NASA, Perseverance in epicly successful entry, descent, and landing at Jezero Crater, Mars

by Chris Gebhardt

After years of hard work and labor, NASA succeeded in the most complex landing ever attempted on another planet.

Perseverance safely touchdown at Jezero Crater at an actual time on Earth of 15:43 EST (20:43 UTC) — with confirmation of landing received 11 minutes 22 seconds later through the Madrid, Spain facility of the Deep Space Network at 15:55 EST (20:55 UTC).

Arriving at Mars, and the challenges thereof

Historically, there is now — with Al-Amal, Tianwen-1, Perseverance’s successes — a better than 50% chance of succeeding in placing a spacecraft at Mars, whether that’s inserting it into orbit or landing it on the surface. 

While that is technically true from a complete historical aspect, it is somewhat misleading based on the technology used for the initial Mars attempts throughout the 1960s and 70s and the newer technology which has seen a far greater success rate in the last two decades. 

Since the start of the 21st century, 18 Mars missions, not counting Perseverance (the success of which cannot be determined until landing), have departed (or tried to depart) for Mars with the purpose of either entering orbit or landing on the planet.  Of those, 14 have been successful and four have failed.

That gives an international, 21st century success rate above 75% — far better than the historical average.

But those numbers themselves do not tell the full story.  Even 21st century missions with well-built and tested autonomous technology have failed during critical events like landing, as was the European Space Agency’s Schiaparelli lander’s fate in October 2016.

Overview of Perseverance. (Credit: NASA/JPL)

What’s more, while Perseverance used some of the same systems that helped land the Curiosity rover on Mars in August 2012, Perseverance also used new technology to execute the most extreme landing ever attempted on another planet.

Perseverance had a very small landing target and carried a need to avoid, in real-time, potentially hazardous terrain inside Jezero Crater — which is itself a 45 kilometer wide impact basin.

During the final stages of descent, Perseverance’s onboard landing systems had to navigate steep cliffs, sand dunes, boulder fields, and smaller impact craters to find a safe location to set the rover down. 

The Terrain Relative Navigation and the Range Trigger, both new technologies incorporated into Perseverance’s landing systems, allowed split-second decision-making as well as an ability to abort landing at one location and fly to a different site nearby if needed.

It was challenging indeed, and while there was no guarantee of success, there were many reasons to believe that NASA engineers, technicians, and mission planners had done everything possible to ensure the rover makes it safely to the surface. 

In Perseverance’s favor were not just the overall 21st century odds but NASA’s overall odds of landing something on the Red Planet — especially when it comes to rovers, for which the U.S. space agency has a stunning 100% success rate.

Moreso, the Sojourner, Spirit, Opportunity, and Curiosity rovers have all enjoyed long scientific careers for their designs, with Opportunity outlasting its 90 day planned mission for a final duration of 14 years 138 days.

Nevertheless, landing on Mars is extremely hard for several reasons, not the least of which being that all commands had to be stored ahead of time as there was no way to communicate in real-time based on the distances between Mars and Earth.

For Perseverance’s landing, it took 11 minutes 22 seconds for a signal from the rover to reach Earth.

The Entry, Descent, and Landing profile for Perseverance. (Credit: NASA/JPL)

By the time teams at the Jet Propulsion Lab receive the confirmation signal that Perseverance has entered the Martian atmosphere, the rover had already landed safely five minutes earlier.

Entry, Descent, and Landing — Seven Minutes of Terror

It was now down to Perseverance’s technology and software to execute the landing program as planned and get the rover safely to the surface.

Overall, there were 16 main sequence events in the Entry, Descent, and Landing choreography that had to reduce Perseverance’s velocity from 19,332.58 km/h to 0 km/h in just six minutes.

The first elements of this sequence included separation of the cruise stage, which has provided power, data, and communication capabilities during the cruise from Earth, followed by de-spin, which stopped the vehicle’s four rpm spin that stabilized it during the interplanetary cruise.

The final step prior to atmospheric entry was jettisoning the cruise balance masses to allow the entry capsule to achieve the proper attitudes during landing.

As atmospheric entry began, the vehicle continued accelerating toward Mars as the tenuous atmosphere around the planet was not yet thick enough to begin reducing the craft’s velocity.

