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Landing on Mars is hard. Only about 40 percent of the missions ever sent to Mars – by any space agency – have been successful. Entry, Descent, and Landing – often referred to as “EDL” – is the shortest and most intense phase of the Mars 2020 mission.
It begins when the spacecraft reaches the top of the Martian atmosphere, travelling nearly 12,500 miles per hour (20,000 kilometers per hour), explains NASA . It ends about seven minutes later, with Perseverance stationary on the Martian surface. To safely go from those speeds down to zero, in that short amount of time, while hitting a narrow target on the surface, requires “slamming on the brakes” in a very careful, creative and challenging way.
Hundreds of things have to go just right during this nail-biting drop. What’s more, Perseverance has to handle everything by itself. During the landing, it takes more than 11 minutes to get a radio signal back from Mars, so by the time the mission team hears that the spacecraft has entered the atmosphere, in reality, the rover is already on the ground. So, Perseverance is designed to complete the entire EDL process by itself – autonomously.
As the spacecraft enters the Martian atmosphere, the drag produced drastically slows it down – but these forces also heat it up dramatically. Peak heating occurs about 80 seconds after atmospheric entry, when the temperature at the external surface of the heat shield reaches about 2,370 degrees Fahrenheit (about 1,300 degrees Celsius). Safe in the aeroshell, however, the rover gets up to only about room temperature.
As it begins to descend through the atmosphere, the spacecraft encounters pockets of air that are more or less dense, which can nudge it off course. To compensate, it fires small thrusters on its backshell that adjust its angle and direction of lift. This “guided entry” technique helps the spacecraft stay on the path to its downrange target.
When an expedition reaches Mars, braking is required to enter orbit. Two options are available – rockets or aerocapture. Aerocapture at Mars for human missions was studied in the 20th century. One of the considerations for using aerocapture on crewed missions is a limit on the maximum force experienced by the astronauts. The current scientific consensus is that 5 g, or five times Earth gravity, is the maximum allowable deceleration
The landing will involve the use of blunt body aerodynamics, deployment of a supersonic parachute and powered descent to safely set the rover down on Mars. Part of the team that developed the Chang’e-3 lunar lander and rover, which successfully soft-landed on the Moon’s Mare Imbrium region in late 2013, is working on the mission, though Mars presents different and greater challenges: notably remoteness, more gravity, the presence of a thin atmosphere and less solar energy reaching the planet.
Parachute Deployment
The heat shield slows the spacecraft to under 1,000 miles per hour (1,600 kilometers per hour). At that point, it’s safe to deploy the supersonic parachute. To nail the timing of this critical event, Perseverance uses a new technology – Range Trigger – to calculate its distance to the landing target and open the parachute at the ideal time to hit its mark. The parachute, which is 70.5 feet (21.5 meters) in diameter, deploys about 240 seconds after entry, at an altitude of about 7 miles (11 kilometers) and a velocity of about 940 mph (1,512 kph). The parachute, which is 70.5 feet (21.5 meters) in diameter, deploys about 240 seconds after entry, at an altitude of about 7 miles (11 kilometers) and a velocity of about 940 mph (1,512 kph).
Mason Peck, an associate professor at Cornell University and former NASA chief technologist says, “The atmosphere of Mars is really inconvenient. It’s too thick to ignore—so, aerothermal heating is important—but too rarefied to offer an easy descent by parachute. So, the lower the elevation, the more atmosphere the spacecraft encounters on its way to the surface, and, therefore, it can decelerate more easily,” Peck says. “If one is unsure of one’s decelerator technology, this is the best-odds approach.”
Twenty seconds after parachute deployment, the heat shield separates and drops away. The rover is exposed to the atmosphere of Mars for the first time, and key cameras and instruments can begin to lock onto the fast-approaching surface below. Its landing radar bounces signals of the surface to figure out its altitude. Meanwhile, another new EDL technology – Terrain-Relative Navigation – kicks in.
Using a special camera to quickly identify features on the surface, the rover compares these to an onboard map to determine exactly where it’s heading. Mission team members have mapped in advance the safest areas of the landing zone. If Perseverance can tell that it’s headed for more hazardous terrain, it picks the safest spot it can reach and gets ready for the next dramatic step.
In the thin Martian atmosphere, the parachute is only able to slow the vehicle to about 200 miles per hour (320 kilometers per hour). To get to its safe touchdown speed, Perseverance must cut itself free of the parachute, and ride the rest of the way down using rockets.
Directly above the rover, inside the backshell, is the rocket-powered descent stage. Think of it as a kind of jetpack with eight engines pointed down at the ground. Once it’s about 6,900 feet (2,100 meters) above the surface, the rover separates from the backshell, and fires up the descent stage engines.
The descent stage quickly diverts to one side or the other, to avoid being impacted by the parachute and backshell coming down behind it. The direction of its divert maneuver is determined by the safe target selected by the computer that runs Terrain-Relative Navigation.
