By 1993, when work on Pathfinder began, the reviewers had told Spear and his team that the only way to avoid smashing through their cost ceiling was to simplify as much as possible. There would be no orbiter; like Surveyor, Pathfinder would fly directly to the surface. (An orbiter launched by NASA last November, Mars Global Surveyor, will reach Mars in September to provide a detailed look at the planet from orbit. But by then Spears hopes Pathfinder will be part of the Martian scenery.)
As much as possible, the design of the lander's heat shield and parachute are borrowed from Viking. But the landing system is entirely different: There is no retrorocket and no radar-controlled vernier engines. Instead, Spear's team chose an approach reminiscent of Ranger's survival package. Pathfinder will simply fall onto the surface, protected not by balsa wood but by a set of pressurized airbags. Airbags had been studied for space probes by JPL engineers as early as 1966. Now, Spear hoped, they would be not only the most rugged solution, but the cheapest.
Like every landing project before them, the Pathfinder team found that nothing was easy. Yes, technology had come a long way since Viking, and Pathfinder's computers and electronic components were cheaper to build. Then, too, the requirement for pre-launch sterilization was dropped, because Viking data suggested that the Martian surface is not only lifeless but chemically hostile to organic matter. And although the Viking heat shield and parachute designs had to be modified for Pathfinder, Spear says using them "saved us a ton of money." But hopes of building Pathfinder without a retrorocket proved too optimistic; without one, calculations showed, the lander would hit the surface too fast for airbags to protect it. A trio of small solid-fuel rockets had to be added, along with a simple radar altimeter to signal when they should ignite.
Meanwhile, work went ahead on the airbags, which turned out to be the biggest headache of all. By the end of 1994 a group led by JPL's Tom Rivellini had worked out their basic design. The spacecraft would be shaped like a tetrahedron, with three sides that opened like flower petals to let it right itself after landing. Each petal would contain an airbag with six spherical lobes that would be deployed just before landing. Fully inflated, the airbags would make Pathfinder resemble a bunch of party balloons. Of course, they would have to be enormously more rugged; calculations showed that the 800-pound lander could strike the surface at 60 mph.
Rivellini's group had selected a material they felt was up to the task: a fabric called Vectran, an exotic cousin of the more familiar Kevlar used in bulletproof vests. To fabricate the bags, they chose the Delaware-based ILC Corporation, whose products include NASA spacesuits. But it didn't take long for frustration to set in. Vectran's incredible resistance to stretching--the source of its strength--made it extremely intolerant of any errors in the shape of a seam. The first bags stitched together at ILC broke apart when they were pressurized.
Even as that problem was solved, it became apparent that Vectran had another weakness, this one potentially fatal. Although tests showed that a Vectran airbag would resist being punctured if a sharp rock were pushed straight into it, a rock dragged across the fabric would disrupt the weave of the fibers, making the bag dramatically more vulnerable to tearing. For that reason, it was crucial that Pathfinder strike the surface with as little horizontal motion as possible. The problem was, the Martian winds might not cooperate. Wind gusts hadn't been a problem for Viking, whose vernier engines were able to counteract them. But Pathfinder would make its final descent suspended from a parachute and aeroshell at the end of a 65-foot tether called a bridle. Blowing winds could make the lander swing like a pendulum, giving it horizontal speed when the bridle is cut. Even worse, a gust could tip the aeroshell at the moment the retrorockets fire, propelling Pathfinder toward the ground at a shallow angle.
Fortunately, there was a tool that no previous landing project had available. Sam Thurman had created a computer program capable of simulating the entire landing sequence. It showed that the parachute-bridle-lander combination would respond to winds not by swinging from side to side but by vibrating like a plucked guitar string, with the lander nearly unaffected. And if the aeroshell tipped, Thurman found, it would quickly right itself.
Still, the risk of horizontal speed at impact continued to cause worry. Says JPL's Rob Manning, the engineer in charge of getting Pathfinder down safely, "We realized we didn't want a regular airbag--we wanted a Michelin tire." Or at least something as strong as a steel-belted radial; the airbags also had to be small enough to be packed with the lander inside its aeroshell, and light enough not to exceed Pathfinder's stringent weight limits.
In late 1994, Rivellini's group began conducting drop tests of the bags with a model lander. To simulate Martian atmospheric surface pressure--only 0.7 percent that on Earth--Rivellini's group turned to the largest vacuum chamber in the country, near Sandusky, Ohio, operated by NASA's Lewis Research Center. Inside the cavernous chamber, a model Pathfinder was suspended 80 feet above the floor, connected to a set of giant bungee cords that would allow it to plunge downward until it crashed into a tilted platform studded with sharp rocks.
Just as Rivellini had feared, the rocks tore the airbags, even though the bags had been covered with a second layer of material to resist abrasion. He still sounds amazed when he talks about how badly the Vectran fared. After one run at speeds approaching those of actual touchdown, "it looked like a bear walked up to this thing with its claws and just shredded the hell out of it," Rivellini recalls.