Failure could take many forms next week when NASA’s next-generation Perseverance rover hits the surface of the Red Planet. Here’s what should go right, and how things can quickly go sideways, as Perseverance tries to make its long-awaited landing.
For NASA, the entry, descent and landing (EDL) of Perseverance Thursday, February 18, presents numerous potential points of failure. NASA has saying that “hundreds of things have to go right” for the rover to survive the seven minutes of terror. We cannot take a safe landing for granted: as NASA points out, only “about 40 percent of missions sent to Mars, by any space agency, have been successful.” What’s up!
Simply put, Perseverance will have to go from speeds that reach 12,500 miles per hour (20,000 km / hr) to a walking pace over the course of several minutes. Also, you will have to do this autonomously, as it takes almost 11 minutes for radio signals to reach Earth. To complicate matters, NASA is debuting two new technologies for the mission, both related to the EDL phase and both untested.
The three phases (entry, descent, and landing) present their own unique challenges.
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The rover, located within the descent stage, will be separated from the cruise stage, which, with its solar panels, radios and fuel tanks, will no longer be needed. The spacecraft will then have to orient itself so that its heat shield faces forward, a task that is made possible by the small thrusters located in the rear casing. During entry into the atmosphere, the spacecraft’s heat shield will have to withstand temperatures reaching 2,370 degrees Fahrenheit (1,300 degrees Celsius). A structural failure at this stage would be catastrophic, ending the mission before it has a chance to begin.
In fact, previous missions to the Red Planet have failed right at the door of Mars. In 1999, NASA’s Mars Climate Orbiter entered an orbit that was too low, causing the spacecraft to burn up in the atmosphere. The failure was finally tracked to a conversion error, in which the imperial units of pound-seconds were not converted to the standard metric Newton-seconds. I hate when that happens.
If the descent stage survives atmospheric input, it will still have to deal with air pockets of varying density that could throw it off course. To avoid this problem, a guided entry will be made, in which the descent stage will fire small thrusters to compensate.
The 21.5-meter (70-foot) wide parachute is then deployed. If the parachute is deployed correctly and does not tangle, the descent stage will abruptly decelerate to 1,000 miles per hour (1,600 km / h), which is still incredibly fast (remember, Mars has a super thin atmosphere). The deployment of this supersonic parachute will depend on a new unproven technology called Range trigger, which will calculate the distance to the landing site and activate the parachute to deploy at the right time. This is expected to occur approximately 240 seconds after atmospheric entry, when the descent stage is about 7 miles (11 km) above the surface. Perseverance will say goodbye to your heat shield about 20 seconds after the parachute has deployed, introducing another potential point of failure.
This is a critical stage, with a regrettable historical record. During the failed landing of ESA’s Schiaparelli mission in 2016, the descent stage prematurely ejected the parachute and heat shield, as a result of a software glitch. An on-board computer thought it was only a few feet off the ground, but in reality the descent stage was somewhere between 1.25 and 2.5 miles (2-4 km) above the surface. You can imagine what happened next. The doomed Schiaparelli lander was traveling at about 185 miles per hour (300 km / h) when it crashed into the Martian regolith.
With the heat shield gone and the rover finally exposed to the Martian atmosphere, another new technology, called Navigation relative to terrain. The correct execution of this tool will be essential, since the chosen landing site, a crater, is quite dangerous.
“Jezero is 28 miles wide, but within that extent there are many potential hazards the rover could encounter: hills, rocky fields, dunes, the crater walls itself, to name just a few,” Andrew Johnson, Principal Robotics System Engineer at NASA’s Jet Propulsion Laboratory, he said in a Press release. “So if it lands in one of those hazards, it could be catastrophic for the entire mission.”
This is how NASA describes the new tool, which should allow the landing craft to determine its position relative to the surface with a degree of accuracy close to 130 feet (40 meters) or less.
Navigation relative to terrain allows the rover to make much more accurate estimates of its position relative to the ground during descent. […] Using images from the Mars orbiters, the mission team creates a map of the landing site. The rover stores this map in its new computer “brain,” specifically designed to support terrain-related navigation. As it descends on its parachute, the rover takes pictures of the rapidly approaching surface. To find out where you are going, the rover quickly compares the landmarks it sees in the images with its onboard map. Armed with the knowledge of where it is going, the rover searches another map aboard safe landing zones to find the safest place it can get to. The rover can avoid dangerous terrain up to 335 meters (1,100 feet) in diameter (about the size of three football fields from one end to the other), swerving to safer terrain.
The parachute should slow down the descent stage to about 200 miles per hour (320 km / h), requiring one last step to slow down: powered descent with eight tiny retro rockets. After ditching the parachute, the rover, still attached to its rear shell, will sail to the surface from an initial altitude of 6,900 feet (2,100 meters).
About 12 seconds before landing, and at a very reasonable speed of 1.7 miles per hour (2.7 km / h), it will be time for the overhead crane to maneuver. The rear shell will lower the rover using three 20-meter (66-foot) long cables, during which time the rover’s legs and wheels will move to their landing position. Perseverance, sensing an imminent landing, will loosen the cables and the descent stage will skyrocket and crash hopefully far away.
Many moving parts, including some projectiles, obviously make this an extraordinarily complicated dance. The heat shield, parachute, and rear shell risk damaging or interfering with landing and / or perseverance performance.
Again, history provides another example of a mission failing at this point, namely NASA’s Mars Polar Lander, which, like the Mars Climate Orbiter, died in 1999 (not a great year for NASA) . According to POT, the “most likely cause of the failure was the generation of false signals when the lander’s legs were deployed during descent”, which “falsely indicated that the spacecraft had landed on Mars when in fact it was still descending”, what caused the engines [to] it shut down prematurely, ”causing the lander to fall to the Martian surface.
If something goes wrong during the landing, Swati Mohan will be one of the first to know, as she is the leader of orientation, navigation and control operations for the Mars 2020 mission. Swill be in NASA mission control tracking the rover’s progress and health during landing.
“Real life can always throw curveballs at you. So we’ll be monitoring everything during the cruise phase, checking the camera power, making sure the data flows as expected, ”Mohan said in a Press release. “And once we get the signal from the rover that says ‘I’ve landed and I’m on stable ground,’ we can celebrate.”
The rover, while inspired by Curiosity, has many new features, including a variety of cameras and the ability to look underneath. the surface with ground penetrating radar. The rover will land in Jezero Crater, where it will look for signs of ancient life. If life ever existed on Mars, a place like Jezero Crater, an ancient lake and river delta, would have been an ideal place for microbes to gather. In addition to this important astrobiological work, Perseverance will also study Martian climate and geology, deploying a small helicopter called Ingenio, and collects samples for a future mission.
NASA will have a live broadcast of coverage of the landing, which is scheduled for February 18 at 3:30 p.m. ET (12:30 p.m. PT). We will be watching and waiting for the better.