Picture what would happen if you threw a digital camera out of your car while barreling down the interstate, and you'll get an idea of the challenge NASA faced in landing a working rover on Mars.
NASA's solution - an almost comical, bouncing bundle of protective airbags - proved itself last month by safely delivering two $410 million rovers to their assigned landing zones.
With those successes, and an earlier one, planetary scientists hope airbag landings will open up rougher, rockier regions of the Red Planet to future robotic explorers.
"We now have had successful airbag landings on Mars three times - Pathfinder [in 1997], Spirit and Opportunity," said Tracy K.P. Gregg, a University at Buffalo volcanologist. "If something happens three times, you've got it. It's in the bag. To me, it suggests we can start pushing the envelope a little bit."
On the plains where Spirit and Opportunity landed, "you're looking at stuff that's today," Gregg said. "If you want to know what's been going on for hundreds of millions, or billions, of years, you've got to go deeper. A cliff is a natural way to see what's inside."
Not everyone agrees.
"Hitting sharp rocks on a steep slope is an airbag engineer's worst nightmare," said Peter H. Smith, a planetary scientist at the University of Arizona. "We're always going to be risk-averse on landing."
Smith is principal investigator for NASA's Phoenix mission, which will be the next to land on Mars, in 2008. His spacecraft will land with thrusters and legs inherited from a canceled 2001 mission. "Both systems are good, and they have their purposes," he said. "We're very happy with what we've got."
The way to get into rougher places, he said, is to learn to make more precise landings on safe terrain that is close to the geologists' targets, and then send in the rovers.
Gregg, who chairs NASA's planetary mapping group, nonetheless plans to propose a future mission that will employ airbags to land on the slopes of a 9,800-foot Martian volcano called Apollinaris Patera ("Apollo's Saucer"). Two billion years of volcanic heat and the presence of liquid water - suggested by gullies carved in the mountain's flanks - might have made the place a sort of Martian "tide pool," ideal for the evolution of life, Gregg said.
The volcano was scratched from previous lists of landing sites because it looked too dangerous. Now, Gregg and others believe that the science community will be more willing to risk a landing there, or places like it.
Airbags, she said, provide the extra margin of error engineers need to survive rough terrain that might topple a legged vehicle like NASA's 1976 Viking landers. For example, she said, on Apollinaris Patera "there are gullies that look like they were cut by water. A lander [with legs] would fall into one of those gullies, and we would lose it. With an airbag, who cares?"
The Soviet Union pioneered airbag landings with its 1966 Luna 9 landing on the moon. The Russians were ready to try it again on the Mars 96 mission, but the spacecraft malfunctioned and fell into the Pacific.
Europe's Beagle 2 lander had airbags, too, but the craft has been missing since its Dec. 25 landing.
NASA has had excellent luck with its airbag systems. "The biggest challenge was finding a material strong enough, yet light enough, to withstand high-speed impacts against very rough surfaces," said Tom Rivellini, lead airbag engineer on Pathfinder and a mechanical systems engineer on the current rovers.
After tinkering with Mylar and piano wire in the 1960s, the space agency considered Kevlar, the fabric used in bulletproof vests. It's strong, Rivellini said, but "it cracks very easily and ... loses a lot of its strength when you fold it."
Then, in the early 1990s, the Jet Propulsion Laboratory in Pasadena, Calif., looked at a new fabric called Vectran, developed by the boating industry as a strong, lightweight and flexible sail cloth. "It has all the same benefits of Kevlar, but had the significant advantage of not being susceptible to flex-cracking," Rivellini said.
Vectran airbags were first used on Pathfinder in 1997. After NASA's Mars Polar Lander failed in a 1999 landing attempt with thrusters and legs, airbags were ordered for the 2004 Mars landings. Manufactured by ILC Dover Inc., of Frederica, Del., they have eight layers of material - six outer layers of a dense weave protecting inner bladders with two lighter layers.
Inflated, the cushions look like a bundle of 24 separate bags, but there are just four, with six lobes each. The gas bladders are interconnected so the landing forces can be shared.
The airbag idea seems simple, but the execution is complex. On the seven-month cruise to Mars, the airbags aboard Spirit and Opportunity were folded tight against the spacecraft. As the two spacecraft entered Mars' atmosphere, their speed was slowed first by friction, then by parachutes.
At a preset altitude, four explosive devices severed the cords that bound the airbags, and three gas generators - small, solid-fuel rockets made by ATK of Elkton - ignited. In 1.5 seconds, they filled the airbags with a mixture of water vapor, carbon dioxide, carbon monoxide and soot.
Five seconds later, and just a few stories above the surface, three ATK retro-rockets attached to the parachute bridle fired, slowing the landers' descent from 150 mph to zero. At that point, the airbag-shrouded landers were cut loose from the parachutes and fell 28 feet to the ground.
Rivellini said you can think of the 1,000-pound lander as "a very fragile bank vault with a very sensitive computer inside, and you're throwing it against a surface at highway velocities."
After nearly a minute of bouncing (Spirit bopped 28 times across Gusev Crater), each of the landers stopped, and the airbags deflated. Because each lander is a pyramid with three sides and a base, engineers knew it would likely come to rest on one of the three "petals" that enclosed the rover.
Sensing which way was down, the lander simply ordered the petal on the bottom to open first. A powerful motor then cranked it open until the lander flopped onto its base.
As successful as they've been on Mars, NASA's airbags would not be practical, say, on airless Mercury, where parachutes are useless, or on low-gravity asteroids where a lander might bounce back into orbit. There are also size limits to the technology, he said. Large payloads would require bigger, more complex airbags that would be too hard to retract. And airbags are heavy, compared with thrusters and legs, limiting the science payload.
But to really open Mars up to exploration, Rivellini said, the airbag is only one of the technologies NASA will need to consider.
To reach the planet's higher elevations, where there is less air to slow a lander's descent, NASA will need better heat shields and parachutes. And to reach the far north or south, where the sunlight is weak or nonexistent in some seasons, future landers might need small nuclear "batteries" to generate electricity.