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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Fortunately, ILC seamstress Eleanor Foraker--who had stitched the moon suits worn by Neil Armstrong and Buzz Aldrin--was on hand to make repairs at a momentÕs notice. That was no easy task; Foraker had to use special ceramic shears for the job. As Rob Manning explains: "You can't use scissors on this stuff." And fabric designed not to be penetrated was "godawful to stitch," Rivellini adds.

The failures continued into the summer of 1995, with time running out to build and test the lander's components. No computer simulation could solve the airbag problems; Rivellini's group had to rely on trial and error as they experimented with different thicknesses and densities of fabric. Finally they hit on a breakthrough: Multiple layers of lightweight fabric worked better than a single heavy layer.

Even as this hopeful sign emerged, a slew of other airbag-related problems demanded solutions. How would the bags be packed into the tiny space between the lander and its aeroshell? At ILC, a former Air Force paratrooper devised a folding scheme. What about inflating them quickly and safely? That called for hot-gas generators, which burn the same type of propellant used for the retrorockets. Rivellini's group knew Pathfinder would land during the Martian nighttime, with temperatures hovering at 112 degrees below zero. As the gas inside the bags chilled, they would lose pressure in moments. To prevent this, a second set of gas generators was added. Of course, the bags would have to be deflated after landing. And finally, Rivellini had to solve a problem that some at JPL predicted would be Pathfinder's undoing: how to retract the deflated bags so that the lander could open its petals and begin operating. After a series of low-tech experiments ("trash bags," Rivellini explains) he devised a set of Vectran cords to be routed through loops attached to the bags' inner walls; after touchdown the cords will be pulled into the lander using a winch.

By the end of 1995 everything had come together. Rivellini and his colleagues had created a bag that didn't break, with up to four layers of Vectran in the most vulnerable places. In August 1996 Pathfinder was shipped to Cape Canaveral to be prepared for its December launch. Its next stop would be the mouth of the Martian channel called Ares Valley, the very place Viking 1 was originally slated to land. All the ingredients seemed to be in place for a bold and ambitious landing, if that is what the space agency is looking for. But as Tony Spear recalls, one high-level NASA manager seemed to have mixed feelings about that: "He told me, 'Don't you dare fail. If you do, I'll shoot you on the [JPL] mall.' "

Jim Martin, who understands that kind of pressure, nevertheless has doubts about whether Pathfinder will succeed. "If I had to put a probability on it," he says, "I'd have a problem." Much of that familiar uncertainty, Martin says, could be avoided if future landers had some kind of hazard avoidance system, borrowing from the technology now used in "smart" bombs and missiles. It would be expensive, Martin realizes, but "having a failed lander mission is not cheap either."

There is much at stake beyond Mars Pathfinder. In 1998 NASA will launch a lander resembling a smaller version of Viking to set down near the ice cap at the planet's south pole. And as early as 2005 will come the Holy Grail of planetary exploration, a mission to retrieve a sample of Martian rock and soil. Now that scientists may have discovered evidence of fossil life inside a meteorite from Mars, there is more interest than ever in a sample-return mission, an engineering task that presents its own knotty problems. The sample-return lander may use some combination of the techniques worked out for Pathfinder and the Mars '98 lander.

But Tony Spear won't be thinking about any of that on July 4, as Pathfinder comes screaming into the Martian atmosphere. Spear, like everyone involved, knows that in many ways Pathfinder is the most complex lander yet devised. After all the testing his team has done, he says, he is confident that the fast-paced events in PathfinderÕs descent will all go off without a hitch, as they must. But when the time comes, they will still seem to take forever, as Pathfinder endures the heat of reentry, deploys its parachute, and lowers itself to the end of its bridle.

Just eight seconds away from impact, the airbags will inflate--explosively, in an instant. Four seconds later the retrorockets will ignite, lighting up the Martian night with their own Fourth of July fireworks, to slow the lander to a halt a little less than 50 feet above the ground. Severing the bridle, Pathfinder will fall onto Mars, bouncing for perhaps two minutes before coming to a stop, its 200 million-mile journey finally over. Just as their predecessors did 21 years before, Spear's team will have to wait long minutes for the radio signals to travel to Earth before they know if they were successful. Hours later, as the sun rises over Ares Valley, Pathfinder will come to life and deploy a 22-pound rover called Sojourner, and the exploration of Mars will begin again.

Will it work? "Yes," says Spear without equivocation. "But I'm still scared to death."

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