Mars 2020 Mission: NASA engineer discusses Martian entry, descent, and landing

mars-2020-mission:-nasa-engineer-discusses-martian-entry,-descent,-and-landing

The Mars 2020 rover mission is set to launch in the coming days. We spoke with a NASA engineer to learn more about the challenges of landing a rover on the Red Planet.

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Image: NASA/JPL

In the coming days, NASA will launch the Mars 2020 Mission to the Red Planet with the Perseverance rover in tow. The mission builds on the legacy laid and the lessons learned from its roving Martian predecessors. The rover is scheduled to enter the Martian atmosphere in February of next year. Needless to say, preparing a craft to travel millions of miles through the cold vacuum of space and successfully land a rover on another planetary body is no cosmic cakewalk. We recently spoke with Keith A. Comeaux, deputy chief engineer on the Mars 2020 Mission, about the challenges involved in landing a craft of the Red Planet.

Comeaux joined the Perseverance team in early 2017 as the project’s deputy chief engineer. Prior to his work on the Perseverance, Comeaux joined JPL’s Curiosity Program in 2006 as part of the rover’s entry, descent, and landing team. He also worked on the Assembly, Test, and Launch Operations (ATLO) team testing the rover as well as the systems involved in delivering the craft to Mars.

“I was flight director on console during [Curiosity’s] entry, descent and landing. That guy that’s jumping up and down in the video that you might have seen. That’s me,” Comeaux said.

(In case you have not seen the video, Comeaux can be seen jumping up and down at the 35: 33 mark.)

On Perseverance, Comeaux focused on supporting the ATLO program, system engineering, and the mission’s testing program. During this time, he helped design tests and simulations to ensure the successful entry, descent, and landing on Mars. These testing parameters are an integral part of mission planning, however, it’s difficult to completely simulate the environment of another planet from millions of miles away.

“We cannot test the entire system here on Earth because Earth just doesn’t present the same environment, both gravity and atmosphere, that we have on Mars. It really is the first time that we’re ever testing the system when we’re doing it for real at Mars,” Comeaux said.

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This photo shows the Mars 2020 Perseverance rover prior to encapsulation inside of its payload fairing.

Image: NASA

Entering the Martian atmosphere: Simulations and approximations

If all goes as planned, the craft will enter the Martian atmosphere at about 12,000 miles per hour. While speed is one factor to consider, the atmosphere is another. The Martian atmosphere is 100 times thinner than Earth’s and this makes slowing down all the more challenging.

NASA’s ASPIRE program focused on creating a specially designed parachute capable of slowing down the craft at these high speeds in the Martian atmosphere. To do so, the team used tests at high altitudes here on Earth and high-speed camera footage to understand the parachute inflation in high-speed and high-altitude environments.

Although the ASPIRE program was designed to closely mimic the entry environment on Mars, these tests do not completely simulate these conditions exactly.

“Even the ASPIRE Program tested in Earth’s atmosphere, which is nitrogen and oxygen. Whereas the density was similar to Mars, it was the wrong gases. At Mars, it’s carbon dioxide. Air versus carbon dioxide can behave differently, especially at supersonic speeds,” Comeaux said.

During the Curiosity mission, the parachute was triggered once the craft was within a certain speed range to ensure the structural integrity of the parachute, according to Comeaux. This time around, NASA is taking a different approach to parachute deployment on Perseverance.

“Now we have a lot more confidence about the conditions at which our parachute can be opened and, rather than triggering on the speed, we’re triggering on our location on Mars relative to where we want to actually land and this gives us a much more precise landing capability on this mission compared to all past missions,” Comeaux said.

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Image: NASA/JPL-Caltech

The Mars 2020 Mission will feature the debut of the Lander Vision System LVS) and Terrain Relative Navigation (TRN) to assist with descent and landing operations. As the parachute slows down the craft, an onboard camera and navigation systems will scan the surface. This system utilizes images stored onboard and compares these maps to the real-time camera information to pinpoint and guide the craft to optimal landing sites.

“We have a camera and a computer onboard, which is taking successive pictures of our landing zones while we’re hanging on the chute, and it’s comparing those pictures to a map that it has stored onboard,” Comeaux said.

The rover is set to land in the Jezero Crater, an area with a “high potential” for detecting evidence of ancient microbial life. Over the years, orbiters have provided detailed images of this region. This area is filled with boulders, cliffs, depressions, and another rugged terrain that could pose a hazard to the descent vehicle system and the rover. This visual system is engineered to avoid these surface structures en route to predetermined landing sites.

“When the time comes to actually release the backshell and the parachute, we fire up the thrusters on the powered descent vehicle, we basically do a maneuver to go target one of those safe areas that it has determined based on pictures that it took,” Comeaux said.

After the descent vehicle has nimbly navigated any barriers in its path, the craft will then be positioned directly above the final landing spot. The vehicle will begin its vertical descent and then slowly lower the rover to the surface using a series of tethers. Once the rover’s wheels come into contact with the Martian surface the craft signals to the overhead vehicle, explained Comeaux.

