Rocket Physics Special: The Physics of Perseverance

Header image: Artist’s impression of Perseverance in powered descent under its skycrane (image source: NASA/JPL-Caltech.)

Perseverance successfully landed on Mars in 2021!

Previous installments of Rocket Physics, the Hard Way have discussed the rocket equation and astrodynamics in the context of sending missions to Mars. Perseverance has completed its journey, but the most dangerous phase of its mission was its perilous arrival at Mars. A one-ton rover the size of an SUV hurtled towards the surface at thousands of kilometers per hour, and everything went exactly as planned or the mission could have ended in disaster. It is no surprise that 60% of all missions to Mars have failed.

After launching from Cape Canaveral on July 30, 2020, an Atlas V booster flung Perseverance onto a transfer orbit to Mars. After just over 200 days in space, trailing behind the Emirati Hope and the Chinese Tianwen-1 missions. Nine hours before landing, it made its last trajectory correction maneuver (or TCM) to fine-tune its landing point.

Then, as it fell towards Mars, it will began the final phase of its journey: Entry, Descent, and Landing (EDL). EDL is also known as the ‘seven minutes of terror’, and for good reason. Data transmission rates between Earth and Mars are extremely slow (slower than dial-up internet) and the distance means that radio signals take a long time to travel between planets. As a result, during EDL, mission controllers could do nothing but wait and hope that the automated systems are working correctly.

For an overview of the whole process, you can watch this NASA animation.

Now, let’s go through each stage, one by one, and learn some of the physics behind them.

Approach

On its journey to Mars, Perseverance was kept alive by its ‘cruise stage’, a ring-shaped device equipped with avionics, thrusters, antennae, and solar panels. It powered the onboard systems, performed TCMs, and maintained communication with Earth.

Ten minutes before entry, the cruise stage was jettisoned to later burn up in the atmosphere. What remained was Perseverance, enclosed in its protective aeroshell. Throughout its journey, the spacecraft was kept at a gentle spin of 2 RPM to keep its alignment stable. Now, its attitude thrusters fired to stop the spin and to align the heatshield in the optimal entry orientation.

During cruise, the spacecraft had to be balanced so that it would spin properly. It did this using two 70-kg ballast weights known as Cruise Mass Balance Devices (CMBDs.) Now, these were jettisoned to shift the centre of mass away from the capsule’s axis, unbalancing it in preparation for entry. These weights smashed into Mars at thousands of kilometres per hour, sending tremors through the ground that were picked up by the InSight lander, heralding the arrival of Perseverance.

Entry

On February 18th at an altitude of 130 kilometres, Perseverance slammed into the Martian atmosphere at nearly 20,000 kilometres per hour. Air couldn’t get out of the capsule’s way rapidly enough, so a shock formed that compressed the oncoming gas in front of it, heating it up to about 1,300 °C. The capsule was equipped with an ablative heat shield, which slowly vaporized throughout entry. This carrieed away heat and protected the rover. This was inspired by how the outer layers of meteors burn away while entering the Earth’s atmosphere, keeping the insides cold.

The fireball surrounding the aeroshell caused the air to become ionized and electrically conductive, blocking radio signals. The blockage of radio signals, combined with the lightspeed delay between Earth and Mars, meant that Perseverance needed to handle every step of EDL autonomously.

The angle at which the capsule moved through the atmosphere had to be just right. Otherwise, it could have deviated too far from its course and miss the landing site. Its entry angle could be too shallow, reducing deceleration, and causing it to ‘skip’ off back into space. It could enter too steeply and fall deep into the atmosphere at high speed, causing it to burn up or crash.

Perseverance had some control over its path using a technique known as a lifting entry. The capsule did not enter with the blunt end directly facing the atmosphere – rather, it did do so at a slight angle. This diverted some of the oncoming air off to one side, generating a sideways lift force.

