The Mars Dilemma

When astronauts finally reach Mars, will they be able to land?

The Mast Camera on NASA’s Mars rover Curiosity captures the ruggedness of the Martian surface in this February 2014 shot of Dingo Gap in Gale Crater. Astronauts would find a barren world with average temperatures of -67 degree Fahrenheit and dust storms that last for months. (NASA/JPL-Caltech)
Air & Space Magazine

Under an exploration program begun in 1993, NASA has sent nine spacecraft to Mars, four to study the planet from orbit and five to land on its surface. Although the program has had its share of failures—two other spacecraft were launched and lost—it has also had spectacular successes. Mars Pathfinder, which landed in 1997, was aptly named. Spirit, Opportunity, and the Mars Science Laboratory Curiosity have all followed. Author Rob Manning, who has worked at the Jet Propulsion Laboratory since 1980, was chief engineer for Pathfinder and Curiosity. The following is excerpted from his recent book (with William L. Simon): Mars Rover Curiosity: An Inside Account from Curiosity’s Chief Engineer. —THE EDITORS

In 2004, the White House announced a far-reaching, long-term vision for space exploration that among other things called for a “human and robotic program to explore the solar system and beyond.” The concept included preparations for the human exploration of Mars. At the time, many people were saying that they already knew how to land humans on Mars. You might think that as the guy who had landed more stuff on Mars than anyone else on the planet, I would know what it would take to land anything there. The truth was that I knew about landing small things, things the size of a dining room table. I knew little about landing anything big enough to carry people.

In response to the White House announcement, NASA began looking for people to lead a series of 15 “capability” roadmap teams—formed around such topics as in-space transportation, human health and support systems, and scientific sensors and other measuring equipment—to help set long-term directions for the agency’s focus in the coming decades. One of the teams was to explore “human planetary landing systems,” and not long after the announcement, I was named to head that panel.

I started by lining up two co-chairs. The first was Claude Graves, who had been a leader in developing the entry systems for the return of Apollo’s astronauts from the moon to Earth. (His death in 2006 was a huge loss to the space program.) My other choice was Harrison Schmitt, known as “Jack.” He had walked on and driven around the moon as part of the Apollo 17 crew, the last Apollo moon mission. He held a doctorate in geology from Harvard University and had also served in Congress as a senator from New Mexico.

For the rest of the panel, the group I rounded up included some core space shuttle specialists in entry, descent, and landing; shuttle astronauts, and people who had built Mars landers, including Viking, the first one to land on the planet, in 1976. I also brought in a few key thinkers from industry and academia, including an old friend, Bobby Braun, a Georgia Tech professor who had helped bring entry-system aerodynamics engineering back from the edge of extinction. Others included Marshall Space Flight Center’s Michelle Monk and old hands like Dick Powell of NASA’s Langley center in Virginia, who had worked on early entry, descent, and landing—EDL—concepts for Mars and would help make sure that our team’s work was solid.

In all, I put together 50 EDL people from around the country. This would be the first time in NASA history that EDL people who built “the little ones” like Pathfinder, Spirit, and Opportunity would be formally putting their heads together with people who built “the big ones”— the spaceships that had carried humans to the moon and back, as well as the shuttle.

The author (seated, with red lanyard) celebrates the landing of the Mars Science Laboratory’s Curiosity rover on August 6, 2012, from the control room at NASA’s Jet Propulsion Laboratory. (NASA )
The skycrane maneuver (illustrated) that put Curiosity on the surface of Mars borrowed landing techniques used on both the Viking missions and the Mars Pathfinder. (NASA )
Curiosity (in a self-portrait taken in 2012 by the Mars Hand Lens Imager) and its predecessors--Sojourner, Spirit, and Opportunity--have encountered the difficulties in traversing the Martian terrain that humans may one day face. (NASA/JPL-Caltech)

In December 2004 , the panel gathered for its first meeting: a day-and-a-half session at the California Institute of Technology, not far from JPL. Human missions to Mars would need to land many tons of gear in order to provide for a few astronauts to survive on the planet. In fact, given the struggle our Mars Science Laboratory team was having with a one-ton rover, I had trouble imagining how it would ever be possible to put on Mars the kind of supplies that would be required.

I am often asked why landing on Mars is so much harder than landing on the moon or on Earth. To land on the moon, the astronauts entered lunar orbit and fired retro-rockets aimed more or less opposite to their direction of travel. As their spacecraft slowed, it descended toward the surface. The landing isn’t trivial, but it’s reasonably straightforward. To bring a lander back to Earth, retro-rockets aren’t needed, because Earth has an atmosphere. Most Earth landers can eliminate more than 99 percent of the speed of orbit simply by slowing down with a heat shield. For the last one percent, we can use parachutes (as did Soyuz) or wings (as did the space shuttle).

Mars is like neither the moon nor Earth, but is annoyingly in between. It has too much atmosphere to land as we do on the moon and not enough to land as we do on Earth. The thickness of the Martian atmosphere at the surface is similar to what a mountaineer on Earth would feel if standing on top of a mountain 130,000 feet high—four and a half times higher than Mount Everest. At that altitude, the space shuttle is still screaming along at over 4,000 mph. How do you slow down quickly enough that by the time you reach the ground you’re not still going over 1,000 mph?

That’s the problem we face when designing our Mars landers. It’s the reason our machines are such Rube Goldberg-esque contraptions and why the seven minutes of entry, descent, and landing are so terrifying. We have to combine all of the tricks we use to land on Earth (heat shields, parachutes) with the techniques we use to land on the moon (retro-rockets, airbags), among many others. For the Mars Science Laboratory (later renamed Curiosity), we had decided to combine two systems, a precision-controlled propulsion system that had been pioneered on Viking and another system designed for Mars Pathfinder.

On Pathfinder, instead of falling away from the entry capsule and landing on legs, the lander was lowered on a rope, and hung under the parachute while falling through the thin Martian atmosphere at more than 150 mph. We kept the lander suspended under the parachute until the last possible second. At about 500 feet above the ground airbags were inflated and three big solid rockets fired for a couple of seconds. The lander came to almost a dead stop some 50 feet up. From there it fell free and bounced over the terrain for the next minute or so until coming to rest.

If a Pathfinder-like lander or rover had more precise control of its rockets, as the Viking spacecraft did, this propulsion system as part of an “entry and descent stage” might be able to lower the lander closer to the ground than Pathfinder’s and slow its descent enough to make a supersoft touchdown. No airbags would be needed. The design was a rover-on-a-rope concept, made more practical by Miguel San Martin, the best guidance-and-control engineer and architect at JPL.

In the same year that I convened the panel studying human landings on Mars, another group evaluated this landing system for MSL. The evaluators included an astronaut and a helicopter pilot as well as experts in mission assurance, propulsion, radar, reliability, systems, guidance, multi-body dynamics, and kinematics. The helicopter pilot pointed out that experienced heavy-lift helo pilots can control both the speed and the position of their suspended loads with exquisite precision. This was a man who had extensive experience in one of the early heavy-lift helicopters, the Sikorsky Skycrane. Afterward we started to call our landing approach the “skycrane maneuver.”

One of the biggest challenges we faced had been how the EDL software could determine when the rover had settled onto the Martian surface. On some of the prior legged-lander Mars missions, each of the three legs had a sensor that signaled the lander’s computer that it had touched down. As soon as two of the legs were down, the computer would turn off the engines. This was a crucial element; if the engines were not turned off quickly enough, the rockets could force the lander to bounce and roll over.


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