“Then we cut bridles, we cut the electrical umbilical, and then the descent stage flies away about several hundred meters away to safely crash on the surface and then the rover is ready to go to work,” Comeaux said.

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The Mars 2020 spacecraft will leverage a specially designed parachute and a descent vehicle for landing. The last leg of this descent utilizes an approach technique known as the “skycrane maneuver,” where the rover is gently lowering the craft to the Martian surface.

Image: NASA

Margins for error and learning from past miscalculations

During the entry, descent, and landing there’s tremendous potential for disaster and virtually no margin for error. The craft will perform numerous reconfigurations, onboard guillotines cut cables to enable progress to the next stage, and any recontact between surfaces during separations could damage the rover.

Let’s not forget about the launch itself and the millions of miles of spacefaring between Earth and Martian entry.

Interestingly enough, NASA has made miscalculations with past rover missions without compromising the mission. Comeaux discussed one mathematical error on Curiosity in particular.

“The problem that I like to recall is the fact that we got gravity wrong on Mars, which is stunning,” Comeaux said.

It all comes back to the idea of simulation and modeling from afar. Engineers can simulate these Martian atmospheres in Earth laboratories, but sometimes teams don’t know what they don’t know.

“We run Newton’s Laws to simulate the entire entry, descent and landing sequence. One of the things that you’ve got to tell the simulation is, ‘What is the gravity of Mars?’ So we assumed we knew what it was and we stuck a number in there, but it turns out that gravity is not constant everywhere on a planet,” Comeaux said.

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The surface terrain such as cliffs as well as subsurface elements affect the overall gravity of a given area, explained Comeaux.

“It varies from place to place, depending on whether you’re nearby mountains, or craters, or large mass concentrations that are underground, that you can’t even see. So our knowledge of Mars’ gravity was not super great,” Comeaux said.

At the time, the team did not factor in the exact gravity of Curiosity’s exact landing area.

“We put the best number that we knew for our landing site, but it turns out we’re landing in a 96 mile crater next to a 15,000-foot mountain, and so the local gravity in that spot was a little bit different,” he continued.

In the time since Curiosity, the team has utilized this knowledge of Martian gravity to enhance entry, descent, and landing operations on the latest Mars rover.

“Now in our simulations for Perseverance, we actually vary the gravity. We do these large Monte Carlo simulations where we run that simulation hundreds of thousands of times and we perturb each one of the inputs, just a little bit, every time to see if there’s any major effect. We didn’t used to do that for gravity at Mars and now we do, because of what we learned on Curiosity,” Comeaux said.

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This NASA illustration depicts Perseverance roving the Red Planet.

Image: NASA

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A first-hand look at the rover’s arrival

The Mars 2020 Mission features other instrumentation firsts to provide the space agency with a better understanding of entry, descent, and landing operations and build on these insights. The mission will include a series of camera on the rover and other decent vehicles to monitor the parachute as it deploys, a first-hand glimpse of the surface as the vehicle approaches, as well as a camera on top of the rover to watch as the skycrane flies away after the successful touchdown on Mars.

“That whole landing sequence, we’ve never really seen it with human eyes. We had telemetry, but that data is limited,” Comeaux said.

NASA has taken a surprisingly low-tech approach to these aforementioned onboard optics. 

“It’s a commercial, off the shelf package that we added to our systems. There’s a chance that it may not work simply because it’s commercial components,” Comeaux said.

Other sensors, such as (MEDLI2) located in the craft heat shield, will gather data related to pressure and temperature to help NASA gain a better understanding of flight dynamics as the craft enters the Martian atmosphere, explained to Comeaux.

“This gives us a profile of the pressure through the vertical direction in the atmosphere and helps us better understand the flight dynamics. After landing, we can reconstruct what actually happened and know a little bit more about how to fly through the Martian atmosphere,” Comeaux said.

Whether it’s fine-tuning the gravity modeling or a better understanding of flight dynamics, each mission builds on the lessons learned in previous programs. In fact, as Comeaux pointed out, the Perseverance mission actually utilizes the same guidance algorithm that was originally designed for the Apollo program. This suite of advanced instrumentation will help NASA plan for missions to Mars in the years ahead.

SEE: Photos: NASA’s latest Mars rover, Perseverance, is headed to the Red Planet (TechRepublic) 

The views from the Jezero Crater

For now, the spacecraft is stowed away in its payload fairing. The Mars 2020 Mission launch window opens on July 30. There’s certainly no shortage of challenges standing between the launchpad at Cape Canaveral and the surface of our distance celestial neighbor.

If all goes as planned, in about seven month’s time, the craft is set to land in the Jezero Crater and provide never before seen views of this strange, alien landscape.

“There’s a fair chance that we could land right next to that river delta, and be looking at it when we open our eyes and start taking pictures. Can you imagine the sight? It would be like landing in some national park when we take those first pictures,” Comeaux said.

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