Using the attitude control thrusters to keep the capsule tilted, however, would cost too much fuel. Instead, the capsule was intentionally unbalanced (by jettisoning the CMBDs mentioned earlier) so that it would naturally tilt due to aerodynamic forces. This technique was used as far back as the Apollo program to provide reentry control.

Accordingly, by simply rotating the capsule, Perseverance’s automatic guidance systems could change the angle of its tilt, thereby changing the direction of lift. This allowed it to steer up if it came in too steep, down if it was too shallow, and left or right if it went off-course depending on where it deflected the air.

As the capsule continued to burn its way through the atmosphere, it slowed down considerably. By the end of the entry phase, Perseverance had slowed from nearly 20,000 kilometres per hour to just under 2,000 kilometres per hour.

Meanwhile, back on Earth, we waited with bated breath to find out if the rover survived.

Descent

At three minutes to landing, six more balancing weights were jettisoned to return the capsule’s centre of mass to its centre axis. This caused it to fly straight again, rather than at an angle. This is known as the ‘straighten up and fly right’ maneuver, or SUFR.

Now, a different type of guidance was needed. Mission planners cannot (yet) achieve pinpoint landings – rather, the spacecraft could reasonably be expected to land somewhere within a region known as a landing ellipse. Essentially, the ellipse represents the uncertainty of the landing point.

It is crucial that the ellipse is free of hazards like slopes and boulders that could damage the rover on landing. So, to maximize flexibility and to minimize driving distances to points of scientific interest, mission planners tried to get the ellipse as small as possible by maximizing the spacecraft’s navigational accuracy. Perseverance demonstrated a suite of new technologies that allowed it to have a 50% smaller landing ellipse than previous Mars missions.

The first new technology was a guidance algorithm called a range trigger. Previous Mars missions deployed their parachute the moment they were slow enough, to minimize the risk of hitting the surface too fast. Conversely, Perseverance intelligently calculated its range to the landing site to deploy the chute at just the right time. It did so using a type of sensor called an inertial measurement unit, or IMU. The IMU measured its deceleration, allowing the onboard guidance computer to predict where it was using integral calculus. Once the guidance computer predicted that it was the right distance from the landing site, the range trigger deployed the parachute.

IMUs are used in numerous everyday applications, such as in your smartphone, which uses one to detect its orientation. IMUs are also vital to aircraft and self-driving vehicles for navigation. All of these systems use a family of accuracy-improving algorithms that were originally developed for use in the Apollo missions, demonstrating the value of spin-off technologies to life on Earth. Learn more about spin-off technologies in the Scientific Discovery and Technological Innovation section of our Why Mars, What About Earth? page.

Four minutes after entry, at an altitude of 11 kilometres and a velocity of about 1,600 kilometres per hour, a small explosive charge blasted the parachute open.

The parachute was 21.5 metres in diameter and was designed to catch the thin Martian atmosphere at supersonic speed. Within twenty seconds, it slowed the rover to under 550 kilometres per hour, and the heat shield was jettisoned. Perseverance tasted the frigid Martian air for the first time.

With the heat shield gone, Perseverance could now demonstrate a second new technology: terrain-based navigation. Since there is no GPS on Mars, this system instead used data from a landing radar and a camera called the Landing Vision System (LVS) to determine its position. The landing radar allowed the guidance computer to estimate the capsule’s altitude while the LVS began snapping pictures of the oncoming terrain. It compared these pictures to pre-programmed maps of the area obtained from orbital imagery, achieving a more refined estimation of Perseverance’s location than the IMU alone can provide. These include hazard maps, in which machine learning algorithms identified unsafe areas to avoid.

During the Apollo missions, all this was done manually by astronauts looking out the window; but now, computers are powerful enough to do it on their own. The guidance computer noted how far off-course the rover is, then planed how the retro-rockets needed to fire to maneuver Perseverance to a safe touchdown spot in the next phase: landing.

Landing

Despite the drag from its 21.5-metre parachute, Perseverance was still falling at 300 kilometres per hour – much too fast for a safe landing. One minute from touchdown, at an altitude of two kilometres, Perseverance jettisoned its backshell, which was carried away by the parachute.

