The Least Crazy Option

Saturday, August 4, 2012
Pasadena, California

JPL’s Adam Steltzner, the lead engineer for MSL’s Entry, Descent and Landing phase, uses a model of the spacecraft to explain hypersonic aero-maneuvering, one of it’s innovative technologies. Credit: Henry Bortman

“It looks crazy,” admitted JPL’s Rob Manning, Flight System Chief Engineer for NASA’s Curiosity rover, as he outlined the spacecraft’s entry, descent and landing sequence at a recent press briefing.

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.

The Curiosity rover touches down on the martian surface in this artist rendition. Curiosity’s landing on Mars will be the first time that the sky crane system is employed. Credit: NASA/JPL-Caltech

About 1.6 km above the surface, the parachute, and the protective backshell to which it is attached, will be jettisoned as well, leaving only the descent stage and the rover.

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.

Gale crater, the landing site for the Mars Science Laboratory (MSL) mission. The ellipse indicates the intended landing area on an alluvial fan. It may take the MSL rover several months to a year to drive to the base of the mound, depending on where the rover lands in the ellipse and how many stops are made along the way. Credit: NASA JPL

Because it is impossible to predict what atmospheric conditions will be like on the day a spacecraft arrives, in the past it was impossible to know at exactly what speed the craft would descend through the atmosphere. “What that’s meant in the past when we’ve gone to Mars is that we don’t know where exactly we’re going to land,” Manning said. “In previous missions we had a footprint of uncertainty that was very, very large, 100 km or more of uncertainty.”

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.