Imagine: You’ve designed and built a billion-dollar rover to investigate another planet and launched it into space. It’s made its way through the darkness on a seven-month journey to Mars, and it’s finally arrived at its destination. Now, you just have to get it to the surface and you can start exploring.
The landing isn’t going to be easy, though. Your craft will be traveling at over 12,000 miles per hour when it hits the martian atmosphere – and that atmosphere is so thin that parachutes work differently there than they do on Earth. Fluctuations in wind speeds and the amount of dust in the atmosphere are extremely hard to predict and can affect the landing. And you need to set down your 2,200-pound rover gently enough not to break anything.
Oh, and on top of all that, Mars is so far away that there’s a communication delay of up to 20 minutes, so you can’t control anything in real time. You have to program the craft to land itself, and once descent begins, you can’t do anything to help it. You can only sit and watch as your precious spacecraft goes hurtling toward the planet’s surface, in a period that engineers call the “seven minutes of terror”.
Overseeing this nail-biting horror is the real-life job of Gregorio Villar, a systems engineer on the the Entry, Descent, and Landing (EDL) team for the Perseverance rover at NASA’s Jet Propulsion Laboratory (JPL). He told us about what it takes to land a rover on Mars.
The journey to Mars
The rover travels to Mars safely cocooned in a spacecraft, in a segment of the mission called the cruise. During this journey, engineers keep watch on the spacecraft and make small adjustments to its flight path to ensure it’s going in the right direction.
As the spacecraft approaches Mars, the engineers have their final opportunities to make any tweaks to its speed and direction. One of the most important tasks for the engineers before landing begins is for them to inform the spacecraft of its position relative to the planet as accurately as possible, so that the landing process can start in exactly the right place.
Five days before landing is scheduled, the command is given for the craft to start its entry, descent, and landing sequence and the autonomous systems begin preparations for the landing. “From that point on, most of what the spacecraft is doing is automated,” Villar said.
The lag in communication times between Earth and Mars means that once the command has been given, the engineers can only hope for the best. “We really can’t control the spacecraft,” he said. “The whole landing sequence is actually done by the spacecraft itself. We don’t touch it at all.”
The EDL system used by Perseverance is similar to the one used for landing NASA’s previous Mars rover, Curiosity, which touched down in 2012. And that gives the team confidence that the system will work again. But the nature of the mission means it’s impossible to test the landing process from start to finish.
“Really, the first time that we practice landing on Mars is at Mars,” he said. “The only real sample point we have is the Curiosity landing, because we know that worked.”
Navigating the atmosphere
Once the spacecraft arrives at the planet, it no longer needs the solar panels, thrusters, and fuel tanks that carried it on its journey. So about 10 minutes before it reaches the atmosphere, it jettisons its cruise stage.
“Now, all you have left is our aeroshell,” Villar said. “It’s basically a capsule that contains the rover.”
This aeroshell protects the rover as it enters the atmosphere at around 12,000 mph, as the friction creates enormously high temperatures. The capsule’s heat shield reaches temperatures of up to 2,370 degrees Fahrenheit (about 1,300 degrees Celsius), but the shielding is so effective that the rover inside stays at a comfortable room temperature.
When the spacecraft’s speed has slowed to around 900 mph, at an altitude of about 7 miles, it deploys a parachute. “Nine-hundred mph is still very, very fast,” Villar said. “So this parachute helps us slow down even more.”
The supersonic parachute is 70 feet in diameter and features some upgrades from the one used by Curiosity. “Our parachute is stronger than what we had on Curiosity. It’s basically the same size, it’s just made of stronger material,” Villar explained.
Approaching the ground
As the spacecraft approaches the surface, another of its new systems comes into play. What is called Terrain Relative Navigation software uses a camera to snap images of the ground below, and correlates information about the terrain with maps it has stored on its onboard computer. Using this information, it can locate a safe area to land, one free from any hazards like large rocks.
Previous missions would try to land “in flat areas – what we call ‘parking lots,’” Villar said. “Really flat, safe deserts on Mars.” These areas present the fewest hazards that would cause problems when landing, but they weren’t the most scientifically interesting areas to explore. Scientists often want to explore rocky areas, and the new landing system allows Perseverance to land in the areas that scientists actually want to visit.
By now, the parachute has slowed the spacecraft down to around 200 mph, but it still needs to slow more to land safely. So the parachute is cut free, and an eight-engine jetpack kicks in. The jetpack’s engines point down toward the ground and fire to slow the descent.
“This jetpack and the rover basically fall from the capsule,” Villar explained, “And when it’s about 60 feet from the ground, the jetpack lowers the rover on cables, slowly. When the rover softly touches the ground, the cables are cut from the rover, and the jetpack flies away.”
This process, called the skycrane maneuver, was a new invention for the landing of Curiosity. As complex as it is, this was an important step forward from previous Mars missions, which used airbags to cushion the landings of rovers like Spirit and Opportunity. These rovers were smaller – and, most importantly, less massive – than Curiosity or Perseverance. “We were able to use airbags because the structure of the airbags was able to tolerate these kind of hard landings with a smaller mass inside,” Villar said. “Curiosity was just way too massive. Airbags would not be able to support its structure bouncing around.”
The other issue with airbags was the accuracy of the landing location. “You drop these airbags, and it bounces around. You’re basically at the whim of wind or velocity in whatever direction you’re going. So where you’re landing is less precise,” Villar said. “With a soft landing [like Perseverance will make], you’re now more able to land precisely where you want to go.”
A new view of the landing process
One exciting, albeit experimental, upgrade made to the Perseverance landing system is a set of cameras that have been fitted to record the landing as it happens. Curiosity had just one EDL camera, which was on the rover pointing down at the ground, but Perseverance has cameras all over the EDL system.
There will be a camera attached to the jetpack looking down, which will hopefully capture the skycrane maneuver as it happens, and there will be a camera on the rover pointed up, to capture the view of the jetpack. Villar said his personal favorites are the three cameras on the top of the capsule, which will record the deployment of a parachute on Mars for the first time.
These EDL cameras are nonessential to the mission, and are fitted on what is called a “best effort basis” – meaning, essentially, that the engineers hope they will work but they aren’t sure – but they could potentially provide incredibly close-up views of the rover landing. NASA plans to publicly share the footage they capture, so we soon may be able to watch the landing for ourselves.
“They’re a cherry on top for the mission,” Villar said.
Fingers crossed for a smooth landing
When landing day arrives on February 18, it won’t be Villar’s first experience of an anxiety-inducing landing. He was also involved in the Curiosity landing in 2012, but he was newer to the project and to JPL then. Having spent seven-and-a-half years since working on the EDL system for Perseverance, he says he’s even more nervous now than he was last time.
“Everything is so much more complicated than I thought back in Curiosity days,” he said. “It’s nerve-wracking because it’s such a complicated system where anything could go wrong.” But he praised the dedication and hard work of his co-workers, and said they were confident that they had done everything they possibly could to give the mission the best chance of success.
“The main thing is that we’re all comfortable with how much we’ve done, and that we’ve done everything we think we can do up to that point,” he reflected. “Whether we have a good day or a bad day, we did the best that we can.”