Next, the powered descent stage of the flight began. The rover hanged under a skycrane equipped with eight retro-rockets, which fired to slow its descent further. This skycrane was similar to that used by Curiosity in 2012. These retro-rockets work by passing hydrazine fuel over a catalyst bed, which caused it to violently decompose into hot nitrogen and hydrogen gas.

For the final approach to the landing site, the skycrane tilted like a helicopter so that a portion of its retro-rocket thrust was directed sideways. It moved over to the safe landing site selected earlier by the Landing Vision System, then canceled out its horizontal velocity to descend straight down. It continued to descend and slow until it was just twenty metres above the ground, at which point half of the retro-rockets shut down. This reduced the thrust generated by the skycrane, allowing it to descend at a constant rate of three kilometres per hour.

At this point, the skycrane began lowering Perseverance using three 7.5-metre nylon bridles and a communications umbilical. This ensured that the retro-rockets were kept far from the ground, reducing the chance that they would kick up rocks and dust that could damage the rover. Perseverance unlocked and deployed its wheels, bracing for landing.

The skycrane then brought the rover to a gentle touchdown. Explosive bolts fired to cut away the bridles and communications umbilical, then the skycrane flew away to crash-land at a safe distance.

Perseverance survived the fire and fury of EDL, and deployed its instruments to radio home its success.

Surface Operations

Perseverance’s is still in operation on the surface of Mars. Like Curiosity, it is nuclear-powered, meaning that it is no longer dependent on the sun for energy. After all, it was a sun-obscuring dust storm that ultimately killed the Opportunity rover. This also broadens the selection of sites the Perseverance can reach to do science.

NASA has stated that it hopes to accomplish four primary objectives during this time (paraphrased from the Perseverance surface operations website):

• Objective A: Search for geological evidence of environments that could have supported past microbial life on Mars.
• Objective B: Search for geochemical evidence of past life, if there was any.
• Objective C: Drill about 30 core samples from potentially valuable rock and “soil” (regolith) targets and cache them on the Martian surface. NASA, in collaboration with the ESA, is planning to retrieve these samples and return them to Earth for further analysis in the 2020-2030 timeframe.
• Objective D: Test the production of oxygen from the Martian atmosphere, to support In-Situ Resource Utilization (ISRU) for future human missions to Mars using the MOXIE instrument.

Perserverance also carries with it the Ingenuity Mars helicopter, which conducted the first powered aircraft flight in another planet’s atmosphere. It acted as a robotic scout and yielded valuable data for the future aerial exploration of Mars.

Perseverance continued the long heritage of rovers searching for past life on Mars, and paved the way for future human explorers to follow. It may also collect the first core samples of Mars ever to be returned to Earth, which will open up boundless opportunities for greater understanding of the Red Planet.

Godspeed, Perseverance.

Be a part of this!

The Mars Society of Canada offers a professional and credible platform for all space advocates to promote Canada’s participation in Mars science and exploration. By becoming a member of our federal not-for-profit, you provide direct support for our educational, public outreach and analog research efforts. We proudly represent the voice of thousands of Canadians who believe in the profound benefit of Mars exploration, and a multi-planetary future for humanity.

Become a Member

Footnotes and further reading

For more information, visit NASA’s website:

• The EDL technology used by Perseverance: https://mars.nasa.gov/mars2020/mission/technology/
• The EDL timeline and an animated simulation: https://mars.nasa.gov/mars2020/timeline/landing/entry-descent-landing

For detailed technical information on Perseverance’s EDL guidance systems and algorithms, read the following paper, available from the Institute of Electrical and Electronics Engineers (IEEE):

A. Nelessen et al., “Mars 2020 Entry, Descent, and Landing System Overview,” 2019 IEEE Aerospace Conference, Big Sky, MT, USA, 2019, pp. 1-20, doi: 10.1109/AERO.2019.8742167.