When the atmosphere became thick enough, active guidance began as the spacecraft encountered pockets of more-dense and less-dense air within Mars’ atmosphere.  This active guidance period kept the craft on course for the landing site by making minor adjustments to the trajectory as it surfs through the atmosphere.

One minute past peak heating, a heading alignment maneuver engaged to correct any additional crossrange error imparted during atmospheric entry that could not be corrected by active guidance. 

Shortly thereafter, the spacecraft began its “Straighten Up and Fly Right,” or SURF, maneuver — ejecting six more balance masses and changing its overall angle of attack to zero to allow the vehicle to begin a much more vertical than horizontal descent.

Parachute deployment then followed at an altitude of 11.94 kilometers, with deployment initiated by the Range Trigger part of the landing system (more detail of which is provided below).

Next, the jettisoning of the heat shield exposed the rover to the Martian atmosphere and allowed its cameras and instruments to begin collecting data as it descends to the surface.

That data collection was immensely important as the Terrain Relative Navigation system began comparing the live images taken by the landing systems to orbital images of the landing site as it attempted to determine if the area the craft is naturally headed for is safe for landing or if it needed to divert away from a potential hazard such as a cliff, boulder, crater, or sand dune.

Once a landing solution from the Terrain Relative Navigation system was found, the backshell separated and the descent stage’s engines — which provided most of the final velocity decrease — ignited.

All eight of the thrusters on the descent stage were not needed for the actual landing. Half were be deactivated just prior to the release of the rover as the landing system’s sky crane cables lowered Perseverance the rest of the way, with the descent stage hovering between 16 and 20 meters above the Martian surface.

As soon as Perseverance’s wheels sensed contact with the ground, signals were sent to sever the descent cable, allowing the descent stage to angle itself for a flyaway maneuver and crash landing onto the surface of Mars a few kilometers away.

Powered descent. Perseverance’s landing system fine-tunes its trajectory to a safe landing location inside Jezero Crater. (Credit: NASA/JPL)

At touchdown, it was 15:53 local mean solar time at Jezero Crater (not to be confused with the near identical local time on Earth in EST time format).

The entire entry descent and landing sequence, including altitude and velocity numbers, are listed in the chart below.  All times are Eastern Standard Time, which is UTC-5.

Event Time of event Earth-Receive Time of event Altitude / Velocity
Cruise Stage Separation 15:26:53 EST 15:38:15 EST Altitude: 1,569 km
Velocity: 16,728.4 km/h
De-spin 15:28:18 EST 15:39:40 EST Altitude: 1,348 km
Velocity: 17,057.4 km/h
Cruise Balance Mass Ejection 15:28:50 EST 15:40:12 EST Altitude: 1,253 km
Velocity: 17,191.8 km/h
Atmospheric Entry 15:36:53 EST 15:48:15 EST Altitude: 131.0 km
Velocity: 19,202.2 km/h
Guidance Start 15:37:47 EST 15:49:09 EST Altitude: 59.6 km
Velocity: 19,293.9 km/h
Peak Heating 15:38:08 EST 15:49:30 EST Altitude: 21.0 km
Velocity: 10,849.5 km/h
Heading Alignment 15:39:12 EST 15:50:34 EST Altitude: 16.1 km
Velocity: 3,697.6 km/h
SURF period begins 15:40:39 EST 15:52:01 EST Altitude: 13.7 km
Velocity: 1,719.2 km/h
Parachute Deployment 15:40:56 EST 15:52:18 EST Altitude: 11.8 km
Velocity: 1,518.9 km/h
Heat Shield Jettison 15:41:17 EST 15:52:39 EST Altitude: 9.7 km
Velocity: 578.7 km/h
TRN Acquisition 15:42:17 EST 15:53:39 EST Altitude: 4.27 km
Velocity: 322.5 km/s
TRN Landing Solution Reached 15:42:24 EST 15:53:46 EST Altitude: 3.6 km
Velocity: 313.3 km/h
Backshell Separation / Descent Stage Ignition 15:42:42 EST 15:54:04 EST Altitude: 2.1 km
Velocity: 293.4 km/h
Descent Stage Throttle Down 15:43:23 EST 15:54:45 EST Altitude: 0.21 km
Velocity: 6.0 km/h
Rover Separation 15:43:27 EST 15:54:49 EST Altitude: 0.16 km
Velocity: 9.9 m/s
Touchdown / Flyaway 15:43:42 EST 15:55:04 EST Altitude: 0.00 km
Velocity: 0.00 m/s

Range Trigger & Terrain Relative Navigation systems

To execute the most daring landing on the Red Planet yet, Perseverance made use of both a Range Trigger element and a Terrain Relative Navigation system. 

