How A Sky Crane Helped NASA Nail Its Most Audacious Landing Yet
"Everyone knows the rover is named Curiosity," says Adam Steltzner. "But we had our own name for the descent stage. We called it Audacity." As the lead engineer for entry, descent, and landing on NASA's most recent mission to Mars, Steltzner was responsible for getting Curiosity through the Martian atmosphere and onto the surface in one piece. Over seven years, he and his team developed a number of interdependent systems to do just that, but their success hinged on one especially ambitious feature: a new and unproven descent-stage component called the sky crane.
The idea for a sky crane first surfaced in 1999, following the crash of the Mars Polar Lander. The device would lower a rover on tethers and place it directly onto a planet's surface. The dynamics were complex, though, and the Martian descent had already wrecked five previous missions. A tethered vehicle would have to contend with pendulum forces and wind shear, too. NASA shelved the idea. Then, in 2003, the agency announced plans for a new rover. It would be too heavy to use landing air bags, which had protected Spirit and Opportunity. Engineers would need a new system. "That fall, we had a three-day brainstorming session," Steltzner says, "and we realized we could use the sky crane. We just had to wait to use it until we were near touchdown, when we were in perfect vertical flight."
Starting in 2007, a team of about 800 began work on the descent stage, a dome-shaped, thruster-powered platform that housed the Curiosity rover and the sky crane. At a predetermined velocity, the descent stage would detach from the entry capsule and fire its eight hydrazine rocket thrusters, slowing itself from 180 mph to 2 mph. At 65 feet above the Martian surface, the sky crane would lower Curiosity from the platform on three nylon tethers. An electrical cable would ferry power and information between the two. After touchdown, cable cutters would slice the tethers, and the platform would fly off.
Unlike the Curiosity rover, whose systems teams at NASA's Jet Propulsion Laboratory could test, the descent stage and sky crane were impossible to prove out in advance. When Steltzner sat at his terminal at 8 p.m. on August 5, the devices were performing for the first time. "If we made a smoking hole on the surface of Mars, there wouldn't be much sympathy," he says. "It wouldn't be, 'Ooooh, you poor thing.' It would be, "You idiot! Of course it failed. It looked totally ridiculous!'"
But that, of course, wasn't the case. On that summer day, the sky crane performed flawlessly, depositing Curiosity at the edge of Gale Crater. For Steltzner, the landing confirmed a bold vision. For NASA, it cemented the sky crane's place in future Mars missions. The device scales up easily, and Steltzner reckons it can handle loads a half ton heavier than Curiosity. "We can deal with whatever the load is," he says. "Just give me the rover, and we'll land it wherever you need it."