The Least Crazy Option
Saturday, August 4, 2012
Curiosity is scheduled to touch down on the martian surface at around 10:30 pm PDT on Sunday, August 5. After hurtling through the martian atmosphere, decelerating, over the course of seven minutes, from a top speed of 13,000 mph to a virtual halt, the one-ton rover, in a configuration NASA refers to as a “sky crane,” will be slowly lowered onto the surface of Mars on a trio of nylon ropes.
Curiosity’s landing on Mars will be a first for the sky crane. Previous landed missions to Mars have used either airbags or legged landers.
Pathfinder/Sojourner, Spirit and Opportunity all used airbags. Each of these spacecraft was enclosed inside a metal structure wrapped in airbags, the whole assemblage hanging down below a set of rockets and a parachute. A few meters from the surface, the airbags were cut free to bounce and roll to a halt. The airbags then deflated, the structure opened and the rover rolled out onto the surface.
But airbags were ruled out for Curiosity, formally known as MSL, or Mars Science Laboratory. “She’s too big,” said Adam Steltzner of JPL. “Here on Earth we don't make material, that we’ve been able to find, that’s strong enough” to make airbags that wouldn’t shred on impact. Steltzner is the Entry, Descent and Landing (EDL) Lead for the MSL mission.
The two Viking landers and Phoenix were legged landers. Each of their scientific payloads plonked down onto Mars atop a tripod of rigid legs.
But “legged landers have some stability challenges. They don't deal with rough terrain very well,” said Steltzner. And they’re top-heavy, a problem made worse when the payload is a 900-kilogram rover.
The solution was to develop a novel strategy, one that will recruit Curiosity itself – or its wheels, at least – into service as a lander.
MSL will go through several different changes in configuration on its descent through the martian atmosphere. Some of the early stages will largely duplicate past mission. But once MSL has slowed enough for its parachute to deploy, and its heat shield to be jettisoned, the rover will be exposed, tucked up under the descent stage of the spacecraft. That’s new.
At 20 meters above the surface, with rockets keeping the spacecraft at a steady velocity of 0.75 m/sec, roughly the pace of a slow, ambling walk, the spacecraft will morph into its sky-crane configuration. The rover will be lowered down below the descent stage on a trio of 20-foot-long nylon ropes.
As the sky crane reels out to its full height, Curiosity’s six wheels, tucked in tight during the long journey to Mars, will unfold downward from the body of the rover.
Curiosity’s wheels, like those of Sojourner, Spirit and Opportunity, are arrayed in a rocker-bogey system, each wheel able to shift independently up and down by as much as a meter and a half. During the rover’s prime mission, this wheel configuration will enable the rover to traverse the erratic martian landscape.
But first Curiosity’s wheels will play a novel role: they will serve as the spacecraft’s initial points of contact with the martian surface. At touchdown, their ability to adapt to variable terrain will provide the stability needed to keep the rover from flipping over.
Another first-time technology deployed by the MSL mission, known as hypersonic aero-maneuvering, will occur high up in the atmosphere, before the parachute deploys.
Natural variations in the temperature, and consequently the density, of the martian atmosphere affect the rate of a spacecraft’s descent. “On a hot day the atmosphere can be lower density, higher, taller, more diffuse, and on a cold day, it can be more concentrated,” Manning explained.
That’s because previous spacecraft didn’t have any way to steer themselves as they hurtled through the atmosphere. MSL’s aeroshell, however, is heavier on one side than on the other, and it has a set of small rockets that enable it to maneuver the heavy side up or down. This, in turn, increases or decreases lift on the spacecraft, causing MSL to speed up or slow down its rate of descent. These small in-flight corrections, which take place under the automatic control of the craft, allow MSL to target a landing ellipse only 20 km long, and thus to reach a scientific target that couldn’t previously be targeted safely.
So although MSL’s landing strategy may seem crazy, it was “the result of careful, long, reasoned thought with rooms full of very, very bright engineering people,” Manning said. And in the end, “it was really the least crazy option.”
Soon, we’ll know whether or not it works.