path to avoid tumbling. The task was daunting. "We were starting essentially from scratch," says engineer Leo Stoolman, who managed Surveyor's design and construction at Hughes Aircraft in El Segundo, California. When the performance of the new Atlas-Centaur launch vehicle fell short of predictions, design teams were forced to trim hundreds of pounds from Surveyor's allowable weight. More than 60 percent of the spacecraft's final 2,200-pound weight went to the braking rocket; what remained was barely enough to carry out a scientific mission after landing.

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