The Range Trigger allowed the vehicle to precisely know its altitude as it headed toward the surface so that its parachute could be deployed at the proper altitude relative to the landing location

After that, the heat shield deployed, and the Terrain Relative Navigation system began acquiring images of the Martian surface to autonomously identify any hazards that must be avoided.

To do this, the Terrain Relative Navigation system was composed of two elements, including the Lander Vision System and Safe Target Selection system.

The entire operational time for the Lander Vision System was just 25 seconds, and it quickly acquired images of the Martian surface to compare them to orbital images of the landing site, with an ability to scrutinize one image per second. 

(Credit: NASA/JPL)

The system’s computers examined the images, searching for contrasting light and dark shadows that would indicate cliffs, craters, boulder fields, and even mountainous peaks. 

Repetition of that process every second provided clarity to the overall system and confidence in an ability to find a safe touch down location.  Once three successful image-to-map comparisons were confirmed, the Lander Vision System switched into Fine Landmark Matching mode, when it will begin searching 125 meter areas of the surface looking for a minimum of 20 matches between day-of collection and orbital, pre-stored images. 

All of those were fed into the Safe Target Selection system, which calculated where the vehicle would land and was be capable of plotting a backup suitable landing location up to 600 meters away from the natural target point where the rover would land without any change imparted from the system.

Mission team members selected and mapped what they believe to be the safest areas to land the rover in relation to its ability to carry out its scientific objectives once on the surface.  However, the two new landing systems will eliminate remaining uncertainty caused by image resolution limits from the orbital surveys obtained.

Phoning home

As with Al-Amal’s arrival at Mars last week, the Deep Space Network facility just outside Madrid, Spain was the primary receiving station for the detailed data telemetry stream coming back to Earth from Perseverance.

The arrays at Goldstone in California serve in a supporting role during landing.

Up until backshell deployment, Perseverance was able to send a constant stream of detailed data directly back to Earth.  However, past that point, the landing site lost its direct line-of-sight to Earth.

To cover that communications gap, NASA’s Mars Reconnaissance Orbiter (MRO) and Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, as well as the Trace Gas Orbiter from the European Space Agency, were within line-of-sight to pick up Perseverance’s signals to collect them and relay them back to Earth.

The primary relay stream was from MRO, which collected the signals, compiled them into 5-second data packets with 16 second latency, and transmitted them to Earth.

MAVEN collected the information and stored the data internally for transmission later while the Trace Gas Orbiter did the same.

In addition to the detailed data stream, Perseverance also sent simpler signals without detailed data in X-band frequency tones and an ultrahigh frequency (UHF) carrier signal.  The UHF signal helped determine the overall functionality of the spacecraft during the landing sequence while the X-band tones revealed completion of key milestones. 

The X-band tones were received through the Deep Space Network dishes while the UHF signal was monitored by the Green Bank Observatory in West Virginia, USA, and the Effelsberg Observatory in Germany.

The use of MRO, MAVEN, and the Trace Gas Orbiter as communication relays and communication hubs is a built-in element of their design, as most of the orbiters sent to Mars are tasked not just with their primary scientific missions but also with serving in this communication relay function for new mission arrivals.

Such international cooperation is usual with Mars planetary arrivals, with the most recent example being the MAVEN spacecraft itself, which served as a communications relay and hub for the United Arab Emirates’ Al-Amal probe which arrived last week.

With the all-critical “I’ve landed safely and await instructions” message received from Perseverance as expected, the rover now begins a planned two Earth year, one Martian year primary scientific mission — though if previous NASA rovers are an indication, Perseverance will in all likelihood far outlast its planned life and will enjoy a prolonged scientific career on the Red Planet.

(Lead image credit: NASA/JPL)

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