Erisa Hines Stilley

Alumna aids rover’s daring descent to another world

By Robert C. Jones Jr.

Alumna aids rover’s daring descent to another world

By Robert C. Jones Jr.
Alumna Erisa Hines Stilley, a NASA engineer on the Mars 2020 mission, helped lead the entry, descent, and landing team for the space agency’s most sophisticated robotic rover.

Everything hinged on the next seven minutes. 

That’s how long it would take for NASA’s newest and most sophisticated robotic rover, Perseverance, to land on the surface of Mars after it pierced the planet’s atmosphere traveling nearly 16 times the speed of sound.

So many things could go wrong during those 420 seconds, dubbed “the seven minutes of terror.” And, as NASA engineer Erisa Hines Stilley sat at her workstation on the second floor of Building 230 of the Jet Propulsion Laboratory (JPL) on February 18, she pondered all the potential pitfalls, knowing that any one of them could doom a mission eight years in the making. 

Would the massive supersonic parachute deploy and slow the rover’s descent as it was designed to do? Would the descent stage rockets ignite, slowing the craft to a 2-mile-per-hour speed just before landing? Would the rover land upside down or on a hillside? Stilley and her team would just have to wait and see. “And that was pretty stressful—the waiting,” she said. 

It can take from seven to 22 minutes to get a signal back from Mars at the speed of light. So, by the time Stilley and her team would learn that Perseverance had reached the top of the thin Martian atmosphere, the rover, in reality, would already be alive or dead on the surface. 

“We can’t joystick anything because of that time delay,” Stilley said. And that’s why the car-sized craft, as it neared the end of its seven-month, 293-million-mile journey, would have to perform the hardest and trickiest part of the mission—entry, descent, and landing or EDL—entirely on its own. 

Months of simulations and troubleshooting helped ensure that it would, and just before 4 p.m. eastern time, Stilley, operations lead for the EDL team, and her colleagues heard the two words they had been waiting for: “Touchdown confirmed.” Perseverance, the heaviest and most advanced astrobiology lab ever sent to another world, had landed successfully on Mars, touching down in Jezero Crater, a 28-mile-wide ancient lakebed and river delta filled with steep cliffs and boulders that may hold clues to past life on the red planet.

 

NASA video of rover landing.

Cheers erupted in JPL mission control in Pasadena, California, and a treat—Klondike ice cream bars supplied by Stilley—awaited the EDL team just outside Building 230, where everyone gathered to relax and talk about the historic landing and what lie ahead. 

For Stilley, who fell in love with the space program after a visit to Kennedy Space Center as a child, Perseverance is the most challenging endeavor of her storied NASA career that has seen the University of Miami College of Engineering graduate work on two Mars missions and the Altair lunar lander. 

Harrowing descent 

Tucked inside a protective aeroshell, Perseverance blasted off for Mars on July 30, 2020, from Kennedy Space Center in Florida—the powerful Atlas V rockets eventually separating shortly after launch to allow the ring-shaped cruise stage, equipped with solar arrays, to take over. During the rover’s 300-million-mile trip to another world, NASA engineers periodically woke it to test its onboard systems. 

Seven months later, came the landing sequence. It started with the craft entering the Martian atmosphere at a blistering 12,000 miles per hour. Atmospheric friction slowed the vehicle somewhat. A parachute modeled after the Mars Viking missions of the 1970s slowed its descent even further. “But at that point, it’s still racing toward the surface at about 170 miles per hour,” Stilley said. It was the rocket-powered descent stage, sort of a jetpack with eight engines, that delicately lowered Perseverance to the ground. 

It all sounds seamless but getting to, and landing on, the red planet is difficult, requiring years of planning and testing. By the time Perseverance lifted off from the Kennedy Space Center last July, Stilley had already been working on the project for seven years. 

To appreciate just how difficult the process is, just consider that whether it be a flyby, orbit, or actual landing on the red planet, Earth’s overall success at Mars is only 40 percent. That’s counting missions by the U.S., Russia, and other countries dating back 60 years. 

