As you read this, the Mars rover Curiosity — or Mars Science Laboratory, as it’s referred to by NASA — should be about halfway to the Red Planet for a planned two-year mission to study the Martian surface and search for organic compounds. While the launch last November 26 at Cape Canaveral was fairly routine, Curiosity’s landing on Mars — planned for August 6 — will be anything but. NASA and the Jet Propulsion Laboratory (Pasadena, CA) have come up with a way to get their two-metric-ton spacecraft and Mini Cooper-sized rover from orbit to the planet’s surface that seems like a scheme that Brains from the old Thunderbirds TV show might have come up with.
Like a number of previous unmanned Mars missions, Curiosity will enter orbit, descend into the planet’s thin atmosphere, and then deploy a parachute to slow its descent. After that, things get a little weird. The parachute will slow the package down to a still-nerve-rattling 180 mph, then rockets will fire and the lander will deploy the six-wheeled rover on a series of cables, where it will hang in midair until the lander slows its descent to about 2 mph and gently deposits the rover on the Martian surface. The cables will then be detached and the lander will nobly sacrifice itself in a crash landing a safe distance away from the rover.
Steven Sell, who holds the title of Powered Flight Systems Lead at the NASA’s Jet Propulsion Laboratory, has been preparing for just that moment for the past five years, and he’s in charge of making sure those rocket engines do exactly what they’re supposed to during what one engineer described as the “six minutes of terror” that will comprise Curiosity’s landing sequence.
According to Sell, there was no way to use the airbag system that was employed for the previous Mars rover missions back in 2004. “That concept was really great and served us well, however, Curiosity is about five times bigger than the MER rovers [Mars Exploration Rovers Spirit and Opportunity], and the airbag landing concept does not scale well once you get a bigger vehicle,” Sell explains. “Once you realize that you can no longer do airbag systems, you’re left with a few choices — like a legged lander, for example, kind of like what the Phoenix or Viking landers used — but in that case you have to have the rover on top of the spacecraft and then you have a complicated egress situation. It’s hard to get the rover off of the lander. You have to be careful about what’s around you. Legged landers also have a feature where shutting off the engines and contacting the surface requires split-second timing, so, just as that first leg touches the surface, you want to shut off the engines and drop the rest of the legs onto the surface. When you have a metric ton of rover there and potentially another metric ton of spacecraft, it becomes a tall order for the structure to handle.”
To overcome this technological hurdle, JPL engineers studied an earthbound transportation system as a model for Curiosity’s landing procedure. “There’s a heavy-lift helicopter called the Erickson Skycrane,” Sell says. “The helicopter stays above the delivery point and lowers its payload to the ground or the top of a building. So we thought we could use a similar approach and, after we did all the analysis, that’s the concept that won.” The system is called the Skycrane landing system (SLS).
Although Curiosity will be using a 21-meter-diameter parachute — the largest ever been propelled to another planet — the Red Planet’s atmosphere density is only 1% of Earth’s, giving the chute very little to grab onto. But Sell doesn’t sound particularly worried about the mission’s highly unorthodox landing methodology. After all, the mission team has already landed the package four million times — in simulation. Factoring in all conceivable variables, the system has performed perfectly — so much so that Sell and his team have been factoring in the inconceivable, just in case. “We can do crazy things like, let’s assume 25 percent of the atmosphere just isn’t there for some reason. What if our engines are giving us 10 percent less thrust than we thought it would? Or what if one of our radar systems loses its antenna? We can do things like that in simulation and find out how our system responds without the downside of testing real hardware. The Skycrane landing system is incredibly robust. You have to start getting into totally unrealistic scenarios before we start to see the thing fall apart.”
Still curious? Go to mars.jpl.nasa.gov/msl