Skycrane Maneuver
As the descent stage levels out and slows to its final descent speed of about 1.7 miles per hour (2.7 kilometers per hour), it initiates the “skycrane” maneuver. With about 12 seconds before touchdown, at about 66 feet (20 meters) above the surface, the descent stage lowers the rover on a set of cables about 21 feet (6.4 meters) long. Meanwhile, the rover unstows its mobility system, locking its legs and wheels into landing position.
As soon as the rover senses that its wheels have touched the ground, it quickly cuts the cables connecting it to the descent stage. This frees the descent stage to fly off to make its own uncontrolled landing on the surface, a safe distance away from Perseverance.
“It’s all very hard,” says Peck. “The smallest errors in orbit maneuvers or failure to correctly model the atmosphere can have catastrophic consequences.” To date, about half of Mars missions have failed, although NASA has a very good track record.
Low-Earth Orbit Flight Test of an Inflatable Decelerator
Low-Earth Orbit Flight Test of an Inflatable Decelerator (LOFTID) is a NASA mission to test inflatable reentry systems. It is the first such test of an inflatable decelerator from Earth-orbital speed.
LOFTID was launched on an Atlas V 401 in November 2022 as a secondary payload, along with the JPSS-2 weather satellite. It deployed successfully and landed in the ocean near Hawaii on November 10, 2022, which NASA stated on November 17 was a “huge success”
It inflates to 6 metres (about 20 feet) in diameter. Its total mass is about 2,400 lb. It is formed from 7 inflatable tori (6 wide and one narrow), with a flexible woven silicon carbide black ceramic fabric thermal protection layer on one side
“In addition to achieving its primary objective of surviving the intense dynamic pressure and heating of re-entry, it appears that the aft side of the heat shield – opposite LOFTID’s nose – was well protected from the re-entry environment.”
The LOFTID demonstration took place Nov. 10. About one hour after launch aboard a United Launch Alliance (ULA) Atlas V rocket, the LOFTID inflatable heat shield deployed and began its demonstration. As LOFTID inflated, the Centaur upper stage rocket spun up and released the heat shield, which then began its intense re-entry journey through Earth’s atmosphere. LOFTID splashed down in the Pacific Ocean at about 7 a.m.
The inflatable heat shield technology tested by LOFTID could improve landing capability on worlds with atmospheres, allowing the landing of heavier payloads and safe touchdown at higher altitudes than are currently accessible. The technology has potential applications for missions to Mars, Venus, Saturn’s moon Titan, and return of large payloads from low-Earth orbit.
NASA Technologies
Studying Spacecraft Materials at a Micro Scale
For decades, doctors have used CT scans – a series of X-ray images – to locate and assess injuries inside the brain and body. More recently, NASA’s Ames Research Center in California’s Silicon Valley has utilized CT imaging for an otherworldly application: Researchers have applied this technology to study how spacecraft are structurally and materially impacted by the extreme temperatures and aerodynamic forces of atmospheric entry when preparing to touchdown on a planet’s surface.
Porous Microstructure Analysis (PuMA) – an open-source software package developed by researchers at NASA Ames – uses micro-CT imaging to assess on a miniscule scale how heat and pressure can affect the lightweight composite structures designed to protect a vehicle during entry, descent, and landing. Material behavior at the microscale is important to generate a physics-based understanding of material performance, empowering engineers to design systems with higher reliability. Looking beyond thermal protection materials, PuMA’s development team has also made significant advancements that enable PuMA to analyze a wide array of materials like parachutes, batteries, and meteorites.
PuMA is the 2022 Software of the Year winner. It was developed under the NASA Entry Systems Modeling project, which is funded by the Game Changing Development Program within NASA’s Space Technology Mission Directorate (STMD).
Making Landings Safer, More Precise
On Earth, GPS is used every day for navigating cars and planes. But in space where GPS is not available, a new laser-based technology has been developed to safely and precisely navigate astronauts to their destinations on the surfaces of other worlds. Navigation Doppler Lidar (NDL), developed with support from STMD at NASA’s Langley Research Center in Hampton, Virginia, determines a spacecraft’s exact velocity and position to softly land at the desired location on planetary surfaces. The technology uses lidar (light detection and ranging), which is a remote sensing method using lasers.
Two NDL units will fly on upcoming Commercial Lunar Payload Services lander flights. Invented at Langley, NDL is also part of the Safe & Precise Landing – Integrated Capabilities Evolution (SPLICE) project, which is led by NASA’s Johnson Space Center to develop, demonstrate, and infuse precision landing and hazard avoidance technologies for NASA and for potential commercial spaceflight missions.
NDL is the 2022 Invention of the Year in the commercial category, which recognizes a NASA technology that has been licensed and has resulted in commercial sales. NDL has been licensed to several companies, including Psionic in Hampton, Virginia. Psionic is customizing NDL for use in both terrestrial and space applications.
References and Resources also include:
https://mars.nasa.gov/mars2020/timeline/landing/entry-descent-landing/