“So much has to go just right,” Stilley said. “Because the physics of Mars is so drastically different, we cannot test our entry, descent, and landing hardware systems fully on Earth. We rely heavily on end-to-end computer simulations, essentially giving the rover an assumption about what its position and velocity will be at a very specific time. It then uses that information as the seed of knowledge to get to the target on the ground that we specified. The better the answer we provide, the more accurately it can fly to the ground.”

Navigation data obtained from Perseverance during its journey through space helped Stilley and her team provide the rover with the critical information it needed for its landing. 

And what a nail-biter that landing turned out to be. “The day Perseverance landed was the absolute first time that our systems were operating in the environment they were designed for. So, that was terrifying,” Stilley recalled. “There were a couple things that showed up in the telemetry that weren’t necessarily expected. Since then, we’ve combed through the data and explained most of it. But in the moment, we started seeing things we were not expecting. We started to worry that it might be an indication that things were going to go wrong.” 

 

They didn’t, though. Perseverance nailed the landing, employing the same “skycrane” maneuver that its predecessor, Curiosity, used to land in Mars’s Gale Crater in 2012. Yet even while Perseverance benefited from some of the technology used by the previous rover, Stilley’s team was still nervous about the mission. “Each time we land on Mars, that’s one data point, one sample of the atmosphere, one version of a navigation solution. It’s one opportunity for the parachute to deploy successfully. We don’t just hit the repeat button. We continually searched for what we may have missed or overlooked. We ask, ‘How can we test the system better, harder, and push it to the limit.’ ” 

Something Perseverance had that Curiosity didn’t was a new terrain-relative navigation system that allowed the rover to use a camera to identify hazardous topography, compare the images to preloaded maps, and adjust its trajectory as needed during descent. 

Also new: onboard cameras that captured the first-ever video of a Mars landing as well as the first audio recorded on the planet. 

Looking for signs of ancient life 

Perseverance has now been on the surface of the red planet for more than 40 Martian solar days, or sols—one of which is nearly 40 minutes longer than an Earth day. It’s already taken its first test drive and is now exploring Jezero Crater, which, from orbit, displays all the signs of a region that could once have sustained life. While Curiosity explored the habitability of Mars, finding nutrients and energy sources that microbes could have used, Perseverance will seek the very signs of past life, collecting rock and soil samples that a future Mars mission hopes to retrieve and bring back to Earth for analysis. 

The four-pound Ingenuity, an experimental rotorcraft that hitched a ride to Mars aboard Perseverance, will soon fly on the planet, marking the first powered flight on another world. 

Though the Mars 2020 surface team has now taken over, Stilley and the rest of the EDL squad are still keeping busy, poring over Perseverance’s landing data to reconstruct exactly what occurred. “To the extent that it’s important to the next mission, it’s an opportunity to catch mistakes,” she said. 

Working on the mission gave her the chance to expand her skillset. “I was given more responsibility and asked to work in an area in which I didn’t necessarily have a lot of background,” Stilley noted. “I had the opportunity to use a lot of my strengths, and when I had successes, they felt really good. But there were other times where I struggled with whether or not I was doing as good a job as I wanted to.” 

But while Perseverance was her biggest challenge, it was not her favorite NASA project to work on. That distinction belongs to Curiosity, which is still roaming around the Martian landscape more than eight years after it landed there. For that project, Stilley served as rover planner and driver, working closely with scientists to map out strategic routes and then executing those drives. 

“EDL is a cool job,” explained Stilley, “but when you’re driving a rover around on Mars, there’s not a lot that can top that. I’ve been in the very enviable and rare position at JPL of having done both, and I try not to take that for granted.” 

She credits NASA’s commitment to diversity for the success of missions like the Mars 2020 Perseverance rover. “The missions we carry out can be so complex and so challenging that it can be easy to convince ourselves that we’re doing something the right way,” Stilley said. “But if we didn’t have someone with a different perspective or different opinion, we’d miss an opportunity to look at ourselves in a multidimensional mirror and ask, ‘Are we doing the right thing the right way, and the best way?’ And that’s why having diversity across the board is so critically important.”